On Laves phase in a 9Cr3W3CoB martensitic heat resistant steel when aged at high temperatures

Abstract

9Cr3W3CoB steels are developed to serve at the temperature range of 620–650 °C, and have been recognized as the most promising martensitic heat-resistant steels for supercritical power plants. Due to the high W and Co contents, the Fe2W Laves phase in such 9Cr3 W3CoB steel possesses some specialties in thermodynamics. In the present research, it was found that even when aged at 800 °C in the 9Cr3W3CoB steel, instead of dissolving, Laves phase formed after 50 h and kept on increasing in size and number density until 1000 h, indicating that the Laves phase was marching for the thermodynamic equilibrium during aging. In this thermodynamic process, the W-rich M3B2 borides in as-received steel and M23C6 carbides were revealed to dissolve, supporting the growth of Laves phase. SEM investigation indicates that Laves phase tended to form clusters, and finally grow as a unit bulk Laves phase with an irregular shape. Besides, Laves phase nucleated next to M23C6 carbides and enwrapped them inside at 800 °C. In addition, the growth processes of Laves phase and M23C6 carbides were a competitive procedure, the coarsening of M23C6 carbides was prior to the growth of Laves phase at 750 °C while the growth of Laves phase was prior to the coarsening of M23C6 carbides at 800 °C.