November 2020 Volume 2

Figure 19: Reconstructed microstructures and EBSD-IQ for SLM 4340 cooled at ( A , B and C ) 0.25C/min and ( D , E and F ) 500C/min.

LAGB CSL HAGB

Figure 21: Grain Boundary Character Distribution for the SLM 4340.

The color scale in Figure 19C and Figure 19F shows the micro-constituents that have lower dislocation density hence higher IQ values represented by purple and blue colors, while micro-constituents with higher dislocation density and lower IQs are shown by the green and yellow colors.

Figure 20 shows the EBSD-IQ phase balance for the SLM samples after cooling at 0.25C/min and 500C/min. The difference in the microstructural components supports the CCT and SEM results.

Pole Figure (IPF), Figure 22b, the IQ map and Figure 22c, a MatLab figure showing the IQ map in a colored scale, all for the cooling rate of 0.25C/min. Figure 22 from (d) to (f) show the same for the cooling rate of 500C/min.

3.4.2. Wrought 4340 Uniform Continuous Cooling For the Wrought 4340, the reconstructed microstructures based on EBSD measurements are shown in Figure 22, while the EBSD-IQ results are shown in Figure 23. Figure 22a shows the Inverse

FORGING RESEARCH AND TECHNOLOGY

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Cr-C-rich Constituents – 11.9% Pearlite – 28.3% Ferrite – 59.8%

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Martensite – 100%

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Figure 20: EBSD-IQ measurements for SLM 4340 cooled at (A) 0.25C/min and (B) 500C/min. The grain boundary character distribution (GBCD) in terms of low angle grain boundaries (LAGB), high angle grain boundaries (HAGB) and coincidence site lattice (CSL) boundaries are shown in Figure 21. As expected, the results clearly indicate that at the slower cooling rates most of the grain boundaries were HAGB, as the cooling rate increased the GBCD became more uniform between LAGB, HAGB and CSL boundaries. This behavior is strongly related to the nature of the transformation mechanisms and final microstructures during the decomposition of Austenite. That is, the GBCD for samples cooled at 0.25C/ min, 1C/min have a higher percentage of HAGB when compared with LAGB due to the absence of low transformation products. At higher cooling rates the volume fraction of LAGB increased due to the formation of low temperature transformation products, i.e. Martensite and/or Bainite. Figure 20: EBSD-IQ measurements for SLM 4340 cooled at ( A ) 0.25C/min and ( B ) 500C/min. The grain boundary character distrib tion (GBCD) in terms of low angle grain boundaries (LAGB), high angle grain boundaries (HAGB) and coi cidence site lattice (CSL) boundaries are shown in Figure 21. As expected, the results clearly indicate that at the slower cooling rates most of the grain boundaries were HAGB, as the cooling rate increased the GBCD became more uniform between LAGB, HAGB and CSL boundaries. This behavior is strongly related to the nature of the transformation mechanisms and final microstructures during the decomposition of Austenite. That is, the GBCD for samples cooled at 0.25C/min, 1C/min have a higher percentage of HAGB when compared with LAGB due to the absence of low transformation products. At higher c oling rates the volume fraction of LAGB increased due to the formation of low temperature transformation products, i.e. Martensite and/or Bainite. 18

( F )

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Figure 22: Reconstructed microstructures and EBSD-IQ for Wrought 4340 cooled at (A, B and C) 0.25C/min and (D, E and F) 500 C/min. Similarly to Figure 19, the color scale on Figure 22C and Figure 22F also shows the micro-constituents that have lower dislocation density hence higher IQ values by purple and blue colors, while phases with higher dislocation density and lower IQs are shown by the green and yellow colors. The GBCD results for samples with cooling rates from 0.25, 1.0, 10, 50, 200 and 500C/min are shown in Figure 24. Figure 22: Reconstructed microstructures and EBSD-IQ for Wrought 4340 cooled at ( A , B and C ) 0.25C/min and ( D , E and F ) 500 C/min. Similarly to Figure 19, the color scale on Figure 22C and igure 22F also shows the micro-constituents that have lower dislocation density hence higher IQ values by purple and blue colors, while phases with higher dislocation density and lower IQs are shown by the green and yellow colors. The GBCD results for samples with cooling rates from 0.25, 1.0, 10, 50, 200 and 500C/min are shown in Figure 24. C-rich Constituents – 3.5% MA - 6.6% Martensite – 19.7% Figure 22: Reconstructed microstructures and EBSD-IQ for Wrought 4340 cooled at ( A , B and C ) 0.25C/min and ( D , E and F ) 500 C/min. Simil rly to Figure 19, the color scale on Figure 22C and Figure 22F also shows the micro-constituents that have lower dislocatio density hence higher IQ values by purple and blu colors, while phases with higher dislocation density and lower IQs are shown by the green and yellow colors. The GBCD results for samples with cooling rates from 0.25, 1.0, 10, 50, 200 and 500C/min are shown in Figure 24. 001 101 0 0.05 0.1 0.15 0.2 0.25

FORGING RESEARCH

Pearlite – 19.9% Ferrite – 50.3%

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C-rich Constituents – 3.5% MA - 6.6% Martensite – 19.7%

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Figure 23: EBSD-IQ measurements for Wrought 4340 cooled at ( A ) 0.25C/min and ( B ) 500C/min.

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Martensite – 100%

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Figure 21: Grain Boundary Character Distribution for the SLM 4340. 3.4.2Wrought 4340 UniformContinuous Cooling For the Wrought 4340, the reconstructed microstructures based on EBSD measurements are shown in Figure 22, while the EBSD-IQ results are shown in Figure 23. Figure 22a shows the Inverse Pole Figure (IPF), Figure 22b, the IQ map and Figure 22c, a MatLab figure showing the IQ map in a colored scale, all for the cooling rate of 0.25C/min. Figure 22 from (d) to (f) show the same for the cooling rate of 500C/min. Figure 21: Grain Boundary Character Distribution for the SLM 4340. Wrought 4340 Uniform Continuous Cooling For the Wrought 4340, the reconstructed microstructures based on EBSD measurements are sho n in Figure 22, while the EBSD-IQ results are shown in Figure 23. Figure 22a shows the Inverse Pole Figure (IPF), Figure 22b, the IQ map and Figure 22c, a MatLab figure showing the IQ ap in a colo d scal , all for the cooling rate of 0.25C/min. Figure 22 from (d) to (f) show the same for the cooling rate of 500C/min. 3.4.2.

Figure 23: EBSD-IQ measurements for Wrought 4340 cooled at (A) 0.25C/ min and (B) 500C/min. Figure 23: EBSD-IQ measurements for Wrought 4340 cooled at ( A ) 0.25C/min and ( B ) 500C/min.

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FIA MAGAZINE | NOVEMBER 2020 92