November 2020 Volume 2

FORGING RESEARCH

FORGING RESEARCH AND TECHNOLOGY

FORGING RESEARCH The results from the Austenite grain coarsening studies are presented in Figure 9. The Austenite grain size in the SLM steel samples did not show any significant grain coarsening up to the maximum reheating temperature of 1150C used in this study. While the Auste i e grains in the Wrought steel samples showed the traditional normal grain growth as the reheating temperature increases. To achieve reliable results, the sizes of 100 random grains were as d and the verage grain size value was determined. This process was repeated for all reheated conditions, the results r shown in Figure 9. The results from the Austenite grain coarsening studies are pre ented in Figure 9. The Austenite grain size in the SLM steel samples did not show any significant grain coarsening up to the maximum reheating temperature of 1150C used in this study. While the Austenite grains in the Wrought steel samples showed the traditional normal grain growth as the reheating temperature increases. To achieve reliable results, the sizes of 100 random gr s w re measured and the average grain size value was determined. This process was repeated for all reheated conditions, the results are shown in Figure 9.

SEM analysis of the Austenite gr boundaries in the SLM samples clea showed the presence of porosity along tri points and along grain boundaries. T presence of porosity provided suffici pinning force preventing the mobility Austenite grain boundaries, henc inhibit grain coarsening. Figure 10 shows example of an Austenite grain af reheating at 1150C for 60 minutes a WQRT. The SEMmicrograph illustrates presence of pores located at the triple poi and along the grain boundaries of Austen

FORGING RESEARCH

average grain size value was determined. This process was repeated for all reheated conditions, the results are shown in Figure 9. SEM analysis of the Austenite grain boundaries in the SLM samples clearly showed the presence of porosity along triple points and along grain boundaries. The presence of porosity provided sufficient pinning force preventing themobility of Austenite grain boundaries, hence inhibiting grain coarsening. Figure 10 shows an example of an Austenite grain after reheating at 1150C for 60minutes andWQRT. The SEMmicrograph illustrates the presence of pores located at the triple points and along the grain boundaries of Austenite. SEM analysis of the Austenite grain boundaries in the SLM am les clearly showed the presence of porosity along triple points and along grain boundaries. The presence of porosity provided sufficient pinning force r venti g the mobility of Austenite grain boundaries, hence inhibiting grain coarsening. Figure 10 shows an example of an Austenite grain after reheating at 1150C for 60 minutes and WQRT. The SEMmicrograph illustrates the presence of pores located at the triple points and along the grain boundaries of Austenite.

misorientation angle between 5° and 15° and the blue lines, high angle boundaries (>15°).

FORGING RESEARCH

0 50 100 150 200 250 300 350 Grain Size (µm) 100 150 200 250 300 350 Grain Size (µm)

( A )

( B )

0 50

850C 850C 6.29 6.29 18.76 18.76

950C

1050C

1150C 17.72 241.13

950C

1050C

1150C 17.72 241.13

SLM 4340

7.38

7.48

( A )

( B )

SLM 4340

7.38

7.48

Wrought 4340

23.18

59.68

Wrought 4340

Holding Temperature (C) 23.18

59.68

Figure 9: Comparison of the Austenite grain coarsening behavior between the Wrought and SLM steels. Figure 9: Comparison of the Austenite grain coarsening behavior between the Wrought and SLM steels. Figure 9: Comparison of the Austenite grain coarsening behavior between the Wrought and SLM steels Holding Temperature (C)

( C )

( D )

111

( C )

( D )

111

101

001

Figure 7: IPF map of the (A) as-received SLM 4340, IQ map of the (B) as received SLM 4340, IPF map of the (C) as-received Wrought 4340 and IQ map of the (D) as-received Wrought 4340. Figure 7: IPF map of the ( A ) as-received SLM 4340, IQ map of the ( B ) as-received SLM 4340, IPF map of th ( C ) as-received Wrought 4340 and IQ map of the ( D ) as-received Wrought 4340. Figure 7: IPF map of the ( A ) as-received SLM 4340, IQ map of the ( B ) as-received SLM 4340, IPF map of the ( C ) as-received Wrought 4340 and IQ map of the ( D ) as-received Wrought 4340. 001 101

0 0.1 0.2 0.3 0.4 0.5

0 0.2 0.4 0.6

Figure 10: SEM micrograph showing the prior Austenite grain size (PAGS) with porosity at the triple points and along the γgb after rehe ating at 1150C and water quenching; etched with 3% Nital. 2 µm

0 0.1 0.2 0.3 0.4 0.5

0 0.2 0.4 0.6

10

Figure 10: SEM micrograph showing the prior Austenite grain size (PAGS) with porosity at the triple points and along the γgb after reheating at 1150C and water quenching; etched with 3%Nital. This observation clearly suggests that the size and volume fraction of the pores in the SLM provides a pinning force (Fp) that is higher than the grain coarsening force (FGC). It is expected that as the reheating temperature increases the volume fraction of the pores will be reduced by the classical grain boundary and volume diffusion mechanisms; hence the Fp will be reduced. At this point the FGC > Fp and grain coarsening will start to take place. Figure 11 shows the change in porosity and the Austenite grain size as function of the reheating temperature for the SLM 4340. The porosity was measured using the Archimedes’ principle. In this method, the sample is fully immersed into liquid. The buoyant force 9 Figure 10: SEM micrograph showing the prior Austenite grain size (PAGS) with porosity at the triple poi and along the γgb f er rehe ating at 1150C and water quenching; tched with 3% Nital. 2 µm IQ

0 20 40 60 80 100 0 20 40 6 80 100

Frequency

Frequency Frequency

0 20 40 60 80 100 0 20 40 60 80 100 Frequency

IQ

IQ

IQ

( A )

( B )

( A )

( B )

Figure 8: EBSD-IQ measurements for (A) as-received SLM 4340 and (B) as received Wrought 4340. The results from the Austenite grain coarsening studies are presented in Figure 9.The Austenite grain size in the SLMsteel samples did not show any significant grain coarsening up to the maximum reheating temperature of 1150C used in this study. While the Austenite grains in the Wrought steel samples showed the traditional normal grain growth as the reheating temperature increases. To achieve reliable results, the sizes of 100 random grains were measured and the High Carbon (Alloyed) Carbides – 16.1% Ferrite – 83.9% Figure 8: EBSD-IQ measurements for ( A ) as-received SLM 4340 and ( B ) as-received Wrought 4340. 9 High Carbon (Alloyed) Carbides – 16.1% Ferrite – 83.9% Figure 8: EBSD-IQ measurements for ( A ) as-received SLM 4340 and ( B ) as-received Wrought 4340. Martensite – 53.1% Tempered Martensite – 46.9% Martensite – 53.1% Tempered Martensite – 46.9%

FIA MAGAZINE | NOVEMBER 2020 87