Characterisation and modelling of precipitation during the early stages of tempering for low carbon low alloy Q&T steels

Ju, Yulin (2019). Characterisation and modelling of precipitation during the early stages of tempering for low carbon low alloy Q&T steels. University of Birmingham. Ph.D.

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Abstract

Carbide precipitation and coarsening in three quench and tempered (Q&T) steels were investigated during tempering at 600 °C up to 16 hours to determine the role of alloying additions of Mo, V, Cr and Si. Kinetic modelling (DICTRA model and mathematical model) for carbide coarsening was carried out and compared with experimental observations. Auto-tempering occurred during water quenching with ε’-carbide and cementite present within the martensite laths in the three steels. Coarsening of inter-lath cementite took place independently from that of intra-lath cementite in the three steels during tempering from 2 hrs to 16 hrs. Finer elliptical secondary Mo-V-rich carbides were observed after tempering for 4 hrs in the Base-Mo-V and Base-Cr-Mo-V-Si steels, but they did not result in any noticeable secondary hardening peak. DICTRA model gave coarsening rates that were too slow and significantly deviated from experimental observations in the Base and Base-Mo-V steels. A mathematical (finite difference) model was established for coupled inter-lath cementite particles in the Base steel, where dissolution of the smaller particle and coarsening of the larger one were treated as a continuous and simultaneous process. The mathematical model predicted the shortest dissolution times for smaller particles in different coupled inter-lath cementite systems.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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