The role of tungsten and molybdenum in high speed steel rolls

Tungsten and molybdenum are the main alloying elements in ordinary high-speed steel rolls. Tungsten and molybdenum are chemically similar, and their effects on the organizational transformation and performance 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 the use of high-speed steel roll manufacturing is to use the excellent red hardness of high-speed steel to improve the high-temperature wear resistance of the roll.

The excellent red hardness of HSS rolls is firstly attributed to the strong anti-aggregation properties of M2C and MC. Large amounts of residual austenite can also be obtained in the quenched organization of normal carbon and low alloy steels. The decomposition of this residual austenite at high temperatures rarely increases the hardness. The austenite in these steels usually decomposes at lower temperatures, and the precipitated Fe3C-type carbides rapidly aggregate at slightly higher temperatures; the aggregation of carbides is the direct cause of softening. In HSS, the precipitation of carbides into very fine particles and the decomposition of residual austenite combine to cause secondary hardening, with the carbides always retaining their fine size, resulting in HSS having good red hardness. In HSS, the elements that influence this phenomenon are tungsten and molybdenum. The atomic size of tungsten and molybdenum in HSS is much larger than that of any other element, and diffusion is slower. In order for carbides to continue to accumulate, diffusion of not only chromium and vanadium, but also tungsten (molybdenum) and carbon is required. Therefore, in order to ensure that HSS rolls have good red hardness and high-temperature wear resistance, it is reasonable to add appropriate amounts of tungsten and molybdenum to the roll structure.
High-speed steel rolls: From the viewpoint of 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 the form of M6C in HSS, which plays an important role in improving the wear resistance of HSS. During high-temperature quenching, a portion of M6C is dissolved in the austenite to improve the hardenability of HSS. Tungsten dissolved in the matrix effectively prevents precipitation during tempering. The tungsten atom has a large radius and high modulus of elasticity. It interacts with dislocations and concentrates on the dislocation line. The dislocations are locked, making it difficult for them to move and creating large solid solution strengthening.

The bond between tungsten and carbon atoms is large, which improves the stability of martensitic decomposition at high temperatures, maintains the martensitic lattice characteristics at high temperatures, and maintains high hardness. Undissolved M6C during quenching and heating prevents austenite grain growth at high temperatures. During high temperature tempering, some of the tungsten is dispersed and precipitated as W2C, leading to secondary hardening and increasing the red hardness of the HSS. It is these properties that make the dispersion strengthening and solid solution strengthening of tungsten-containing HSS 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 HSS. High-speed steel rolls: the effect of tungsten on the organization and main properties of high-speed steel is not proportional to its content. High-speed steel containing 7% -8% of W 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 can not be too high, otherwise it will produce a very coarse grain, strength and toughness will be significantly reduced. If the tungsten content continues to increase, there will be an increase in the amount of M6C produced, which will significantly improve the superheat stability of the steel. However, if the tungsten content is too high, the amount of leucite in the roll structure will increase, and the carbide particles will be large and non-uniform, which will adversely affect the thermal fatigue properties of the rolls.

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