Development of an austenitic/martensitic gradient steel connection by additive manufacturing

This PhD work concerns the problem of bimetallic austenitic/martensitic steel connections. This research action focuses on a 316L austenitic steel (X2 CrNiMo 18-12-02)/Fe-9Cr-1Mo (X10 CrMo 9-1) martensitic steel connection. The objective is to understand the metallurgical problems related to the assembly of these two steels and to evaluate the possibilities of using powder metallurgy and additive manufacturing to produce austenitic/martensitic steel transitions. A weld obtained by electron beam is used as a reference for this study which focuses on the interest of powder metallurgy to achieve a transition between two steels. Materials with a chemical composition gradient have been consolidated by HIP and SPS and show very good mechanical properties and an excellent junction between the two steels. By additive manufacturing (DED-LB or PBF-LB), we also obtain very good bonds between the two steels, but the microstructures are much more complex. Curiously, we observe that the higher the cooling rate, the higher the ferrite fraction in the martensitic steel. Different calculations based on the nucleation and growth of the austenitic phase have made it possible to propose a coherent scenario to explain the phase fractions present in the materials. The transition zone between the two steels shows strong variations in hardness. These variations are explained by changes in chemical composition, leading to modifications in phase change temperatures, and the particular thermal cycles seen during building. From a technological point of view, materials obtained by additive manufacturing have tensile performances very similar to those obtained by electron beam welding. It is shown that additive manufacturing also makes it possible to control the composition gradient between a martensitic and an austenitic steel. (author)