An analysis of grain boundary cohesion in precipitation hardened stainless steel

Abstract

17-4 precipitation hardened (PH) stainless steel belongs to the family of precipitation hardening martensitic stainless steel (AISI 630), containing 17%Cr, 4%Ni and 4%Cu as the main alloying elements; carbon content in the steel is normally maintained below about 0.05 wt.%. The authors have recently attempted an assessment of 17-4 precipitation hardened martensitic steel in terms of processing steps and heat treatment with a view to obtain a rationale of the quantitative effects of grain boundary segregation processes. Auger electron spectroscopy (AES) studies enabled an understanding of a striking variation in impact toughness as a function of carbon content to emerge on the basis of Nb-C-P interaction processes. In order to understand the factors responsible for the large variation in the impact toughness of the industrially important 17-4 PH stainless steel, a series of steels with varied % of carbon (0.02 to 0.09 wt %) but about the same levels of the Nb (0.28wt%) and P(0.03 wt%) was selected, since it was felt that Nb/C ratio may play a pivotal role in influencing the impact toughness. Two distinct regimes were delineated (a) a low toughness regime (Nb/C ratio > 6 but less than about 20) characterized by high grain boundary concentrationmore » of phosphorus and formation of NbC in the grain interior owing to strong Nb-C interaction, (b) a high toughness regime (Nb/C <6), where grain boundary P is displaced by C through site competition. In this paper the authors attempt thermodynamic analysis in terms of free energy of segregation for the aforementioned two regimes.« less