Tensile behaviour of duplex stainless steel at low temperatures

Abstract

The tensile behaviour of the ferrite and austenite phases of Fe–22Cr–5Ni (wt-%) duplex stainless steel containing a maximum of 17·2% austenite was investigated in the temperature range 65–298 K. The results indicate that mechanical twinning occurred in the testing temperature range, and that austenite impeded the growth of twinning. Mechanical twinning in ferrite was well decorated with a ‘dislocation shell’, and the density of dislocations at the coherent twin boundary and within a twin was much higher than in the matrix above the ductile–brittle transition temperature (DBTT). This supported the occurrence of slip localisation next to coherent twin boundaries. Dislocations in the material with no austenite tested below the DBTT were characterised by coplanar slip dislocation on the { 110} plane, and both coplanar slip on { 110} and cross-slip dislocations were observed above the DBTT. Dislocation in ferrite was negligibly affected by the presence of austenitic particles. Strain induced martensite transformation occurred in austenitic particles at or below 220 K, and the characteristics of the transformation were essentially similar to those in type 304 stainless steel. The DBTT of the material was lowered from ∼140 to 110 K in the presence of austenite, independent of the volume fraction of austenite. This suggests that the decrease in the DBTT of the material was mainly due to austenite scavenging carbon and other interstitial elements from the ferritic matrix. The fracture of the material at low temperatures was primarily controlled by the fracture of twin boundaries in ferrite.