Metastable austenitic stainless steels can be susceptible to delayed cracking after forming processes. The objective of this research was to explain the high susceptibility of low-Ni high-Mn austenitic stainless steels to delayed cracking, and to clarify the role of the different contributing factors, i.e. solute hydrogen, strain-induced α'-martensite and tensile residual stresses, in the phenomenon. Susceptibility of seven austenitic stainless steels, both conventional Fe-Cr-Ni grades and low-Ni grades, to delayed cracking was investigated by means of deep drawing Swift cup tests. Residual stresses in the cups were measured with X-ray diffraction and a ring slitting method. Volume fraction of strain-induced α'-martensite was determined with a Ferritescope. Hydrogen content of the test materials was analysed with hot extraction, melt extraction and thermal desorption spectroscopy. Hardness of α'-martensite and austenite phases was measured using nanoindentation. Additionally, a constant load tensile testing arrangement was developed and applied for systematic study on the role of different contributing factors in delayed cracking kinetics. The presence of α'-martensite was a necessary prerequisite for delayed cracking to occur in austenitic stainless steels with typical internal hydrogen concentrations (<5.5 wppm). Cracking proceeded through α'-martensite phase. Martensitic transformation substantially increased the magnitude of residual stresses in deep-drawn cups. Alloying elements of the stainless steels influenced the sensitivity to delayed cracking through their effect on the austenite stability and properties of α'-martensite. According to nanoindentation measurements the hardness of α'-martensite correlated with the level of residual stresses. Critical combinations of residual stress, α´-martensite and hydrogen content for delayed cracking were specified for each test material. Explanations for the high susceptibility of low-Ni grades were their high residual stresses after forming, and high internal hydrogen content. Lowering the hydrogen content by annealing markedly lowered the risk of delayed fracture. Constant load tensile testing results demonstrated the role of α'-martensite as a medium for hydrogen diffusion. Hydrogen content seemed to have the strongest effect on time to fracture, which supports the assumption that cracking requires hydrogen accumulation at regions of high stress. The results of this research work can be utilized in the design of demanding forming applications, to avoid the risk of delayed fracture. Increased understanding of the delayed cracking phenomenon enables the development of novel cost-efficient, high-strength stainless steels and wider usage of current austenitic stainless steels.
|Translated title of the contribution||Viivästynyt murtuminen metastabiileissa matalanikkelisissä austeniittisissa ruostumattomissa teräksissä|
|Publication status||Published - 2015|
|MoE publication type||G5 Doctoral dissertation (article)|
- metastable low-nickel austenitic stainless steels
- delayed cracking
- strain-induced martensite
- residual stress