Influence Of Surface Martensite On Hydrogen Embrittlement Of Fe-Mn-C-Mo Steels In Wet H₂S Environment

Hydrogen sulfide?H₂S?is one of the harmful medium with the most corrosive effect in oil and gas industry,which seriously affects the oil and gas exploration and transportation and restricts the development of oil and gas equipment material.With the increasing demand of oil and gas,the development of oil well pipe and pipeline steels with higher strength level has become more urgent.However,the higher strength of the materials,the more sensitive it will be to hydrogen embrittlement.Thus,the resistance of the material to hydrogen brittleness reduces.While enhance the material strength level,therefore,we must take its resistance to hydrogen embrittlement into account.Twinning induced plasticity?TWIP?steel is a new generation of high strength steel which is actively researched and developed currently at home and abroad with high strength,high plasticity and high strain hardening ability etc.In addition,as the solubility of hydrogen is big and diffusion is slow in austenitic TWIP steel?FCC?,it has good resistance to hydrogen embrittlement.As a result,high manganese TWIP has been seen as important material in the field of future energy,especially in the exploration and transportation of oil and gas well containing H₂S which will have a broad application prospect.This study investigated the effect of thermally induced surface martensite layer on hydrogenembrittlementofFe-16Mn-0.4C-2Mo?wt.%??16Mn?and Fe-25Mn-0.4C-2Mo?wt.%??25Mn?steels through slow strain rate stress corrosion cracking testing and proof ring testing in wet H₂S environment.The 16Mn steel had a surface layer of less than 150?m in depth containing?-martensite,??-martensite and austenitic twins.The martensite layer is found to reduce the hydrogen embrittlement resistance of the steel.In comparison,the 25Mn steel developed a full??-martensite surface layer,which exhibited practically nil effect on the hydrogen embrittlement resistance of the steel.The lower hydrogen embrittlement resistance of the 16Mn steel is partly attributed to the lower corrosion resistance of its surface layer in wet H₂S environment,which produces more hydrogen and thus more severe condition for hydrogen embrittlement failure.The?-martensite provides much larger interface areas with the mechanical twins of the austenite in the 16Mn steel than the??-martensite/austenite interfaces in the 25Mn steel.These interfaces are hydrogen trapping sites and are prone to initiate surface cracks,as observed in the scanning electron microscope.The formation of the cracks is attributed to hydrogen concentration at the?-martensite and austenitic twin interfaces,which accelerates material fracture.