Springer Science and Business Media LLC

The microstructure and mechanical properties evolution of AISI 301LN metastable austenitic stainless steels during cold rolling were investigated. A wide range of cold thickness reduction (10% — 80%) was carried out in a four-high rolling mill at ambient temperature. The X-ray and Feritscope MP30 were used to identify the straininduced α’-martensite phase and its volume fraction, respectively. The microstructure was observed by optical micrograph and the mechanical properties were determined by tensile tests and microhardness. The results show that the strain-induced α’-martensite nucleated at the shear bands intersections and the growth of α’-martensite occurred by the repeated nucleation of new embryos. The volume fraction of strain-induced α’-martensite increased with increasing the cold rolling reduction. In addition, the percentage increased in the tensile strength is the same as that of hardness. The ratio between the average tensile strength and the average microhardness was found to range between 2.82 and 3.17.

The mechanical properties of welded joints in resistance spot welding of DP780 steel were tested, and three different types of welding cracks in welded joints were investigated by optical microscopy, scanning electron microscopy and electron back-scattered diffraction. Finally, the failure mode of the welded joints in shear tensile test was discussed. It is found the shear tensile strength of welded joints can be greatly improved by adding preheating current or tempering current. The surface crack in welded joint is intergranular fracture, while the inner crack in welded joint is transgranular fracture, and the surface crack on the edge of the electrode imprint can be improved by adding preheating current or tempering current. The traditional failure mode criterion advised by American Welding Society is no longer suitable for DP780 spot welds and the critical nugget size suggested by Pouranvari is overestimated.