November 2020 Volume 2

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 and, for the wrought steel, the average grain size is 176.5±75.8µm. 3.6.2. Water Quenched Condition After Deformation and with the MTEX the prior austenite grains reconstructed for both the WQ SLM and wrought 4340 samples. The sample 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. FORGING RESEARCH AND TECHNOLOGY 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

FORGING RESEARCH

3.6.3 Sv Measurements and KAM factor Comparisons between the two WQ conditions were done. First, the Sv value was measured either before and after deformation. Figure 35 shows these measurements.

FORGING RESEARCH

measured either before and after deformation. Figure 35 shows these measurements.

( B ) measured either before and after deformation. Figure 35 shows these measurements. ( C )

100 150 200 250 300 350 400 450

395.84

( A )

111

001 100 150 200 250 300 350 400 450

226.76

395.84

101

Sv (mm-1)

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. 27 Figure 32: ( A ) Inverse Pole Figure (IPF), ( B ) grayscale IQ-map and grain boundary distribution, and ( C ) reconstruct d PAGB, for the WQ SLM 4340 after deformation. FORGING RESEARCH 226.76

31.45

0 50

11.3

SLM 4340

Wrought 4340

SLM 4340

Wrought 4340

Prior to Deformation

After Deformation

Figure 35: Sv measurements for both alloys. These measurements were done assuming linear the temperature during hot deformation, also assuming the temperature of non recrystallization (Tnr) as 850°C and the deformation was performed under Tnr. The micrographs presented in Figure 34 and the increased value of the Sv clearly confirms that the deformation was done below the Tnr. The main objective of hot deformation in modern steel industry are to control the shape and microstructure to meet different requirements. In this study, the hot deformation was also used to eliminate porosity in SLM 4340 in order to improve mechanical properties. The increase of nucleation sites for γ-α transformation depends on the amount of deformation in non-recrystallization region, expressed by effective interfacial area per unit volume (Sv). If hot deformation takes place under recrystallization temperature, the Sv is comprised of two terms: the area of elongated austenite grain boundaries per unit volume (Sv.gb) and the area of deformation bands and twins per unit volume inside austenite grains (Sv.IPD). Sv for hot deformation below the recrystallization temperature is determined by the following equation: Figure 35: S v measurements for both alloys. (Sv). If hot deformation takes place under recrystallization temperature, the Sv is comprised of two terms: the area of elongated austenite grain b undaries per unit volume (Sv.gb) and the area of deformation bands and twins per unit volume inside austenite grains (Sv.IPD). Sv for hot deformation below the recrystallization temperature is determined by the following equation: = {1.67( − 0.1) + 1} ( 2 ) + 63( − 0.3) [1,2,3] Second, the Kernel Average Misorientation (KAM) factor map was extracted from the EBSD analysis in order to create a deeper understanding about local grain misorientation, see Figure 36 for the SLM 4340 and Figure 37 for the W oug t 4340. (Sv). If hot deformation takes place under recrystallization temperature, the Sv is comprised of two terms: the re of elonga ed aust nite g ain boundari s per unit volume (Sv.gb) and the area of deformation bands and twins per unit volume inside austenite grains (Sv.IPD). Sv for hot deformation below the recrystallization temperature is determined by the following equati = {1.67( − 0.1) + 1} ( 2 ) + 63( − 0.3) [1,2,3] Second, the Kernel Average Misorientation (KAM) factor map was extracted from the EBSD analysis in order to create a deeper understanding about local grain 31.45 SLM 4340 Wrought 4340 After Deformati n Second, the Kernel Average Misorientation (KAM) factor map was extracted from the EBSD analysis in order to create a deeper understanding about local grain misorientation, see Figure 36 for the SLM 4340 and Figure 37 for the Wrought 4340. 29 These measurements were done assuming linear the temperature during hot deformation, also assuming the temperature of non-recrystallization (Tnr) as 850°C and the deformation was performed under Tnr. The micrographs presented in Figure 34 and the increased value of the Sv clearly confirms that the deformation was done below the Tnr. The main objective of hot deformation in modern steel industry are to control the shape and microstructure to meet different requirements. In this study, the hot deformation was also used to eliminate porosity in SLM 4340 in order to improve mechanical properteies. The increase of nucleation sites fo γ - α transformatio pends on the amount of deformation in non-recrystallization region, expressed by effective interfacial area per unit volume 11.3

Sv (mm-1)

0 50

SLM 4340

Wrought 4340

FORGING RESEARCH

( C ) Prior to Deformation

( A )

( B )

111

Figure 35: S v measurements for both alloys.

Figure 33: (A) Inverse Pole Figure (IPF), (B) grayscale IQ-map and grain boundary distribution, and (C) reconstructed PAGB, for the WQ wrought 4340 after deformation. Specifically, for these samples, the EBSD mapping was performed under lower magnifications and bigger scan areas. This was due to the different grain size distribution for the alloys studied. Even after deformation, big differences can be noticed on the PAGS. For the SLM, a fine deformed grain size distribution can clearly be seen. For the wrought 4340, the PAGS are much coarser and twin boundaries can also be seen. Again, to complement the EBSD analysis, a special etching was used to reveal the PAGB (prior austenite grain boundaries) under optical microscopy characterization. Figure 34 shows a comparison for both alloys after deformation. Figure 33: ( A ) Inverse Pole Figure (IPF), ( B ) grayscale IQ-map and grain boundary distribution, and ( C ) reconstructed PAGB, for the WQ wrought 4340 after deformation. Specifically, for these samples, the EBSD mapping was performed under low magnifications and bigger scan areas. This was due to the differ nt grain size distribution for the alloys studied. Even after deformation, big differences can be noticed on the PAGS. For the SLM, a fine deformed grain size distribution can clearly be seen. For the wrought 4340, the PAGS are much coarser and twin boundaries can also be seen. Again, to complement the EBSD analysis, a special etching was used to reveal the PAGB (prior austenite grain boundaries) under optical microscopy characterization. Figure 34 shows c mparison for both alloys ter deformation. ` ( B ) Figure 34: OM micrographs (1kX mag) showing the PAGB for ( A ) SLM 4340, and ( B ) wrought 4340 after deformation. 20 µm 20 µm ( A ) ( B ) ( C ) Figure 33: ( A ) Inverse Pole Figure (IPF), ( B ) grayscale IQ-map and grain boundary distribution, and ( C ) reconstructed AGB, for the WQ wrought 4340 aft r deforma ion. Specifically, for these samples, the EBSD mapping was performed under lower m g ifications and bigg r scan areas. This was due to the different grain size distribution for the alloys studied. Even after def rmation, big differences can be noticed on the PAGS. For the SLM, a fine deformed grain size distribution can clearly be seen. For the wrought 4340, the PAGS are much coarser and twin boundaries can also be seen. Again, to complement the EBSD an lysi , a special etching was used to reveal the PAGB (prior austenite grain boundaries) under optical microscopy characteriz tion. Figure 34 shows a comparison for both alloys after deformation. 111 001 101 These measurements were done assuming linear the temperature during hot deformation, al o assuming the temperature of non-recrystallization (Tnr) as 850°C and the deformation was performed under Tnr. The micrographs resented in Figure 34 and the increased value of the Sv clearly confirms that the deformation was done below the Tnr. The main objective of hot deform tion in modern steel i dustry are to control the shape and microstructure to meet different requirements. In this study, the hot deformation was als used to eliminate porosity in SLM 4340 in order to improve mechanical properteies. The increase of nucleation sites for γ - α transformation depends on the amount of deformation in non-recrystallization region, expressed by effective interfacial area per unit volume Figure 34: OMmicrographs (1kX mag) showing the PAGB for (A) SLM 4340, and (B) wrought 4340 after deformation. After deformation, the aspect ratio for the SLM4340 is 2.9±1.2 and, for the wrought steel, the aspect ratio is 4.2±1.6. ( B ) Figure 34: OM micrographs (1kX mag) showing the PAGB for ( A ) SLM 4340, and ( B ) wrought 4340 after deformati n. After deformation, the aspect ratio for the SLM 4340 is 2.9±1.2 and, for the wrought steel, the aspect ratio is 4.2±1.6. 3.6.3. S v Measurements and KAM factor Comparisons between the two WQ conditions were done. First, the S v value was 28 ( A ) After deformation, the aspect ratio for the SLM 4340 is 2.9±1.2 and, for the wrought steel, the aspect ratio is 4.2±1.6. 3.6.3. S v Measurements and KAM factor Comparisons between the two WQ conditions were done. First, the S v value was 001 101 ( A ) ` 20 µm 20 µm

misorientation, see Figure 36 for the SLM 4340 and Figure 37 for the Wrought 4340.

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FIA MAGAZINE | NOVEMBER 2020 96