The present work concerns the kinetics of hardness loss during induction heat treating of a AISI 4340 steel. Induction hardening is used in industry as it allows superficial hardening of parts in a very short time and it is “green” manufacturing process. The vast majority of induction hardening is done on ferrite-pearlite structures and low core hardness. However, in this study, the core material is a tempered martensite with hardness of 400-450 Hv for mechanical purposes. Metallurgically, there is a big difference between these two cases as ferrite-pearlite structure is thermodynamically stable, whereas martensite is metastable. Because of the excessive temperature experienced just below the transformed region, the martensite of this region softens and this region is qualified as “over-tempered”. Indeed, this martensitic layer is heated to very high temperature levels without being austenitized and tends to a stable state, i.e., ferrite with dispersed carbides. This return to equilibrium leads to hardness loss. The aim of this study is to predict the affected depth and to quantify the final hardness of this over-tempered zone according to a given thermal cycle, i.e. very short times and high temperatures. Thanks to an experience campaign, the influence of time, temperature and hardness / initial microstructure on the kinetics of softening of the low alloy steel has been studied. Abacus obtained were described using 3 different models and then implemented in a FEM simulation to compare the calculated hardness of the over-tempering area versus the real hardness.
| Date | 25 Mar 2010 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Philippe Bocher (Supervisor) |
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Ducassy, C. (Author),
Bocher (Supervisor),
25 Mar 2010Student thesis: Master's thesis › Master in Engineering: Mechanical Engineering