Toughness and Ductile/Brittle Fracture

Michael Muldoon, Mechanical Engineering

Materials Science Lab, Section 04

10/7/2014

Group Members: Full Class Lab

Abstract: In this lab impact toughness tests were done on five different steel specimens. It was done on three 4140 alloy steel specimens, a 1045 carbon steel and a 1018 carbon steel specimen. This lab was conducted to observe the effects temperature and alloying elements have on a material’s toughness. This lab was also done to observe ductile and brittle fractures and to show the relationship between hardness and toughness. All five specimens were tested at room temperature (~22°C) and at about -55°C. The ambient specimens were also tested for hardness. As expected there was an inverse relationship between toughness and hardness. The alloying elements in the 4140 steel specimens made the materials toughness less biased to changing temperature in comparison to the 1045 carbon steel. Change in temperature did always affect toughness but affected each material differently. The materials weren’t as tough at the lower temperature as they were at the ambient temperature. Generally all of the fractures did illustrate the type of fracture they were based on the data collected with a few exceptions. Both the 1045 and 1018 steel specimens appeared slightly more ductile than the data exemplified for the cold impact toughness test. Overall this lab did demonstrate the effects of alloying elements and temperature on toughness and the differences between ductile and brittle fractures.

Toughness and Ductile/Brittle Fracture

Michael Muldoon, Mechanical Engineering

Materials Science Lab, Section 04

Objective: This lab was directed to observe the effect alloying elements and temperature have on material toughness. This lab was also done to detect the relationship between a material’s toughness and hardness and to observe the differences between ductile and brittle fracture.

Procedure:

Experimental Procedure:

1)Five different steel specimens were tested.

  1. Plain Carbon Steels
  2. White: 1018 (0.18 wt% C)
  3. Blue: 1045 (0.45 wt% C)
  4. 4140 Alloy Steels (0.40 wt% C, 0.95wt% Cr, 0.88 wt% Mn, 0.22 wt% Si)
  5. Yellow: Quench Hardened then tempered to HRC30 (@~1200°F)
  6. Red: Quench Hardened then tempered to HRC40 (@~800°F)
  7. Green: Quench Hardened then tempered to HRC47 (@~400°F)
  8. All specimens are held to ASTM standard E-23 the standard test methods for notched bar impact testing of metallic materials.

2)Toughness Test

  1. The SATEC systems model SI-1c3 impact tester was used to test the materials.
  2. First the lever was pulled under the latch.
  3. The hammer was raised into the locking position.
  4. The pointer was then set.
  5. The specimen was then placed in the machine using the special tongs.
  6. The tester would say clear to avoid people from being in the line of fire.
  7. Once the hammer swung down and broke the specimen the lever would be pulled to apply the brake.
  8. The energy would then be read off the machine and recorded.
  9. Lastly the broken pieces would be collected.
  10. This test was done according to ASTM standard E23
  11. Cold specimen testing slightly deviated from the ASTM standard because the specimen could not be tested within five seconds of being removed from the alcohol bath.

3)Hardness Test

  1. The Clark CR – 3E Rockwell tester was used to determine the hardness of these materials.
  2. The specimens were tested according to ASTM-E18, the standard for Rockwell testing materials.
  3. Once the specimens were impact tested they were placed on the base of the machine and preloaded by hand.
  4. The test was then conducted as the tester pushed the indenter into the material. The hardness at the given point was recorded after the test.
  5. This was repeated three times for every specimen and the results were averaged and recorded.

Specific Test Procedure:

1)First the impact tester was calibrated.

2)The toughness test was conducted on all the specimens at room temperature (~22°C).

3)The toughness test was then done on all the specimens in the alcohol bath. These specimens were at about -55°C.

4)Data for all the tests were measured and recorded on the data sheet.

5)After the room temperature specimens were brought to the tester,the tester was calibrated to make sure it was operating correctly.

6)The tester was then set to 30T for the steel specimens.

7)A specimen was then put on the base of the tester and the indenter was visually centered somewhere along the specimens length.

8)The specimen was then preloaded to the designated force and the test began.

9)The tester then ran its test and returned data for the Rockwell hardness.

10)This procedure was repeated three times for every specimen.

Results:

Data Table

Specimen ID / Material / Temperature (°C) / Impact Energy (J) / Hardness (RC)
White / 1018 Steel / Ambient (~22°C) / 64 / 18.7
Blue / 1045 Steel / 24 / 27.3
Yellow / 4140 Steel / 71 / 29.7
Red / 4140 Steel / 30 / 38.6
Green / 4140 Steel / 6 / 48.1
White / 1018 Steel / ~-55°C / 21
Blue / 1045 Steel / 5
Yellow / 4140 Steel / 54
Red / 4140 Steel / 18
Green / 4140 Steel / 4

Figure 1.1

Fracture Images

Figure 2.1: Plain carbon steel specimens (white and blue) at ambient temperature.

Figure 2.2: Alloy steel specimens (yellow, red and green) at ambient temperature.

Figure 2.3: Plain carbon steel specimens (white and blue) at about -55°C.

Figure 2.4: Alloy steel specimens (yellow, red and green) at about -55°C.

Data Plots

Figure 3.1: Impact toughness for carbon steels versus temperature.

Figure 3.2: Impact toughness for 4140 alloy steels versus temperature.

Figure 3.3: Impact toughness against hardness for carbon steels and alloy steels at ambient temperature.

Figure 3.4: Impact toughness against hardness for carbon steels and alloy steels at about -55°C.

Observations:

Fractures and Toughness: The fractures shown in figure 2.1 indicate ductile fractures for both white and blue specimens. The actual data from the test also indicate ductile breaks for both specimens although the white specimen is much more ductile than the blue. The breaks shown in figure 2.2 show a very ductile break for the yellow specimen, a somewhat ductile break for the red specimen and an almost perfectly brittle break for the green specimen. The data collected from the experiment confirms this observation. The fractures shown in figures 2.4 also agree with the data concluded from the experiment. The fractures shown in figure 2.3 show somewhat ductile breaks while the data suggests otherwise. This difference could be because these materials are generally ductile at room temperature and had the tendency to break in a ductile manner while actually being brittle at -55°C. This difference could also be from camera angle and not being able to properly analyze the fracture.

Data Patterns: As shown in figure 3.1 the blue specimen has a gentle slope suggesting it is a high strength material because of its low ductility and small change of toughness with temperature. The white specimen has a steeper slope showing that it is a low strength steel with a BCC structurebecause of its high variability of toughness with changing temperature. Displayed in figure 3.2 the red and green specimens have gentle slopes with low ductility showing they are high strength materials. The yellow specimen has a gentle slope but a high ductility showing that it is a low strength metal with FCC or HCP structure.

Influence of Alloying Elements: The alloying elements present in the 4140 steels tended to make the steels toughness less subjective to change in temperature. Compared to the 1045 (Blue) specimen all of the 4140 alloy steel specimen’s toughness’s change less due to the same temperature difference.

Conclusion: This lab experiment shows that allowing elements affecta materials toughness and that temperature can affect toughness drastically or slightly depending on the type of material. This lab also demonstrates an inverse relationship between toughness and hardness of materials.

References:

1)"ASTM Material Specification A109-13- Cold-Rolled Mild Carbon Steel Strip." N.p., 4 Feb. 2014. Web. 16 Sept. 2014.

2)"ASTM E23-12c Standard Test Methods for Notched Bar Impact Testing of Metallic Materials." ASTM International. N.p., n.d. Web. 2 Oct. 2014.

Appendix: