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

Massimo Pellizzari*, Vasco Menegante, Matteo Villa, Marcel A. J. Somers

*Corresponding author for this work

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Abstract

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.
Original languageEnglish
Publication date2019
Number of pages6
Publication statusPublished - 2019
Event26th IFHTSE Congress of Heat Treatment and Surface Engineering - Expocenter Fairgrounds , Moscow, Russian Federation
Duration: 17 Sep 201919 Sep 2019

Conference

Conference26th IFHTSE Congress of Heat Treatment and Surface Engineering
LocationExpocenter Fairgrounds
CountryRussian Federation
CityMoscow
Period17/09/201919/09/2019

Keywords

  • Tool steel
  • Cryogenic treatment
  • Retained Austenite
  • Dilatometry
  • Differential Scanning Calorimetry
  • XRD
  • Synchrotron

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