Forged steel blocks of large dimensions are widely used for plastic injection moulding, used in the automotive industry. The in-service properties of these materials are very sensitive to the conditions of the production process.
The heat treatment (quenching-tempering) of the steels currently used is no longer sufficient as thermomechanical treatments have a decisive influence on the properties to meet the growing requirements of the casting industry. The interactions between the chemical composition, characteristics of steel, and the multiple final parameters of the manufactured parts make it difficult to obtain the desired properties. For this work, it was necessary to control the parameters influencing the hardening process.
First, the effect of the heating rate was investigated. More precisely, the kinetics of austenite formation in the surface and centre regions of a large forged ingot was studied by high-resolution dilatometry. The starting microstructures of the surface and central regions have different proportions of bainite and residual austenite as well as different sizes of primary austenite grains. Two heating rates representing the actual heating rates in the surface (5°C/s) and central (0.5 °C/s) regions of large forged blocks were used. The dilatometric curves revealed only one transformation step for austenite formation for both heating rates, regardless of grain size or phase proportion. The kinetic parameters of austenite formation were determined from dilatometry data using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The JMAK coefficients, n and k, were determined for each condition of the steel studied. Calculations indicated that the nucleation and growth of austenite in the surface region was accelerated more than 10,000 times due to the average austenite grain size being one-quarter smaller, the stability of the retained initial austenite and the accumulation of coarse carbides on the surface.
Secondly, the formation of austenite maintained at different austenitizing temperatures and its transformation into martensite and/or bainite at two different cooling rates are studied. The selected cooling rates represent the actual cooling rates of the surface and centre areas of a large forged ingot during the cooling cycle. It can be seen that by increasing the austenitizing temperature from 850 to 950 °C, martensite becomes dominant on the surface and its fraction increases from 2% to 100% respectively. On the other hand, although bainite is dominant in the center but its fraction decreases from 87% at 850 °C to 76% at 950 °C. The calculation of the equilibrium carbon content of austenite at different austenitizing temperatures and its relationship to the experimental results shows that the nominal content (0.35 wt %) will be obtained by austenitizing at 950 °C. In addition, the surface cooling rate is higher than the critical rate required to activate the martensitic transformation. For this reason, the formation of martensite on the surface becomes possible by austenitizing at 950°C.
Finally, the influence of austenitizing time on phase transformations was studied on the basis of the actual heating and cooling conditions of the processes. The results first showed that after 5 h of holding and more, a mixed microstructure composed of bainite and martensite no longer exists. The volume fraction of martensite increases between 0.5 to 12 h of surface holding time, then decreases during 24 h of holding time. The volume fraction of the retained austenite decreases in surface area and the minimum value is reached at 12 h then starts to increase again. Therefore, a holding time of 12 h would be appropriate for an austenitizing time at 900 °C for a surface region with less austenite retained. For the central region, the volume fraction of bainite decreases and the volume fraction of austenite continuously increases. However, the carbon content of martensite and bainite becomes almost similar and around 0.4% by weight, thus less distortion due to the different solid phase transformations at the center and surface. The hardness values remain constant for both regions after 5 h. The difference in values between the surface (614 HV) and the centre (438 HV) is due to the grain size and carbon content. The kinetic study of bainitic transformation for different austenitizing times showed that for a holding time of 12 h or more, the nucleation site and growth rate do not change significantly and must be avoided.
| Date | 23 Apr 2019 |
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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) & Mamoun Medraj (Co-supervisor) |
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Ben Fredj, E. (Author),
Jahazi (Supervisor) & Medraj (Co-supervisor),
23 Apr 2019Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering