Effect of Internal Hydrogen on Delayed Cracking of Metastable Low-Nickel Austenitic Stainless Steels
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METASTABLE austenitic stainless steels that undergo a strain-induced martensitic transformation due to plastic deformation may be susceptible to delayed cracking after certain forming processes, e.g., deep drawing. Delayed cracking, also called internal hydrogen embrittlement, can occur in formed metallic components at static stress below the yield strength of the material. Cracking occurs unexpectedly after a certain time from a successful forming operation. Delayed cracking is related to coexistence of internal hydrogen, strain-induced a¢-martensite, and tensile residual stresses.[1–3] The initiation and advance of cracks involve hydrogen diffusion to regions of high tensile stress. Because of the process being diffusion-controlled, there is a delay or incubation time before the cracking occurs. Low-nickel austenitic stainless steel grades, such as 201, often have higher susceptibility to delayed cracking than 300-series Fe-Cr-Ni grades. That is an important obstacle restricting wider industrial application of these cost-effective stainless steels. Susceptibility of austenitic stainless steels to internal hydrogen embrittlement is strongly dependent on their nickel content, as nickel SUVI PAPULA and OLGA TODOSHCHENKO, Doctoral Students, and HANNU HA¨NNINEN, Professor, are with the Department of Engineering Design and Production, Aalto University School of Engineering, P.O. Box 14200, 00076 Aalto, Finland. Contact e-mail: suvi.papula@aalto.fi JUHO TALONEN, Director R&D, is with the Outokumpu Oyj, P.O. Box 140, 02201 Espoo, Finland. Contact e-mail: suvi.papula@aalto.fi Manuscript submitted February 14, 2014. Article published online July 18, 2014 5270—VOLUME 45A, OCTOBER 2014
plays an important role in the deformation mechanisms that affect hydrogen-assisted fracture.[1,4–6] The sensitivity of austenitic stainless steels to hydrogen embrittlement and delayed cracking is generally considered to be a function of austenite stability—the larger the strain-induced a¢-martensite content the higher the risk for fracture.[7,8] The existence of a¢martensite, which is a harder and intrinsically more brittle phase than austenite, offers potential crack initiation sites and can increase markedly the embrittling effect of hydrogen.[9–11] If present, a¢-martensite serves as a fast diffusion path for hydrogen to crack initiation sites, because the diffusivity of hydrogen in bcc phases, such as a¢-martensite or ferrite, is significantly greater than in fcc phases, such as austenite.[4,12] a¢-martensite phase accelerates hydrogen diffusion, but the solubility of hydrogen in a¢-martensite is much lower than in austenite. Hydrogen tends to gather in the phase boundary between a¢-martensite and austenite causing local strain and enhancing delayed cracking initiation.[13,14] Austenite stability is strongly dependent on alloying and the stacking fault energy (SFE) of the material. Low SFE decreases the thermodynamical stability of austenite and leads to higher amount of strain-induced a¢martensite in forming.[15–17] Decreasing the nickel cont
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