Hardness of rolls is considered to be of prime importance
for roll users these days rather it has become the only defining factor to
judge the quality of the rolls.
This has been proved to be incorrect.
There are a set of important parameters for rolls which together define the quality of rolls
1
The composition i.e.
the carbon content and the alloy factor are of utmost importance. The carbide
formation (MC) is dictated by the carbon content which varies from rolls to
roll and its application in the rolling mill. Addition of alloying metals like
Ni, Cr, Mo, etc. increase the alloy factor of rolls which further increases the
wear resistance.
2
The Micro-structure i.e.
the way the Fe atoms align in the roll. This parameter is the most difficult to
control within a roll; standardization of the micro-structure is what
differentiates the good roll makers from the normal ones. The micro-structure
could be one which imparts strength to absorb shocks or have a fine structure
which imparts polish, etc.
3
Hardness of rolls is a property of its cooling
rate during casting along with the carbide content in the rolls. So, a Roll
with high carbide formation and fast rate of cooling would have a higher
hardness than slow cooling rate or lower carbide formation. On the same lines, Cast
iron roll with higher carbon content would have more carbide formation and
hardness than steel base rolls with lower carbon content. eg. DPIC cast iron or Acicular SG iron (high carbon) material compared to Adamite and SG Steel (low carbon)
In a rolling mill the above 3 parameter
should vary as below for the different areas in a mill are required as below: e.g. in a roughing mill the carbon content required is low, rolls like Adamite rolls, SG Steel Rolls, while for a finishing mill it should be higher for better carbide formation and better hardness, rolls like DPIC Rolls, High Chrome rolls, Acicular SG Iron rolls
ROUGHING MILL INTERMEDIATE MILL FINISHING MILL
COMPOSITION: LOW ------------------ CARBON
CONTENT ------------------ HIGH
MICRO STRUCTURE: HIGH -------------------- STRENGTH -------------------- LOW
MICRO STRUCTURE: LOW -------------------- FINE
STRUCTURE -------------------- HIGH
HARDNESS: LOW -------------------- HARDNESS -------------------- HIGH
For more information check SARJIT ROLLS
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