Microstructure and Dissolution of Eutectic Precipitates in Electroslag Remelted 15Cr–30Ni–2Ti–3Cu Valve Alloy

The microstructure and eutectic precipitates in a novel 15Cr–30Ni–2Ti–3Cu valve alloy produced by electroslag remelting, and the evolution of eutectic precipitates and dissolution kinetics of Laves phase during homogenization were studied. The microhardness of the valve alloy reaches a value of 247.8 HV after the homogenization at 1273 K (1000 °C)/6 hours + 1413 K (1140 °C)/5.5 hours, and the hardness uniformity reaches a maximum after the homogenization at 1223 K (950 °C)/6 hours + 1413 K (1140 °C)/5.5 hours. The secondary dendrite arm spacing in the as-cast ingot is 48.75 μm, and the area fractions of Fe₂Ti Laves phase, (Ti,Nb)C, Sigma phase and γ′-Ni₃(Ti,Al) phase are 2.83, 0.92, 0.09 and 0.01 pct, respectively. The Laves phase dissolved obviously even at the lowest homogenization temperature 1223 K (950 °C). The eutectic (Ti,Nb)C phase begins to dissolve at 1223 K (950 °C), and the dissolution degree is much lower than that of Laves phase. The homogenization degree of alloying elements after single-stage homogenization is much lower than that of dual-stage homogenization. The dissolution kinetics of Laves phase established based on the Johnson-Mehl-Avrami-Kolmogorov model shows that the activation energy of Laves phase in alloy is 275.8 kJ/mol. The reverse diffusion of Mo from Laves phase into matrix is the determining step of Fe₂Ti Laves phase dissolution.