Tungsten and molybdenum in high-speed steel rolls Role of

　　Tungsten and molybdenum are the main alloying elements in ordinary high-speed steel rolls. Tungsten and molybdenum have similar chemical properties, and their effects on the microstructure transformation and properties of high-speed steel rolls are almost the same. The difference is that molybdenum causes structural transformation at a lower temperature. The main reason for using high-speed steel to manufacture rolls is to utilize the excellent red hardness of high-speed steel to improve the high-temperature wear resistance of the rolls. The excellent red hardness of high-speed steel rolls is primarily attributed to the strong anti-agglomeration of M2C and MC. A large amount of retained austenite can also be obtained in the quenched microstructure of ordinary carbon steel and low-alloy steel. The decomposition of these retained austenites at high temperatures rarely increases hardness. The austenite in these steels usually decomposes at lower temperatures, while the precipitated Fe3C-type carbides agglomerate rapidly at slightly higher temperatures. The agglomeration of carbides is the direct cause of softening. In high-speed steel, carbide precipitation into very fine particles and the decomposition of retained austenite together lead to secondary hardening, and the carbides always maintain their fine size, so high-speed steel has good red hardness. In high-speed steel, the elements that affect the formation of this phenomenon are tungsten and molybdenum. The atomic size of tungsten and molybdenum in high-speed steel is much larger than that of any other element, and the diffusion rate is slower. In order for the carbides to continue to accumulate, not only the diffusion of chromium and vanadium is needed, but also the diffusion of tungsten (molybdenum) and carbon. Therefore, in order to ensure high-speed steel rolls good red hardness and high-temperature wear resistance, it is reasonable to add an appropriate amount of tungsten and molybdenum to the roll structure.

　　High-speed steel rolls: From the development history of high-speed steel, tungsten is also an element that improves the tempering stability and red hardness of high-speed steel. Tungsten mainly exists in high-speed steel in the form of M6C, which plays an important role in improving the wear resistance of high-speed steel. During high-temperature quenching, part of the M6C dissolves in austenite to improve the hardenability of high-speed steel. The tungsten dissolved in the matrix can effectively prevent precipitation during tempering. Tungsten atoms have a large radius and high elastic modulus. It interacts with dislocations and concentrates on dislocation lines. Dislocations are locked, making them difficult to move, forming large solid solution strengthening. The strong binding force between tungsten atoms and carbon atoms improves the stability of martensite decomposition at high temperatures, maintains the martensite lattice characteristics at high temperatures, and maintains high hardness. Undissolved M6C during quenching and heating can prevent austenite grain growth at high temperatures. During high-temperature tempering, part of the tungsten is dispersed and precipitated in the form of W2C, leading to secondary hardening and improving the red hardness of high-speed steel. It is these characteristics that make the dispersion strengthening and solid solution strengthening of tungsten-containing high-speed steel improve with the increase of tungsten content during heating and insulation, which determines that tungsten has a strong ability to improve the thermal stability of high-speed steel. high-speed steel rolls ：The effect of tungsten on the microstructure and main properties of high-speed steel is not proportional to its content. 7%-8% W in high-speed steel can obtain satisfactory secondary hardness and thermal stability, but at this time, the carbide phase contains too much M23C6 and too little M6C. Therefore, the quenching temperature cannot be too high, otherwise, very coarse grains will be produced, and the strength and toughness will be significantly reduced. If the tungsten content continues to increase, the generated M6C will increase, which will significantly improve the overheating stability of the steel. However, if the tungsten content is too high, the amount of ledeburite in the roll structure will increase, and the carbide particles will be large and uneven, which will have an adverse effect on the thermal fatigue performance of the roll.