CONTACT STRESS SPALLS CONTINUED COLD MILL WORK ROLLS CHARACTERISTICS

Transcription

1 CONTACT STRESS SPALLS CONTINUED COLD MILL WORK ROLLS CHARACTERISTICS Contact stress spalling can occur on cold mill work rolls via both modes described previously. Instantaneous - This mode can be characterized as an area that appears crushed and does not show any distinct initiation site. High Cycle Contact Stress Fatigue Fracture - This mode has different characteristics depending on whether the stress was located on the body or the body edge. Roll Body - High cycle fatigue fracture on the work roll body is usually not directly responsible for the spalling that occurs. In this case, the contact stress fatigue cracks that form below the surface do not propagate towards the surface but rather propagate radially and circumferentially forming a fatigue wreck path. It is this mode of crack propagation that eventually causes spalling to occur (see Spalling - Surface Initiation). High cycle contact stress fatigue is difficult to diagnose when it occurs on a work roll body because the cracks that initiated the fatigue wreck path are not readily seen on the fracture face but are hidden in the sub-surface. Roll Body Edge - High cycle contact stress fatigue fracture on the work roll body edge can be present as a small crumbly type fatigue spall. The crumbly fatigue spall is usually located at the contact point between the work roll and back-up roll body edge, strip edge or at the work roll body edge. The crumbly spall is usually small in area and no greater than deep. In many cases, a fatigue wreck path originating from the area of contact stress fatigue is also present (see Spalling - Surface Initiation).

2 EXAMPLES OF CONTACT STRESS SPALLING IN WORK ROLLS INSTANTANEOUS EXAMPLE 1 Arrows highlight the fracture face of a work roll body edge that spalled instantaneously. In this case, the contact stresses applied to the body edge were large enough to exceed the compressive strength of the material and subsurface crack initiation and propagation occurred instantaneously. EXAMPLE 2 Fracture face of a work roll body edge that spalled instantaneously. In this case, the contact stresses applied to the body edge were large enough to exceed the compressive strength of the material and subsurface crack initiation and propagation occurred instantaneously.

3 EXAMPLES OF CONTACT STRESS SPALLING IN WORK ROLLS EXAMPLE 1 Fracture face from a portion of spall that occurred on the body of a cold mill work roll. Arrow highlights the initiation site associated with high cycle contact stress fatigue. Note the presence of several distinct fatigue wreck paths originating from the area of contact stress fatigue. EXAMPLE 2 Transverse view of the spall shown in Example 1. Arrow highlights the high cycle contact stress fatigue cracks that were formed below the surface.

4 EXAMPLES OF CONTACT STRESS SPALLING IN WORK ROLLS EXAMPLE 3 Network of high cycle contact stress fatigue cracks present in the sub-surface of the spall shown in Example 1. EXAMPLE 4 Cold mill work roll exhibiting a body edge spall.

5 EXAMPLES OF CONTACT STRESS SPALLING IN WORK ROLLS EXAMPLE 5 Fracture face of the body edge spall shown in Example 4. Note the distinct fatigue wreck path responsible for the major portion of spalling. Arrow highlights the direction of fatigue propagation.

6 EXAMPLES OF CONTACT STRESS SPALLING IN WORK ROLLS EXAMPLE 6 Close up of the fatigue wreck path initiation point that resulted in the spall shown in Example 4. Arrows highlight small crumbly spalls associated with high cycle contact stress fatigue. The fatigue path can be seen initiating from the area of high cycle contact stress fatigue spalling. EXAMPLE 7 Cold mill work roll exhibiting a body edge spall initiating from the edge of the bevel.

7 EXAMPLES OF CONTACT STRESS SPALLING IN WORK ROLLS EXAMPLE 8 Cold mill work roll exhibiting crumbly contact stress fatigue spalling on the body edge.

8 EXAMPLES OF CONTACT STRESS SPALLING IN WORK ROLLS EXAMPLE 9 Cold mill work roll exhibiting plastic deformation resulting from contact stress that exceeded the compressive yield strength.

9 MECHANISM Instantaneous Contact Stress Spalling: As was described in the General Mechanism section of Contact Stress Spalling, the maximum resultant shear stress is located a short distance below the surface. A sudden mill stop, or a skid in extreme circumstances, can cause the maximum resultant shear stress to exceed the compressive strength of the roll. This sudden rise in shear stress can cause sub-surface cracks to form instantaneously. In extreme cases, the contact stress is severe enough to cause the subsurface cracks formed to also propagate to spalling instantaneously. High Cycle Contact Stress Fatigue: Body - Due to the high hardness, high cycle contact stress fatigue on a work roll body is usually the result of a point load being induced on the roll. This point load can be a small piece of grit stuck in the roll bite, a strip weld being passed through the roll bite or anything that would act to concentrate stresses at a single point on the roll body. If the stress is greater than the compressive strength of the material small cracks can initiate in the sub-surface which over time propagate via a fatigue wreck path mode and spalling is inevitable. Body Edge - Areas on the work roll body such as the strip edge, contact zone between the work roll and the back-up roll edge or the body edge act as stress concentration factors when the roll is put into service. With each revolution of the roll, if the maximum resultant shear stress, located just below the roll surface (see General Mechanism section of Contact Stress Fatigue), exceeds the compressive fatigue strength of the material, cracks will form at that location. Further stress cycles will propagate the cracks toward the surface where small crumbly spalling then occurs. In many cases, fatigue wreck paths will initiate at locations where sub-surface contact stress fatigue cracks have formed. The fatigue wreck path can then propagate radially and circumferentially until the strength of the surrounding material is reduced to such a degree that large spalling occurs.

10 PREVENTION Instantaneous Contact Stress Spalls Avoid mill incidents such as skids, mill wrecks, metal wrap etc. High Cycle Contact Stress Fatigue - Several steps can be taken to prevent the occurrence of high cycle contact stress fatigue on cold mill work rolls: Avoid increases in maximum resultant shear stress above the compressive fatigue strength of the roll by the passing of grit or welds etc. through the roll bite. Ultrasonic inspection techniques using a dual probe ( pitch/catch ) and surface wave transducer on every roll after completion of the grinding operation. This will insure that every 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 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. Develop a surface profile (work roll/back-up roll crowning practice) to insure a uniform contact stress pattern along the entire work roll/back-up roll (intermediate roll) contact zone. Change the roll design from a body bevel to a body radius to reduce the stress concentration on the roll body edge. Change the back-up roll body edge design to a body radius and proper relief (approximately 0.5 ) to reduce the stress concentration on the work roll body at the point of contact between the work roll and the back-up roll body edge.