Influence of long heat treatments on the laves phase nucleation in a type 444 ferritic stainless steel

Composition, morphology and nucleation of Laves phase particles containing Mo, Nb and Si were investigated in a type 444 (18%Cr-0.4%Nb-0.1%Ti-0.5%Si-2.0%Mo) ferritic stainless steel heat treated at 650 °C for up to 120 h with and without a heat treatment and dissolution treatment at 1050 °C during which all Laves phase dissolve. In situ and extracted precipitate particles were analysed using scanning and transmission electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. An (FeCrSi)2(MoNb) Laves phase, titanium nitrides and niobium carbides were present in the as-received sheet prior to heat treatment but during treatment at 650 °C the number density and size of Laves phase particles increased substantially within 120 h. The morphology of the hexagonal (FeCrSi)2(MoNb) Laves phase was characterized as either plate-like or equiaxed depending on whether the particle had an orientation relationship with the matrix or not, i.e. whether the precipitate nucleated during the heat treatment or was present in the as-received material.