Influence of Carbonitriding Process on Phase Transformation during Case Hardening, Retained Austenite and Residual Stresses

The carbonitriding process is a surface hardening technique with an ultimate goal of improving surface hardness, fatigue properties and resistance to wear of highly stressed parts. As opposed to carburizing process which enriches engineering components with carbon atoms only, carbonitriding introduces both carbon and nitrogen atoms in the surface layer. The presence of nitrogen stabilizes austenite and depending on the level of carbon and nitrogen content reached, as high as 70 mass-% of austenite can be retained. The thermal and mechanical stability of such high amount of retained austenite is vital as retained austenite should remain stable to avoid shape and dimensional changes especially in close fittings. Moreover, such high amount of retained austenite affects the nature, magnitudes and distribution of residual stresses which can influence the service properties. In the present work, the influence of carbonitriding process on the phase transformation during case hardening, retained austenite and residual stresses were investigated. In particular, the following points were taken into consideration: (1) characterization of the state after carbonitriding, (2) analysis of the state during and after tempering, (3) investigation of the state after tempering coupled with cryogenic treatment, (4) investigation of the state after thermal stabilization, and (5) investigation of the mechanical stability of carbonitrided samples. Five carbonitriding variants with different carbon and nitrogen contents were considered. The phase compositions and residual stress analysis was carried out using X-ray diffraction. For each variant, the amount of retained austenite was dependent on the level of carbon and nitrogen reached which in turn depends on the carbon and nitrogen potential in the carbonitriding atmosphere. Besides the misfit between the case and the core, the amount and distribution of retained austenite in the case affects the nature, magnitudes and distribution of residual stresses in both retained austenite and martensite phase. The thermal stability of retained austenite and residual stress relaxation during the process of tempering was captured in situ, using a diffractometer equipped with a position sensitive detector with high resolution and a heating system. This study establishes the range of thermal stability of retained austenite and its kinetics of decomposition during continuous heating and isothermal holding. Further, it helped to quantify the magnitudes and kinetics of residual stress relaxation. Analysis of state after cryogenic treatment revealed that indeed tempering prior cryogenic treatment does stabilize retained austenite which then becomes difficult to transform to martensite during cryogenic treatment. The new martensite formed during the cryogenic treatment enhances significantly the compressive residual stresses in the martensite phase. Via shot-peening treatment it could be revealed that retained austenite was mechanically unstable and readily transforms; consequently high compressive residual stresses in both retained austenite and martensite phase are resulting.