Microstructure and mechanical properties of Fe-Cr-C-Nb white cast irons

摘要

The microstructure and properties of 17 wt.% Cr-2.9 wt.% C cast irons with up to 3.17 wt.% Nb additions, in both as-cast and heat treated state, have been studied. Also the influence of titanium and cerium on the structure and properties of 17 wt.% Cr-2.9 wt.% C-2 wt.% Nb alloys are examined. NbC carbides present in the structure of tested alloys, due to their characteristic morphology, show higher wear resistance and toughness than M_7C_3 carbides. Increasing amount of this type of carbides, caused by the increase of niobium in Fe-Cr-C-Nb alloys, contributes to the improvement of wear resistance and dynamic fracture toughness. The alloy containing approximately 3% Nb gives the best compromise between wear resistance and fracture toughness. This alloy shows about 30% greater dynamic fracture toughness and about 30% greater abrasion wear resistance than the basic Fe-Cr-C alloy. Titanium and cerium affect the crystallisation process of Fe-Cr-C-Nb alloys and the transformation of austenite during the cooling after solidification. The addition of 0.26% Ti results in a substantial change in the morphology and distribution of NbC carbides. The secondary carbides which precipitate in the matrix regions of the tested 17 wt.% Cr-2.9 wt.% C-2 wt.% Nb white iron containing titanium has an impact on the abrasion behaviour and fracture toughness. The alloy containing 0.28% Ti and 0.19% Ce has pearlite-austenitic matrix microstructure in as-cast condition. The pearlite, due to its high microhard-ness, improves the wear resistance under low-stress abrasion conditions, but drastically reduces the toughness of tested alloy.