Qingzhi Shi

Relationship between austenite stability and martensite formation in modified 9Cr-1Mo steel

Abstract Aimed at investigating the relationship between austenite stability and martensite formation in modified 9Cr-1Mo steel, the fully austenitized steel was cooled to a temperature above or below the start temperature of martensite transformation (Mso, 406 °C), which occurs upon continuous cooling from the austenitizing temperature to room temperature, and held for different times (10 min, 25 min, and 45 min) followed by cooling to room temperature. The concept of primary and secondary martensite is introduced to indicate that two different, sequential, martensites formed in this processing. When cooled to 650 °C or 450 °C for different times, the austenite stability decreased with prolonged holding time, and during the subsequent cooling the primary martensite start temperature (Ms) was raised to a value higher than Mso. In contrast, Ms was reduced to a level lower than Mso when cooled to 420 °C or 410 °C for 45 min. On the other hand, as the steel was isothermally quenched at temperatures below Mso, during the subsequent cooling a secondary martensite reaction could be observed, and the secondary martensite start temperature (Ms′) decreased with holding time. Retained austenite in the form of thin films along martensite lath boundaries was obtained when cooled to 380 °C or below for different times.

A JMAK-like approach for isochronal austenite–ferrite transformation kinetics in Fe–0·055 wt-%N alloy

Abstract The kinetics of the isochronal austenite γ to ferrite α transformation of Fe–0·055 wt-%N alloy was investigated for cooling rates in the range of 5–15 K min− by high resolution dilatometry. In accordance with thermodynamic characteristics of γ→α transformation investigated in this study and previous kinetic theory, a Johnson–Mehl–Avrami–Kologoromov (JMAK)-like approach for the kinetics of isochronal phase transformations was developed that incorporates three overlapping processes: site saturation nucleation, alternate growth modes (transition from interface- to diffusion-controlled growth) as well as impingement for random distribution nuclei. The JMAK-like approach has been employed to fit the experimental results, and the fitting results show that the γ→α transformation of Fe–0·055 wt-%N alloy does have two stages: a first, short interface-controlled growth stage and a second, long diffusion-controlled growth stage. In addition, soft impingement effect has been recognised to become serious in the later part of the second stage.