Kinetics analysis of two-stage austenitization in supermartensitic stainless steel

Abstract

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Formation and stabilization of reverted austenite in supermartensitic stainless steel

“…Since austenite forms by diffusion at low heating rates and thus also during isothermal annealing [3,17], it can be assumed that the composition of austenite in the experimental annealing is close to the predicted equilibrium composition in Figure 8b. In order to evaluate the stability of reversed austenite at different annealing temperatures, the Ms-temperature of reversed austenite with equilibrium composition as a function of temperature is calculated with two empirical formulas.…”

Austenite reversion in low-carbon martensitic stainless steels – a CALPHAD-assisted review

“…At low temperature, enhanced partitioning of Ni is required to form austenite which, together with slow substitutional diffusion kinetics, significantly limits the kinetics of austenite reversion [27,62,68]. Nevertheless, the kinetics of austenite reversion at low temperature were measured to be significantly faster than predicted by modelling of bulk-diffusion, suggesting that grain boundary diffusion and diffusion along dislocations may be important mechanisms that significantly increase the transformation kinetics at these temperatures [27,33,68]. Further, the increased driving force for austenite formation in this temperature range renders also incoherent interfaces [56] [57].…”

“…Figure 2) by bulk-diffusion at temperatures between 600 and 700 °C were assessed in steps of 25 °C by kinetics modelling of diffusion with DICTRA [94] (see Ref. [27] for further details on the kinetics model). Kinetics modelling assumes purely diffusion controlled martensite-to-austenite transformation and local equilibrium at the martensite/austenite interface [95].…”

Kinetics modeling of delta-ferrite formation and retainment during casting of supermartensitic stainless steel

“…This requires diffusion of Cr from ferrite into austenite and diffusion of Ni from austenite into ferrite. It is well known that diffusion of Ni and Cr occurs slower in fcc than in bcc [9,[44][45][46][47]. The decrease of the diffusion rate during cooling thus kinetically freezes the transformation, so that δ-ferrite is retained.…”

“…The tempering treatment leads to formation of reversed austenite in a finely dispersed lamellar morphology on grain boundaries of lath martensite. This is accompanied by diffusion of austenite stabilizing elements into austenite, which stabilize this phase to room temperature [8][9][10][11][12][13]. Since the good mechanical properties of the alloy depend on this stabilization of reversed austenite, it is vital to control the compositional homogeneity of the initial martensitic microstructure prior to tempering.…”