Kinetics Of Isochronal Austenization In The Modified High Cr Ferritic Heat-resistant Steel

Austenization, a important part of heat treatment, makes great influences on thefollowing hot working process, final structure, and properties of the metals. Althougha rapid development in steel research has been sustained in the past years, the kineticsof austenization was scarcely discussed, two competing mechanisms (shear anddiffusion-type) of the growth of austenite grains in the martensite (α)-to-austenite (γ)transformation have been controversial for years. Furthermore,Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is usually used for analyzing thekinetics of binary and ternary-alloy system. Hence, it is necessary to do severalexploratory researches for a multicomponent alloy containing a complicatedcomposition.In this paper, the martensite (α)-to-austenite (γ) phase transformation and ferrite(α)-to-austenite (γ) phase transformation of the modified high Cr ferritic heat-resistantsteel, subjected to various heating rates, were investigated by differential thermalanalysis (DTA), microstructural observation and JMAK model.The results indicate that heating rate makes greater influence on the endtemperatures than the onset temperatures. Higher heating rate would result in highersuperheat as well as larger velocity of transformation. Considering the influences ofdifferent initial microstructures on the austenization, it is found that the onsettemperature and the end temperature of αâ†'γ are higher than that of αâ†'γ under thesame heating rate. Microstructural observation shows that the αâ†'γ phasetransformation brought in grain refinement and improved performance. The activationenergy in αâ†'γ phase transformation is smaller than that in αâ†'γ phase transformation,yet the austenite-formation rate of αâ†'γ phase transformation is larger than that inαâ†'γ phase transformation.Only one peak occurred in the austenite-formation rate curves of the αâ†'γ (αâ†'γ)phase transformation, which indicates that the transformation is normal. A JMAK-likeapproach for the kinetics of isochronal αâ†'γ and αâ†'γ phase transformationsinvolving three overlapping processes: site saturation nucleation, diffusion-controlled growth and impingement correction for random distribution of nuclei.Thermodynamic analysis and kinetics analysis indicate that the mechanisms of theaustenization process in the modified high Cr ferritic heat-resistant steel isdiffusion-type. The growth rate of austenite grains was determined by the diffusionrates of C and other interstitial atoms. The content of chromium, which performs asthe rate-limiting factor for austenization, is relatively large. Cr can promote theformation of carbide precipitates which act as pins hindering the growth of austenitegrains, and thus affect the activation of carbon atoms immensely.