This thesis establishes the impact of heat treatment parameters on metallurgical phenomena. It describes the microstructural characteristics of a low alloy steel used for making molds dedicated to automotive components. This investigation is focused on the austenitization, the quenching but above all the tempering which will determine the microstructure and its evolution in terms of phases as well as precipitates.
The heat treatments were produced using a dilatometer to ensure detection of even the slightest transformation. The analysis begins with the determination of the phases formed during cooling at several rates from the austenitization temperature of 900°. Cooling above 0.7°C/s results in martensite and those below give bainite.
Once the CCT diagram is obtained, the impact of tempering, through the heating rate and the temperature, is observed on bainite and martensite. The heating is compared for a slow rate of 0.2°C/s and a fast rate of 5°C/s to visualize the dissimilarities in the precipitation. There are more precipitates in a low-speed heated microstructure for martensite and little or none for bainite.
Then the results obtained at 550°C and 620°C are studied to see the behaviour of the carbides, the decomposition of martensite-retained austenite (M/A) blocks and the evolution of the matrix (bainite or martensite). The decomposition of the M/A blocks is incomplete for both temperatures but is more advanced at 620°C. The transformed part of the M/A becomes chromium carbides and during the cooling of the tempering the remainder turns into martensite. A higher temperature makes the lath more equiaxial and changes the carbides characteristics at the grain boundaries.
An industrial cycle was tested with austenitization at 900°C for 24 hours and tempering at 550°C for 44 hours. The objective is to compare it with the microstructure deemed optimal to decide on the parameters to be modified in order to reduce the total heat treatment time.
| Date | 11 Jan 2022 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Mohammad Jahazi (Supervisor) |
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Hurel, V. (Author),
Jahazi (Supervisor),
11 Jan 2022Student thesis: Master's thesis › Master in Engineering: Mechanical Engineering