1: Brittle fracture
The fracture shape of this type of roll is relatively flat, and the surface of the roll body around the fracture is relatively neat;
2: Ductile fracture
The fracture shape of such rolls is mostly “mushroom head”, and the roll body near the fracture is crushed. Comparing the two, it is found that the form of roll breakage in this roll breaking accident is ductile fracture.
Both brittle and ductile fractures are caused by roll stress exceeding core strength. The reasons for this are related to the residual stress of the roll itself, the mechanical stress during rolling and the thermal stress of the roll, especially when the temperature difference between the surface and the core of the roll body is large.
This temperature difference can be caused by poor roll cooling, interruptions in cooling, or overheating of the roll surface at the start of a new rolling cycle. This huge temperature difference between the surface and the core of the roll causes large thermal stress, and when the large thermal stress, mechanical stress and residual stress of the roll exceeds the core strength of the roll, it causes the roll to break. For example, when the temperature difference between the surface of the roll and the core is 70°C, the roll will increase the longitudinal thermal stress of 100MPa. The greater the temperature difference, the greater the increased thermal stress. Compared with rolls with brittle fractures, the core material of rolls with ductile fractures has better toughness and is less prone to fracture. There are four types of stresses that cause roll failure:
- Residual stress in the manufacturing process;
- Mechanical stress during rolling;
- The tissue stress of the roll during the rolling process;
- Thermal stress caused by the temperature difference between the inside and outside of the roll.