Martensitic Transformation in High Carbon Steels - Martensite Tetragonality and Impact of Cooling Rate

Abstract: Case-hardened gears are an essential part of our every day life in their role of power transmission in cars and heavy-duty trucks. The increasing use of low pressure carburizing (LPC) for case-hardening in combination with high pressure gas quenching (HPGQ) for cooling opens new possibilities in the design of heat treatments for improved fatigue performance. The use of slower cooling rates through the martensitic transformation regime of the case-hardened gear surface layer has been shown to result in significant fatigue improvements of up to 20 %. The goal of this work is to understand the underlying mechanism of this fatigue improvement. The work is split into investigations of the martensitic transformation in model carbon steels with a homogeneous carbon content and an industrial scale investigation of fatigue performance of LPC and HPGQ case-hardened gears with an in-depth characterization.The impact of three cooling rates, 15, 5 and 0.5 ºC/s, and martensite start temperature (Ms) on the evolution of the martensite tetragonality and phase fraction during cooling is studied by in-situ high energy X-ray diffraction (HEXRD) and electron backscatter diffraction (EBSD). This was done for two high carbon steels, 0.54 wt% C and 0.74 wt% C, to better understand the transformation in different parts of the case-hardened layer. The martensite tetragonality shows a heterogeneous distribution for all samples with a stronger reduction in tetragonality for slower cooling and higher Ms temperature indicating that autotempering occurs during cooling.The correlation of the local martensite tetragonality determined by EBSD and carbon content in atom probe tomography (APT) is investigated for the 0.74 wt% C samples to clarify if autotempering has taken place during cooling. A novel approach is used for the selection of the site-specific lift-out position of the APT tips based on EBSD maps of local martensite tetragonality with low and high tetragonality values. The average bulk carbon content of low and high tetragonality regions have similar values compared to the nominal composition, but there are clear differences in the carbon distribution. The low tetragonality regions have a stronger heterogeneous carbon distribution with homogeneously distributed carbon enriched or depleted volumes, while the high tetragonality regions show a more homogeneous carbon distribution with few elongated volumes with carbon agglomeration. The clear correlation of the local martensite tetragonality with the local carbon distribution shows that autotempering takes place during cooling.The industrial scale investigation of two steel grades, 20MnCr5 and 17NiCrMo6-4, reproduced the positive effect of slower cooling during HPGQ on the fatigue performance. The surface hardness, case-hardness depth and martensite variant pairing can be excluded as possible mechanisms of the fatigue improvement based on the characterization results, while only small differences are observed for the core hardness, retained austenite content and martensite unit size distribution. The residual stress profile provides no consistent explanation for the improvement since a clear change is only observed for 20MnCr5, whereas 17NiCrMo6-4 shows no effect. The possible effect of different degrees of autotempering for slow and fast cooling on the carbide precipitation kinetics resulting in the fatigue improvement is discussed based on the results from the model steel studies and literature.Finally, the early stages of the martensitic transformation in the 0.74 wt% C samples is studied due to the observation of a two-step process in the transformation kinetics. A heat treatment with isothermal hold at the end of the first stage slightly below Ms temperature is used to achieve a mixed microstructure consisting of tempered martensite formed before the isothermal hold and fresh martensite formed afterwards. The tempered martensite units are determined based on their tetragonality in EBSD and a good agreement with conventional light optical microscopy (LOM) is observed. This opens new possibilities for in-depth statistical analysis. The early formed martensite units are formed and grow dominantly along the prior austenite grain boundary (PAGB). Additionally, the large martensite units are mainly formed in the first transformation step with a strong plate martensite crystallographic character.Overall, this thesis studies the martensitic transformation in high carbon steels with a focus on martensite tetragonality and the impact of the cooling rate during HPGQ. Clear influences of the cooling rate on martensite tetragonality and fatigue performance are observed. In addition, new experimental approaches for the local correlation of martensite tetragonality and carbon content as well as for the determination of tempered martensite in a mixed microstructure are presented.Keywords: High carbon steel, Martensitic transformation, Martensite tetragonality, Fatigue performance 

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