Analysis for mill roll fracture problems

The the mill roll journal fracture photos were sent by our customer from their other suppliers, let's analyze the fracture reasons

The above roll neck fracture photos were sent by our customer from their other suppliers, and the customer want us to analyze the fracture reasons. We give reasons from four factors: material, design, machining and working conditions, to provide technical reference for troubleshooting. 
Material and internal defects: the main cause of fracture
1.Poor spheroidization effect: The poor spheroidization effect will reduce the number of graphite balls in the core of the mill roll, and the graphite balls size will be uneven, and the proportion of graphite in flakes or other irregular forms will increase. Flake graphite will split the metal matrix, and the toughness and strength of the mill roll core will be significantly reduced, making it prone to failure forms such as fracture during use.
2.Metallurgical defects such as inclusions
The inclusions produced by smelting reduce the purity of the material and become a source of cracks. When mill roll stressed, stress concentration occurs in the inclusion area, which causes crack propagation and journal fracture, and the origin of the defect is often visible at the fracture.
3.Defects in the structure of the casting hot spot
The casting hot spot is cooled slowly, the coarse structure reduces the bearing capacity by about 30%, and it is easy to be brittle when mill roll stressed, and ultrasonic flaw detection can identify hot spot.
The material brands and requirements of the mill rolls produced by our company can be referred to BMK Glycidate
Design defects: the main cause of stress concentration
In the part where the stress of the mill roll journal is more concentrated, especially at the root of the mill roll, the design of the transition arc is too small, which is easy to lead to the fracture of the journal.
1.The transition arc is too small, resulting in a high concentration of stress in this part, and when the mill roll is loaded, the stress value in these stress concentration areas is much higher than the average stress,the stress of the material exceeds its yield strength, then cracks are generated,and leads to journal fracture.
2.The mill roll journal usually bears alternating load during machining, and the stress concentration caused by too small transition arc will significantly reduce the fatigue strength of the journal under the action of alternating stress, accelerate the initiation and expansion of fatigue cracks, and cause the journal to fracture easily in long-term operation.
Machining and manufacturing deviation: hidden danger of fracture
1. Residual stress superimposed failure
The residual tensile stress generated by heat treatment / machining is not eliminated and is superimposed with the working stress and exceeds the strength limit, resulting in crack propagation.
2.Insufficient machining accuracy
The surface roughness of the mill roll journal is high, the size deviation is large, or there are machining knife marks, etc., resulting in stress concentration, reducing fatigue strength, and easy to fracture after long-term operation.
Abnormal operating conditions: Trigger fracture condition
1. Overload fracture
Excessive depression/low overloading temperature (≤900°C) and rolling force exceeding the rated rating by 1.5 times, making the rolling force exceed the bearing capacity of the journal, which will cause the journal to overload and fracture.
2.Shock, vibration, fatigue
The impact and vibration during rolling, such as the impact force at the moment of biting the steel, or the vibration caused by the unstable operation of the equipment, accumulate the fatigue damage of the mill roll journal and cause fatigue fracture。
3.Brittle fracture under thermal stress
Cooling uneven temperature difference ≥ 100°C, cooling interruption, or surface overheating at the beginning of the rolling cycle,large temperature difference between the surface of the journal and the interior result in thermal stress, superimposed with other stresses exceed the resistance limit, resulting mill roll fracture.
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Journal fracture is caused by multiple factors such as material, design, machining and working conditions. In the production, it is necessary to pay attention to the purity of molten iron, reduce slag inclusion, improve the spheroidization effect of ductile iron mill rolls, and reduce the casting defects; In the design, some unreasonable transition arc sizes are corrected in time to avoid the problem of stress concentration; Pay attention to surface accuracy, residual stress and other issues during machining; At the same time,during rolling production and using mill roll, pay attention to the use specifications of mill roll, avoid emergencies during rolling steel, maintain mill roll in time to avoid fatigue accumulation caused by the transitional use of mill roll.When the roll is grinded by the machine, ensure the rolling fatigue layer is completely removed.