CONTACT STRESS SPALLS CONTINUED HOT MILL WORK ROLLS (THERMAL PITTING) CHARA CTERISTICS Contact stress fatigue spalling on hot mill work rolls can be characterized as small “pits” that may be random or clustered on the roll body at areas of stress concentration. Individual pits can exhibit evidence of fatigue arrest marks (beach marks) and erosion on the rolling surface of the spall. EXAMPLES EXAMPLE 1 Hot mill work roll exhibiting contact stress fatigue in the form of thermal pitting on the roll body surface.
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CONTACT STRESS SPALLS CONTINUED
HOT MILL WORK ROLLS(THERMAL PITTING)
CHARACTERISTICS
Contact stress fatigue spalling on hot mill work rolls can be characterized as small “pits” that may be random or clustered on the roll body at areas of stress concentration. Individual pits can exhibit evidence of fatigue arrest marks (beach marks) and erosion on the rolling surface of the spall.
EXAMPLES
EXAMPLE 1Hot mill work roll exhibiting contact stress fatigue in the form of thermal pitting on the roll body surface.
EXAMPLE 2Close-up view of the thermal pitting condition exhibited by the hot mill work roll shown in Example 1.
MECHANISM
In 4-high mills, thermal pitting typically initiates at a sub-surface location defined by the maximum shear stress (see General Mechanism section of Contact Stress Fatigue). During rolling, any one point on the hot mill work roll surface will cycle between high temperatures (when in contact with the hot strip surface) and low temperatures (when cooled with the sprays). As the material cycles in temperature, it undergoes dimensional changes. Cracks formed at the sub-surface location of maximum shear stress can then propagate, via fatigue resulting from the thermally induced dimensional changes, along shear planes both toward the surface and deeper into the roll until the strength of the surrounding material is reduced to such a degree that spalling occurs. Surface initiated thermal pitting can also occur in 2 high mills where cracks form at the roll surface and propagate via fatigue into the interior until spalling occurs. Foreign material (oxide) “sticking” to the roll surface can accelerate the pitting process by a localized increase in the contact stress.
PREVENTION
Thermal pitting of hot mill work rolls can be prevented by the following:
• Proper selection of roll material (alloy), heat treatment and hardness to optimize the thermal fatigue strength of the roll material.
• Use of “scratch brushes” to clean the roll surface and prevent debris from entering the work roll/back-up roll contact zone where a localized increase in the maximum shear stress can occur.
• Use of spray cooling to insure proper coverage of the roll body. Nonuniform and/or inadequate spray cooling can lead to excessive heating of the roll body during service and increase the probability of thermal fatigue spalling in the affected area.
• Chrome plating the roll body to improve the lubricity and resistance to “oxide pick-up”
• Ultrasonic inspection techniques using a dual probe (“pitch/catch”) and surface wavetransducer on every roll after completion of the grinding operation. This will insure thatevery roll that is returned to the mill for service is free of both surface and sub-surface cracks.
• Sufficient stock removal during the grinding operation to either assure the removal of any cracks formed in the sub-surface or to relocate those cracks further away from the zone of maximum resultant shear stress. Relocating the cracks will subject them to a lower stress state where they will be less likely to propagate.
• Shorten campaign times to decrease the number of stress cycles the roll is subjected to. Repeated application of stress above the compressive and thermal fatigue strength of the roll will lead to sub-surface crack initiation if the number of stress cycles is sufficient enough.
• Reduce the rolling pressures to reduce the maximum resultant shear stress.
