November 2020 Volume 2

be seen, especially for the cooling rate of 20°C/min. In this condition, martensite, bainite, granular bainite and MA could be observed throughout the sample. At the fastest cooling path, most of the SLM sample exhibited martensite and bainite, except for the presence of carbides-free bainitic ferrite. As expected, the water quenched samples were 100% martensitic. For the wrought 4340, martensite and bainite were observed at cooling rates

behavior as the selective laser melted sample, according to phase distributions, i.e. ferrite, pearlite and MA. Again, as expected, for the water quenched samples, only martensite could be seen. Figure 28 shows a comparison for both alloys cooled at 20°C/min. The main difference between the samples is the presence of granular bainite in the SLM sample.

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Figure 29: ( A ) Inverse Pole Figure (IPF), ( B ) grayscale IQ-map and grain boundary distribution, and ( C ) reconstructed PAGB, for the WQ SLM 4340 prior to deformation.

Bainite

Martensite

Bainite

Martensite

Granular Bainite

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Figure 28: SEM micrographs comparing the (A) SLM 4340 and (B) Wrought 4340 cooled at 20°C/min. The microstructural results observed clearly indicates the strong effect of the fabrication technique, microstructural condition and uniformity of the chemical composition of the steels used in this program. It is suspected that the carbon content and micro segregation have a strong effect on the microstructural changes after continuous cooling paths, mainly after deformation. The starting microstructure also has an important effect on the final microstructure. 3.6 Advanced Microstructural Analysis after Hot Compression Electron Backscattered Diffraction (EBSD) - Image Quality (IQ) analysis was performed on all the samples investigated for this report. This analysis provides a quantitative percentage of the microstructural components observed. This technique and its principles has been discussed extensively in the literature and presented in the previous two reports program. 3.6.1Water Quenched Condition Prior to Deformation Before studying the impact of deformation and after controlled cooling rates on the transformation behavior of austenite, it is important to understand the austenite grain size prior to transformation. Therefore, the EBSD inverse pole figure was plotted and combined with IQ analysis and with the MTEX prior austenite grains reconstruction, of the WQ SLM and wrought 4340. Samples were reheated at 1100˚C held for 2 minutes and air cooled until 900°C, and then, water quenched. Figure 29 shows the EBSD-IQ analysis for the SLM 4340, while Figure 30, for the Wrought 4340. 25 ( B ) Figure 28: SEM micrographs comparing the ( A ) SLM 4340 and ( B ) Wrought 4340 cooled at 20°C/min. The microstructural results observed clearly indicates the strong effect of the fabrication technique, microstructural condition and uniformity of the chemical composition of the steels used in this program. It is suspected that the carbon content and icro segregation have a strong effect on the microstructural changes after continuous cooling paths, mainly after deformation. The starting microstructure also has an impo tant effect on the final microstructure. 3.6. Advan ed Microstructural Analysis after Hot Compression Electron Backscattered Diffraction (EBSD) - Image Quality (IQ) analysis was performed on all the samples investigated for this report. This analysis provides a quantitative percentage of the microstructural components observed. This technique and its principles has been discussed extensively in the literature and presented in the previous two reports program. 3.6.1. Water Quenched Condition Prior to Deformation Before studyi g the impact of eformati n and after controlled cooling rates on the transformation behavior of austenite, it is important to understand the austenite grain size prior to transformation. Therefore, the EBSD inverse pole figure was plotted and combined with IQ analysis and with the MTEX prior austenite grains reconstruction, of the WQ SLM and wrought 4340. Samples were reheated at 1100˚C held for 2 minutes and air cooled until 900°C, and then, water quenched. Figure 29 shows the EBSD-IQ FORGING RESEARCH ( A )

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Figure 30: (A) Inverse Pole Figure (IPF), (B) grayscale IQ-map and grain boundary distribution, and (C) reconstructed PAGB, for the WQ wrought 4340 prior to deformation. It is important to notice the difference in the PAGS when both alloys are compared. The SLM, has a fine grain size distribution, while the PAGS in the wrought 4340 are much coarser. To corroborate with the EBSD analysis, a special etching was used for revealing the PAGB under optical microscopy characterization. Figure 31 shows a comparison of the PAGS for both alloys. 26 Figure 30: ( A ) Inverse Pole Figure (IPF), ( B ) grayscale IQ-map and grain boundary distribution, and ( C ) reconstructed PAGB, for the WQ wrought 4340 prior to deformation. It is important to notice the difference in the PAGS when both alloys are compared. The SLM, has a fine grain size distribution, while the PAGS in the wrought 4340 are much coarser. To corroborate with the EBSD analysis, a special etching was used for revealing the FORGING RESEARCH PAGB under optical microscopy characterization. Figure 31 shows a comparison of the PAGS for both alloys.

20 µm

20 µm

Figure 31: OMmicrographs (1kX mag) showing the PAGB for (A) SLM 4340, and (B) wrought 4340. For the un-deformed SLM 4340, the average grain diameter is 8.7±3.9µm and, for the wrought steel, the average grain size is 176.5±75.8µm. 3.6.2Water Quenched Condition After Deformation After deformation the samples were WQ and similar type of PAGS analysis was performed. The EBSD inverse pole figure was plotted and combined with IQ analysis and with the MTEX the prior austenite grains reconstructed for both the WQ SLM and wrought 4340 samples. The sample was reheated at 1100˚C held for 2minutes and air cooled until 900°C, then the samples were deformed 50% in one hit. Then, they were rapidly quenched. Figure 32 shows the EBSD-IQ analysis for the SLM 4340, while Figure 33, for the Wrought 4340. ( B ) Figure 31: OM micrographs (1kX mag) showing the PAGB for ( A ) SLM 4340, and ( B ) wrought 4340. For the un-deformed SLM 4340, the average grain diameter is 8.7±3.9µm nd, for the wrought steel, the average grain size is 176.5±75.8µm. 3.6.2. Water Quenched Condition After Deformation After deformation the samples were WQ and simil r type of PAGS analysis was performed. The EBSD inverse pole figure was plott d and combi ed with IQ analysis and with the MTEX the prior austenite rains reconstructed for both the WQ SLM and wrought 4340 samples. The sampl was reheate d at 1100˚C held for 2 minutes and air cooled until 900°C, then the samples were deformed 50% in one hit. Then, they were rapidly quenched. Figure 32 shows the EBSD-IQ analysis for the SLM 4340, while Figure 33, for the Wrought 4340. ( A ) ( B ) ( C ) ( A )

analysis for the SLM 4340, while Figure 30, for the Wrought 4340.

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Figure 32: ( A ) Inverse Pole Figure (IPF), ( B ) grayscale IQ-map and grain boundary distribution, and ( C ) reconstructed PAGB, for the WQ SLM 4340 after deformation.

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Figure 29: (A) Inverse Pole Figure (IPF), (B) grayscale IQ-map and grain boundary distribution, and (C) reconstructed PAGB, for the WQ SLM 4340 prior to deformation. Figure 29: ( A ) Inverse Pole Figure (IPF), ( B ) grayscale IQ-map and grain boundary distribution, and ( C ) reconstructed PAGB, for the WQ SLM 4340 prior to deformation.

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FIA MAGAZINE | NOVEMBER 2020 95