November 2020 Volume 2

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A comparison of the GBCD between the Wrought steel samples (Figure 24) to those from the SLM (Figure 21) reveals the presence of higher volume fraction of LAGB in the Wrought steel at low cooling rates. This supports the observations that the Wrought steel has higher hardenability than the SLM steel under the experimental conditions used in this study. As the cooling rate increases the GBCD balance becomes very similar for both steels.

3.4.3. Reconstruction of Prior Austenite Grain Size (PAGS) The reconstruction of the PAGS from the Martensitic structures, using the MTEX technique is displayed in Figure 25. Figure 25 from (A) to (C) compare the PAGS corresponding to the SLM 4340 and from (D) to (F) for to the Wrought 4340. All the reconstructed PAGS maps were performed based on the Inverse Pole Figure Maps (IPF) for each condition.

FORGING RESEARCH

FORGING RESEARCH AND TECHNOLOGY

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

FORGING RESEARCH

Grain Boundary Fraction

0.25C/min 1C/min

10C/min

50C/min 200C/min 500C/min

( A )

( B )

( C )

LAGB CSL HAGB

Figure 24: Grain Boundary Character Distribution for the Wrought 4340. A comparison of the GBCD between the Wrought steel samples (Figure 24) to those from the SLM (Figure 21) reveals the presence of higher volume fraction of LAGB in the Wrought steel at low cooling rates. This supports the observations that the Wrought steel has higher hardenability than the SLM steel under the experimental conditions used in this study. As the cooling rate increases the GBCD balance becomes very similar for both steels. 3.4.3 Reconstruction of Prior Austenite Grain Size (PAGS) The reconstruction of the PAGS from the Martensitic structures, using the MTEX technique is displayed in Figure 25. Figure 25 from (A) to (C) compare the PAGS corresponding to the SLM 4340 and from (D) to (F) for to the Wrought 4340. All the reconstructed PAGS maps were performed based on the Inverse Pole Figure Maps (IPF) for each condition. Figure 24: Grain Boundary Char r Distribution f r the Wrought 4340. A comparis n of the GBCD between the Wrought steel samples (Figure 24) to those from the SLM (Figure 21) reveals the presence of higher volume fraction of LAGB in the Wrought steel at low cooling rates. T is upports t e observations t t the Wrought steel has higher hardenability than the SLM steel under the experimental conditions used in this study. As the cooling rate increases the GBCD balance becomes very similar for both steels. 3.4.3. Grain Size (PAGS) The reconstruction of th PAGS from the Martensitic structures, using th MTEX technique is displayed in Figure 25. Figure 25 from (A) to (C) compare the PAGS corresponding to the SLM 4340 and from (D) to (F) for to the Wrought 4340. All the reconstructed PAGS maps were performed based on the Inverse Pole Figure Maps (IPF) f r eac condition. ( D ) ( E ) 111 001 101 Reconstruction of Prior Austenite

( F )

( D )

( E )

( F )

111

101

001

Figure 25: PAGB reconstruction for (A) 10C/min – SLM 4340, (B) 50C/min – SLM 4340, (C) 500C/min – SLM 4340, (D) 10C/min – Wrought 4340, (E) 50C/min – Wrought 4340 and (F) 500C/min – Wrought 4340. A comparison of the PAGS clearly shows two major points: 1) the SLM steel samples have finer Austenite grains compared to the Wrought steel samples; and 2) the average grain size distribution is also thinner in the SLM samples compared to theWrought samples. Table 3 shows a comparison of the different microstructures observed of the transformation of austenite after dilatometry and uniform continuous cooling at 0.25C/min and 500C/min. Figure 25 : PAGB reconstruction for ( A ) 10C/min – SLM 4340, ( B ) 50C/min – SLM 4340, ( C ) 500C/min – SLM 4340, ( D ) 10C/min – Wrought 4340, ( E ) 50C/min – Wrought 4340 and ( F ) 500C/min – Wrought 4340. A comparison of the PAGS clearly shows two major points: 1) the SLM steel samples have finer Austenite grains compared to the Wrought steel samples; and 2) the average grain size distribution is also thinner in the SLM samples compared to the Wrought samples. Table 3 shows a comparison of the different microstructures observed of the transformation of austenite after dilatometry and uniform continuous cooling at 0.25C/min and 500C/min. Table 3: icrostructural Summary afte Dilatometry testing As-received 0.25°C/min 500°C/min SLM 4340 53.1% Martensite 46.9% Tempered Martensite Cr-C-rich Constituents – 11.9% Martensite – 100%

( B ) A comparison of the PAGS clearly shows two major points: 1) the SLM steel samples have finer Austenite grains compared to the Wrought steel samples; and 2) the average grain size distribution is also thinner in the SLM samples compared to the Wrought samples.

Table 3 shows a comparison of the different microstructures observed of the transformation of austenite after dilatometry and uniform continuous cooling at 0.25C/min and 500C/min.

( A )

( C )

Pearlite – 28.3% Ferrite – 59.8%

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

High Carbon (Alloyed) Carbides – 16.1% Ferrite – 83.9%

Wrought 4340

Table 3: Microstructural Summary after Dilatometry testing As-received 0.25°C/min

Martensite – 100%

Pearlite – 19.9% Ferrite – 50.3%

500°C/min

21 3.5. Cr-C-rich Constituents – 11.9%

from the microstructural characterization of the 4340 SLM and 4340 wrought samples will be described in the following sections. The results showed differences in terms of microstructural constituents, phase

Microstructural Analysis after Hot Compression The transformation behavior of deformed austenite under continuous cooling conditions provided a wealth of microstructural information. The results

53.1% Martensite 46.9% Tempered Martensite

SLM 4340

Martensite – 100%

Pearlite – 28.3% Ferrite – 59.8%

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

High Carbon (Alloyed) Carbides – 16.1% Ferrite – 83.9%

Wrought 4340

Martensite – 100%

Pearlite – 19.9% Ferrite – 50.3%

3.5. Microstructural Analysis after Hot Compression The transformation behavior of deformed austenite under continuous cooling conditions provided a wealth of microstructural information. The results

FIA MAGAZINE | NOVEMBER 2020 93

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