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Computer simulation of heat treatment processes has improved significantly over the past two decades, relating to both the material models and the database accuracy. Simulations are being used more aggressively in part and process design rather than just as a trouble shooting agent, meaning heat treat simulation is maturing as an accepted technology. In this paper, an induction heating and spray quenching process of a steel gear is optimized using the commercial heat treatment software DANTE. As a hardening process, induction hardening of steel parts is gaining popularity due to: (1) the process is consistent on a part-to-part basis, making it easier for quality control; (2) the process is more environmentally friendly than oil or polymer immersion quenching because a water spray is the typical cooling agent; and (3) fatigue life can be improved due to higher and deeper residual compression in the hardened surface. Two steel grades, AISI 5120 and AISI 5130, may be used in a thin-walled spur gear. The simplified heat treatment process includes vacuum carburization, controlled cooling to ambient, induction heating, and spray quenching. During induction heating, the internal heat generated by eddy currents is used as direct input to drive the thermal model. A sensitivity based optimization method is combined with heat treatment simulation to optimize the delay and spray quenching practice after induction heating. The objective function is defined to minimize the distortion of the gear tooth while satisfying the residual stress, gear surface temperature, and final microstructure requirements. The results of a two-dimensional (2D) plane strain single tooth model are evaluated during the optimization process, the quenching practice variables are adjusted, and the quenching model is re-run until the optimization function is satisfied. The residual stresses predicted after induction hardening are then mapped to a three-dimensional (3D) whole gear model as the initial stress state of a tooth loading model for predicting gear stresses. The significant effect on fatigue behavior due to residual stresses from heat treatment is addressed.
Procedures are presented for analyzing the cyclic stability and influence on fatigue resistance of residual stress patterns arising from mechanical and thermal surface processing treatments such as shot peening and induction hardening. Cyclic properties and behavioral trends developed using smooth, axial specimens of steels simulating the various microstructures found in surface processed componentry are used to develop criteria to predict cyclic stability, the rate of relaxation for prescribed straining levels, the failure initiation point (surface or subsurface), and expected fatigue lifetime. The validity of the approach is verified using experimental data from the literature. Finally, the incorporation of these procedures in modern computer-based fatigue analysis routines, and opportunities for further enhancements, are discussed.
The work in this thesis examines the effect of mean stress on the fatigue behaviour of very hard (Rockwell C 60) steels (AISI 8822, 8620, 9310, and cold-worked pre-stressing wire). In the mean stress tests, the minimum stress in the fatigue cycle was varied from test to test over a range from -1200 MPa to a value approaching the true fracture stress of each material. The results are not adequately explained by current theories for the effect of mean stress on fatigue behaviour in the region of compressive mean stresses. All current theories suggest that the maximum stress at the fatigue limit decreases with decreasing minimum stress. The results of this study shows that instead of continuing to decrease with decreasing minimum stress the maximum stress at the fatigue limit remains constant indicating an insensitivity to the minimum stress in the fatigue cycle for minimum stresses below the value in a fully reversed fatigue test. The theory proposed by the author corrects this error by maintaining the maximum stress at the fatigue limit constant with decreasing minimum stress in the region of negative mean stresses. The results are of interest to designers of components in which high negative residual stresses are introduced into materials hardened by, for example, carburizing, nitriding, or induction hardening to improve the fatigue strength of components. The present work allows considerably higher design stresses for operating stresses in the negative mean stress region than previous theories permit.