On the Influence of Deep Cryogenic Treatment on Tempering Transformations in AISI D2 Steels

The aim of this paper is to investigate the effect of deep cryogenic treatment (DCT) on the tempering behaviour of AISI D2 steel. Dilatometry samples were austenitized at 1030°C for 30min and thereafter quenched at 120°C/min. One set of samples was isochronally tempered at a rate of 6°C/min until 800°C to distinguish four tempering stages: i. precipitation of transition carbides (RT-220°C), ii. precipitation of cementite (220-350°C), iii. decomposition of retained austenite (600-650°C) and iv. precipitation of secondary alloy carbides (450-700°C). In-situ Synchrotron X-Ray Diffraction (S-XRD) investigation of the same tempering cycle shows a reduction of the martensite tetragonality in stage i. In stage ii., retained austenite experiences a marked lattice expansion, which can be ascribed to relaxation of phase-specific compressive stresses (stresses of type II) that had built-up during quenching. Stages iii. and iv. start at approx. 450°C with C depletion of retained austenite, as revealed by an anomalously low thermal expansion of the austenite lattice at T>450°C, and continues with a further relaxation of the martensite structure starting at 540°C. Simultaneously, austenite decomposes into ferrite and secondary alloy carbides, which is completed at 650°C. The effect of DCT, carried out by immersion in boiling nitrogen and soaking at -196°C for various times, can be summarized in a) a shift of transition carbide precipitation to higher temperature, b) a more pronounced volume reduction associated with precipitation of cementite and c) a less pronounced volume expansion associated with (partial) decomposition of RA. Very limited effects (if any) of storage time in boiling nitrogen (i.e. 5min vs 29 h or 72 h) could be observed on dilatometry curves and on the structural parameters of austenite and martensite.