February 2022 Volume 4

FORGING RESEARCH

1. Executive Summary The present report is intended to provide a review of the activities conducted during the last Stage 6 (March 2021 – September 2021) of this program. The major activities accomplished during this stage were directly related to comparing the response of the microstructure and mechanical properties of the 4340 SLM and the 4340 wrought steels to two different thermomechanical processing (TMP) conditions. The activities were divided in three distinct areas: 1. Laboratory Hot Rolling experiments and post-processing heat treatments 2. Advanced microstructural characterization including crystallographic texture; 3. Mechanical testing assessment – Tensile and Charpy tests. The microstructural response to TMP and post-processing of the wrought 4340 and the Selective Laser Melted (SLM) steels was compared according to their prior austenite grain size (PAGS), final overall microstructure, crystallographic texture and mechanical properties. The results were documented and will be presented in this report. In the previous report, the tempering study of these samples was studied and reported. This post-heat treatment was then applied to the steels for mechanical evaluation. Only one of the selected temperatures, 1150˚F was used. This temperature was used to simulate the tempering conditions that steel 4340 is subjected under 2. Experimental Procedure 2.1.

commercial conditions, according to our industrial partners. This current study included the effect of heating rate during tempering and its effect to the final microstructure and properties was assessed. The effect of heating rate during tempering on the microstructural changes of the steel samples was conducted with the aid of electron optic techniques (OM, SEMand EBSD). In addition, XRD analysis was conducted to evaluate and compare the changes in crystal lattice as function of heat treatment. Values such as dislocation density and micro-strain were assessed and compared between the wrought and SLM 4340 steels. The prior austenite grain size (PAGS) was evaluated using the MTEX technique and the number of intercrystalline defects (Sv) parameter was determined from well-known equations published in the literature. Similarly, the crystallographic texture after TMP of the steels was evaluated and compared. Overall hardness measurements were taken in each of the samples. The results will be presented and discussed in this report.

FIA MAGAZINE | FEBRUARY 2022 85 The hot rolling experiments were performed in two steps, see Figure 1a and igure 1b. In the first step the samples were re- heated up to 2012˚F (1100°C) for 6 0 minutes. Then the samples 2. Experimental Procedure 2.1. Materials and Processing Materials and Processing Table 1 shows the chemical composition of the two steels studied in this program. The compositions are very similar, except for the higher content in the nitrogen and oxygen in the SLM 4340, c mpared to the wr ught steel. The pro essing of the as-received samples used in this study has been previously documented and reported. Table 1: Chemical Composition of 4340 steels (.wt%). C Mn P S Si Ni Cr Mo V Cu Al N O Wrought 4340 0.42 0.75 0.013 0.008 0.31 1.75 0.85 0.27 0.006 0.22 0.021 - - Laboratory Hot Rolling One of the major objectives accomplished during the previous Stages 4 and 5 of this project was to examine the effect of laboratory hot rolling on the microstructural refinement of austenite for both steels and compare the prior austenite grain size (PAGS) and the number of intercrystalline defects per unit volume (S V ). These results will be reviewed, with the addition of the new results from TMP adopted during this stage 6. For both rolling experiments. the samples were sectioned from the as-received plates. The samples used during the first laboratory hot rolling experiments had the approximated geometry of 5 in. (127 mm) in length x 3 in. (76.2 mm) in width x 2 in. (50.8 mm) in thickness. The hot rolling mill used is a fully integrated computer-controlled system housed at the US Steel Technical Research Center. Table 1: Chemical Composition of 4340 steels (.wt%). 2.2. Laboratory Hot Rolling One of the major objectives accomplished during the previous stages 4 and 5 of this project was to examin effect of laboratory hot rolling on the microstructural refinement of austenite for both steels and compare the prior austenite grain size (PAGS) and the number of intercrystalline defects per unit volume (SV). These results will be reviewed, with the addition of the new results from TMP adopted during this stage 6. For both rolling experiments, the samples were sectioned from the as-received plates. The samples used during the first laboratory hot rolling experiments had the approximated geometry of 5 in. (127 mm) in length x 3 in. (76.2 mm) in width x 2 in. (50.8 mm) in thickness. The hot rolling mill used is a fully integrated computer- controlled systemhoused at the US Steel Technical ResearchCenter. The hot rolling experiments were performed in two steps, see Figure 1a and Figure 1b. In the first step the samples were re-heated up to 2012˚F (1100°C) for 60 minutes. Then the samples were hot rolled using 4 equal deformations to a final thickness of 0.900 in. (22.9 mm). The finishing rolling temperature (FRT) was about 1600˚F (871°C) and then the sample was air cooled to room temperature (ACRT). At this stage the samples were expected to have similar hot rolled microstructures. The hot rolled plates were sectioned in two equal lengths. Meaning, two plates for each steel processing starting condition (wrought and SLM) were produc d for further hot rolling. To continue reading, visit https://www.forging.org/producers-and- suppliers/technology/metallurgy/ferrous. Table 1 shows the chemical composition of the two steels studied in this program. The compositions are very similar, except for the higher content in the nitrogen and oxygen in the SLM 4340, compared to the wrought steel. The processing of the as-received samples used in this study has been previously documented and reported. SLM 4340 0.41 0.78 0.010 0.004 0.33 1.87 0.88 0.24 - - - 0.02 0.07 2.2.