On the effects of core microstructure on distortion behavior and mechanical properties of case-carburized steel parts

Controlling the heat treatment-induced distortion of power transmission components have constantly been a serious challenge for engineers of the field and is continuously gaining importance by current trends of downsizing and lightweight construction. Distortion gives rise to unequal distribution of load, noise generation, premature damage and fracture of these key engineering parts. Distortion of case-carburized steel components necessitates additional costly processes such as hard grinding to correct the distortion and meet the severely restricted dimensional requirements. This not only imposes additional expenses, but also removes a part of the hardened surface, thus, significantly affects the total production efficiency and cost. Hence, it is of crucial importance to gain more insight into the underlying factors governing the distortion mechanisms. Controlling and minimizing the distortion becomes even more important when considering the strict regulations applied by automotive sector to decrease engine size, increase power density and extend the lifetime of driveline systems.Distortion can be caused by numerous factors, some of them can be controlled to avoid or minimize the distortion, whereas the others are inevitable. Furthermore, some of the distortion-causing factors are rooted in the initial manufacturing steps and are transferred along the process chain, the so-called carriers of distortion potential, as per the terminology adopted in the Collaborative Research Center SFB 570 Distortion Engineering. On the other hand, the other factors are only distortion potentials introducing distortion in an individual step and are not transferred to the following steps. The present research aims at understanding the effects of hardening parameters (hardening temperature and duration) following the carburizing segment on microstructural changes of steel parts, the resulting distortion and mechanical properties as well as the corresponding bending fatigue behavior. The research objectives were targeted to achieve in the course of three consecutive stages whose outcomes were appeared as research papers.In the first step, a case hardening steel was designed which was then laboratory melted to fabricate testing samples. The corresponding alloying elements were adjusted in a way so that the ferrite stabilizing elements such as Si and Mo and the austenite stabilizing elements such as Mn and Ni are set to the maximum and minimum permissible limits, respectively, which are common in the currently-in-use commercial case hardening grades. Navy C-ring specimens were fabricated and employed to investigate the effects of the hardening parameters on resulting distortion. The specimens underwent two different case hardening routes. In the first route, the low-pressure carburized specimens subjected to high pressure gas (N2) quenching from a hardening temperature (860 °C) at which the microstructure of the specimens’ core (which contains the base carbon content) is in a fully austenitic region. In contrast, the specimens of the second cycle were gas-quenched from a lower hardening temperature (775 °C) where ferrite is thermodynamically stable and can develop up to around 30% in the core region. The alloying elements of the investigated steel were basically designed in a way so that by decreasing the hardening temperature, carbides (particularly cementite) which are susceptible to form on the carburized surface are suppressed. It is shown that the lowered hardening temperature results in 27% less distortion as compared to the other cycle while retaining the hardness properties in core and case of both cycles similar. Comprehensive microstructural investigations including light optical microscopy (LOM), Electron Backscatter Diffraction (EBSD), various hardness measurements including microhardness and nanoindentation tests, also mini-tensile tests and X-Ray diffraction (XRD) measurements were also carried out to better correlate the microstructural variations and the resulting distortional and mechanical properties. Additionally, the experimental investigations were supported by thermodynamic-based calculations using the software Thermo-Calc/Dictra®. The results of this part of the research were compiled in paper No. 1.In continuation, the impact toughness of the microstructures developed in the Navy C-ring specimens’ core were examined by employing the mini-Charpy V-notch specimens. Comprehensive microstructural and hardness investigations were additionally carried out. It was shown that due to the formation of bainitic microstructure and the associated higher fraction of retained austenite in the specimens quenched from the higher hardening temperature, better impact toughness properties can be achieved. Besides, performing the multi-phase field modelling using the software Micress® revealed that the higher fraction of retained austenite in the specimens of the first cycle (860 °C) is attributed to the higher level of carbon enrichment in the adjacent austenite during the formation of bainitic ferrite. The results of these investigations were appeared in paper No. 2.The heat treatment-microstructure-fatigue performance correlations were studied in a further step. V-notched specimens subjected to the investigated heat treatment cycles and underwent 4-point cyclic plane bending experiments using a hydraulic testing machine with a loading ratio of 0.5 at 30 Hz frequency and at room temperature for 5 million cycles. The fatigue crack propagation beneath the notch root were also monitored and quantified using an optical measuring system Aramis®. The bainitic-martensitic core microstructure of the carburized V-notched specimens indicated a slightly better bending fatigue performance as compared to the ferrite-containing counterparts. The better fatigue properties of the former are mostly attributed to the fatigue crack-deflecting effect of bainitic microstructures associated with the numerous high-angle grain boundaries. Additionally, the presence of retained austenite in the core microstructure, which was confirmed by the electron probe microanalysis (EPMA), assist to alleviate the stress level ahead of the crack tip (TRIP effect), hence, slows down the fatigue crack growth rate. Moreover, it is further argued that the ferrite-martensite interfaces in the core microstructure of the second group (775 °C) act as preferred sites for crack nucleation due to sharp hardness differences, hence a degraded fatigue performance. Paper No. 3 details the finding of these investigations.In short, it was shown that by proper selection of alloying elements and heat treatment parameters the case hardening-induced distortion can substantially be suppressed down to 27%, the hardness properties can be maintained similar and without significant loss in the bending fatigue properties. Impact toughness of the core microstructure however degrades