August 2024 Volume 6

MATERIALS

The first step in the experiments is to investigate each material's temper back resistance. This was conducted with starting hardness for Uddeholm Unimax at 57 HRC, and Uddeholm Skolvar at 58 HRC and AISI H13 of 52 HRC. Samples were heat treated to reach the set hardness values. Samples then were placed in a furnace set to 600°C (1110 ⁰F). After ten hours, the samples were removed from the furnace to measure hardness drop in comparison with start hardness.

The second step in the experiment was to conduct the wear test. Wear testing was performed using a pin on abrasive paper Al2O3 (800 Mesh), where the pin martial was Uddeholm Skolvar, Uddeholm Unimax and AISI H13 ESR. Samples were held against the abra sive papers using dead weights for a prescribed amount of time and the relative wear resistance was calculated from the mass loss, lower the mass loss, higher the resistance to wear. Tests were conducted in each hardness condition twice on the same sample and on a parallel sample, totaling four times on the same material to ensure repeat ability.

Figure 4: Equipment of wear test, pin on disc.

The third test was to investigate the hot strength of the material, the tests were conducted through tensile testing at elevated temperatures. The samples were heated to 600 ⁰C (1110 ⁰F) before the tensile test started. Results and Discussion The results of the temper back resistance showed in Figure 5 that after 10 hours in 600 °C (1110 ⁰F) Uddeholm Skolvar had minimum loss in hardness, only four HRC. In comparison with the other grades, Uddeholm Skolvar had the highest start hardness and the lowest loss of hardness after being exposed to high heat. This shows that Uddeholm Skolvar keeps its hardness for longer time in a forging application where heat exposure to the die surface is happening.

Wear test results, see Figure 6, showed that Uddeholm Skolvar had the lowest mass loss hence the best abrasive wear resistance. The results indicate the importance of hard phase carbides and not only the hardness of the materials.

Figure 6: Results of wear test, pin on abrasive paper Al2O3 (800 Mesh), lower mass loss higher wear resistance. The result of hot strength (see Figure 7), showed that Uddeholm Skolvar also had the highest hot yield strength. In a forging operation, the die surface will not be at room temperature but at elevated temperature, therefore it is important for the tool steel to have a high hot strength to withstand the load without starting to plastic deform.

Figure 5: Results of temper back resistance in dark blue compared with original hardness in light blue.

FIA MAGAZINE | AUGUST 2024 40