DEFECTS IN METAL ROLLING

1. BY:
ANIL SHARMA, ROLL SHOP ENGINEER,
AFRICAN FOUNDERY LTD. NIGERIA.

3. A wide variety of defects are possible in metal rolling manufacture.
Surface defects commonly occur due to impurities in the material,
scale, rust, or dirt. Adequate surface preparation prior to the metal
rolling operation can help avoid these. Most serious internal
defects are caused by improper material distribution in the final
product. Defects such as edge cracks, center cracks, and wavy
edges, are all common with this method of metal manufacturing.

4. Another interesting defect that can occur
in flat rolling is alligatoring,
where the work being rolled actually splits
in two during the process.
The two parts of the work material travel
in opposite directions relative to their
respective rolls.

5. In hot rolling, if the temperature of the
workpiece is not uniform the flow of the
material will occur more in the warmer
parts and less in the cooler. If the
temperature difference is great enough
cracking and tearing can occur

6. • Symmetrical edge wave - the edges on both sides of the workpiece are "wavy"
due to the material at the edges being longer than the material in the center.
• Asymmetrical edge wave - one edge is "wavy" due to the material at one side
being longer than the other side.
• Center buckle - The center of the strip is "wavy" due to the strip in the center
being longer than the strip at the edges.
• Quarter buckle - This is a rare defect where the fibers are elongated in the
quarter regions (the portion of the strip between the center and the edge).
This is normally attributed to using excessive roll bending force since the
bending force may not compensate for the roll deflection across the entire
length of the roll.

7. Hot tearing is considered as a phenomenon linked to the
inadequate compensation of solidification shrinkage by melt flow
in the presence of thermal stresses, there are more factors that
could be involved in the formation of cracks at supersolidus
temperatures. Hot cracking susceptibility and the composition of
an alloy-- has been established on many occasions. A large
freezing range of an alloy promotes hot tearing because such an
alloy spends a longer time in the vulnerable state in which thin
liquid films exist between the dendrites. The liquid film
distribution is determined by wetting of grain boundaries, i.E., By
surface tension between liquid and solid phases. When the surface
tension is low and wetting is good, the liquid tends to spread out
over the grain boundary surface, which strongly reduces the
dendrite coherency, weakens the mush, and may promote hot
tearing.

8. .
The reduction in sulfur content for minimizing the formation
and intrusion of complex iron oxides were suggested
The increase of rolling passes and the decrease of reduction
ratio for homogeneous rolling of billets
Reduce the billet movement speed.
Check the clearance between the rollers

9. Check for the chemical analysis of the billet. Check for Manganese, Sulphur,
Phos content. Also check if the metal is getting completely killed in the
furnace. Insufficient degassing of the metal and high sulphur and phos
contents are usually the main culprits for this problem.If sulphur and phos is
high say about in the range of 0.140 and above cumulatively and nothing can
be done about it owing to the raw material composition inthe furnace then
increase manganese content in the metal. Also check if aluminium being
used to kill the final metal is being used in a manner to make the killing
effective or else use ferro aluminium as it ensures better killing as it negates
the chances of manual errors in method of dipping into the furnace if
melting is taking place in an induction furnace