Characterization of delayed cracking in deep-drawn Swift cups of metastable austenitic stainless steels

摘要

Fracture behaviour of metastable austenitic stainless steels (Cr-Ni grade 301 and low-Ni high-Mn grades 201 and 204Cu) that suffer of delayed cracking after deep drawing was examined. The phenomenon of delayed cracking is known to be related to the coexistence of internal hydrogen, strain-induced α'-martensite and tensile residual stresses. The objective of this study was to maximize the information attainable from the common Swift cup test, which is used to evaluate the formability and delayed cracking susceptibility of sheet metals. Fractures in deep-drawn Swift cups were analysed macroscopically, by fracture surface study and electron backscatter diffraction analysis. Residual stresses in the cups were measured with X-ray diffraction and α'-martensite content using a Ferritescope. Estimation of fracture toughness of the materials was made using two different approaches: measuring the width of shear lips and relating it to the plastic zone size, and measuring the crack-tip opening angle CTOA. On the basis of the results, grade 301 presents the highest fracture toughness, and 204Cu the lowest. Toughness of the materials seems to correlate with their nickel content. The order between the studied materials is similar when considering the susceptibility to delayed cracking.