Tungsten and molybdenum are the main alloying elements in ordinary high-speed steel. The chemical properties of tungsten and molybdenum are similar. Their effects on the transformation and performance of high-speed steel are almost the same. The only difference is that the temperature at which molybdenum causes the transformation of the structure is lower.
The main reason for using high-speed steel to manufacture rolls is to use the excellent red hardness of high-speed steel to improve the high-temperature wear resistance of rolls. The excellent red hardness of high-speed steel is first attributed to the strong anti-agglomeration ability of M2C and MC. A large amount of retained austenite can also be obtained in the quenched structure of ordinary carbon steel and low alloy steel. At high temperature, the decomposition of these retained austenite can rarely increase the hardness. The austenite in these steels is usually at a lower temperature. Decomposition, and the precipitated Fe3C carbides gather rapidly at a slightly higher temperature, and the aggregation of carbides is the direct cause of softening. In high-speed steel, the precipitation of carbides into very fine particles and the decomposition of retained austenite cause secondary hardening, and the carbides always retain their fine size, thus making high-speed steel have good red hardness.
In high-speed steel, the elements that have the greatest influence on 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 speed is slow. To carry out, not only the diffusion of chromium and vanadium is required, but also the diffusion of tungsten (molybdenum) and carbon. Therefore, in order to ensure that high-speed steel rolls have good red hardness and high-temperature wear resistance, it is reasonable to add an appropriate amount of tungsten and molybdenum to the roll structure.
From the perspective of the development history of high-speed steel, tungsten has always been the preferred element to improve the stability of high-speed steel against tempering and red hardness.
Tungsten mainly exists in the form of M6C in high-speed steel, which has a great effect on improving the wear resistance of high-speed steel. During high-temperature quenching, part of M6C dissolves into austenite to improve the hardenability of high-speed steel. The dissolution of tungsten into the matrix can effectively hinder the precipitation during tempering. The tungsten atomic radius is large and the elastic modulus is high. It interacts with dislocations and segregates on the dislocation line, locking the dislocations so that they are difficult to move and form Great solid solution strengthening.
The strong bonding force between tungsten atoms and carbon atoms improves the stability of martensite pyrolysis, maintains the characteristics of martensite lattice at high temperature, and maintains high hardness. The undissolved M6C in the process of quenching and heating can prevent the growth of austenite grains at high temperature. During high-temperature tempering, a part of tungsten is dispersed and precipitated in the form of W2C, causing secondary hardening and improving the red hardness of high-speed steel. It is the above characteristics that make the tungsten-containing high-speed steel increase in dispersion strengthening and solid solution strengthening with the increase of tungsten content in the process of heating and heat preservation, which determines that tungsten has a strong ability to improve the thermal stability of high-speed steel.
The influence of tungsten on the structure and main properties of high-speed steel does not change in proportion to its content. The high-speed steel contains 7% to 8% W to obtain satisfactory secondary hardness and thermal stability, but at this time the carbide phase contains too much M23C6 and too little M6C, so the quenching temperature should not be too high, otherwise it will Very coarse grains are produced, and the strength and toughness are obviously reduced. Continue to increase the tungsten content, and the M6C produced 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 unevenly distributed, which will affect the thermal fatigue performance of the roll. produce adverse effects.
The role of platinum is similar to that of tungsten, and the iron angle of the Fe-Mo-C phase diagram and the Fe-W-C phase diagram are very similar. Molybdenum also forms M6C carbides. Compared with M6C in tungsten-containing high-speed steel, its lattice is the same, and the lattice parameters are almost the same, but the density is lower. Since the relative atomic mass of molybdenum is smaller than that of tungsten (about half), in general, 1% Mo can replace 2% W, and (W + 2Mo)% is called tungsten equivalent. Usually, the tungsten equivalent of high-speed steel is 16%–20%. From the perspective of saving resources, it is hoped that the tungsten equivalent should be as low as possible. Uchida and others believe that the higher the tungsten equivalent, the better the wear resistance. In addition, the lower the tungsten equivalent, the higher the toughness. Therefore, the tungsten equivalent of the high-speed steel roll should be controlled below 15%.