Effect of Repeated Tempering on Hardness and Retained Austenite of High Chromium Cast Iron Containing Molybdenum

Abstract

Two series of hypoeutectic high chromium cast iron specimens containing 16 mass% Cr and 26 mass% Cr with and without Mo were prepared to study the variation of hardness and volume fraction of retained austenite (Vγ) during repeated tempering. In the as-hardened state, the hardness did not change in the 16 mass% Cr cast irons, but it increased gradually in the 26 mass% Cr cast irons as their Mo content increased. The Vγ rose gradually with an increase in the Mo content of both series of cast iron specimens. In the tempered state, the hardness decreased and then increased with a successive number of tempering cycles. In other words, a secondary hardening was evident due to the precipitation of secondary carbides in the austenite and the transformation of destabilized austenite into martensite during cooling. The hardness increased with rising tempering temperature. The Vγ decreased gradually with repeated tempering and increasing tempering temperature. The maximum tempered hardness (H Tmax ) was obtained at 723-798 K after triple tempering. The H Tmax increased proportionally with a rise in Mo content in both the 16 and 26 mass% Cr cast irons. The highest values of H Tmax were 900 HV30 in 16 mass% Cr and 950 HV30 in 26 mass% Cr cast irons, respectively.