The dissolution behavior of δ-ferrite in two commercial stainless steels, 15-5PH and M-154, was studied. In this work, a new approach combining hot deformation and additional post-treatment was investigated and compared with conventional annealing heat treatments for the dissolution of δ-ferrite. The results showed the acceleration in the dissolution of δ-ferrite using the new methodology. Samples from each steel were subjected to conventional annealing heat treatments at 1000 °C and 1150 °C, with soaking times of 1, 2, and 3 h. A second set of samples was subjected to hot compression experiments at 900 °C, under different strain rates, followed by post-processing heat treatments at 1000 °C and 1150 °C, while keeping the holding time constant for 10 min. Advanced microstructural characterization techniques such as Scanning Electron Microscopy (SEM), and Electron Probe Micro-Analysis (EPMA) were employed to investigate δ-ferrite dissolution in terms of changes in area fraction and chemical composition. The results indicated a strong correlation between the dissolution behavior of δ-ferrite and the processing parameters. In addition, thermodynamic calculations using Thermo-Calc software (version 2021.2.87071-368) were used to assess the diffusion of elements during the dissolution of δ-ferrite as a function of temperature and time.

This study compares austenite grain growth and its continuous cooling transformation (CCT) behavior between selective laser melting (SLM) 4340 and conventional wrought 4340 steels. Standard dilatometry tests were used to determine the austenite decomposition behavior at different cooling rates. The analysis of the data from the controlled cooling paths was used to generate CCT diagrams for the two steels investigated in this study. Advanced microstructural characterization techniques, Scanning Electron Microscopy and Orientation Imaging Microscopy, were employed to support the CCT diagrams. Electron Backscatter Diffraction (EBSD) and a special EBSD-Image Quality characterization technique were used to assess the percentage of the microstructural components observed. Three important observations were made: (1) the hardenability of the wrought 4340 steel was higher than the SLM 4340 steel; (2) the presence of granular bainite was observed after slow cooling conditions in both steels; (3) the SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied. MTEX, a MatLab program, was used to reconstruct the prior austenite grains. The SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied.