Effects of Isothermal Treatment on Amount of Retained Austenite in Cr Cast Irons

Abstract

  Four types of alloys were prepared, in which C and Cr contents were about 2 and 8%, respectively, while Ni contents were changed in the range of 0 to 2%, and Mo contents were 1 or 2%. The previous paper showed that there exists a window between pearlite and bainite transformation at around 773K in the TTT diagrams for these cast irons. The specimens were heat treated at 1273K for 7.2 × 10³s, cooled down to the temperature of 773K with a cooling rate of 0.3K/s, and then isothermally tempered various times. The hardness of the specimens increased with increasing holding time and then decreased after peaking at 24.5 × 10⁴s. The increase in hardness was mainly attributed to the formation of martensite.

  Although the amount of retained austenite decreased with the holding time, the increase in the area fraction of pearlite was observed in the prolonged treatment specimens. The addition of Ni and Mo led to increased hardness with suppression of pearlite transformation during treatment, while the amount of retained austenite also increased.