Changes in the fine structure of the austenite retained when high-speed steel is tempered

Abstract

1. At tempering temperatures of 200–600° C, the diffusionless increase in the lattice parameter of retained austenite is replaced by a diffusional decrease in this parameter. 2. The maximum displacement of the iron atoms from their equilibrium positions caused by three-dimensional compression of the retained austenite amounts to 0.008 A. 3. The decrease in hardness in the first stage of tempering is accompanied by a diffusionless increase in the retained austenite lattice parameter, a breakdown of the mosaic blocks and a reduction in the 2nd-order distortions. 4. During tempering, until the steel reaches maximum hardness, there is a continuous process of simultaneous hardening and softening of the retained austenite, due to precipitation-hardening, distortionless strain hardening by the martensite, and recrystallization. This is accompanied by an increase in the 2nd- and 3rd- order distortions and mosaic block size. The degree of strain hardening depends on the relative magnitudes of these three factors. 5. The reduction in the lattice parameter of the retained austenite at temperatures above 540° C is due to loss of carbon, and partially to loss of alloying elements. These details confirm the theory about the aging of retained austenite, which causes a secondary martensite transformation during cooling. 6. The temperature at which the martensite transformation begins during multiple tempering depends on the magnitudes of the 3rd-order static distortions in the retained austenite, and does not depend on the 2nd-order distortions or the unit cell dimensions; the greater the distortions, the more does the retained austenite become stabilized.