Effects of retained austenite on abrasion wear resistance and hardness of hypoeutectic high Cr white cast iron

Abstract

A hypoeutectic white cast iron of 2.3%C—26%Cr—l%Ni—0.5%Mo was heat-treated with three different methods in order to obtain the specimens with different levels of retained austenite(Vr) in the matrix. Then, the effect of the Vr value on the abrasion wear resistance, and the macro- and micro-hardness of the iron was investigated to derive a mutual relationship among them. While the Vr value is 70.2% in the as-cast specimen, those of the heat-treated specimens range from 0.3 to 65.4%, depending upon the types of heat-treatment. In the scratching abrasion wear test, a linear relationship between wear loss and wearing time was obtained after the primary stage of unsteady wear in all the specimens. Therefore, the rate of wear(Rw: mg/s), equivalent to the slope at the portion of straight line, was adopted as an index of wear resistance. The Rw values range from 2.77×10−2 to 4.12×l0−2, the lowest Rw being obtained in the as-cast specimen with the highest Vr value and the highest Rw in the homogenized specimen with the lowest Vr value. The Rw value decreases with an increase in the Vr value probably due to a transformation of austenite into martensite during abrasion wear test. While the range of micro-hardness is from HV 358 to HV 756, the macro-hardness ranges from HV 529 to HV 785 due to the involvement of eutectic M7C3 carbides in measurements. The highest hardness value is obtained in the specimen heat-treated by fan-air hardening with austenitizing condition of 1323K for 7.2ks. On the other hand, the lowest hardness is obtained in the as-cast specimen due to the presence of large amount of austenite and of less martensite, and the absence of secondary carbides. Mutual relationships among Rw, Vr and hardness show a good correspondence.