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How to avoid spalling for rolls
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Spalling is usually found in the area of highest contact pressure between the work and back up rolls. It is desirable, therefore, to provide a uniform contact pressure distribution across the faces of the rolls. However, due to such factors as mill design, roll wear, chamfers, mechanical crowns, and thermal crowns roll bending forces etc. the pressure is generally not uniform. Spalling are generally caused due to :
A) Residual Stresses
B) Thermal Gradients
C) Contact Fatigue
D) Hydrogen Embrittlement
E) Local Overstressing or Roll Surface
2. How to Avoid Spalling.
Chipping off, flaking off, dismembering, of a portion of roll from the roll barrel is known as spalling. Spalling can occur both in work rolls as well as back up rolls and is usually the end effect of crack generation and propagation.
Is a difficult advise to make, still one should take care of the following:-
a. Coolant should be sufficient and should of good quality.
b. Sufficient stock removal is recommended to remove work hardened layer. Hardness should be brought back to original level.
c. Nital test to be performed after every 3 or 4 regrinding to ensure that rolls is free of micro cracks.
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