Maximizing Hardness and Toughness in 8620 Steel Emily Anderson, Rachel Freer, Josh Kubiak, Allison Perna, Katie Sullivan 1
Introduction Totten, G., Funatani, K., Xie, L. Handbook of Metallurgical Process Design. Technology & Engineering, 2004, 460. 2
Introduction Tempering forms ferrite and carbide, increases toughness Higher temperatures increase toughness, decrease hardness Rapid quenching increases surface hardness martensite http://cml.postech.ac. kr/2008/steel_microstructure/picture 1.png 3
Introduction Above 700 C Grain coarsening Loss of mechanical properties Below 300 C Little effect Focus Range 300 C-700 C 4
Introduction Fracture toughness Increases up to 500 C Hardness Slightly decreases up to 500 C Rapidly decreases above 530 C Optimal temperature between 500 C-530 C Lee, S.; Ho, W. The Effects of Surface Hardening on Fracture Toughness of Carburized Steel. Metall. Trans. A, 1989, 20A, 519-525. 5
Experimental Procedure: Overview 6
Experimental Procedure Austenitized all Charpy and tensile bars at 926 C for 1 hour Heat treatment of 3 Charpy bars for 1 hour at each temperature: 400 C, 450 C, 500 C, 550 C, 600 C Slow cool in furnace and water quench from 926 C for 3 Charpy bars each Analyze microhardness after polishing (120 grit for about 5mm to 0.3 alumina slurry) and etching (5% nital for 30 seconds) to determine optimized temperatures Temper tensile bars and additional 3 Charpy bars at 550 C and 575 C Optical microscopy on cross section and SEM of fractured Charpy bars using ASPEX Explorer with secondary electron (SE) detector 7
Results: Impact Toughness Annealed samples exhibit intermediate toughness between Slow Cooled (SC) and As Quenched (AQ) samples Except for 575 C, toughness increased w/ tempering temperature Original 550 C tempered sample = 51 J, discovered to be 4140 8
Results: Fracture 250x, deformed surfaces in SC and tempered samples. AQ 450 C 500 C 550 C (4140) 550 C 575 C 600 C SC 9
Results: Fracture SC 8620 4140 10
Results: Microhardness (Strength) Tempered samples have hardness between SC and AQ samples Hardness decreased with increasing tempered temperature Little variation with specimen depth due to slow quenching AQ 4140 SC 11
Results: Preliminary Material Index Toughness from Charpy test Microhardness is average of 1mm, 3mm, and 5mm from surface Increases with temperature except for 575 C Original 550 C sample=186,000(106 N2/m), discovered to be 4140 12
Results: Tensile Testing Corrected true stress vs. true strain 575 C tempered yielded higher toughness than 550 C tempered Ultimate Strength (MPa) Yield Strength (MPa) Modulus (GPa) Toughness (MPa) 550 C 859 715 234 65.8 575 C 1140 991 209 69.6 13
Results: Final Material Index Tensile Toughness (MPa) Charpy Toughness (J) Hardness (MPa) Material Index -Tensile((MPa)2) 550 65.8 18.0 2929 193,000 52,700 575 69.6 13 3288 229,000 43,000 600 -- 21 2848 -- 60,000 SC -- 26.3 2279 -- 59,900 Tempering Temp. ( C) Material Index Charpy(N2/m *106) 14
Discussion: Types of Microstructures Martensite Formed by quenching austenite Hard, brittle, low toughness Carbon is trapped Cementite (Iron Carbide) Precipitated carbon Hard and brittle Bramfitt, B. L. Structure/Property Relationships in Irons and Steels. Materials Selection and Design. 1997, 20, 357-382. 15
Discussion: Types of Microstructures Ferrite Low carbon High toughness Pearlite Cementite and ferrite in lamellar structure High strength and hardness Poor ductility and toughness University of California, Santa Barbara. Carbon Applications. http://research.mrl.ucsb. edu/~barton/materials.html (accessed Dec 1, 2014). 16
Discussion: Experimental Microstructures As Quenched Slow Cooled Largely martensitic (black) with regions of ferrite and/or carbide (white). Extensive regions of ferrite (white) with regions of pearlite (black). 17
Discussion: Experimental Microstructures General increase in ferrite with increasing tempering temperature Cannot tell if this increase exists for 500 C to 575 C 450 C 550 C 500 C 575 C 600 C 18
Discussion: Tempered Martensite Microstructure approximately follows expected trend for tempered martensite Gowelding. The Metallurgy of Carbon Steel. http://www.gowelding.com/met/carbon.htm (accessed Nov 28, 19
Discussion: Slow Cooled 0.2% Carbon Looks similar to those in literature Slow cooled sample is visually between the 0.1% and 0.25% in terms of ferrite to pearlite (white to black ) ratio Ferrite is tough and pearlite 0.1% Carbon provides some hardness. Slow cooled has most ferrite Supported by Charpy data (highest toughness) 0.25% Carbon Top image is our slow cooled sample (in air from 900 C). Lower are two Ferrite-Pearlite samples from literature. Bramfitt, Bruce L. "Structure/Property Relationships in Irons and Steels." 20 N.p.: n.p., n.d. N. pag. Print.
Discussion: Toughness Expect highest toughness for smallest grains and most ferrite. Grain size increases 600 550 575 ~ 500 Cannot conclude which has most ferrite. Conclusion: 550 C sample s microstructure indicates higher toughness than 575 C s and 500 C s, and data matches. Uneven furnace temperature may have caused the 575 C discrepancy 500 C 550 C 575 C 600 C 21
Discussion: Hardness Trend Hardness in a higher carbon steel decreased then increased after tempering temperature of 500 C. This is caused by combined effects of Molybdenum and Chromium on hardness. Microhardness data has similar trend shifted by ~75 C. Conclusion: This is a potential reason for our sudden increase in hardness for the 575 C sample Lee, S.; Ho, W. The Effects of Surface Hardening on Fracture Toughness of Carburized Steel. Metall. Trans. A, 1989, 20A, 519-525. 22
Conclusions As-quenched shows martensite phase Increasing amounts of ferrite as tempering temperature increased Higher material index under low strain rate for 575 C than 550 C Highest material index for 600 C and slow cooled based on Charpy Toughness generally increases with tempering temperature Hardness generally decreases with tempering temperature 23
Future Recommendations Use a higher range of temperatures Use a statistically relevant number of samples Be more careful and constantly check samples Ensure doors of furnace remain closed during heating Have more consistent quenching process 24
Acknowledgements We would like to acknowledge Professor Marc DeGraef, Bill Pingitore, and the lab TAs for their support and help throughout the laboratory experiments. 25
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References The Tempering of Martensite: Part One. Key to Metals. http://steel.keytometals. com/articles/art127.htm (accessed Dec 2, 2014). Three Planes. Martensite. http://www.threeplanes.net/martensite.html (accessed Nov 30, 2014). University of Portland. Steel Microstructure. http://faculty.up. edu/lulay/egr221/steelmicrostruct.pdf (accessed Nov 30, 2014). DeGraef, M.; Rollet, A. Experiment No. 2A: Optimizing Microstructure-Sensitive Mechanical Properties in Steel. Carnegie Mellon University, Pittsburgh, PA, 2014. Lee, S.; Ho, W. The Effects of Surface Hardening on Fracture Toughness of Carburized Steel. Metall. Trans. A, 1989, 20A, 519-525. Penha, R.; Canale, L.; Vatavuk, J.; Lampman, S. Tempering of Steels, Steel Heat Treating Fundamentals and Processes. ASM Handbook, 2013, 4A, 327 351. Hong, S.; Lin, H.; Yang, C.; Tseng, L.; Lin, K. Effect of Heat Treatment and Composition Modification on SAE 8620 Steels. Mater. Sci. Form. 2007, 539, 44524457. 27
Supplemental Slides 28
ImageJ Analysis 1. Image Adjust Threshold 2. Check Dark background 3. Cover the white portions of the image with red manually 4. Analyze Analyze Particles 5. Save data to Excel 6. Sum areas of white 29
Charpy Data Treatment As Quenched 450 C Tempered 500 C Tempered 550 C Tempered (4140) Charpy Toughness (J) Sample 1 6.4 2 11.2 3 6.8 1 11.5 2 11.4 3 -- 1 14.0 2 16.7 3 14.4 1 55.0 2 3 Average (J) Stand. Dev. (J) 8 Treatment 550 C Tempered Repeat 1 17.9 2 18 3 18.1 575 C Tempered 1 13.8 2 12.1 3 14 1 20.0 2 20.0 3 24.0 1 26 46.1 2 27 52.4 3 26 11.5 15 51 3 Charpy Toughness (J) Sample -- 1 5 600 C Tempered Slow Cooled Average (J) Stand. Dev. (J) 18.0 0.1 13 1 21 2 26.3 0.6 30
Microhardness Data Microhardness (MPa) Treatment 1mm from surface 3mm from surface 5mm from surface Average As Quenched 5010 5291 5597 5300 450 C Tempered 3738 3806 3711 3752 500 C Tempered 2989 3169 3344 3167 550 C Tempered (4140) 3608 3792 3557 3652 550 C Tempered Repeat 2914 2859 3008 2927 575 C Tempered 3332 3321 3212 3288 600 C Tempered 2798 2877 2869 2848 Slow Cooled 2239 2342 2258 2279 31
Instron Data *550 Sample B, was originally tested is "fast" mode on the frame resulting in failure of the test. The sample was then reevaluated at the proper strain rate, but the sample may have been plastically deformed during the first test. Ultimate Strength (MPa) Yield Strength (MPa) Modulus (GPa) Toughness (MPa) 550 C Tempered A 859.1 714.9 234.3 65.77 550 C Tempered B* 712.0 531.4 204.2 53.25 550 C Average* 785.6 623.1 219.2 59.51 575 C Tempered A 1139 1025 229.3 70.33 575 C Tempered B 1132 956.1 188.9 68.83 575 C Average 1135 990.6 209.1 69.58 Sample 32
Bainite Bramfitt, B. L. Structure/Property Relationships in Irons and Steels. Materials Selection and Design. 1997, 20, 357-382. 33
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