Effect of nitrogen content on the precipitation and growth behavior of eutectic carbides M7C3 in electroslag remelted 7Cr13 steel

Abstract

The effect of nitrogen content on the type, distribution, morphology, precipitation, composition, growth and aggregation behavior of eutectic carbides in 7Cr13 steel ingots were studied. The results show that the eutectic carbide type (all M7C3) and composition do not change when the nitrogen content in the 7Cr13 steel varies from 0.043% to 0.164%. The increase of N content in the steel refines the eutectic carbides, but the heterogeneous nucleation effect of (Ti,V)N particles increases the number density of eutectic carbides precipitated. As the N content increases, the eutectic carbide area fraction at the edge of the ingot gradually increased. However, the area fraction of carbides first decreases and then increases at other locations in the ingot. Eutectic carbides in steel consist of aggregates of carbide particles of different morphologies and orientations. The higher supersaturation of Cr atoms at the beginning of carbide precipitation in high-N steels will favor the formation of brain-like carbides. The morphology of the eutectic carbide changes from blocky-fibrous (closely linked to the blocky carbide) - spherical to brain-like as the N content increases from 0.043% to 0.164%. The refinement of secondary dendrites and the inhibition of eutectic carbides precipitation by nitrogen make the space for eutectic carbide growth smaller with increasing nitrogen content in the steel, which reduces the precipitation of fibrous carbides (independent existence) that serve as a bridge between different carbides in low-nitrogen steels. This allows the network carbide structure to gradually disappear and promotes the uniform distribution of carbides.