Laser Beam Direct Energy Deposition of graded austenitic-to-martensitic steel junctions compared to dissimilar Electron Beam welding

Abstract

This article presents the Laser Beam Direct Energy Deposition (DED-LB) process as a method to build a graded austenitic-to-martensitic steel junction.Builds were obtained by varying the ratio of the two powders during DED-LB processing. Samples with gradual transitions were successfully obtained using a high dilution rate from one layer to the next. Long austenitic grains are observed on the 316 L side while martensitic grains are observed on the Fe–9Cr–1Mo side. In the transition zone the microstructure is mainly martensitic.Characterisations performed after building and after a tempering heat treatment at 630 °C for 8 h were compared to dissimilar Electron Beam (EB) welds. Before heat treatment the DED-LB graded area has high hardness (values of around 430 HV) due to fresh martensite formed during building. Tempering heat treatment reduces this hardness to 300 HV.EDS measurements indicate that the chemical gradient between 316 L and Fe–9Cr–1Mo obtained by DED-LB is smoother than the chemical change obtained in EB welds. Microstructures in DED-LB are quite different from those obtained by EB welding. Hardness values in DED-LB samples and in welds are similar; the weld metal and the Fe–9Cr–1Mo heat-affected zone are relatively hard after welding because of fresh martensite, as found in the DED-LB transition zone; both are softened by tempering heat treatment.Tensile tests show that DED-LB samples and EB welds have similar behaviour with failure in 316 L base metal at 20 °C and 400 °C and failure in Fe–9Cr–1Mo base metal at 550 °C. DED-LB samples have comparable mechanical properties to EB welds.