Effect of Strain-Induced Martensite on Tensile Properties and Hydrogen Embrittlement of 304 Stainless Steel

Abstract

Room temperature tensile tests have been conducted at different strain rates ranging from 2 9 10 À6 to 1 9 10 À2 /s on hydrogen-free and hydrogen-charged 304 stainless steel (SS). Using a ferritescope and neutron diffraction, the amount of strain-induced martensite (SIM) has been in situ measured at the center region of the gage section of the tensile specimens or ex situ measured on the fractured tensile specimens. The ductility, tensile stress, hardness, and the amount of SIM increase with decreasing strain rate in hydrogen-free 304 SS and decrease in hydrogen-charged one. Specifically, SIM that forms during tensile tests is beneficial in increasing the ductility, strain hardening, and tensile stress of 304 SS, irrespective of the presence of hydrogen. A correlation of the tensile properties of hydrogen-free and hydrogen-charged 304 SS and the amount of SIM shows that hydrogen suppresses the formation of SIM in hydrogen-charged 304 SS, leading to a ductility loss and localized brittle fracture. Consequently, we demonstrate that hydrogen embrittlement of 304 SS is related to hydrogen-suppressed formation of SIM, corresponding to the disordered phase, according to our proposition. Compelling evidence is provided by the observations of the increased lattice expansion of martensite with decreasing strain rate in hydrogen-free 304 SS and its lattice contraction in hydrogen-charged one.

The Role of Hydrogen in Hydrogen Embrittlement of Metals: The Case of Stainless Steel

“…The reduction of SIM by hydrogen decreased the hardness of 316 SS to 345 HV when compared to that when tested in air (393 HV) ( Table 1). This result agrees perfectly with our previous observations [2], revealing that hydrogen suppresses SIM formation in austenitic stainless steels.…”

“…As shown in Table 1, the amount of SIM was 43 vol.% in air, but it decreased considerably to 15 vol.% in hydrogen. Given that hydrogen-suppressed formation of SIM has been observed in a 304 SS irrespective of the magnitude of tensile strain [2], a reduced amount of SIM in the 316 SS when tested in hydrogen is related primarily to the hydrogen effect. The reduction of SIM by hydrogen decreased the hardness of 316 SS to 345 HV when compared to that when tested in air (393 HV) ( Table 1).…”

“…To check if SIM formation was suppressed by hydrogen during the tensile tests, the hardness and amount of SIM at and near the fracture surfaces of the 316 SS were determined, respectively, using a ferritescope (Helmut Fisher Korea Co, Feritscope-MP30, Korea) and a Vickers hardness tester (Shimadzu, HMV 2000, Kyoto, Japan). The detailed procedure to determine the amount of SIM has been reported elsewhere [2]. Vickers hardness measurements were performed using a Shimadzu hardness tester (HMV 2000, Kyoto, Japan), according to ASTM E92-82 [9].…”

“…Furthermore, considering that hydrogen-suppressed formation of SIM occurred in 316 SS during RT deformation in high-pressure hydrogen, as shown in Table 1, it is evident that hydrogen whether it is cathodically charged or hydrogen gas restraints formation of SIM in austenitic SS. As SIM is a disordered phase, hydrogen-suppressed formation corresponds to hydrogen-enhanced ordering whose solid evidence is lattice contraction of SIM in hydrogen-charged 304 SS during deformation at RT [2].…”

“…Recently, we observed that strain-induced martensite (SIM) formed during tensile tests underwent lattice expansion in hydrogen-free 304 stainless steel (SS) but lattice contraction in hydrogen-charged 304 SS [2], and that SIM formation was suppressed by hydrogen, leading to hydrogen embrittlement. As a result, we have proposed that SIM is a disordered phase created by the destruction of short-range ordered DO 3 -(Fe, Cr) 3 Ni and that hydrogen promotes ordering in 304 SS, leading to suppression of SIM formation, the latter of which was enhanced with decreasing strain rate.…”