T-TRIP: Investigation of transformation induced plasticity during precipitation formation in quenched and tempered steels and maraging steels

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The T-TRIP project aims to investigate more closely the so far not systematically studied transformation induced plasticity (TRIP) during the precipitation formation in steels under mechanical stresses. For this purpose, a selection of steels from the class of martensitic aging (maraging) steels and tempering steels are investigated as example materials. The choice of these steel classes results from important technological applications. Heat-treated steels are usually tempered after hardening, resulting in carbide formation. Since residual stresses due to manufacturing are commonly present in steel components, the precipitation reaction occurs under the influence of an internal load. This applies in particular to modern surface hardening methods by means of electro-magnetic induction or lasers. Martensitic aging steels are used, for example, in welding processes or in additive manufacturing. These processes result in extremely high cooling rates and associated thermally induced residual stresses. Due to the layer wise processing, re-heating of already solidified areas occurs and a precipitation formation under the influence of stresses and increased temperatures is initiated. From both classes steel compositions which differ in the volume content of the precipitates will be selected for this investigation. The experiments include dilatometry for the measurement of the TRIP effect at various applied tensile and compressive stresses as well as isothermal and continuous temperature control. In addition, the influence of the applied stress on the resulting mechanical properties of the material is to be investigated. As indicators for this, the yield strength and the hardness after the heat treatment are to be determined. In the next step, the transferability between the steel classes will be examined in order to determine the influence of the precipitation type (carbides in tempered steels as well as intermetallic phases in martensitic aging steels) on the transformation induced plasticity. The properties of the matrix as well as the morphologies of the precipitates will be characterized by means of X-ray and electron microscopic methods.The findings of the mechanical experiments are then to be interpreted and evaluated with the help of the results from the microscopic characterization in order to give a more detailed insight into the mechanisms during martensitic aging and tempering. The determined dependencies of the measured transformation induced plasticity will be described by means of suitable models for both steel classes. In particular, the dependence on the precipitated volume content for the respective steels should be correlated with the transformation plasticity constant characteristic of the TRIP effect. Finally, from the results and created models the transfer to the technological application in the field of heat treatment and component distortion can be made possible.