Effect of initial microstructural conditions and tempering parameters on the carbide characteristics and hardness of alloyed quenched and tempered steel

Goodall, Aimee Louise (2020). Effect of initial microstructural conditions and tempering parameters on the carbide characteristics and hardness of alloyed quenched and tempered steel. University of Birmingham. Ph.D.

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Abstract

Commercial production of high strength steel plates by the quenching and tempering (Q&T) route requires control of alloy design and heat treatment parameters to achieve the desired strength and toughness through thickness. For thick plate the difference in cooling rate through thickness affects the as-quenched microstructure with martensite, auto-tempered martensite and lower and/or upper bainite being present. The different as-quenched microstructures can show a different response to tempering which affects the final strength and toughness.

The starting microstructure of a low alloy 0.17 wt. % C Q&T steel has been varied using isothermal heat treatment to create mixed martensite and lower bainite microstructures. The effects of tempering at 600 °C for times between 0.5 and 100 hours on the carbide precipitates and hardness of the mixed microstructures have been investigated and compared to the tempering response of single phase (martensite and lower bainite) microstructures. It has been found that the hardness decrease due to tempering is larger in the martensitic structure than the bainitic structure due to more rapid carbide coarsening. The as-quenched hardness of the mixed microstructures can be predicted by a rule of mixtures using the single-phase properties. The tempering response of the mixed microstructures is discussed.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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