Microstructure transformation and twinning mechanism of 304 stainless steel tube during hydraulic bulging

Abstract At room temperature, the hollow shaft of AISI 304 stainless steel tubes was produced by a hydraulic bulging process. The behavior of strain-induced austenite to martensite transformation and the twin crystallographic nature of AISI 304 stainless steel tubes at different positions after hydraulic bulging were discussed. The results have demonstrated that strain-induced austenite to martensite transformation occurred in AISI 304 stainless steel tubes during hydraulic bulging, resulting in the formation of the α ′-martensite phase, and the volume fraction of martensite gradually increased with an increase in strain. The austenite and α ′-martensite phases maintained lattice coherency throughout and followed the Kurdjumov–Sachs (K-S) relationship in terms of lattice coherency. During the deformation process, de-twinning occurred in the austenite and the deformation twins were formed in α ′-martensite. With the increase in strain, the volume fraction of the annealing twins gradually reduced until complete disappearance in the austenite. The volume fraction of the deformation twins increased in the martensite with an increase in strain, and finally reached saturation.