November 2020 Volume 2

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FORGING RESEARCH AND TECHNOLOGY

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Figure 40: Inverse Pole Figure (IPF), and grayscale IQ-map and grain boundary distribution, for the Wrought 4340 cooled at ( A, B ) 1C/min, ( C, D ) 20C/min and ( E, F ) 100C/min; after deformation.

4.0 Conclusions The results of this study examined the impact of processing on the Austenite grain coarsening behavior between a SLM and a conventional Wrought 4340 steel grade. In addition, this study provided a better understanding of the transformation behavior of Austenite for these types of steel under uniform cooling conditions with and without deformation. The presence of porosity in the SLM 4340 provides the pinning force to inhibit the grain coarsening of prior Austenite grain boundaries (PAGB) from 900 to 1150C. The stability of the PAGS and the presence of porosity in the SLM causes this steel to have lower hardenability than the conventionalWrought steel. This was confirmed by the CCT and SEM and EBSD-IQ characterization techniques. Also, the MTEX technique provided a good understanding of the PAGS behavior prior to transformation. The presence of Granular Bainite in these steels was observed at the slower cooling rates (0.25C/min, 0.5C/min and 1C/min).The CCTs published in the open literature have not reported the presence of GB in this grade of steel. References 1. Koh-ichi Sugitomo, Tomohiki, and, Yuta Mizuno, “Torsional Fatigue Strength of newly Developed Case Hardening TRIP– Aided Steel”, Metals 2017, 7(9), 375; https://doi.org/10.3390/ met7090375 2. Hebsur, M.G. “Recent attempts of improving the mechanical properties of AISI 4340 steel by control of microstructure —A brief review”. JMES 4, 28–37 (1982). https://doi.org/10.1007/ BF02833379 3. Sharma, Sathyashankara et al. “Mechanical Property Assessment of Austempered and Conventionally Hardened AISI 4340 Steel.” (2014). https://perma.cc/2STV-QRUT 4. Niazi N, Aqueel Shah SN. “Austempering Heat Treatment of AISI 4340 Steel and comparative analysis of various physical properties at different parameters”. Int J Multidiscip Sci Eng, 2014, 5(10). https://perma.cc/3SJV-ULP2 5. Torres MAS, Voorwald HJC. “An evaluation of shot peening, residual stress and stress relaxation on the fatigue life of AISI 4340 steel”. Int J Fatigue, 2002, 24(8):877-86. https://doi. org/10.1016/S0142-1123(01)00205-5 6. J,E, Pembertion, A.I. Guy, Properties and Selection: Irons Steels and High-Performance Alloys, ASM Handbook, vol. 1,(ASM International, 1990) 7. Majeed, A., Iqbal, A. & Lv, J. “Enhancement of tool life in drilling of hardened AISI 4340 steel using 3DFEMmodeling”. Int J Adv Manuf Technol 95, 1875–1889 (2018). https://doi org.pitt.idm.oclc.org/10.1007/s00170-017-1235-8 8. Jelis, E., Hespos, M.R. & Ravindra, N.M. Process Evaluation of AISI 4340 Steel Manufactured by Laser Powder Bed Fusion. J. of Materi Eng and Perform 27, 63–71 (2018). https://doi. org/10.1007/s11665-017-2989-8 9. D D Gu, W Meiners, K Wissenbach & R Poprawe. “Laser 34

0 0.1 0.2 0.3 0.4

C-rich constituents – 3.3% MA – 5.6%

Pearlite – 20.3% Ferrite – 70.8%

Frequency

0 20 40 60 80 100

IQ

( A )

0.6

0.4

Martensite – 81.5% Bainite – 18.5%

0.2

0

Frequency FORGING RESEARCH

0 20 40 60 80 100

IQ

( B )

0 0.1 0.2 0.3 0.4 0.5

Martensite – 82.1% Bainite – 17.9%

Frequency FORGING RESEARCH

0 20 40 60 80 100

IQ

Figure 41: EBSD-IQ analysis for the SLM 4340 cooled at (A) 1C/min, (B) 20C/min and (C) 100C/min after deformation. Table 4 shows a comparison of the different microstructures observed of the transformation of austenite after deformation and uniform continuous cooling at 1C/min, 20C/min and 100C/min; between the SLM and the wrought steels. The main observation at this level of characterization is the presence of granular bainite in the SLM 4340, corroborating with what was seen after SEM analysis. This behavior may be due to the starting condition after hot deformation of the austenite (microstructure, PAGS and composition). Chemical composition changes could also provide an explanation. This effect will be studied more deeply on the further stages of the project. Electron Probe Microanalysis (EPMA), will be use to study the difference in carbon content and micro-segregation between the samples. ( C ) Figure 41: EBSD-IQ analysis for the SLM 4340 cooled at ( A ) 1C/min, ( B ) 20C/min and ( C ) 100C/min after deformation. Table 4 shows a comparison of the different microstructures observed of the transformation of austenite after deformation and uniform continuous cooling at 1C/min, 20C/min and 100C/min; between the SLM and the wrought steels. The main observation at this level of characterization is the presence of granular bainite in the SLM 4340, corroborating with what was seen after SEM analysis. This behavior may be due to the starting condition after hot deformation of the austenite (microstructure, PAGS and composition). Chemical composition changes could also provide an explanation. This effect will be studied more deeply on the further stages of the project. Electron Probe Microanalysis (EPMA), will be use to study the difference in carbon content and micro- egregation between the samples. Table 4: Microstructural Summary after hot compression. 1°C/min 20°C/min 100°C/min Martensite – 82.1% B init – 17.9% ( C ) Figure 41: EBSD-IQ analysis for the SLM 4340 cooled at ( A ) 1C/min, ( B ) 20C/min and ( C ) 100C/min after deformation. Table 4 shows a comparison of the differen microstructures observed of t e transformation of austenite after deformation and uniform continuous cooling at 1C/min, 20C/min and 100C/min; between the SLM and the wrought ste ls. The main observation at this level of characterization is the presence of granular bainite in the SLM 4340, corroborating with what was seen after SEM analysis. This behavior may be due to the starting cond tion afte ho defo ma ion of the austenite (microstructure, PAGS and composition). Chemical composition changes could also provide an explanation. This ffect will be studied more deeply on the further stages of the project. Electron Probe Microanalysis (EPMA), will be use to study the difference in carbon content and micro-segregation between the samples. 0 0.1 0.2 0.3 0.4 0.5 0 20 40 60 80 100 Frequency IQ SLM 4340

Martensite – 79.6% Granular Bainite – 15.9% Carbides-Free Bainitic Ferrite – 4.5%

MA – 3.5% Martensite – 25.7% Granular Bainite – 34.5% Bainite – 36.6%

MA – 14.2% Pearlite – 23.3% Ferrite – 62.5%

Table 4: Microstructural Summary after hot compression. 1°C/min 20°C/min

100°C/min

C-rich constituents – 3.3% MA – 5.6%

Martensite – 79.6% Granular Bainite – 15.9% Carbides-Free Bainitic Ferrite – 4.5%

MA – 3.5% Martensite – 25.7% Granular Bainite – 34.5% Bainite – 36.6%

Wrought 4340

Martensite – 81.5% Bainite – 18.5%

Martensite – 82.1% Bainite – 17.9%

Pearlite – 20.3% Ferrite – 70.8% MA – 14.2% Pearlite – 23.3% Ferrite – 62.5%

SLM 4340

C-rich constituents – 3.3% MA – 5.6%

Wrought 4340

35 under uniform cooling conditions with and without deformation. The presence of porosity in the SLM 4340 provides the pinning force to inhibit the grain coarsening of prior Austenite grain boundaries (PAGB) from 900 to 1150C. The stability of the PAGS and the presence of porosity in the SLM causes this steel to have lower hardenability than the conventional Martensite – 82.1% Bainite – 17.9% under uniform cooling conditions with and without deformation. The presence of porosity in the SLM 4340 provides the pinning force to inhibit the grain coarsening of prior Austenite grain boundaries (PAGB) from 900 to 1150C. The stability of the PAGS and the presence of porosity in the Martensite – 81.5% Bainite – 18.5%

4.0. Conclusions The results of this study examined the impact of processing on the Austenite grain coarsening behavior between a SLM and a conventional Wrought 4340 steel grade. In addition, this study provided a better understanding of the transformation behavior of Austenite for these types of steel Pearlite – 20.3% Ferrite – 70.8% 4.0. Conclusions The results of this study examined the impact of processing on the Austenite grain coarsening behavior between a SLM and a conventional Wrought 4340 steel grade. In addition, this study provided a better

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