Optimization Of Heat Treatment Process For High Speed Seamless Steel Tubes

High-speed seamless steel pipe is a high-carbon, high-alloy cold-worked seamless steel pipe. When heated at a conventional quenching temperature, due to the dissolution of the alloy carbide, the carbon content and the alloying element content in the matrix are high, the strength and hardness are high, but the toughness is insufficient, and brittle fracture and early failure are liable to occur during use. To improve the overall performance of high-speed seamless steel tubes, improving the heat treatment process is a key technology.

1. Control the austenitizing temperature.
Refining the grains is the only way to increase the strength of the material and increase the toughness of the material. For high-speed seamless steel tubes and high-carbon, high-alloy cold-worked steels, the tendency to fragile cracking can be reduced by appropriately reducing the quenching temperature. Because the quenching temperature is lowered, on the one hand, the M6C and MC type carbides can be controlled to dissolve into the matrix, so that the average carbon content in the austenite is lowered, the Ms point is increased, and the number of twinned martensite after quenching is reduced. The quenched structure dominated by lath martensite improves the toughness of the steel.

On the other hand, since the quenching temperature is lowered, the austenite grain size is refined, which is also advantageous for the improvement of toughness. The test results show that the grain size is 11.5 when the austenitizing temperature is 1200 and 1180 °C, and the grain size is 12 when the austenitizing temperature is 1160 °C, indicating that the austenite can be reduced by appropriately reducing the quenching temperature. The grains are refined. In addition, as the austenitizing temperature is lowered from 1200 °C to 1160 °C, the hardness of W6Mo5Cr4V2 high-speed steel is slightly decreased, and the impact absorption energy is improved, and the impact absorption energy test value is increased from 4.62J to 4.96J.

2. Use a graded tempering process.
The purpose of tempering of seamless steel tubes is to obtain a stable structure by the dispersion of alloy carbides and the sufficient transformation of retained austenite. The tempering process is the key to high redness, wear resistance and toughness of high speed seamless steel tubes. The tempering process recommended by foreign literature generally tends to use two tempering methods. Graded tempering has also been used by some domestic factories to replace the conventional three-stage tempering process. The W6Mo5Cr4V2 high-speed steel adopts the graded tempering process, that is, the treatment of increasing 350 °C during high temperature tempering (350 °C × 1 h + 560 °C × 1 h), and the result is better toughness than conventional tempering.

The test results show that the wear resistance and red hardness of W6Mo5Cr4V26 high speed steel after classification and tempering are better than conventional tempering at the same austenitizing temperature. Metallographic examination showed that the graded tempered W6Mo5Cr4V2 high-speed steel has more uniform distribution of cementite particles and more dispersed carbides along the twin boundaries. Graded tempering can make W6Mo5Cr4V2 high speed steel have better performance than conventional tempering. This is because the high-temperature tempering increases the classification treatment of 350 ° C, so that the high-speed seamless steel pipe first precipitates a uniformly dispersed cementite mass, which can promote the precipitation of M2C-type alloy carbides in the subsequent high-temperature tempering.

It is these diffused, finely distributed carbides that make the secondary hardening effect of graded tempered high speed steel more pronounced, resulting in higher hardness, red hardness and better wear resistance than conventional tempered high speed steel. At the same time, it is difficult to improve the toughness due to the difficulty in crack initiation and expansion along the twin boundary. The performance tests are compared as follows: Hardness: Graded tempered sample is 64.6 HRC, conventional tempered sample is 63.8 HRC; Red Hardness: Graded tempered sample is 59.3 HR, and conventional tempered sample is 58.6 HR.

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