ROLLS: Pearlite, Bainite and Martensite Structures

By SARJIT ROLLS

Keywords: Adamite, SG Iron, Alloy steel, DPIC Solid, Enhanced carbide ICDP, SG Acicular, hot or cold rolling

At the very first instance these words could sound very tedious and overwhelming but the actual scientific mechanism is very simple.

Pearlite structure is one which is mainly achieved in static castings as the rate of cooling of the metal is slow i.e. a temperature below 600 degC is achieved only after 3 – 4 hours or more after pouring of molten material for rolls. The main characteristic of this structure is that it imparts strength with its equiaxed matrix to the rolls and is used to manufacture rouging mill rolls as in Alloy steel, Adamite, SG Iron grade. The hardness that can be achieved is in the range of 40 – 70 sH.

Bainite Structure can be achieved by both in static casting, using thicker molds, and in centrifugal casting as the rate of cooling of metal is relatively faster i.e. a temperature below 600 degC is achieved within 15 – 20 minutes after pouring of molten material for rolls. The main characteristic of this structure is that it imparts polish on the rolled product due its fine matrix. Mainly used for finishing mill rolls as in Duplex DPIC rolls, Enhanced carbide ICDP rolls, alloyed barrels and SG acicular rolls. The hardness that can be achieved is in the range of 65 – 80 sH

Martensite Structures can only be casted using centrifugal mechanism as the rate of cooling of metal is very fast i.e. a temperature below 600 degC is achieved within 3-4 minutes after pouring of molten material for rolls. The structure of the material formed is very fine in nature but overall it does not have any useful industrial application as faster the cooling more the risk of chipping of material.

NOTE: There is no fine line between the above structures. Due to variation in cooling at any point either 2 or even three of the above could exist in the same casting. But the major portion could be one of them

For any queries please respond to www.sarjitrolls.com

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