Precipitation of carbonitrides and high-temperature strength in heat-affected zone of high-Nb containing fire-resistant steel

The morphology and structure of precipitates in the heat-affected zone (HAZ) of fire-resistant steel were studied by transmission electron microscopy, while the simulated elevated temperature strength studies were carried out to elucidate the effect of precipitation state on strength at high temperature. The precipitates were identified as (Nb, Ti)(C, N) with FCC structure. A high density of fine (Nb, Ti)(C, N) precipitated in subcritical HAZ. As the peak temperature of thermal cycle was increased, the density and size of particles were observed to decrease. During austenisation, precipitates were dissolved mostly in the coarse-grained heat-affected zone and dissolved totally in the fusion line. The higher strength at 600 degrees C was present in coarse-grained heat-affected zone and lower in intercritical heat-affected zone. The different precipitate condition in sub-HAZs led to the different evolution behaviour of carbonitrides on heating and thus high-temperature strength.