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Reason for fracture

1. Brittle fracture: The fracture surface of this type of roll is relatively flat, and the surface of the roll body around the fracture is relatively smooth;

2. Ductile fracture. The fracture surface of such rollers often presents a "mushroom head" shape, with the roller body near the fracture surface being crushed into powder.

Upon comparing the two, it was found that the form of roll fracture in this incident was ductile fracture. Both brittle fracture and ductile fracture are caused by the stress on the roll exceeding the strength of the core.

The causes of this phenomenon are related to the residual stress of the roll itself, the mechanical stress during rolling, and the thermal stress of the roll. It is particularly prone to occur when there is a large temperature difference between the surface and the core of the roll body. This temperature difference may be caused by poor roll cooling, cooling interruptions, or overheating of the roll surface at the beginning of a new rolling cycle. The significant temperature difference between the surface and the core of the roll induces large thermal stress. When the thermal stress, mechanical stress, and residual stress of the roll exceed the core strength of the roll, roll breakage occurs. For example, a temperature difference of 70°C between the roll surface and the core increases the longitudinal thermal stress of the roll by 100 MPa. The greater the temperature difference, the greater the increased thermal stress. Compared to rolls with brittle fractures, rolls with ductile fractures have better toughness in the core material and are less prone to fracture.

There are four types of stress that contribute to the failure of rolling mill rolls:

1. Residual stress during the manufacturing process;

2. Mechanical stress during the rolling process;

3. Structural stress of the roll during the rolling process;

4. Thermal stress caused by temperature difference between the inside and outside of the roll.

If the fracture is due to excessive residual stress during manufacturing, it typically occurs during the first few times the roll is used on the machine, and affects the first few pieces of rolled material. The roll that fractured this time has been used for four times on the machine, with the working layer worn down by 14mm. Therefore, it should not be a fracture caused by residual stress during manufacturing.

If the fracture is caused by mechanical stress, a significant amount of mechanical stress is required. Rough calculations indicate that for a high-chromium cast steel roll with such a large cross-section to break due to mechanical stress, a tensile force of over 100MN would be necessary, which is impossible for the rolling mill where this roll operates. The part of the roll that experiences the highest stress is the roll neck at the drive end. If the mechanical properties of the material are insufficient, the roll neck at the drive end will be the first to fail under normal rolling conditions. Based on actual rolling practices and roll breakage scenarios, the fracture of the roll body is not caused by mechanical stress.

The most significant factor affecting organizational stress is the content of residual austenite in the outer layer of the tissue. Under the alternating effects of rolling temperature, rolling pressure, and water cooling, residual austenite undergoes a transformation from austenite to martensite or bainite. Due to the smaller specific volume of austenite and the larger specific volume of martensite, the process of organizational transformation is accompanied by volume expansion, which can lead to greater compressive stress in the working layer of the roll and greater tensile stress in the core. Once the core stress exceeds the strength of the material, it will inevitably result in roll fracture. Considering the impact of residual austenite on organizational stress and the working conditions of hot strip mill rolls, generally controlling the residual austenite content of the roll to less than 5% can ensure safe use. The residual austenite content in the outer layer of the fractured roll is less than 1%, so the organizational stress can be neglected.

The fracture of the roll may also be related to thermal stress caused by uneven temperature. During the use of the roll in the machine, due to its close contact with the rolled material, the surface temperature of the roll rises rapidly, while the temperature of the roll core rises more slowly. At this time, the temperature difference between the roll surface and the roll core is at its maximum, and the thermal stress caused by the temperature difference is also at its maximum. If the thermal stress of the roll and the residual stress of the roll are superimposed and exceed the strength limit of the roll core, an accident of roll fracture may occur.

Methods to prevent fracture

To prevent fracture, efforts should be made to reduce residual stress, mechanical stress, structural stress, and thermal stress during manufacturing.

Generally, most of the manufacturing residual stress will be eliminated during the heat treatment process, and it will gradually diminish as the storage time of the rollers increases. Therefore, storing new rollers for a period of time before use can reduce the risk of roll breakage. The main method to avoid large mechanical stress is to avoid over-cooled steel. The method to reduce structural stress is to control the residual austenite content in the working layer of the roller body to less than 5% through heat treatment. The way to reduce thermal stress is to properly cool the rollers during the steel rolling process.

The main causes of fracture in high-chromium steel rollers are manufacturing residual stress, mechanical stress, organizational stress, and thermal stress. Good heat treatment, rolling conditions, and cooling can effectively prevent the fracture of high-chromium steel rollers.