Optimization of Heat Treatment Process of High Speed Seamless Steel Pipe

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

First, controlling austenitizing temperatureRefinement of the grain is the only way to increase both the strength of the material and the toughness of the material. For high-speed seamless steel pipe and high-carbon, high-alloy cold-work die steels, the tendency to brittle cracking can be reduced by appropriately lowering the quenching temperature. Because the quenching temperature is reduced, on the one hand, it is possible to control the dissolution of M6C and MC carbides into the matrix, so that the average carbon content in austenite decreases, resulting in an increase in the Ms point and a decrease in the number of twin martensite after quenching. Lath martensite-based quenching structure, thereby enhancing the toughness of the steel; On the other hand, due to lower quenching temperature, austenite grain size is refined, but also conducive to the improvement of toughness. The test results show that when the austenitizing temperature is 1200 and 1180°C, the grain size is 11.5; when the austenitizing temperature is 1160°C, the grain size is 12 grades, which means that the appropriate reduction of the quenching temperature can make the austenite. Grains are refined. In addition, with the decrease of austenitizing temperature from 1200°C to 1160°C, the hardness of W6Mo5Cr4V2 high-speed steel slightly decreased, while the impact absorption work increased, and the impact energy absorption test value increased from 4.62J to 4.96J.

Second, the use of graded tempering processThe purpose of tempering of high-speed seamless steel pipe is to obtain a stable structure through the dispersion of alloy carbides and the full transformation of retained austenite. Tempering process is the key to obtaining high red hardness, wear resistance and toughness of high-speed seamless steel tubes. The tempering process recommended by foreign literature tends to adopt two tempering methods. Graded tempering is also used by some domestic factories to replace the conventional three tempering processes. The graded tempering process for W6Mo5Cr4V2 high-speed steel, that is, the 350°C increase in tempering at high temperatures, results in better strength and toughness than conventional tempering. The test results show that at the same austenitizing temperature, the wear resistance and red hardness of W6Mo5Cr4V26 high speed steel after classifying and tempering are better than the conventional tempering. Metallographic examination showed that the W6Mo5Cr4V2 high speed steel after graded tempering had more evenly distributed cementite particles and dispersed carbides precipitated 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 increased by 350 °C classification process, so that high-speed seamless steel pipe first precipitated uniformly dispersed cementite particles, can promote the subsequent high-temperature tempering M2C alloy carbide precipitation. It is these dispersed and finely distributed carbides that make the secondary hardening effect of graded and tempered high speed steel more pronounced, and thus have higher hardness, red hardness and better wear resistance than conventional tempered high speed steel. At the same time, it is also difficult to improve the toughness because the crack initiation and propagation along the twin boundaries is difficult. The performance test is compared as follows:Hardness: The graded tempering specimen is 64.6HRC, and the conventional tempering specimen is 63.8HRC;Red hardness: The graded tempering specimen is 59.3 HR, and the conventional tempering specimen is 58.6 HR.

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