Characterisation of precipitation and carbide coarsening in low carbon alloy Q&T steel during the early stages of tempering

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Authors

Colleges, School and Institutes

External organisations

  • Warwick Manufacturing Group, Institute of Digital Healthcare, University of Warwick

Abstract

In order to ensure that appropriate tempering conditions are used to obtain desired strength and toughness for low carbon low alloy quench and tempered (Q&T) steel plates in a range of thickness, it is desirable to be able to predict the effect of composition and tempering conditions (time and temperature) on the microstructure and hence the hardness evolution for these steels. In this paper, carbide precipitation and coarsening behaviour in three low carbon low alloy Q&T steels have been investigated during tempering at 600 °C up to 16 h to determine the role of alloying additions of Mo, V, Cr and Si. It has been found that auto-tempering occurs during water quenching with ε’-carbide and cementite being present within the martensite laths for all three steels. In the Base steel, cementite becomes the stable second phase after 2 h tempering and has an elliptical (sometimes spherical) shape, which coarsens with time during tempering from 2 h to 16 h. However, in the Base-Mo-V and Base-Cr-Mo-V-Si steels, elliptical and needle-shaped cementite (shown to contain Mn, Mo and Cr) both exist during tempering; furthermore, finer elliptical secondary Mo-V-rich carbides are observed after tempering for 4 h. The coarsening of cementite contributing to the softening process in the three steels has been quantified with, most significantly, the inter- and intra-lath carbides coarsening independently. Although fine secondary alloy carbides are observed after 4 h tempering, they do not result in any noticeable secondary hardening peak in the Base-Mo-V and Base-Cr-Mo-V-Si steels.

Details

Original languageEnglish
Pages (from-to)174-189
Number of pages16
JournalMaterials Science and Engineering A
Volume738
Early online date15 Sep 2018
Publication statusPublished - 19 Dec 2018