Tingzhen Ming

Wei Li

Qingchun Yuan

Philip Davies

Renaud De Richter

Chong Peng

Qihong Deng

Yanping Yuan

Sylvain Caillol

Nan Zhou

Perspectives on removal of atmospheric methane

Keywords: methane removal, photocatalysis, chlorine atoms, hydroxyl radicals, zeolites, methane mitigation, methane remediation, enhanced atmospheric methane oxidation Highlights

 Action on methane is the easiest and faster way to slow down global warming  At COP26 105 countries signed 'the methane pledge' for -30% CH 4 emissions by 2030  CH 4 removal (remediation) is complementary with reducing CH 4 emissions (mitigation)  Atmospheric CH 4 removal methods are described and early costs estimates provided  Co-benefits include: human health, agriculture, economy, and ozone layer recovery.

Introduction A spotlight on methane versus carbon dioxide

While the atmospheric stock of carbon dioxide (CO 2 ) in the atmosphere has increased by about 50% since preindustrial time (417 vs 278 ppm), that of methane (CH 4 ) has more than doubled (1879 vs 722 ppb) [START_REF]Global Monitoring Laboratory -Eath System Research laboratories[END_REF]. Although the importance of CH 4 as a greenhouse gas has been known about for many years, as reflected by the Kyoto protocol of 1997 [START_REF] Zhou | United Nations Framework Convention on Climate Change -The Kyoto protocol[END_REF], until recently public attention focused mainly on CO 2 . Most mitigation and remediation proposals targeted CO 2 .

Recently, however, more attention is being given to CH 4 . Thus, in November 2021, at the UN Climate Change Conference (COP 26) held in Glasgow, 105 participating countries signed "The Global Methane Pledge" committing to a 30% reduction in emissions from oil and gas by 2030 relative to 2020 [3]. Moreover, the new contribution of Working Group 1 to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC WR1 AR6) released on August 2021 [START_REF] Masson-Delmotte | The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF] highlights the need to quickly reduce global CH 4 emissions to slow warming [START_REF]Reduce methane or face climate catastrophe, scientists warn[END_REF][START_REF] Cnn | Scientists say this invisible gas could seal our fate on climate change[END_REF] and "buy us time" [START_REF]Climate change: Curbing methane emissions will 'buy us time[END_REF].

Previously, on July 2021, a U.S.-Russia Joint Statement Addressing the Climate Challenge expressed their intent to work together bilaterally to address climate change, including emissions reductions from non-CO 2 greenhouse gases (GHGs), including CH 4 [START_REF]Russia Joint Statement Addressing the Climate Challenge[END_REF]. Methane featured in the agenda of the recent U.S. Leaders' Climate Summit. Meanwhile, the Chinese 14th Five-Year Plan presented in March 2021 was expanded to include CH 4 and other non-CO 2 gases [START_REF] You | China to Reduce Emissions of Methane, a Powerful Greenhouse Gas[END_REF], and China's biggest gas and oil producer is targeting a 50% reduction in CH 4 emission intensity by 2025 [START_REF] Reuters | China's CNPC targets 50% slash in methane emission intensity by 2025[END_REF]. The United Nations Environment Program (UNEP) and the Climate and Clean Air Coalition in their "Global Methane Assessment" [START_REF]Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions[END_REF] as well as the International Energy Agency [START_REF]IEA calls on companies, governments and regulators[END_REF] are calling for urgent action to cut CH 4 emissions, and scientists and non-governmental organisations (NGOs) are calling for atmospheric CH 4 removal [START_REF]Scientists' Statement on Lowering Atmospheric Methane Concentrations[END_REF] [START_REF]Declaration on Reducing Atmospheric Methane[END_REF].

These calls and decisions are timely. In the absence of further climate action, by the end of the century, global-mean warming due only to CH 4 emissions could contribute to about 0.9°C (±0.2°C), compared to a warming of about 0.5°C (±0.1°C) currently, due to historical CH 4 emissions [START_REF] Ocko | Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming[END_REF], and compared to the Paris agreement target of less than 2°C global warming including all GHGs.

The atmospheric concentration of CH 4 is rising

In 2020, despite Covid-19 shutdowns, the annual increase in atmospheric CH 4 was the largest recorded since 1983, when systematic measurements began [START_REF]Despite pandemic shutdowns, carbon dioxide and methane surged in[END_REF]. Since the preindustrial era, tropospheric concentrations of CO 2 and CH 4 have increased by 47% and 167%, respectively [START_REF]Global Monitoring Laboratory -Eath System Research laboratories[END_REF]. Since 2007, tropospheric CH 4 has been rising rapidly, with an average annual growth rate of 9.3 parts per billion (ppb) (approximately 0.4% year -1 ) between 2014 and 2019 [START_REF]Global Monitoring Laboratory -Eath System Research laboratories[END_REF]. Over a longer timescale, CH 4 concentrations have multiplied 3.28 times from a minimum of 570 ppb, reached 5,000 years ago [START_REF] Ruddiman | plagues, and petroleum: how humans took control of climate[END_REF]. The IPCC [START_REF] Stocker | The Physical Science Basis, Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF] predicts that, over the next 10 to 20 years, CH 4

and CO 2 will have similar global warming impacts, as measured by heat absorbed (global warming potential, GWP) and temperature rise (Global Temperature change Potential, GTP). See Figure 1. Besides slowing global warming, CH 4 removal also can help protect the ozone layer, because one of the indirect effects of the rising concentrations of CH 4 is the increasing amounts of water in the stratosphere, which participates in ozone layer depletion [START_REF] Revell | The role of methane in projections of 21st century stratospheric water vapour[END_REF].

Whereas the natural capacity of the atmosphere to remove GHGs remains roughly constant [START_REF] Rigby | Role of atmospheric oxidation in recent methane growth[END_REF][START_REF] Turner | Ambiguity in the causes for decadal trends in atmospheric methane and hydroxyl[END_REF], CH 4 emissions from the major natural and anthropogenic sources are increasing [START_REF]Despite pandemic shutdowns, carbon dioxide and methane surged in[END_REF]. The biogenic sources include tropical wetlands [START_REF] O'connor | Possible role of wetlands, permafrost, and methane hydrates in the methane cycle under future climate change: A review[END_REF], lakes [START_REF] Wik | Climate-sensitive northern lakes and ponds are critical components of methane release[END_REF],

ponds, hydroelectric reservoirs [START_REF] Beaulieu | Eutrophication will increase methane emissions from lakes and impoundments during the 21st century[END_REF], and rivers [START_REF] Wilkinson | Continuous seasonal river ebullition measurements linked to sediment methane formation[END_REF]; and human-made sources include fossil fuels (coal mines, oil and gas wells); agriculture (livestock and rice cultivation); landfills; and some of the biomass burning due to intentional wildfires [START_REF] Saunois | The global methane budget 2000-2017[END_REF].

CH 4 emissions from the fossil fuel industry are uncertain but recently shown to be approximately 40% higher than previously estimated [START_REF] Hmiel | Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions[END_REF]. They include venting, flaring, and fugitive emissions of global diesel and gasoline [START_REF] Pieprzyk | Influence of methane emissions on the GHG emissions of fossil fuels[END_REF], as well as leaks in gas distribution and use [START_REF] Schwietzke | Upward revision of global fossil fuel methane emissions based on isotope database[END_REF].

Due to warming oceans and surface air temperatures, several scientists perceive a risk of massive release of CH 4 by destabilization of seafloor methane-hydrates [START_REF] Andreassen | Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor[END_REF][START_REF] Cardoso | Increased methane emissions from deep osmotic and buoyant convection beneath submarine seeps as climate warms[END_REF][START_REF] Archer | Ocean methane hydrates as a slow tipping point in the global carbon cycle[END_REF] and Arctic permafrost thaw [START_REF] Schuur | Climate change and the permafrost carbon feedback[END_REF][START_REF] Holgerson | Large contribution to inland water CO 2 and CH 4 emissions from very small ponds[END_REF]. Some observed CH 4 fluxes from 5 to 24 grams per square meter per day (gm -2 day -1 ) were observed in 2013 [START_REF] Shakhova | The East Siberian Arctic Shelf: towards further assessment of permafrostrelated methane fluxes and role of sea ice[END_REF].

However, some consider such risks to be lower, mainly thanks to microbial CH 4 consumption by methanotrophs [START_REF] Cooper | Limited contribution of permafrost carbon to methane release from thawing peatlands[END_REF][START_REF] Knoblauch | Methane production as key to the greenhouse gas budget of thawing permafrost[END_REF][START_REF] Singleton | Methanotrophy across a natural permafrost thaw environment[END_REF][START_REF] Ruppel | The interaction of climate change and methane hydrates[END_REF] whose populations rise with the increasing abundance of CH 4 . Meanwhile, increasing CH 4 emissions due to warming of wetlands and landfills, eutrophication of lakes [START_REF] Beaulieu | Eutrophication will increase methane emissions from lakes and impoundments during the 21st century[END_REF], and fossil fuel extraction [START_REF] Dalsøren | Discrepancy between simulated and observed ethane and propane levels explained by underestimated fossil emissions[END_REF] adds revenue [START_REF]Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions[END_REF].

The rapid rise of atmospheric CH 4 concentration requires action to try to limit global warming well below 2°C as targeted by the COP21 Paris Agreement, bearing in mind that the GWP of CH 4 is nearly 28 times higher than that of CO 2 on a 100-year basis, and 84 times higher on a 20-year basis [START_REF] Stocker | The Physical Science Basis, Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF]. Moreover, because CH 4 depletes the atmospheric reservoir of hydroxyl radicals responsible for removing CH 4 itself, large additions of CH 4 to the atmosphere extends the lifetime and GWP of the CH 4 already in the atmosphere [START_REF] Holmes | Future methane, hydroxyl, and their uncertainties: key climate and emission parameters for future predictions[END_REF][START_REF] Voulgarakis | Analysis of present day and future OH and methane lifetime in the ACCMIP simulations[END_REF].

In the next section the importance of focusing in CH 4 reduction is explained. The following sections review the infrastructure needed for large-scale CH 4 removal with the methods and strategies proposed so far, summarizing the advantages and disadvantages of those methods, and their potential costs compared to some CO 2 removal technologies. Then some expected co-benefits are discussed, before the concluding remarks.

2.

Actions are needed against methane emissions and its atmospheric stock Methods have been proposed to limit and reduce CH 4 emissions from several anthropogenic sources [START_REF] Stolaroff | Review of methane mitigation technologies with application to rapid release of methane from the Arctic[END_REF][START_REF] Nisbet | Methane mitigation: methods to reduce emissions, on the path to the Paris agreement[END_REF][START_REF] Johannisson | A Structured Approach for the Mitigation of Natural Methane Emissions-Lessons Learned from Anthropogenic Emissions[END_REF]: examples include better management of landfill (separate biodegradable waste) and coal, oil and gas fields (reduce leakage, recovering instead of flaring, capturing instead of venting, etc.) [START_REF]Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions[END_REF][START_REF] Ocko | Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming[END_REF], as well as dietary changes to reduce emissions from livestock populations [START_REF] Kebreab | Strategies to Reduce Methane Emissions from Enteric and Lagoon Sources[END_REF]. But few of these can be adapted to natural emissions, which are diffuse over large areas of thousands to millions of square kilometers and/or partly inaccessible. Unfortunately, most CH 4

emissions come from such diffuse natural biogenic sources [START_REF] Nisbet | Methane mitigation: methods to reduce emissions, on the path to the Paris agreement[END_REF], which by top-down estimations represent about 40% of total emissions [START_REF] Saunois | The global methane budget 2000-2017[END_REF]. Biogenic emissions come mainly from wetlands (31%) and a smaller part (7%) from freshwater systems, oceans, estuaries, permafrost, termites, wild animals and vegetation. Other estimates of the natural CH 4 emissions are of about 50% [START_REF] Rosentreter | Half of global methane emissions come from highly variable aquatic ecosystem sources[END_REF], as well as 50% by bottom-up estimates [START_REF] Saunois | The global methane budget 2000-2017[END_REF].

Removal or remediation methods seem even more technologically challenging, because they need to deplete CH 4 already released to the atmosphere, where it has been diluted about 0.5 million times in an air volume of some 1.4 billion cubic kilometers. In addition, the relatively short life cycle of atmospheric CH 4 (about 10 years) means that removal would have to be ongoing to reduce the concentration by a target amount. This contrasts with CO 2 , for which a one-off removal achieves an almost permanent reduction. Further, CH 4 molecules are about 200 times scarcer in the atmosphere than CO 2 molecules. Unlike CO 2 , however, CH 4 and other GHGs can be removed by in-situ oxidation, to products with no GWP or with much lower GWP than CH 4 ; for instance, CO 2 is obtained from CH 4 and nitrogen and oxygen from nitrous oxide (N 2 O) without the need of capture, separation, or storage. The oxidation reaction is exothermic and exergonic, and as such requires no minimum energy input once the activation energy is overcome. This differs from CO 2 , which requires at least 18 kJ mol -1 for its separation from the atmosphere [START_REF] Wilcox | Assessment of reasonable opportunities for direct air capture[END_REF], and more energy still if CO 2 is to be reduced to carbon or organic compounds such as industrial polymers [START_REF] Liu | Highly Efficient Synthesis of Functionalizable Polymers from a CO2/1, 3-Butadiene-Derived Lactone[END_REF].

Removal of CH 4 may be considered an acceleration of the natural oxidation processes, as once in the atmosphere it finally ends-up as CO 2 .

3.

Infrastructure needed for treating large volumes of air

As CH 4 is a well-mixed GHG, and its life expectancy in the troposphere is approximately 10 years, in order to have an impact on global warming, some authors have proposed that at least one-tenth of the atmosphere has to be processed every year, to compete with natural sinks [START_REF] Stolaroff | Review of methane mitigation technologies with application to rapid release of methane from the Arctic[END_REF]. In reality, any process of a portfolio of technologies allowing the atmospheric CH 4 concentration to start decreasing (while the imbalance between sources and sinks is currently increasing) will reduce its direct global warming impacts, as well as its indirect impacts (due to tropospheric ozone generation) on heath, food production, and primary productivity [START_REF] Ainsworth | The effects of tropospheric ozone on net primary productivity and implications for climate change[END_REF][START_REF] Feng | Economic losses due to ozone impacts on human health, forest productivity and crop yield across China[END_REF]. proposed to directly oxidize it in-situ by a variety of possible methods, including bioinspired aqueous-phase catalytic oxidation, bio-reactors containing methanogens, enzymatic systems, and catalysts made of precious metals. But these authors did not provide any specific details about how to process the very large volume of air in the atmosphere, concluding that these ideas were speculative.

Lockley [START_REF] Lockley | Comment on "Review of Methane Mitigation Technologies with Application to Rapid Release of Methane from the Arctic[END_REF] proposed several additional mitigation or removal techniques, such as: ignition of CH 4 at point sources, lake sealing with impermeable covers or with nonbiodegradable foaming agents, ducting CH 4 bubble streams from underwater sources, and others. Again, in the absence of specific details, these ideas seem speculative.

Later, researchers proposed adding to the atmosphere CH 4 depleting agents such as chlorine (Cl) atoms generated by iron salt aerosols in the air [START_REF] Stolaroff | Review of methane mitigation technologies with application to rapid release of methane from the Arctic[END_REF][START_REF] Oeste | Climate engineering by mimicking natural dust climate control: the iron salt aerosol method[END_REF] or using human made infrastructure devoted to another use. These could include:

 solar updraft chimneys (SUT) [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF], which produce CO 2 -free renewable electricity and each unit can process on the order of 6,000 cubic kilometers (km 3 ) of air per year;

 direct air capture (DAC) systems [START_REF] Keith | A process for capturing CO2 from the atmosphere[END_REF] that are being developed to capture CO 2 from air [START_REF] De Richter | Iron Salt Aerosol and Photocatalytic Solar Chimneys: Two innovative breakthrough technologies to remove greenhouse gases[END_REF][START_REF] Jackson | Methane removal and atmospheric restoration[END_REF].

Existing infrastructure could be used for CH 4 removal. For example, titanium dioxide in self-cleaning windows or other photocatalytic coatings such as paintings on buildings could contribute to CH 4 removal and attract carbon credits and certificates, thus helping to finance future removal infrastructure. Aircrafts, wind turbines, or other structures already in contact with large quantities of air might also provide reaction surfaces to oxidize CH 4 .

Principal methods for enhancing atmospheric methane removal

Enhancing the main natural CH 4 sinks

Currently, in the troposphere, the principal natural CH 4 sinks are the hydroxyl radicals (which remove nearly 90% of the CH 4 ) [START_REF] Naik | Preindustrial to present-day changes in tropospheric hydroxyl radical and methane lifetime from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP)[END_REF], chlorine atoms (which remove about 2.5% of the CH 4 ) [START_REF] Hossaini | A global model of tropospheric chlorine chemistry: Organic versus inorganic sources and impact on methane oxidation[END_REF], minerals in soils and dust [START_REF] George | Heterogeneous photochemistry in the atmosphere[END_REF], soil microbes, plants and trees.

Enhancing those natural sinks can be a strategy to increase atmospheric CH 4 removal.

Enhancing the hydroxyl radical °OH and targeting in majority point sources

Atmospheric natural self-cleansing and volatile organic compound (VOC) removal is mainly due to hydroxyl radicals [START_REF] Rohrer | Maximum efficiency in the hydroxyl-radical-based self-cleansing of the troposphere[END_REF]. Hydroxyl radical generators, as well as ozone generators, are commercially available, and indoor VOC pollution can be efficiently controlled by short-wave ultraviolet (UVC) light in closed systems [START_REF] Johnson | Gas-phase advanced oxidation for effective, efficient in situ control of pollution[END_REF]. Some coauthors of this article are working on methods to enhance the °OH sink of CH 4 and reduce its lifetime, although still more research is needed to be able to provide cost estimates and quantitative estimates of efficacy [START_REF] Ming | Atmospheric methane removal by enhancing the hydroxyl radical sink[END_REF]. Still, the increasing efficiency and lifetime and the decreasing costs of ultra-violet light-emitting diodes (UV-LEDs) are promising. Research conducted at the University of Copenhagen has led to the creation of start-ups such as Infuser and AirLabs, which already apply this technology to point sources [START_REF]Strartups[END_REF]. Several other possible strategies to generate °OH radicals and apply them in the open atmosphere might be possible, based on the numerous and complex mechanisms by which they are produced [START_REF] Zhao | Inter-model comparison of global hydroxyl radical (OH) distributions and their impact on atmospheric methane over the 2000-2016 period[END_REF]. Knowing the intensity of sunlight UV, the °OH radical concentration can be predicted [START_REF] Wennberg | Radicals follow the sun[END_REF]. But care has to be taken not to expose human beings, animals, and plants to dangerous UV radiation and to ozone.

Enhancing the natural chlorine sink of CH 4 -at the molecular level

In 2017, some of the coauthors of this article proposed to deplete the atmospheric CH 4 directly in the lower troposphere with Cl atoms [START_REF] Oeste | Climate engineering by mimicking natural dust climate control: the iron salt aerosol method[END_REF][START_REF] De Richter | Iron Salt Aerosol and Photocatalytic Solar Chimneys: Two innovative breakthrough technologies to remove greenhouse gases[END_REF], mainly but not exclusively under the marine boundary layer. Recent scientific research from 2015-2017 proved that Cl atoms can be generated in large amounts from the sodium chloride (NaCl) content of natural sea-spray aerosols, thanks to an iron(III)/iron(II) sunlight photo-catalyzed reaction [START_REF] Wittmer | Iron (III)-induced activation of chloride from artificial sea-salt aerosol[END_REF][START_REF] Wittmer | Photochemical activation of chlorine by iron-oxide aerosol[END_REF]. Acidity (pH<3) found over coastal areas is naturally generated from NaCl by acid displacement with biogenic sulfate and nitrate [START_REF] Fong | Effect of relative humidity on HCl formation from the reaction of H 2 SO 4 and HNO 3 with NaCl particles[END_REF]. However, it also can be enhanced by anthropogenic pollution due to combustion sources, where nitrogen oxides and sulfur oxides are further oxidized in the atmosphere into nitric and sulfuric acids that react with sea salt to generate hydrochloric acid and sodium nitrate and sodium sulfate salts. Figure 2 illustrates a possible way by which CH 4 is already being removed by enhancing its chlorine sink.

Over polluted coastal areas, the chlorine sink destroys up to 11% of CH 4 [START_REF] Sommariva | Multiphase halogen chemistry in the tropical Atlantic Ocean[END_REF]. It is worth noting that authors in favor of this method do not propose enhancing acid air pollution, and do not target the enhancement of the Cl atom generation over populated areas or coasts [START_REF] Oeste | Climate engineering by mimicking natural dust climate control: the iron salt aerosol method[END_REF][START_REF] Ming | A nature-based negative emissions technology able to remove atmospheric methane and other greenhouse gases[END_REF]. As CH 4 is a well-mixed GHG, it can be removed anywhere at atmospheric concentrations (about 1.9 ppm), consequently they propose to do so in remote unpopulated areas. They also plan to address point sources of CH 4 , where it is more concentrated, to deplete it before it becomes diluted and mixed in the global atmosphere, and in this case the Cl atom generation will be carried out in closed systems such as the existing ventilation systems of coal mines, with very low risks.

Enhancing the natural mineral sink of CH 4 -on surfaces

In 2017, it was proposed to perform CH 4 depletion by large scale photocatalysis [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF] using solar updraft chimneys (SUT), which are structures able to process very large volumes of air, as illustrated by Figure 3. As an example, a hypothetical 400-MW SUT would process 38,000 km 3 of air yearly [START_REF] De Richter | An unusual renewable energy device for atmospheric removal of GHGs[END_REF][START_REF] Schindelin | Entwurf eines 1500m hohen Turms eines Solar-Aufwindkraftwerkes Parameteruntersuchung zur Geometrieoptimierung (Design of a 1500m high tower of a Solar Updraft Chimney: Parameter study of the geometry optimization[END_REF].

The proposed photocatalyst for CH 4 depletion is a semiconductor metal oxide: a zinc oxide (ZnO) doped with 0.1% silver (Ag) [START_REF] Chen | Photocatalytic oxidation of methane over silver decorated zinc oxide nanocatalysts[END_REF]. It is expected to remove 50% of the CH 4 from the air processed thanks to sunlight at ambient temperatures [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF]. Then in 2018, DAC devices were proposed to remove atmospheric CH 4 alongside CO 2 , using the same ZnO-0.1%Ag photocatalyst illustrated by Figure 4. The enormous energy costs of fan-driven DAC make it less attractive than passive generation of large airflows by the SUT devices proposed above. Nonetheless, it is anticipated that the DAC technology will develop rapidly to remove CO 2 from the atmosphere. Once DAC plants exist, profiting from this existing infrastructure by upgrading it to also remove CH 4 could enhance the capture yields in terms of CO 2 -eq by 20%. Depending on the CH 4 oxidation yields and costs, it could be advantageous, especially since the removal of other GHGs also seems feasible [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF][START_REF] Ming | Fighting global warming by greenhouse gas removal: destroying atmospheric nitrous oxide thanks to synergies between two breakthrough technologies[END_REF][START_REF] De Richter | Fighting global warming by GHG removal: destroying CFCs and HCFCs in solar-wind power plant hybrids producing renewable energy with no-intermittency[END_REF] and does not introduce a pressure drop requiring more energy for the fans, and no additional CO 2 capture capacity would be required [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF].

In 2019, other scientists proposed the use of DAC devices dedicated to CH 4 capture by zeolites and then removal of CH 4 using a thermal catalyst [START_REF] Jackson | Methane removal and atmospheric restoration[END_REF]. No published data was found on the pressure drop across the zeolite, making it difficult to know if the DAC device might be attractive to capture CO 2 also. Lackner [START_REF] Lackner | Practical constraints on atmospheric methane removal[END_REF] commented on this proposal by pointing to the extreme dilution of CH 4 in air (200 times more dilute than CO 2 ) which (based on the Sherwood law) may cause a three-order of magnitude energy penalty in using fan-driven DAC systemsthus suggesting that approaches should take advantage of natural air flow and use passive methods. Both the comment and the response [START_REF] Jackson | Reply to: Practical constraints on atmospheric methane removal[END_REF] mention that it would be more interesting to remove N 2 O, the third most important GHG by its radiative forcing, with an atmospheric lifetime estimated to 114 years and a GWP 100 nearly 300 times higher than of CO 2 .

Such a proposal was made in 2016 [START_REF] Ming | Fighting global warming by greenhouse gas removal: destroying atmospheric nitrous oxide thanks to synergies between two breakthrough technologies[END_REF]. [START_REF] De Richter | Iron Salt Aerosol and Photocatalytic Solar Chimneys: Two innovative breakthrough technologies to remove greenhouse gases[END_REF]. Reproduced from John Bradley [START_REF] Bradley | Environmental Visionaries: The Big Gun[END_REF].

Two proposals [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF][START_REF] De Richter | Iron Salt Aerosol and Photocatalytic Solar Chimneys: Two innovative breakthrough technologies to remove greenhouse gases[END_REF] also suggested removal of other GHGs (like N 2 O, as well as many halogenated GHG gases) with very high global warming potentials, which also damage the stratospheric ozone layer and are included in the Montreal Protocol. A long list of possible photocatalysts was proposed [START_REF] Ming | Fighting global warming by greenhouse gas removal: destroying atmospheric nitrous oxide thanks to synergies between two breakthrough technologies[END_REF][START_REF] De Richter | Fighting global warming by GHG removal: destroying CFCs and HCFCs in solar-wind power plant hybrids producing renewable energy with no-intermittency[END_REF], mainly titanium dioxide derivatives, all acting at ambient temperature and activated by sunlight.

Consequently, not only CH 4 , but almost all non-CO 2 GHGs are targeted by the photocatalytic method [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF].

In table 1, the principal methods of CH 4 removal are summarized. costs might be reduced, with a target costrange of $100 ton -1 of CO 2 -eq [START_REF]Methane-Action[END_REF].

In case of use of hybrid CO 2 DAC plants upgraded to also capture and oxidize CH4, the costs might be lower, as almost all the infrastructure and the air-flow already exist.

Photocatalysis on surfaces

Point sources such as ventilation systems; and global atmosphere

The estimated cost ton -1 CO 2 -eq is $166

by 2030 with a target of $100 by 2040 [START_REF]Methane-Action[END_REF]. For CH 4 removal using chlorine atoms, directly in the troposphere, mimicking natural processes costs estimates range from $ 54 to as low as $1.7 ton -1 CO 2 -eq [START_REF] Johnson | Methane Removal and Emerging Technologies[END_REF]. A startup targets costs of about $1.6 ton -1 CO 2 -eq [START_REF] Amr | Atmospheric Methane Removal[END_REF].

Those estimations of the costs look very 1 Although it is yet too early in the development process to have accurate cost estimates for CH 4 removal directly in the troposphere, initial estimations have been provided 2 Provided for illustration purposes only, as mitigation is out of the scope of this review and has been reviewed elsewhere [START_REF] Ocko | Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming[END_REF][START_REF] Stolaroff | Review of methane mitigation technologies with application to rapid release of methane from the Arctic[END_REF][START_REF] Nisbet | Methane mitigation: methods to reduce emissions, on the path to the Paris agreement[END_REF][START_REF] Johannisson | A Structured Approach for the Mitigation of Natural Methane Emissions-Lessons Learned from Anthropogenic Emissions[END_REF][START_REF] Kebreab | Strategies to Reduce Methane Emissions from Enteric and Lagoon Sources[END_REF].

Enhancing the sinks by the use of plants, trees, and microbes

Mitigation of landfill CH 4 emissions using soil amendments such as biochar [START_REF] Huang | Enhancement of the methane removal efficiency via aeration for biocharamended landfill soil cover[END_REF], and microbial CH 4 oxidation processes with bio-covers [START_REF] Scheutz | Microbial methane oxidation processes and technologies for mitigation of landfill gas emissions[END_REF] or bio-trickling filters are wellestablished methods [START_REF] Yoon | Feasibility of atmospheric methane removal using methanotrophic biotrickling filters[END_REF]. Adding methanotrophs to flooded paddy soil also mitigates CH 4 emissions [START_REF] Davamani | Mitigation of methane gas emissions in flooded paddy soil through the utilization of methanotrophs[END_REF]. Enhancing methanotrophic activity is among the mitigation methods proposed to prevent CH 4 from reaching the atmosphere [START_REF] Nisbet | Methane mitigation: methods to reduce emissions, on the path to the Paris agreement[END_REF][START_REF] Johannisson | A Structured Approach for the Mitigation of Natural Methane Emissions-Lessons Learned from Anthropogenic Emissions[END_REF]. Similar methods also might be possible for greenhouse gas removal (GGR), as it has been

shown that an important CH 4 sink can be created by cropland reforestation [START_REF] Wu | Afforestation enhanced soil CH4 uptake rate in subtropical China: Evidence from carbon stable isotope experiments[END_REF]. One can probably imagine that for afforestation projects and for the "one trillion trees" initiative, planting trees that absorb tropospheric CH 4 [START_REF] Covey | Methane production and emissions in trees and forests[END_REF] in addition to CO 2 (instead of plants and trees that emit CH 4 [START_REF] Covey | Methane production and emissions in trees and forests[END_REF]), as recently observed in the seasonally flooded Amazon floodplain, is a good ideaespecially if using local trees species, and if biodiversity is preserved or restored, without competing with agricultural land.

Airborne microbes are abundant in the atmosphere [START_REF] Smets | Airborne bacteria in the atmosphere: presence, purpose, and potential[END_REF] and subject to long-range transport [100], so it might be possible to enhance the amount of CH 4 consuming microorganisms (methanotrophs) [101] for instance by enhancing the amount of methanotrophs already present on the bark of tree trunks [102]. Currently, plants, trees, and microbes represent 5%-6% of the sinks for atmospheric CH 4 , but to our knowledge apart a brief mention [START_REF] Boucher | New directions: atmospheric methane removal as a way to mitigate climate change?[END_REF], no large-scale strategy has yet been proposed

to take advantage of these sinks.

Discussion

The mail benefit of returning to CH 4 pre-industrial levels will be to reduce global warming by up to 0. ), many people would consider them significant. When "net-zero" is reached in the second half of this century, renewable energy will be preponderant.

Without knowing much about potential energy costs, and costs of scaling up, it is very difficult to understand and predict how feasible CH 4 removal technologies can be. But in our opinion, even if during the first decade following their invention, a very smallscale effective efficiency is obtained at very high cost, technologies able to avoid GHG emissions or able to remove GHGs already in the atmosphere deserve attention as soon as their scalability and globalization seems possible. Otherwise, the criterion of "at a climatically significant scale" may lead to too many options being dismissed and too few remaining, while the scale of the global warming problem requires a large portfolio of methods and technologies to be developed. A significant scale might be achieved as the sum of many contributions that are not individually very significant.

There are several direct and indirect co-benefits to reduced CH 4 atmospheric concentrations. The rapid climate benefits of reducing the concentration of CH 4 in the atmosphere are significant [START_REF] Ocko | Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming[END_REF] for agriculture and the economy [109, 110] as the tropospheric ozone burden enhanced by CH 4 will also be reduced [103]. Lower surface ozone concentrations will increase crops yields and global photosynthesis, potentially allowing some CO 2 removal [START_REF] Feng | Economic losses due to ozone impacts on human health, forest productivity and crop yield across China[END_REF]. The co-benefits for human health are numerous [111], as it will reduce hospitalizations, asthma and pulmonary diseases and premature deaths due to the linked ozone pollution [START_REF]Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions[END_REF]. Development of CH 4 removal methods is still in its infancy and requires more research, development and funding [START_REF] Jackson | Atmospheric methane removal: A research agenda[END_REF].

Some of the CH 4 enhanced oxidation methods proposed will have other co-benefits.

As °OH and Cl atoms are very reactive and not very selective, by enhancing their generation several other GHGs and atmospheric pollutants will be removed faster than CH 4 . This includes, for instance VOCs, whose removal will help reduce CH 4 lifetime [112]. Such VOCs include also organo-halogens, human made hydrofluorocarbons and hydrochlorofluorocarbons, as well as natural biogenic halogenated-methane compounds produced mainly by oceanic plankton and bacteria [113]. The latter are not considered to be GHGs, but they participate in the stratospheric ozone layer natural cycle of destruction [114]. By reducing faster the amount of natural biogenic halogenated-methane compounds in the lower troposphere, as well as by reducing the water content of the stratosphere due to CH 4 oxidation [START_REF] Revell | The role of methane in projections of 21st century stratospheric water vapour[END_REF], the ozone layer might recover faster.

Of course, the main benefit of returning to CH 4 pre-industrial levels will be to reduce global warming by up to about 0.5°C [103], which can help reduce a temperature overshoot above 2°C by mid-century [104].

6.

Concluding remarks

This perspective article has discussed different strategies (some already proposed and several new ones) to accelerate the removal of already emitted CH 4 , reducing its radiative forcing by direct and indirect effects. A reduction of the atmospheric CH 4 burden might help the ozone layer to recover faster and will have rapid climate benefits together with significant co-benefits for agriculture, human health and the economy.

Unlike CO 2 removal methods, the CH 4 removal methods described here do not require capture and long-term geological sequestration as for CO 2 , as they only accelerate the natural oxidation processes that will anyway occur with the products remaining in the atmosphere. By returning to CH 4 pre-industrial levels, the increase of atmospheric CO 2 resulting from CH 4 oxidation is small compared to global CO 2 annual emissions, while the reduction of the radiative forcing could be significant. Net

Figure 1 :

 1 Figure 1: Equivalent emissions of the principal GHGs, on a GWP and GTP basis, compared over a time horizon of 10, 20, and 100 years, from the IPCC[START_REF] Stocker | The Physical Science Basis, Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF]

Figure 2 :

 2 Figure 2: A container ship powered by bunker fuel mixed with commercially availableiron additives[START_REF] Ergin | An experimental investigation on the effects of fuel additive on the exhaust emissions of a ferry[END_REF][START_REF] Debbarma | Effects of iron nanoparticle fuel additive on the performance and exhaust emissions of a compression ignition engine fueled with diesel and biodiesel[END_REF] which are sold to reduce black carbon and carbon monoxide (CO) emissions and to reduce fuel consumption. In the exhaust plume, the iron compounds react with sea salt to produce iron chloride (FeCl 3 ), which under sunlight generates Cl atoms[START_REF] Wittmer | Photochemical activation of chlorine by iron-oxide aerosol[END_REF] that oxidize CH 4 16 times faster than °OH radicals[START_REF] Sommariva | Multiphase halogen chemistry in the tropical Atlantic Ocean[END_REF].

Figure 3 :

 3 Figure 3: A photocatalytic solar updraft tower concept to remove atmospheric CH 4 , nitrous oxide (N 2 O), and other GHGs, from [55].

Figure 4 :

 4 Figure 4: A hypothetical industrial DAC device with added photocatalyst, proposed to oxidize CH 4 into CO 2 [57]. Reproduced from John Bradley [82].

  favorable but are uncertain prior to demonstration, waiting for field trails which cannot start before a full-scale environmental assessment has been conducted.[START_REF] Oeste | Climate engineering by mimicking natural dust climate control: the iron salt aerosol method[END_REF][START_REF] Ming | A nature-based negative emissions technology able to remove atmospheric methane and other greenhouse gases[END_REF] For CH 4 mitigation at point sources, for instance in ventilation systems of coal mines where CH 4 is more concentrated, costs might even be lower as the infrastructure and the air-flow already exist and the generation of Cl atoms can be made by photolysis of Cl 2 gas, produced by the well-established chlor-alkali industrial process.Possible co-benefit: iron salt aerosols provide iron to depleted oceans, with possible CO 2 capture in the oceans at costs about $1 ton -1 of CO 2[START_REF] Ming | A nature-based negative emissions technology able to remove atmospheric methane and other greenhouse gases[END_REF] based on the "Redfield ratio" of oceanic C-N-P-Fe stoichiometry and assuming 10% sequestration in the bottom of the oceans.

  warming could be reduced by about 0.5°C.Although CH 4 enhanced oxidation methods can be applied both to global tropospheric CH 4 and to some local concentrated sources, removal and mitigation strategies and methods do not necessarily target the same sources and are complementary. In case of an abrupt acceleration of CH 4 emissions from natural sources (e.g. submarine methane-hydrates, or wetlands), the availability of effective and proven techniques would constitute an assurance to avoid a rapid acceleration of global warming.Those innovative methods deserve more attention from the scientific community to help evaluate their potential risks, costs, public acceptability, and societal appropriation. Together with CO 2 and CH 4 mitigation and with CO 2 removal, CH 4 removal methods can help fight climate change, win time by slowing down warming and thus meet the targets of the Paris Agreement with limited temperature overshoot.

Table 1 :

 1 Summary of the methane removal methods, technologies and estimated costs. For comparison, some methane mitigation strategies are also briefly described.

	Method	Principal targets Description, possible costs and	Ref.
			comments
		Both for removal and mitigation of CH 4	1
	Zeolites +	Point sources	For removal, using air moving devices	[85]
	catalysis	such as ventilation	such as Direct CO 2 Air Capture plants
		systems; and	associated with zeolites and a catalyst, if
		global atmosphere	the DAC plant is only devoted to CH 4
			capture and oxidation, costs might be
			slightly higher than for CO 2 capture [80] as
			tropospheric CH 4 is more than 200 times
			more diluted than CO 2 and air handling will
			consume more energy. But if passive
			systems are used and as on zeolites some
			catalysts can oxidize CH 4 at room
			temperature [83] and as no capture, no
			purification, no compression, no transport
			and no long-term storage is required in the
			case of CH 4 compared to CO 2 , then global

  as being of interest, for a global potential of 0.13 Gt CO 2 removal per year and of only 0.02 Gt CO 2 yr -1 in the United States. After a half-century of development, solar photovoltaics and wind turbines are now cost-competitive with other electricity generating technologies. In 2018, PV and wind turbines avoided respectively 0.15 and 0.37 gigatons of CO 2 -equivalent per year (Mt CO 2 -eq yr -1 ) [107, 108], compared to only a few tons per year five decades ago. Even though these savings are still small compared to global CO 2 emissions (currently about 40 gigatons CO 2 -eq yr -1

	5°C [103], which can help reduce a temperature overshoot above
	2°C by mid-century [104].
	The expression "at a climatically significant scale" often appears in discussions about
	GHG removal, but lacks precise definition. Perhaps one benchmark could be the
	amount of GHG removal achieved up to now. According to the International Energy
	Agency [105], after 10 years of development, existing DAC installations captured just
	9,000 tons CO 2 yr -1 in 2019. The report of three U.S. National Academies [106] on
	negative emissions technologies (NETs) considers "coastal blue carbon"
	(mangroves)

Acknowledgement

This research was supported by the National Key Research and Development Plan (Key Special Project of Inter-governmental National Scientific and Technological Innovation Cooperation, Grant No. 2019YFE0197500), National Natural Science Foundation of China (Grant No. 51778511), the European Commission H2020 Marie S Curie Research and Innovation Staff Exchange (RISE) award (Grant No. 871998), Hubei Provincial Natural Science Foundation of China (Grant No.2018CFA029), Key Project of ESI Discipline Development of Wuhan University of Technology (Grant No. 2017001), and the Fundamental Research Funds for the Central Universities (Grant No. 2019IVB082).


Benjamin Gerin
email: benjamin.gerin@ensam.eu

Etienne Pessard

Franck Morel

Catherine Verdu

Influence of surface integrity on the fatigue behaviour of a hot-forged and shot-peened C70 steel component

Keywords:

A C T

Hot-forging is a process commonly used in the manufacturing of automobile parts such as connecting rods. Most hot-forged components are shot-blasted after forging in order to clean off the forging scale. The shotblasting process is akin to shot-peening and greatly affects the surface integrity of the components by introducing hardening and residual stresses. This study focuses on the influence of the surface integrity on the fatigue behaviour of a hot-forged C70 steel connecting rod. Specimens were machined out of the connecting rods in order to perform fatigue testing on the forged surface. Several surfaces states, including shot-peened surfaces, were studied in order to quantify the influence of the surface integrity.

A thorough characterisation of the surface integrity of the specimens was first performed. The hardness, residual stresses and microstructure gradients were analysed, and the surface of each specimen was scanned using a profilometer. The specimens show complex networks of surface defects introduced during forging, and the shot-blasting process introduces important surface residual stress and microstructure gradients. High cycle fatigue tests in plane bending were then performed on the specimens, with the surface scans helping to identify the critical defect on which crack initiation occurred. The fatigue results, presented in the form of a Kitagawa diagram, are analysed in order to determine which surface integrity parameters are the most influential on fatigue behaviour. The forging defects have an important negative effect on fatigue strength. After shot-blasting, the fatigue strength increases considerably because of the large compressive residual stresses introduced by the shot-blasting process.

Introduction

Hot-forging is a process commonly used in the manufacturing of automobile parts. The expensive dies used for this process mean it is mainly used for mass production. Connecting rods for an internal combustion engines are a typical example of components manufactured with this process.

Hot-forging requires heating the material to high temperatures in order to achieve sufficient ductility (typically 1000 °C for steel). The subsequent phase transition during cooling means that the resulting microstructure shows little to none of the hardening or the residual stresses introduced during forming.

However, the high temperature increases oxidation and often a layer of oxides, called forging scale, is formed on the surface of the components. This layer must be removed before the next steps of the manufacturing process can be performed (sizing, machining, etc).

A very common method for cleaning off the scale is the shotblasting process [START_REF] Lange | Handbook of metal forming[END_REF]. Several hundred components are placed in a rotating barrel while steel shot is propelled into the barrel (Fig. 1a). Shot-blasting is therefore similar to shot-peening but does not have the same purpose, as shot-peening is used to introduce compressive residual stresses and also to change surface roughness (Fig. 1b). Both processes (shot-blasting and shot-peening) introduce local plastic strain with each shot impact, resulting in hardening and internal stresses near the surface of the components.

During forging, defects can appear on the surface of the components. Between two strikes, scale can get stuck on the dies' surface, generating surface defects the size of which can reach several millimeters in length and several hundred micrometers in depth. Shotblasting does not remove the largest of these forging defects. Surface defects are potentially very harmful in fatigue, which is why it is important to study their impact on fatigue behaviour [START_REF] Pessard | The effect of quenching and defects size on the hcf behaviour of boron steel[END_REF].

The oxidation during forging can also lead to decarburisation of the surface: the carbon content decreases which directly impacts the mechanical characteristics, most notably hardness. Gildersleeve [START_REF] Gildersleeve | Relationship between decarburisation and fatigue strength of through hardened and carburising steels[END_REF] studied the influence of surface carbon content on the fatigue strength of steel specimens. Using controlled decarburisation to modify the carbon content, he found that fatigue strength was proportional to surface hardness and followed the empirical law suggested by Garwood et al. [START_REF] Garwood | Interpretation of Tests and Correlation With Service[END_REF]:

σ = 1.5 × H D V .
McKelvey & Fatemi [START_REF] Mckelvey | Surface finish effect on fatigue behavior of forged steel[END_REF] performed fatigue tests on hot-forged specimens, studying the effects on fatigue of decarburisation and shotcleaning of the surface. They showed that the as-forged surface has a much lower fatigue strength compared to polished specimens (50% to 70% drop) because of the combined effects of surface roughness and decarburisation. The severity of the decarburisation depends on the heating process, with induction heating leading to less decarburisation than gas heating. They also showed that the use of shot-cleaning on the as-forged surface improves fatigue strength, independently of decarburisation.

Numerous studies have been conducted on how shot-peening affects surface integrity. Gariépy et al. [START_REF] Gariépy | Experimental and numerical investigation of material heterogeneity in shot peened aluminium alloy AA2024-T351[END_REF] conducted a thorough characterisation of the surface of a shot-peened aluminium, measuring the hardness, microstructure and residual stresses gradients generated near the surface. They also showed that shot-peening increases roughness and creates surface micro-defects.

The influence of shot-peening on fatigue behaviour has also been extensively studied. Bhuiyan et al. [START_REF] Bhuiyan | The influence of mechanical surface treatments on fatigue behavior of extruded az61 magnesium alloy[END_REF] investigated shot-peened magnesium alloy specimens, some of which were stress-relieved after peening. They showed that the fatigue strength depends on the combined effects of the compressive residual stresses (which improve the fatigue strength), and the roughness (which decreases fatigue strength).

Sakamoto et al. [START_REF] Sakamoto | Effect of fibprocessed sharp flaw on fatigue limit of shot peened medium carbon steel[END_REF][START_REF] Sakamoto | Effect of surface flaw on fatigue strength of 250 10 100 1000 shot-peened medium-carbon steel[END_REF] performed fatigue tests on steel specimens, polished or shot-peened. Focused ion beam (FIB) was used to introduce small surface notches (50 µm) in some of the specimens. The fatigue results showed that the notches reduced the fatigue limit for the polished specimens, but not the shot-peened ones. The shotpeened specimens had multiple crack initiations, on the notches and on cracks generated by the shot-peening. These results show that surface defects have no effect on the fatigue behaviour of a shot-peened surface if they have the same size as the cracks generated by the shot-peening.

Fathallah et al. [START_REF] Fathallah | High cycle fatigue behavior prediction of shot-peened parts[END_REF] studied the effect on fatigue behaviour of shotpeening coverage. They performed characterisations and fatigue tests on steel specimens with 100% or 1000% surface coverage. The found that 1000% coverage had bigger surface defects and lower surface residual stresses than 100% coverage. To model the fatigue behaviour, they performed 2D finite element simulations, taking into account the specimen geometry, the surface defects, the surface damage, the residual stresses and the hardening introduced by shot-peening. Their model correctly predicted the experimental fatigue limits and showed that ignoring the effects of the defects and the surface damage would lead to predicting crack initiation not on the surface but at the depth where the residual stresses reach zero.

These articles show that the surface integrity of forged components is the combination of different aspects that can all have an effect on fatigue behaviour. These aspects result from the hot-forging or the subsequent shot-blasting.

From forging:

• Large surface defects • Decarburisation

From shot-blasting:

• Surface roughness • Hardening gradient • Residual stresses • Microstructure gradient • Surface micro-defects
These aspects all influence fatigue behaviour differently, and are not easily studied independently, especially in the case of shot-blasting where all are introduced simultaneously. Very few articles have managed to decouple the various effects of the surface integrity.

The goal of the present study is therefore to analyse the fatigue strength and the crack initiation mechanisms for specifically chosen surface integrity conditions. Fist, the various aspects of the surface integrity will be thoroughly characterised, then fatigue tests will be conducted in order to quantify their effect on fatigue behaviour. The fatigue results will serve to decouple the various surface integrity factors affecting fatigue behaviour.

Experimental procedure for studying various surface integrities

The component chosen for this study is a hot-forged connecting rod produced by Atelier des Janves (ADJ) and used in certain Renault engines. The connecting rods are produced from round bars of C70 steel. These bars are hot-rolled and air-cooled resulting in a pearlitic microstructure.

Mechanical properties of the C70 steel

The chemical composition of the C70 steel are given in Table 1 and its mechanical properties after forging are given in Table 2. Hardness was measured along the cross-section using a microdurometer.

Studied industrial component: hot-forged connecting rod

The steps for the production of the connecting rods are:

• Cutting the steel bar • Induction heating of the workpiece • Cross rolling to obtain the preform • Stamping of the connecting rod in three passes • Deburring (while hot) • Sizing (while hot)

• Controlled air-cooling on a belt conveyor • Shot-blasting • Quality control After forging, the connecting rods also have a pearlitic microstructure.

The study is restricted to the flat central part of the connecting rod, with fatigue specimens machined out of the connecting rod beam using spark machining (Fig. 2a). Restricting the study to a flat area makes analysing the surface much easier and avoids local stress concentrations due to specimen geometry. Fig. 2b shows the geometry of the specimens.

The specimens were machined so as to leave the surface of the middle section unaltered. The corners of the specimens were rounded and the machined sides were polished to prevent crack initiation outside of the studied surface.

Studied batches

In order to quantify the influence of the various aspects, it is necessary to study different surface states and to compare them. ADJ delivered 688 shot-blasted connecting rods and 485 as-forged (without shot-blasting), taken from their production line. The forging scale was still present on the as-forged connecting rods so those used in the study were manually cleaned with a metal brush in order to avoid damaging the surface or introducing residual stresses. It was decided to use shotpeening to obtain specific surface states, different from that of shotblasting. Shot-peening is similar to shot-blasting but with a high degree of control on the peening parameters meaning that it is possible to obtain specific surface states. Two shot-peening batches were chosen: the first replicates the shot-blasting surface state and the second has a lower roughness and a shallower residual stress profile. The shotpeening was performed by the Metal Improvement Company on specimens machined out of as-forged connecting rods (manually cleaned of scale). In order to have a reference for the fatigue tests, some specimens were ground and mirror-polished so as to have a surface state with no hardening, no defects.

The various surface states used in this study are as follows:

• As-forged surface, manually cleaned of scale • Shot-blasted, the surface state of industrially produced connecting rods

• Shot-peened with shot diameter 800 µm, Almen intensity 3060 A with 200% coverage. This shot-peening was performed on as-forged specimens and was chosen so as to be as close as possible to the shot-blasted surface.

• Shot-peening with shot diameter 400 µm, Almen intensity 2030 A with 200% coverage. This shot-peening was performed on as-forged specimens and was chosen so as to have a lower roughness and a shallower residual stress profile.

• Polished, used as the reference in fatigue Fig. 3 shows specimens from each batch and details how the specimens were prepared.

Surface defects, residual stresses and microstructural characteristics for the different surface integrities

The various batches have different surface states but have all the same core properties, determined with tensile tests and microhardness measurements. The surface state of each batch was analysed as follows:

• Microhardness measurements • X-ray diffraction analysis of the residual stresses • Surface topography measurements with a profilometer • Scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) imaging of the microstructure

Microstructure and residual stresses

The as-forged specimens have a pearlitic microstructure, which shows no gradient from the core to the surface. Furthermore, the microstructure is the same for the long and transverse directions. The microstructure of the specimens is therefore homogeneous and isotropic, which is generally the case in hot-forging. SEM images and EBSD maps of the polished cross-section of as-forged specimens are shown in Fig. 4. EBSD gives a visual representation of the ferrite grain orientations, showing the impact of the extrusion on the grains. Characterisation was performed on a Zeiss Supra VP55 SEM with a field-emission gun, operated at 15 kV and equipped with an Oxford-Instruments EBSD camera. A step size between 0.4 and 0.1 µm was used depending on magnification. It should be noted that the cementite is not visible in the EBSD maps, meaning that only the ferrite grains are mapped.

The SEM images show the surface microstructure of these specimens remains pearlitic, even just beneath residual scale. These observations indicate that no decarburisation occurred during the forging process. This is due to ADJ controlling the forging and cooling parameters in order to reduce decarburation as much as possible.

After shot-blasting and shot-peening, the specimens show a clear microstructure gradient near the surface of shot-blasted specimens (Fig. 5). This surface microstructure can be divided into four layers:

1. A surface layer 5-10 µm deep where the grains are extremely small 2. A second layer around 20 µm deep with heavily deformed grains 3. A transition layer 100-150 µm deep where the grains are progressively less deformed 4. The unaffected core material (not shown in Fig. 5) These layers are the same as those observed by Gariépy et al. [START_REF] Gariépy | Experimental and numerical investigation of material heterogeneity in shot peened aluminium alloy AA2024-T351[END_REF] on a shot-peened aluminium alloy.

Fig. 6 shows SEM and EBSD images of a surface fold generated during shot-blasting, with trapped scale. The scale was not detected during EBSD mapping and appears white in the image. Additionally, cracks running parallel to the surface were detected in the shot-blasted surface, likely resulting from grain delamination during impact. Feng et al. [START_REF] Feng | Formation of short crack and its effect on fatigue properties of ultrasonic peening treatment S355 steel[END_REF] have observed similar microdefects in ultrasonic peening and have shown that they can have a negative impact on fatigue behaviour. Fig. 7 shows examples of the shot-blasted surface integrity which all have a potential effect on fatigue behaviour.

Fig. 8 shows the surface hardness gradient up to a depth of 1 mm for all batches. For the as-forged batch, the surface hardness is close to the core value which confirms the absence of decarburisation. The shotblasted and shot-peened batches show a clear increase in hardness near the surface, with a surface value around 50 H V higher than the core value.

The residual stresses were analysed using X-ray diffraction. A PROTO iXRD machine was used, equipped with a Cr anode vacuum tube and two PSD detectors. The Kα radiation of the Cr target has a wavelength of λ = 0.22909 nm. The diffracting planes are the {211} planes of the BCC structure of the ferrite, with a Bragg angle of θ 2 = 156.4. The crystallographic elastic constants used were: S = -1.28 × 10 MPa . The first measure (zero depth) is made on the specimen surface, with deeper measurements obtained by locally removing thin surface layers through electropolishing. Fig. 9 shows the residual stress profiles of all batches. The as-forged surface shows negligible residual stresses, which is expected in hot-forging. Shot-blasting produces a residual stress profile which does not have the standard bell-shape seen in shotpeening. Instead, the highest value is located on the surface (-500 MPa) and the profile gradually decreases to reach a value close to zero at a depth of around 500 µm. Both shot-peened surfaces have the expected profiles: shot-peening with ∅800 μm shot has a profile very close to that of the shot-blasting, and shot-peening with ∅400 μm shot has the same surface value but half the depth. Shot-blasting and shot-peening do not favour specific orientations, so the stress values are similar in both directions.

Roughness and surface defects

3D surface scans of the specimens were made using a Bruker ContourGT-K0-X profilometer. All specimens were scanned before and after fatigue testing, in order to easily analyse the crack initiation location. Fig. 10 shows a 2×2 mm close-up for each batch. Each batch has an unique topography which makes it easily distinguishable from the others. The forging defects are clearly visible on the as-forged surface, and the defects are not completely erased after shot-blasting or shot-peening. Individual shot impacts are visible on both shot-peened surfaces, and the difference in shot diameter can be seen clearly. In the case of the shot-blasted surface, the individual impacts are not easily visible. This is because for shot-blasting, the shot is not often replaced, instead the same batch is re-used repeatedly. This means that while the shot diameter is initially 1 mm, wear and fragmentation during shotblasting lead to very varied shot sizes.

The surface scans were also used to measure roughness values for each batch. The results are given in Table 3. The roughness values are very similar for the as-forged and the shot-peened ∅400 µm shot surfaces, despite the fact that the two surfaces are easily distinguished visually. The same is also true for the shot-blasted and the shot-peened ∅800 µm shot surfaces. This means that the roughness values do not accurately reflect the differences in surface topography between the various batches. This is due to the forging defects which have a high roughness value even before shot-peening.

Fatigue strength and crack initiation mechanism

Plane bending fatigue tests

The fatigue tests were performed in plane bending with a load ratio of R = -1. This load ratio was chosen as it creates the highest loading stress on the specimen surface and therefore favours crack initiation in the studied surface and not the machined sides of the specimens. Fatigue tests were performed on a RUMUL Cracktronic resonant fatigue testing machine, at a frequency of 80 Hz. The surface of each specimen was scanned before fatigue testing and after crack propaga- tion. This, combined with SEM observation of the fracture surface, allowed to confirm that crack initiation had occurred on the studied surface and to locate the crack initiation on the undamaged surface scan.

g n i p p a m D S B E e g a m i M E S
A staircase method was performed on 15 specimens for the polished batch. The staircase method [START_REF] Dixon | A method for obtaining and analyzing sensitivity data[END_REF] determines the fatigue limit using multiple specimens. The first specimen is cycled with a load close to the expected fatigue limit. If a specimen survives the 2. 10 6 cycles, the loading value is increased for the next specimen. If a specimen fails, loading is decreased for the next specimen. Loading is increased or decreased by a fixed value chosen so as to be small enough to accurately reflect the fatigue dispersion without leading to too many load levels (generally 10-25 MPa). After all specimens have been cycled, a statistical analysis determines the fatigue limit.

For the batches other than the polished specimens, the "Locati" step method was used in order to determine a fatigue strength value for each specimen. This method was previously used for a similar study performed on cold-forged components [START_REF] Gerin | Beneficial effect of prestrain due to cold extrusion on the multiaxial fatigue strength of a 27MnCr5 steel[END_REF] and also by other authors [START_REF] Pessard | A new approach to model the fatigue anisotropy due to non-metallic inclusions in forged steels[END_REF].

Residual stress analysis during fatigue testing

The residual stresses potentially have a large impact on fatigue behaviour, which is why it is important to know if stress-relief occurs during fatigue cycling. Kang et al. [START_REF] Kang | Effect of prestrain on and prediction of fatigue limit in carbon steel[END_REF] have shown that for high-cycle fatigue, the surface residual stresses have only minimal stress-relief. In order to quantify stress-relief, a shot-blasted specimen with a large surface defect was chosen; surface residual stresses were analysed in the centre of the specimen and in the defect. The specimen was then loaded in fatigue at a high stress value of 475 MPa, and the test was periodically stopped to perform new measurements of the residual stresses. Fig. 11 shows the location of the residual stress analysis on the surface scan of the specimen, and the results of the measurements with the corresponding number of cycles. The specimen failed at 139, 400 cycles, with the crack initiation located at the defect. For the first 100,000 cycles, the surface residual stresses show almost no change in value, both at the centre of the specimen and in the defect. These results show that no stress relief occurs during fatigue cycling, the Residual stresses (MPa)

As-forged -long.

As-forged -tvs.

Shot-peening 400 µm shot -long.

Shot-peening 400 µm shot -tvs.

Shot-peening 800 µm shot -long.

Shot-peening 800 µm shot -tvs.

Shot-blasting -long.

Shot-blasting -tvs.

Distance from surface (µm) residual stresses are therefore assumed to keep their initial value during fatigue testing.

Fatigue testing results

In addition to bending fatigue tests, some tension fatigue tests were also performed, using a hydraulic testing machine.

In order to quantify the effect of the residual stresses on fatigue behaviour, some specimens were stress-relieved by applying a 1% elastic deformation in tension. Residual stress analysis before and after deformation were made to confirm that the deformation had relieved most of the residual stresses in the longitudinal direction. The residual stresses in the transverse direction were unaffected.

Not all specimens resulted in successful fatigue tests, as some crack initiations occurred in the specimen corners or on the machined sides. All the specimens which did not have a crack initiation located on the studied surface were discarded.

Only one of the tension tests was valid, performed on a stressrelieved shot-peened ∅800 µm shot. This test was the only valid stressrelieved test, as all the others had a corner crack initiation.

Table 4 details the number of valid specimens for each batch, with their respective fatigue strength range. The average value from the staircase method on the polished specimens is the reference fatigue strength of the material: σ = 424 MPa D 0 . Additional tests were performed on specimens which were ground prior to shot-peening, in order to remove the forging defects. Some of these specimens were also stress-relieved. Because of the absence of the forging defects, the crack initiation for these defects was always occurred in the specimen corners (special care was taken to the peening of the corners during shot-peening). A staircase of five specimens was performed on ground and shot-peened ∅400 µm shot specimens, giving a fatigue strength of 488 MPa. Another staircase was performed on five stress-relieved specimens (ground and shotpeened ∅400 µm), resulting in a fatigue strength of 388 MPa. The fatigue results for these staircases specimens cannot be directly exploited, however, comparing the results of these two batches shows a 100 MPa drop in fatigue strength after stress-relief.

Crack initiation mechanism

For all the valid specimens (except the polished batch), crack initiation was located on a large forging defect. This defect is called the critical defect because it is the defect which led to crack initiation first. Fig. 12 shows examples of the fracture surface with the critical defect for specimens of each batch. Crack initiation was always located on the surface, for all batches (Fig. 13). For the shot-blasted and shotpeened specimens, the crack initiation is therefore influenced by both the large forging defects and the microdefects introduced by the shotblasting and shot-peening processes.

Each specimen having been scanned prior to fatigue testing, the critical defect can be identified on the surface scans using the fracture surface analysis. Fig. 14 shows the surface scan and a close-up of the critical defect of the as-forged specimen shown in Fig. 12.

Analysis and discussion

Fatigue trend for each batch

Defect size is represented using the square root of their surface area, projected along the loading direction [START_REF] Murakami | Effects of hardness and crack geometries on K Δ th of small cracks emanating from small defects[END_REF]. The fracture surfaces can be used to calculate the projected surface area of each critical defect, which is generally used to represent defect size. All the defects have a projected area of similar shape, with the width much bigger than the depth (defects are typically 500-2000 µm wide and 50-200 µm deep). Calculating the projected area is done using the SEM fracture surface images: the total width and maximum depth of the defects are measured and their projected surface area is calculated by supposing that they have a semi-elliptical shape:

πa c area = 2 (1)
with a being half the defect's width (total width is a 2 ) and c its depth. As suggested by Murakami, the defect width is limited by a threshold of 10 times the depth when calculating the projected area. The fatigue results are represented in a Kitagawa diagram [START_REF] Kitagawa | Applicability of fracture mechanics to very small cracks or cracks in the early stage[END_REF] (Fig. 15).

• As-forged specimens: the fatigue strength is always lower than the reference fatigue strength σ = 424 MPa D 0

. This is due solely to the forging defects, as these specimens have no hardening or residual stresses. This batch has a high dispersion in fatigue strength, however a bigger defects tend to have a greater impact on fatigue strength. The lowest fatigue strength is 140 MPa lower than the reference fatigue strength, which is a 34% drop.

• Shot-blasted and shot-peened specimens: the fatigue strength is generally much higher than the reference, which means that shotblasting and shot-peening have a very positive effect on fatigue behaviour. The highest fatigue strength is 150 MPa higher than the reference (35% increase). However, this does not negate the influence of the forging defects: the biggest defects also have the lowest fatigue strengths. At equivalent defects sizes, shot-blasted and shot-peened specimens have a fatigue strength around 150 MPa higher than the as-forged specimens, which is an increase of roughly 50%. The shot-blasted specimens all have large defects because all the other specimens in the batch had corner crack initiations and were thus discarded. For equivalent defect sizes, the shot-blasted and shot-peened ∅400 µm shot specimens have the same fatigue strength. The two shot-peened batches do not have quite the same fatigue strength, with the ∅400 µm shot being around 50 MPa lower.

Some shot-peened specimens were scanned before and after shot-peening in order to analyse the influence of shot-peening on defects. Two (non critical) defects of similar size and shape were chosen. Fig. 16 shows the surface scans before and after shot-peening.

The after shot-peening surfaces have been altered the general shape of the defects remains unchanged. However, it seems that the shot-peening ∅800 µm shot has a greater impact on defect geometry.

• Stress-relieved specimen: only one stress-relieved specimen was valid, of the shot-peening ∅800 µm shot batch, loaded in tension. The fatigue strength for this specimen is 431 MPa, which is, for the same defect size, roughly 70 MPa lower than the non-stress-relieved specimens (800 µm batch) and 100 MPa higher than the as-forged specimens. This drop of the fatigue strength after stress-relief has been confirmed with the ground and shot-peened specimens, which show a consistent drop of around 100 MPa after stress-relief. These results show that the residual stresses have a very beneficial influence on fatigue behaviour. However, relieving the residual stresses in the loading direction was not enough to lower the fatigue strength to the level of as-forged specimens. This could be due to the hardening generated by the shot-peening which was not affected during stress-relief. However, it is also possible that the residual stresses in the transverse direction, which remained unchanged, could also have an influence in fatigue.

Murakami's model

Murakami's model [START_REF] Murakami | Effects of hardness and crack geometries on K Δ th of small cracks emanating from small defects[END_REF] can be used to predict the fatigue strength using the defect size and the hardness value:

σ A area = (H + 120) ( ) D V 1/6 (2)
with A = 1.43 for surface defects. The projected area is defined in Eq. (1). However, this model shows a very conservative prediction compared to the experimental values for the as-forged batch (Fig. 17). The model can be fitted to the as-forged values using A = 1.91

As forged -

. This fitted curve shows that the as-forged batch follows the 1/6 slope of the model. This model predicts a critical defect size of 41 µm, which cannot be experimentally confirmed as no specimen had a crack initiation on a defect smaller than this value. Using this model with the hardness value of the shot-blasted (or shot-peened) surface (350 H V ) shows that the increased hardness is not enough to correctly predict the shot-blasted and shot-peened fatigue strength values (Fig. 17).

These results show that taking into account the hardening introduced by the shot-blasting and shot-peening is not enough and that the residual stresses must be included in order to accurately model the fatigue strength values.

Murakami's model considers that the defects with the greatest projected area have the most influence in fatigue. However, the experimental results show that this is not always the case. Fig. 18 shows the surface scan of the critical defect of an as-forged specimen. Next to the critical defect are two larger defects which did not initiate a crack. The projected area for each defect is given in Table 5. The values show that defect n°3 has a greater projected area, which means that according to Murakami's model, this defect should have been the critical defect. A different criterion is therefore needed in order to explain why defect n°1 is the critical defect. Peterson [START_REF] Peterson | Stress Concentration Factors[END_REF] suggested a way of determining the stress concentration factor of an elliptical notch:

K t ρ = 1 + 2 t ( 3
)
where t is the depth of the notch and ρ is it's curvature radius in the loading direction. Where the two models differ the most is that Murakami considers the defect size only in the direction perpendicular to the loading direction, while Peterson uses the geometrical properties along the loading direction. The K t values for the three defects (Table 5) show that the critical defect has the highest K t . The influence of a defect is therefore determined not just by its size, but also by its sharpness in the loading direction. Further work must be done in order to determine the defect's geometric parameters which are the most influential on fatigue behaviour.

Conclusion

This experimental study was performed on specimens machined out of industrial components. The first part of the study was the thorough characterisation of the surface integrity of the specimens. The as-forged surface shows complex networks of defects which appear during forging.

Shot-blasting and shot-peening heavily alter the surface integrity, generating a microstructure gradient. Surface hardness is increased by around 60 H V and highly compressive residual stresses are also introduced (-500 MPa on the surface). The forging defects are not erased but the shot impacts change the surface topography.

Fatigue tests were then performed to quantify the effect of the surface integrity on fatigue behaviour. The fatigue results show that the surface integrity has a major impact on fatigue behaviour: large forging and shot-blasting both affect fatigue strength. The forging defects are detrimental in fatigue and lower the fatigue strength with larger defects having a greater impact. The fatigue results show a large dispersion because of the varied surface defects.

Shot-blasting is used to clean the specimens of the forging scale, but has a large beneficial impact on the fatigue strength. A large part of this effect is due to the residual stresses introduced during shot-blasting (and shot-peening).

The fatigue strength of the shot-peened specimens is quite close to that of the shot-blasted specimens. This shows that for the shotpeening parameters chosen for this study, shot-peening and shotblasting have an equivalent effect on fatigue behaviour.

The experimental tests are the first part of a larger study: the results will be used in subsequent studies to develop a fatigue model taking into account the effect of both the defects and shot-peening.

Fig. 1 .

 1 Fig. 1. a) Diagram of the shot-blasting process. b) Photos illustrating the shot-peening process, source: Metal Improvement Company.

Fig. 2 .Fig. 3 .

 23 Fig. 2. a) Connecting rod with spark-machined specimen. b) Engineering drawing of the specimens machined out of the connecting rods.

Fig. 4 .

 4 Fig. 4. SEM image and EBSD mapping of the surface microstructure for the as-forged batch.

Fig. 5 .Fig. 6 .

 56 Fig. 5. Microstructure gradient of a shot-blasted specimen.

Fig. 7 .Fig. 8 .

 78 Fig. 7. SEM images showing the various aspects of the surface integrity of shot-blasted specimens.

Fig. 9 .Fig. 10 .

 910 Fig. 9. Residual stress profiles for the various batches, in the direction of the specimen axis (long.), and the transverse direction (tvs.).

Fig. 11 .

 11 Fig. 11. Surface residual stresses analysis performed during the fatigue test of a shot-blasted specimen. The location of the measurements are indicated on the surface scan of the specimen.

Fig. 12 .

 12 Fig. 12. SEM images of fracture surfaces showing the critical defect at the origin of crack initiation for each batch.

Fig. 13 .

 13 Fig. 13. SEM images of fracture surfaces showing the crack initiation location for each batch.

Fig. 14 .

 14 Fig. 14. Surface scan of the as-forged specimen from Fig. 12, showing a close-up of the critical defect. Crack initiation and crack path are indicated.

Fig. 15 .

 15 Fig. 15. Kitagawa diagram of the fatigue tests, with fatigue strength and critical defect size of each valid specimen.

Fig. 16 .

 16 Fig.[START_REF] Murakami | Effects of hardness and crack geometries on K Δ th of small cracks emanating from small defects[END_REF]. Surface scans of a forging defect, before and after shot-peening.

Table 1

 1 Chemical composition C70 steel, in weight percentage.

	Element C	Si	Mn	S	P	Ni	Cr	Mo
	Min	0.690 0.150 0.530 0.0600			0.100
	Value	0.707 0.170 0.569 0.0660 0.015	0.051	0.177	0.024
	Max	0.730 0.250 0.600 0.0700 0.030	0.180	0.200	0.050
	Element Cu	Al	Sn	V	Te	Se	N 2
	Min				0.030			0.01200
	Value	0.086 0.000 0.008 0.034	0.0030 0.0000 0.01420
	Max	0.250 0.010 0.025 0.040	0.0030 0.0050 0.00170

Table 2

 2 Mechanical characteristics of the C70 steel (measured on as-forged specimens).

		σ YS	σ UTS	A t	Hardness H V 0.1 kg
		MPa	MPa	%	Average ± Std. deviation
	C70 steel	598	1020	15.6

Table 3

 3 Roughness values for each batch. Measuring length: 12.5 mm; cut-off length: 2500 µm.

	Surface state	R a and std. deviation	R z	S a	S z
		(µm)	(µm)	(µm)	(µm)
	As-forged	6.44 ± 2.01	46.11	10.08	122.33
	Shot-blasting	7.83 ± 0.92	58.77	10.66	160.40
	Shot-peening ∅800 µm shot	7.64 ± 0.81	64.86	11.62	153.28
	Shot-peening ∅400 µm shot	6.34 ± 1.64	49.44	11.41	138.61

Table 4

 4 Overview of the number of valid tests and fatigue strength range for each batch.

	Batch	Test type	Nbr. of specimens	σ D (MPa)
	Polished	Staircase	15	424
	As-forged	Locati	34	280-400
	Shot-blasting	Locati	8	420-500
	Shot-peening ∅800 µm shot	Locati	10	500-580
	Shot-peening ∅400 µm shot	Locati	18	400-500
	Tension SP ∅800 µm shot stress-	Locati	1	431
	relieved

Acknowledgements

This work has been performed within the ANR (National Research Agency) DEFISURF project, in a partnership including several industrial (Ascometal, CETIM, Renault, Transvalor, Atelier des Janves, Gévelot Extrusion) and academic (INSA Lyon MATEIS, ENSMP-CEMEF, Arts et Métiers ParisTech LAMPA) institutions.