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Spheroidal Graphite Iron Rolls
 
Spheroidal Graphite Cast Iron (S G Iron) or Ductile Iron is considered as one of the most versatile roll materials now a days. It is produced by a small proportion of magnesium added to the melt as nickel-magnesium or alternative alloy, or as pure magnesium. In S G Iron Roll, the free carbon takes the shape of spheroids or nodules, thereby eliminating the notch effect of flake graphite and improving upon the mechanical properties of the cast iron.
S.G Iron Rolls are so superior in wear resistance to that of Cast Steel Rolls that they are specially adapted for roughing and intermediate plate mills and rod or bar mill roughers. As a result of the spheroidal form of the graphite, S G Iron Rolls are much stronger than rolls of the clear-chill type and the gradual fall in hardness is an added advantage. As such, these rolls are particularly suitable for Skelp and Strip Mills, also Bar Billet Mills, and are being increasingly used for other applications.
The structure of S G Iron Rolls consists of transformed austenite (pearlite) eutectic carbide and spheroidal graphite. Spheroidal Graphite Iron Rolls may have a pearlitic matrix or, if relatively large amount of alloy elements are present the structure may be accicular or martensitic.
   
Pearlitic Rolls  
Pearlitic
These are specifically good for small section and flat rolling. There is also some use of pearlitic S G Iron Rolls in conditions in which the first essential is toughness, rather than wear-resistance, e.g. rolls for heavily loaded roughing stands.
X 200
Accicular (Bainitic) Rolls
Accicular
X200
In many cases S G Rolls with an accicular matrix is preferred, because of its wear-resisting qualities. To this end, the iron is alloyed with nickel and molybdenum, to permit formation of an accicular structure in the as-cast condition.

These rolls are most suitable for heavier section where added strength and depth of hardness are required. Sand-cast accicular S G iron is employed for certain special-purpose rolls, e.g. pilger rolls.
Properties
Hardness Graph
 
CHEMICAL COMPOSITION MECHANICAL PROPERTIES
U.T.S. Impact Hardness
Rolls C Si Mn Cr Ni Mo kg/mm² kg.m/cm² Shore°C
Pearlitic 2.90-3.40 1.40-2.50 0.50-0.70 0.15-0.35 1.50-2.00 0.25-0.60 38-62 0.50-0.80 45-55
Pearlitic 3.20-3.50 1.20-1.90 0.60-0.90 0.25-0.50 1.50-2.50 0.25-0.60 38-62 0.50-0.80 55-65
Accicular 3.20-3.50 1.20-1.70 0.50-0.70 0.30-0.70 2.00-3.00 0.70-1.10 40-65 1.00-2.00 65-75
Accicular 3.25-3.55 1.20-1.50 0.50-0.70 0.30-0.80 2.80-4.00 0.70-1.20 40-65 1.00-2.00 75-80
   
 
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