Monday, October 3, 2016

Rolling mills: Roll Hardness and it characteristics

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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