Maximizing Hardness and Toughness in 8620 Steel

Emily Anderson, Rachel Freer, Josh Kubiak, Allison Perna, Katie Sullivan

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2Totten, G., Funatani, K., Xie, L. Handbook of Metallurgical Process Design. Technology & Engineering, 2004, 460.

Introduction

Introduction● Tempering forms ferrite

and carbide, increases toughness

● Higher temperatures increase toughness, decrease hardness

● Rapid quenching increases surface hardness

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http://cml.postech.ac.kr/2008/Steel_Microstructure/Picture1.pngmartensite

Introduction● Above 700˚C

○ Grain coarsening○ Loss of mechanical properties

● Below 300˚C○ Little effect

● Focus Range 300˚C-700˚C

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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

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Lee, S.; Ho, W. The Effects of Surface Hardening on Fracture Toughness of Carburized Steel. Metall. Trans. A, 1989, 20A, 519-525.

Experimental Procedure: Overview

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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

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● 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

Results: Impact Toughness

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● 250x, deformed surfaces in SC and tempered samples.

450°C 550°C (4140)

600°C575°C

500°C

550°C

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AQ

SC

Results: Fracture

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Results: Fracture

4140SC 8620

● Tempered samples have hardness between SC and AQ samples● Hardness decreased with increasing tempered temperature● Little variation with specimen depth due to slow quenching

Results: Microhardness (Strength)

AQ

SC

4140

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● 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

Results: Preliminary Material Index

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● Corrected true stress vs. true strain● 575°C tempered yielded higher toughness than 550°C tempered

Results: Tensile Testing

Ultimate Strength (MPa)

Yield Strength (MPa)

Modulus (GPa)

Toughness (MPa)

550°C 859 715 234 65.8

575°C 1140 991 209 69.6

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Results: Final Material Index

Tempering Temp. (°C)

Tensile Toughness (MPa)

Charpy Toughness (J)

Hardness (MPa)

Material Index -Tensile((MPa)2)

Material Index -Charpy(N2/m *106)

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

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Discussion: Types of Microstructures

Martensite● Formed by quenching austenite● Hard, brittle, low toughness ● Carbon is trapped

Cementite (Iron Carbide)● Precipitated carbon● Hard and brittle

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Bramfitt, B. L. Structure/Property Relationships in Irons and Steels. Materials Selection and Design. 1997, 20, 357-382.

Ferrite● Low carbon● High toughness

Pearlite● Cementite and ferrite in lamellar structure● High strength and hardness ● Poor ductility and toughness

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Discussion: Types of Microstructures

University of California, Santa Barbara. Carbon Applications. http://research.mrl.ucsb.edu/~barton/materials.html (accessed Dec 1, 2014).

Slow CooledAs Quenched

Extensive regions of ferrite (white) with regions of pearlite (black).

Largely martensitic (black) with regions of ferrite and/or carbide (white). 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

600°C575°C

500°C

550°C

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Discussion: Experimental Microstructures

● Microstructure approximately follows expected trend for tempered martensite

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Discussion: Tempered Martensite

Gowelding. The Metallurgy of Carbon Steel. http://www.gowelding.com/met/carbon.htm (accessed Nov 28, 2014).

Discussion: Slow Cooled● 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 provides some hardness.

● Slow cooled has most ferrite● Supported by Charpy data

(highest toughness)

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0.2% Carbon

0.1% Carbon 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." N.p.: n.p., n.d. N. pag. Print.

● 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

575°C500°C

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550°C 600°C

Discussion: Toughness

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● 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

Discussion: Hardness Trend

Lee, S.; Ho, W. The Effects of Surface Hardening on Fracture Toughness of Carburized Steel. Metall. Trans. A, 1989, 20A, 519-525.

● 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

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Conclusions

● 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

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Future Recommendations

We would like to acknowledge Professor Marc DeGraef, Bill Pingitore, and the lab TAs for their support and help throughout the laboratory experiments.

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Acknowledgements

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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, 4452-4457.

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Supplemental Slides

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ImageJ Analysis

1. Image → Adjust → Threshold2. Check “Dark background” 3. Cover the white portions of the image with

red manually4. Analyze → Analyze Particles5. Save data to Excel6. Sum areas of white

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Charpy Data

Treatment SampleCharpy Toughness

(J)Average

(J)Stand. Dev.

(J)550°C Tempered Repeat

1 17.9 18.0 0.12 183 18.1

575°C Tempered

1 13.8 13 12 12.13 14

600°C Tempered

1 20.0 21 22 20.03 24.0

Slow Cooled 1 26 26.3 0.62 273 26

Treatment SampleCharpy Toughness

(J)Average

(J)Stand. Dev.

(J)As Quenched

1 6.4 8 32 11.23 6.8

450°C Tempered

1 11.5 11.5 --2 11.43 --

500°C Tempered

1 14.0 15 12 16.73 14.4

550°C Tempered(4140)

1 55.0 51 52 46.13 52.4

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Microhardness Data

Treatment

Microhardness (MPa)1mm from

surface3mm from

surface5mm from

surface AverageAs Quenched 5010 5291 5597 5300450°C Tempered 3738 3806 3711 3752500°C Tempered 2989 3169 3344 3167550°C Tempered (4140) 3608 3792 3557 3652550°C Tempered Repeat 2914 2859 3008 2927575°C Tempered 3332 3321 3212 3288600°C Tempered 2798 2877 2869 2848Slow Cooled 2239 2342 2258 2279

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*550 Sample B, was originally tested is "fast" mode on the frame resulting in failure of the test. The sample was then re-evaluated at the proper strain rate, but the sample may have been plastically deformed during the first test.

Instron Data

Sample

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

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Bainite

33Bramfitt, B. L. Structure/Property Relationships in Irons and Steels. Materials Selection and Design. 1997, 20, 357-382.

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