Heat treatment behavior of high chromium cast iron for abrasive wear resistance

Abstract

Plain high chromium cast irons containing 10 mass%, 16 mass%, 20 mass% and 26 mass% Cr and eutectic and hypoeutectic 16 mass% and 26 mass% Cr cast irons with single addition of Ni, Cu, Mo and V were prepared in order to investigate their heat treatment behaviors. In plain high chromium cast iron, hardness in as-hardened state does not change much but volume fraction of retained austenie (V[subscript gamma]) varies greatly at the same chromium level depending on carbon content. The V[subscript gamma] is high at higher austenitizing temperature, and it gradually decreases with an increase in Cr/C value when the Cr/C value is more than 4.0. Curve of tempered hardness shows a secondary hardening. The precipitation hardening is greater in the iron hardened from higher austenitizing temperature. The tempering termperature to obtain the maximum hardness (H[subscript Tmax]) shiffs to the long time side when the3 austenitizing temeprature rise. The H[subscriptTMas) can be uniformly related to the V[subscript gamma] value in as-hardened state irrespective of austenitizing temperature and it increases gradually up to the highest value of around 800 HV30 at 20% V[subscript gamma], and the H[subscript Tmax] is settled even if the V[subscript gamma] increases over 20%. In eutectic high chromium cast iron with alloying element, in as-hardened state, hardness is changed remarkably by V[subscript gamma] which is closely related to kind and amount of alloying element. Ni and Cu decreased and Mo increased the hardness, but V increased it in 16 mass% Cr iron and reduced it in 26 mass% Cr iron. V[subscript gamma] rose with an increase in Ni, Cu and Mo content but it was reduced by increasing V content. An increase in austenitizing temperature made V[subscript gamma] more in all the as-hardened specimens. Curve of tempered hardness showed an evident secondary hardening due to both the precipitation of special carbides formed by carbide reaction and the transformation of destabilized austenite into martensite. The degree of the precipitation hardening was much larger in cast irons with carbide forming elements than that in the irons free of them, except for the irons with V which had lower V[subscript gamma]. The degree of precipitation hardening was greater in the irons hardened from higher austenitizing temperature, and largest in irons with Ni and smallest in the irons with V. In 16 mass% and 26 mass% Cr hypoeutectic cast irons, in as-hardened state, hardness changed remarkably depending on the V[subscript gamma], Ni and Cu decreased hardness and Mo increased it. V increased hardness in 16 mass% Cr cast iron but decreased it in 26 mass% Cr cast iron. The V[subscript gamma] increased with Ni, Cu and Mo content but diminished with increasing V content in 16 mass% Cr cast iron. In 26 mass% Cr cast iron, the V[subscript gamma] increased with Ni and Mo content but decreased with increasing Cu and V content. Higher austenitization caused more V[subscript gamma], Curves of temperedhardness showed an evident secondary hardening due to precipitation of special carbides and transformation of destabilized austenite into martensite. High tempered hardness was obtained in the specimens with high V[subscript gamma] in as-hardened state. Maximum tempered hardness (H[subscript Tmax]) was obtained when V[subscript gamma] was less than 20% and it increased with an increase in Mo content. The H[subscript Tmax] slightly increased with V content in 16 mass% Cr cast iron but decreased in 26 mass% Cr cast iron. Ni and Cu did not show significant effects on H[subscript Tmax]. The highest value of H[subscript Tmax] was obtained in the cast irons containing Mo.