The hardness of adamite roll materials for hot work rolls depends on their microstructures; the amount and hardness of coarse carbides and the hardness o f matrix. The high-hardened adamite roll materials are excellent in wear resistance when the amount of coarse carbides is about 8 %, but the wear resistance deteriorates when the amount of coarse carbides exceeds a critical level.
What is the hardness of rolling mill rolls?
- It is well-known that adamite roll materials used for work rolls in hot rolling mills possess higher toughness compared to cast iron roll materials, but they have lower wear resistance due to their hardness ranging from Hs 40 to 60.
- Recently, as the conditions for hot rolling have become more demanding, there is an increased need for rolls with higher wear resistance. If adamite rolls can be hardened to improve their wear resistance, it is anticipated that their use for hot work rolls will rise.
- Adamite roll materials can be considered as composite materials composed of two phases: coarse carbides and matrix. Consequently, the hardness of these roll materials is significantly influenced by the amount and hardness of the coarse carbides. However, increasing the amount of coarse carbides beyond a critical value can deteriorate the wear resistance of the roll materials. Therefore, the microstructures of these roll materials should contain an optimal amount and distribution of coarse carbides to achieve a good balance of high hardness and wear resistance.
Wear Resistance of the Conventional Roll Materials
for Hot Work Roll
- High-chromium and high-nickel grain iron roll materials, along with adamite roll materials, are commonly used for hot work rolls. The wear resistance of these roll materials at high temperatures is significantly influenced by various wear conditions, such as temperatures, rolling speeds, loads, and slipping rates during testing.
- In previous studies, the dependence of wear resistance on these testing conditions was examined using a high-temperature wear testing machine. For instance, Fig. 12 illustrates the relationship between the wear resistance of these roll materials and rolling speeds over the same rolling distance. It was found that the wear resistance of adamite roll materials is more significantly affected by rolling speeds compared to other iron roll materials.
- Specific wear loss at high rolling speeds decreases when the surface is covered with a black oxide layer, which has good wear resistance. The following experiments were conducted using the worn surface without the black oxide layer.
Relation between the Hardness and the Wear Resistance
- Figure 13 demonstrates the impact of hardness on wear resistance in both short-term and long-term wear testing at high temperatures. The results indicate that wear resistance improves with increasing hardness over both testing durations.
- These findings suggest that the wear resistance of these roll materials can vary based on the hardening mechanisms. For instance, an increase in the amount and hardness of coarse carbides, as well as an increase in the hardness of the matrix, can enhance wear resistance even if the overall hardness of the roll materials remains unchanged.
The amount of coarse carbides in hardened adamite roll materials is needed to be optimized at about 8 % to ensure good wear resistance. For this purpose, the C content in adamite roll materials must be limited at about 2 %. Furthermore, the adamite roll materials must be hardened by the hardening of coarse carbides with Cr addition in proportion to C content. It is also necessary that the matrix is hardened by an addition of Mn and Ni.
The high-hardened adamite rolls with about 8 % of coarse carbides have a hardness from Hs 65 to 70 and provide a satisfactory result in application to the work rolls in hot rolling mill.