Mechanical and Microstructural Characterization of Plasma Welded Joints of 18 Ni-300 Maraging Steels Subjected to Repairs

Abstract

Among the ultra-high strength structural steels, 18Ni-300 Maraging steel is one of the most important for the naval and aerospace industries. For example, in the manufacture of motor envelopes Plasma Arc Welding-PAW processes are used with the keyhole technique. The joints are regions of high responsibility and undergo a non-destructive inspection according to the standardized technical criteria. If there is an unacceptable defect, the weld is partially removed and repairs are made to the original weld. The objective of this work is to analyze the weld repairs in 18Ni-300 Maraging steel sheets from a microstructural, corrosion and mechanical behavior point of view. Plasma Arc Weld (PAW) repairs, up to three repairs, were carried out in the same location and the modifications produced in the Fusion Zone (FZ), in the Heat Affected Zone (HAZ) and surroundings were evaluated. Microstructural changes were observed due to thermal cycling, such as: Phase transformations, formation of precipitates and reversed austenite, mainly altering the Heat-Affected Zone (HAZ) and the transition region to the Base Metal (BM). There is a reduction in hardness in this region with a consequent decrease in tensile mechanical properties. The application of aging heat treatment partially restored the microstructure affected by the welding and repairs; however, the fractographic analyzes show that the most vulnerable region remains the FZ and the transition region between it and the HAZ. Despite this effect, the loss in tensile strength is low and the ductility of the material is preserved, indicating the feasibility of using welding by the PAW process and the use of up to three repairs to the structure.