Reasons of Cracks In Cold Rolling Mill Rolls

There can be many reasons due to which cracks are generated in rolling mill rolls under perating conditions. This can be due to quality of raw material or product defects. But the prime factors that govern the crack generation is rolling mill rolls are :

A) Residual Stresses
 
Develop in roll manufacturing process such as heat treatment and roll grinding. Crack propagation could be rapid when rolling under stress and rapid premature failure takes place in case of extremely high residual stresses.

B) Thermal Gradients

Cracks develop in regions between top hardened surface and sub surface below, during grinding stage. Gradually crack develops and propagates.

C) Contact Fatigue

Crack initiates due to compressive stresses experienced during contact between roll and strip while rolling. Mostly, cracks grow under fatigue conditions and failure results.

D) Hydrogen Embrittlement

Under inadequate supply of coolant, hydrogen is released due to decomposition of coolant/ lubricant, when temperature between roll surface and strip becomes high. This hydrogen is picked up by roll surface where structure is martensite. Roll pressure facilitates hydrogen pick up and crack formation.

E) Local Overstressing or Roll Surface.

Local overstressing of roll surface can be caused due to several reasons such as :-

a. Stopping the roll under screw down pressure or by excessive roll face pressure during starting the mill.
b. Excessive body pressure caused by never camber ir by rolling too close to the end of the roll body.
c. Skidding during rolling.
d. Laps & Laminations lead to local overloading of the rolls.
e. Shifting and folding of strip during rolling. Strip rupture.
f. Local over heating.
g. Rolling narrow width strip on one rolling path.
h. Uneven cooling of roll.
i. Excessive work hardening.


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