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Induction hardening response of a carburized 4121 steel
Tanous, Benjamin H.
Tanous, Benjamin H.
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2024
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Abstract
Carburizing and induction hardening are both surface heat treatments that are commonly used to increase fatigue performance and wear resistance, especially in the automotive industry for parts subject to cyclical torsional loading such as constant-velocity joints, steering shafts, and driveshafts. It is known in industry that applying an induction hardening treatment to previously carburized parts can potentially increase fatigue performance, and this work aims to provide further understanding as to why.
SAE 4121 steel modified using an industrially relevant modification of 0.84 wt pct Cr was chosen for this study due to industrial relevance and high hardenability. The cylindrical torsional fatigue specimens underwent a carburizing heat treatment to either 1.0 mm or 1.5 mm defined case depth, followed by a single shot induction hardening heat treatment to either 0 mm, 2.0 mm, or 3.0 mm defined case depth.
Microstructural characterization techniques include light optical and scanning electron microscopy, including a quantitative analysis of near surface prior austenite grain size measurements and identification of non-martensitic transformation products near surface intergranular oxidation. Mechanical property characterization techniques include post-heat treatment dimensional analysis of the fatigue specimens, radial hardness profiles, carbon gradient analysis, torsional fatigue testing, and residual stress vs depth measurements. Finally, a fracture study was conducted to elucidate differences in fracture behavior and/or fracture toughness between the as-carburized and carburized plus induction hardened conditions.
The results of the torsional fatigue testing show that the as-carburized conditions exhibited better fatigue performance than the carburized plus induction hardened conditions, which is opposite to what is observed in industry. The residual stress analysis revealed that the carburized plus induction hardened conditions exhibited lower magnitude surface compressive residual stresses than the as-carburized conditions, and the fracture study revealed that the carburized plus induction hardened conditions tended to exhibit intergranular fracture in the overload failure zone while the as-carburized conditions tended to exhibit primarily transgranular fracture. Therefore, the residual stress profiles and potential grain boundary embrittlement of the carburized plus induction hardened conditions are believed to be the primary explanations behind the observed torsional fatigue results.
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