As a rule, induction surface hardening is carried out industrially by employing polymer solutions since these ensure a more homogeneous quench than immersion cooling in water. Besides reproducing the quenching process, the intention here is to minimise the hardening defects and the distortions arising from the heat treatment. Polymer solutions also have a few disadvantages which include, among others, poor environmental compatibility and handleability. Quenching by means of spray cooling provides an effective alternative. The purpose of the current investigation is to substitute the polymer solution by a water‐air spray in induction hardening equipment for surface hardening spur gearwheels made of 42CrMo4 hardening and tempering steel. The suitability of spray cooling was assessed by means of hardness measurements, residual stress conditions, distortion measurements and by metallographic examinations. Based on the analyses currently carried out, it was possible to show that the two‐phase spray cooling represents an alternative quenching method which produces comparable component properties.
Modified press‐hardening processes are very attractive for manufacturing safety‐relevant vehicle body parts from steels with martensitic–ferritic microstructures. In the process developed, the formation of the two‐phase microstructure and the hot sheet forming simultaneously occur subsequently to an intercritical annealing. By contrast, previously used process chains do not integrate setting of a multi‐phase microstructure within the forming step. In order to successfully combine the intercritical annealing with the actual forming, comprehensive knowledge of the microstructural evolution and the resulting mechanical properties is needed. Specifically, different heat‐treating routes are used to obtain different microstructures of ferritic–martensitic dual‐phase steels and partial martensitic steels. As a result of intercritical annealing in the temperature range of Ac1–Ac3, it is possible to vary the martensite volume fractions from 7 to 96 vol%. The data obtained can be employed for numerically describing the microstructural transformation and for designing the heat treatment process. It is demonstrated that this combined process allows for designing steels that feature properties that are similar to complex‐phase steels.
The use of air‐hardening steels in the manufacture of automobile body components shortens the corresponding process chain by means of eliminating the immersion or gas quenching operation since hardening occurs in still air. To choose the optimal forming and heat‐treatment parameters, the development of the microstructure needs to be considered as a consequence of the thermomechanical production processes. The object of this work is to investigate the effects of forming and the subsequent heat‐treating on the changes of the microstructural parameters for the air‐hardening steel LH800. For this purpose, specimens were processed under different forming and heat‐treating conditions and subsequently examined by means of SEM and mechanically tested using tensile tests. Based on this, an empirical equation was derived which describes the influence of both the martensite's bundle thickness and the original austenite's grain size on the yield stress. In this way, microstructural parameters can be identified which lead to good mechanical properties. The measured correlation can thus be used for modeling the LH800 steel's forming and heat‐treatment processes.
In recent years, high strength steels, particularly press‐hardening steel, have been more extensively employed for manufacturing safety‐relevant structural components in vehicle bodies. These applications require contrasting material properties such as extremely high strengths as well as high forming ductility. Owing to the purely martensitic microstructure, the residual ductility of the conventional press hardened steels is low. Quenching and partitioning heat treatments can fulfil the requirement of an increased residual ductility by stabilizing the retained austenite. Moreover, if the quenching and partitioning heat treatments are carried out after intercritical annealing treatment, then the steel's mechanical properties can be tailored by defining the volume fraction of ferrite in the microstructure. In order to determine the potential of an 1‐step quenching and partitioning heat treatment combined with intercritical annealing processes, elevated contents of retained austenite are produced in ferritic‐martensitic microstructures using the low‐alloyed 22MnB5 steel grade. In addition to a tempered martensitic microstructure, secondary martensite, and a low fraction of retained austenite, the microstructure consists of remaining fractions of ferrite; thus, providing an additional increase in ductility. For this optimized microstructure, a yield stress of 513 MPa and a tensile strength of 1045 MPa are measured with a total elongation of 10.8%.
The application of water-air spray cooling in the process of induction surface hardening according to the simultaneous dual-frequency technology represents a hitherto unexploited and equivalent alternative to conventionally employed polymer solutions. The reason for this is that the selection of the optimum parameters is associated with a high experimental outlay and analytical effort for performing the tests and evaluating the results, respectively, and the parameters must be selected for a specific application. In order to reduce this effort, a numerical model was developed to formulate the coupled thermal, microstructural, and mechanical processes during quenching by means of water-air spray cooling of induction heated spur gearwheels made from 42CrMo4 hardening and tempering steel. The model was implemented in the commercial simulation software ANSYS Workbench 13.0 and verified using simulation results for temperature development, hardness distribution, residual stresses, and distortion. A comparison of the simulated and experimental results show that the model introduced here is suitable for predicting the hardening results during quenching using spray cooling subsequent to induction heating.
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