17-4PH stainless steel finds application in the aerospace industry owing to its good mechanical properties and corrosion resistance. In the literature, this steel is described as good for welding, but research shows that it may be problematic due to the formation of defects. In this study, the welded joints were made by the robotic TIG welding method with various welding speeds (2 and 3 mm/s). The joints were subjected to non-destructive testing and were free from defects. The microstructure was observed by light microscopy and scanning electron microscopy. Changes in the microstructure of the heat affected zone were observed and discussed. Based on the observation of the microstructure and the change in the hardness profile, the heat affected zone was divided into 4 characteristic regions. δ-ferrite and NbC were observed in the martensite matrix. The welded joints were subjected to heat treatment consisting of solution and aging in 550°C for 4 h. The microstructure of the heat affected zone become homogenized as a result of the heat treatment. The content of stable austenite in the welded joint after the heat treatment was about 3%.

The study was intended to determine the effect of the input condition of the 17-4PH steel on the microstructure, mechanical properties and stress state of welded joints. The steel adopted for testing was in the solution condition at 1040°C, the aged condition at 550°C/4h and the overaged condition at 760°C/2 h + 620°C/4 h. Samples of 17-4PH steel, after heat treatment processed with different parameters, were electron beam welded (EBW). The microscopic observation (LM, SEM/EDS) showed that the microstructure of the weld consisted of martensite with a δ-ferrite lattice. In the heat-affected zone (HAZ), transformed martensite was found with evidence of niobium carbides. The results of hardness testing revealed the different nature of the hardness profile with the condition the material before the EB welding process. The hardness profile of the HAZ of the welded samples in the as-solution (ES2) and overaged (ES12) condition was varied (from about 340 HV to 450 HV). However, in the aged condition specimen of 17-4PH steel (ES22) showed a similar hardness level, at around 370 HV. The solution condition (ES2) had the highest strength properties R_m 1180.6 MPa with the lowest elongation A 7.6% of all samples tested. The aged welded specimen (ES22) retained high strength R_m 1103.4 MPa with a better relative elongation A 10.1%, whereas the overaged welded specimen (ES12) saw a reduction of strength R_m 950.4 MPa with an improvement in plastic properties A 18.8%. Obtained results showed a significant effect of the input steel condition on the obtained EB welded joints.