Thermomechanical Treatment of High-alloyed Martensitic Stainless Steels for Complex Parts

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In addition to their corrosion resistance, high alloy martensitic stainless steels are characterized by a high hardness and strength. The manufacturing of component parts made of these steels is mainly performed by cold forming of the steel in soft-annealed condition. Only after forming, the material is quenched and tempered to adjust the desired final mechanical properties. However, using cold forming, the maximum achievable degree of forming is often not high enough to manufacture parts with complex geometries. This limitation can be overcome by a thermomechanical treatment (TMT), i.e. a hot forming process with an accurate control of forming and temperature regime. The TMT poses technological and material-scientific challenges for the user, since various physical metallurgical phenomena (e.g. phase transformation, recrystallization or formation of chromium carbides) are activated, which have a strong impact on the formability and the resulting mechanical properties (hardness, strength, corrosion resistance). Yet these effects for the martensitic stainless steels have not been investigated sufficiently.In this project, the effect of TMT processing parameters (austenitizing temperature, holding time, degree and temperature of deformation and strain rate) on the formability as well as mechanical and corrosive properties of these steels will be investigated systematically. One aim is to determine basic requirements for a stable TMT process. Furthermore, physical metallurgical phenomena and the microstructural changes activated during TMT, as well as their influence on the kinetics of precipitation and phase transformation, have to be identified. In the work plan, Continuous Cooling Transformation (CCT) diagrams considering the influence of hot prestrains (DCCT) and austenitizing temperature (TTA) will be determined by dilatometry, metallography and instrumented hardness testing. Moreover, the resulting microstructures will be characterized by optical as well as electron microscopy. The experimental program is complemented by thermodynamic calculations. Formability will be evaluated by hot flow curves, the performance of cupping and deep drawing tests as well as the determination of Flow Limit Diagrams (FLD). Hardness measurements and tensile tests will be carried out to characterize the resulting properties. The corrosion behavior will be evaluated on the basis of potentiodynamic polarization testing. With the aid of statistical design of experiments, the experimental program will be defined and the relationships between process parameters and final properties will be identified. On this basis, phenomenological models will be created to be used in a multivariable optimization to define process windows for the TMT in order to develop specific mechanical and anticorrosive properties. The framework of this research project aims on the establishment of stable TMT processes that lead to an enhanced applicability of martensitic stainless steels.