Sigma-phase precipitation upon industrial-like heating of cast heat-resistant steels

In order to investigate the formation of sigma-phase in two cast heat-resistant steels of wide industrial use, two ferrous alloys containing 25.02Cr–4.26Ni–0.53C and 31.23Cr–6.08Ni-0.36C were prepared by casting techniques and then subjected to sigma precipitation by heating the alloys, for increasing periods of time, at a temperature which resembles the average thermal level usually encountered in service conditions of these materials. Initially, the phase nucleates along ferrite–austenite boundaries, but with more time sigma precipitates inside the ferrite grains. Furthermore, both alloys showed a behavior that resembles the well-known Johnson–Mehl–Avramiʹs mechanism, stated for a nucleation and diffusion-controlled growth process in metallic systems. Hence, the phenomenon reached asymptotic values of approximately 25 and 55 vol.% in the HC- and HD-grade, respectively, after 120 h heating at 1053 K. The experimental results may help in controlling the behavior of these materials when they are to be utilized in industrial applications at elevated temperatures.