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1. SPALLING
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.
Normally, factors responsible for crack generation are five fold.
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.
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.
How to Avoid Spalling.
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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