- Tungsten and molybdenum are the key alloying elements in high speed steel rolls. Their chemical properties are similar, and their effects on the structure changes and properties of high speed steel rolls are almost the same. The difference is that molybdenum causes the structural transformation at a lower temperature. The main reason for using high speed steel to manufacture rolls is to take advantage of its excellent red hardness to improve the high temperature wear resistance of the rolls. The reason why high speed steel rolls have excellent red hardness is first because M2C and MC have strong anti-aggregation.
Tungsten and Molybdenum for High Speed Steel Roll Quality
- A large amount of residual austenite can also be obtained in the quenched structure of ordinary carbon steel and low alloy steel. However, the decomposition of these residual austenites at high temperatures rarely increases hardness. The austenite in these steels is usually decomposed at a lower temperature, and the precipitated Fe3C carbides will rapidly aggregate at slightly higher temperatures, and the aggregation of carbides is the direct cause of softening. In high speed steel, the precipitation of carbides and the decomposition of residual austenite together lead to secondary hardening, and the carbides always maintain their small size, so the high speed steel has a good red hardness. In high-speed steel, the elements that have an effect on 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 slow. In order for the carbides to precipitate continuously, the diffusion of not only chromium and vanadium, but also tungsten (molybdenum) and carbon is required. Therefore, in order to ensure that the high speed steel roll has good red hardness and high temperature wear resistance, it is reasonable to add an appropriate amount of tungsten and molybdenum to the roll structure.
Understanding Tungsten’s Impact on High Speed Steel Rolls
- From the development history of high speed steel, tungsten is a key element to enhance the tempering stability and red hardness of high speed steel. In high-speed steel, tungsten mainly exists in the form of M6C and plays an important role in improving the wear resistance of the steel. During the high-temperature quenching process, a part of the M6C is dissolved in austenite to boost the hardenability of the high-speed steel.
Tungsten: The Key to High-Speed Steel Hardness and Stability
The tungsten atom has a large radius and high elastic modulus, and it interacts with the dislocation and concentrates on the dislocation line, making the dislocation difficult to move, resulting in significant solid solution strengthening. The binding force between tungsten atoms and carbon atoms is very strong, which improves the stability of martensite decomposition at high temperatures, maintains the characteristics of the martensite lattice at high temperatures, and maintains high hardness. The undissolved M6C during quenching and heating prevents austenite grains from growing at high temperatures. During the high-temperature tempering process, part of the tungsten is dispersed and precipitated in the form of W2C, leading to secondary hardening and increasing the red hardness of the high-speed steel.
The influence of tungsten on the structure and main properties of high speed steel is not proportional to its content. High speed steel containing 7%-8% 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 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 content in the roll structure will increase, and the carbide particles will become large and uneven, which will adversely affect the thermal fatigue performance of the roll.