Heat treatment response of additively manufactured 17-4PH stainless steel

Unlike conventional 17–4PH stainless steels where thermal treatment (solution annealing + aging) leads to ≈95% martensite and ≈5% retained austenite, additively manufactured 17–4PH stainless steels are not responsive to conventional thermal treatment and considerable fraction of retained and/or reversed austenite is formed during solution treatment and aging. In the present study, we have investigated how applying various thermal treatments (i.e., solutionising within the temperature range of 1040 °C–1240 °C from 0.25 h to 4 h and aging at temperature of 480 °C from 1 h to 8 h) affect the microstructure, volume fraction of retained/reversed austenite, hardness, and tensile properties of thermally treated 17–4PH stainless steel manufactured by selective laser melting (SLM) process. For comparison, additively manufactured parts were subject to direct aging without initial solution treatment. The results showed the occurrence of significant partitioning of Ni and Cu in reversed austenite as both elements are strong FCC stabilizer. The presence of inclusions inherent from powder metallurgy (SiO2 and MnS) acts as high-energy interface promoting the heterogenous nucleation of reversed austenite. The results showed that direct aging without initial solution treatment continuously decreased the volume fraction of retained austenite which can be related to the precipitation of Cu-rich precipitates and change in chemical stability of the austenite. It was observed that in the solutionised sample at 1040 °C, the volume fraction of retained austenite increased with solution time at the beginning and reached a maximum value of ≈44% after 1 h and then it decreased to ≈11.5% after 4 h. However, this trend was not the case for the solutionised samples at 1240 °C. The increase of solution temperature from 1040 °C to 1240 °C resulted in decreasing the volume fraction of retained austenite to less than ≈5%. The distribution and enrichment of Ni and Cu in the austenite affecting the chemical stability of austenite and presence of inclusions are major factors controlling the volume fraction of retained/reversed austenite at different thermal treatment conditions. The lowest amount of austenite (≈4.3%) was measured in the solutionised alloy at 1240 °C-4 h followed by aging at 480 °C for 1 h. Direct aging at 480 °C for 3 h increased the hardness by ≈45% and further increase of aging time to 8 h decreased the hardness. For solutionised samples, conventional solution treatment at 1040 °C regardless of solution treatment time (0.25 h to 4 h) did not result in peak hardness during aging due to the considerable volume fraction of retained and/or reversed austenite and low diffusivity of Cu in the austenite. The ultimate tensile strength (UTS) of the solutionised sample at 1240 °C-4 h followed by aging at 480 °C-1 h (UTS ≈ 1227 MPa) and directly aged sample at 480 °C-3 h (UTS ≈ 1211 MPa) without initial solution treating were found to be ≈30% higher than that of as-printed alloy (UTS ≈ 945 MPa) due to the presence of least amount of austenite in the matrix (≈5–8%).