Heat Treatment Process for Improving the Service Life of Extruder Auger Shafts

To obtain extruder auger shafts with high strength and toughness, 40CrMnMo steel is mostly used as the material. This material is a low-carbon alloy structural steel and is used in the quenched and tempered condition. Then, after quenching and tempering (quenching + high-temperature tempering), does 40CrMnMo steel possess the best wear resistance in addition to high strength and toughness?

1 Effect of Heat Treatment Process on the Wear Resistance of 40CrMnMo Steel

The heat treatment technical requirements for 40CrMnMo steel in the tests are shown in Table 1, and the data obtained after wear testing are shown in Table 2.

Table 1 Technical Requirements for Heat Treatment Processes

Table 2 Wear Mass Loss and Hardness

From Table 1, it can be seen that the designed heat treatment processes are:
a. Quenching only; b. Quenching + low-temperature tempering; c. Quenching + medium-temperature tempering; d. Quenching + high-temperature tempering.

From Table 2, it can be seen that the hardness of 40CrMnMo steel varies with tempering temperature in accordance with the general rule, i.e., as the tempering temperature increases, the hardness gradually decreases. However, the wear resistance does not change in parallel with hardness as the tempering temperature changes. The wear mass loss of the quenched + low-temperature tempered specimens is lower than that of the quenched-only and quenched + medium- and high-temperature tempered specimens. This indicates that after quenching only, while the hardness of the material increases, brittleness is also produced. Therefore, under hard abrasive wear conditions, brittle spalling is likely to occur, reducing wear resistance. After low-temperature tempering, the hardness decreases only slightly, but toughness is significantly improved, thereby enhancing wear resistance. As the tempering temperature continues to increase, hardness gradually decreases, and wear mass loss gradually increases. During medium-temperature tempering, the supersaturated carbon atoms in the quenched martensite gradually precipitate, the hardness of the matrix decreases, and a large amount of highly dispersed carbides precipitate in the matrix, which still contributes significantly to improving wear resistance. During high-temperature tempering, the carbide size increases, the matrix hardness drops substantially, and thus wear mass loss increases significantly, while wear resistance decreases.

2 Conclusion

After quenching + low-temperature tempering and quenching + medium-temperature tempering, 40CrMnMo steel has relatively high wear resistance. After low-temperature tempering, the material has the best hardness and wear resistance; after medium-temperature tempering, the strength, toughness, and wear resistance of the material are all balanced; after high-temperature tempering, the material has the best strength and toughness, but its wear resistance decreases. Considering that the extruder auger shaft requires not only high wear resistance but also good strength and toughness, it is concluded that the optimal heat treatment process for the extruder auger shaft is: 850°C quenching + 400°C medium-temperature tempering.

Selected from Brick and Tile 2000(02)