Trade-off between strength, ductility and toughness of laser directed energy deposition repaired 34CrNiMo6 steel with different heat treatment schedules

In this work, the wrought 34CrNiMo6 steel has been repaired by the laser directed energy deposition and then heat treated by two different schemes: the quenching & tempering (QT) and the tempering (T). The cross-regional microstructure evolution has been examined by various characterization methods. The tensile and shear tests were conducted to evaluate the quality of the repaired samples. The results indicated that the inferior ductility and interfacial toughness of the as-repaired steel were due to the bainite and martensite within the deposition region and heat affected zone (HAZ). After the QT treatment, all heterogeneous microstructure has transformed to the tempered sorbite, which improves the ductility and interfacial toughness at the cost of slightly sacrificed strength. Furthermore, the T treatment can also soften the hard & brittle structures, thereby avoiding the premature failure. However, due to the lack of austenitization, the heterogeneous microstructure has also slightly deteriorated the ductility and toughness as compared to those of the QT treated repaired steels. This work provides new insights into tailoring the microstructure and mechanical properties of the repaired structural steels.