Influence of Stabilizing Elements on Ductile-Brittle Transition Temperature (DBTT) of 18Cr Ferritic Stainless Steels

The influence of different stabilizing elements (Nb, Ti and Zr) which are added to stabilize interstitial elements in the ferrite matrix on the ductile-brittle transition temperature (DBTT) of 18Cr ferritic stainless steels was systematically investigated at different temperatures ranging from -25 degrees C to 75 degrees C. Hot-rolled sheets of 18 mass% Cr ferritic stainless steels containing a trace amount (0.1 atomic%) of Nb, Ti or Zr were studied. The added stabilizing elements formed precipitates of carbides/nitrides in all the steels tested in the present study. The particle diameter of the precipitates observed in the Nb-added steel was much smaller than those in the other steels. The measured DBTTs of the Base steel and the Nb-added, Ti-added and Zr-added steels were 39 degrees C, 3 degrees C, 39 degrees C and 11 degrees C, respectively. Although initiation of cleavage cracks in each steel occurred independent of the kind of stabilizing element, the Nb-added steel exhibited a relatively higher Charpy impact value due to ductile plastic deformation following cleavage crack initiation, resulting in a lower DBTT (superior impact toughness). The occurrence of plastic deformation in the Nb-added steel is considered to be due to the fine carbide and/or nitride precipitates which were homogenously distributed in the material. Detailed analyses of the fracture surfaces were also performed for the other steels, and the results were used as a basis for discussing the effects of each stabilizing element on the DBTT of 18Cr ferritic stainless steels in terms of the precipitation morphologies observed in the ferrite matrix.