 |
|
|
|
| |
Double Poured or Composite Iron
Rolls
The endeavour to obtain roll with high wear resistance and toughness resulted in the development of Double Poured Rolls. Such rolls are basically composite rolls having
very hard shell and sufficiently tough core. By double pouring technique, it is possible to obtain a bainitic or martensitic structure on the surface upto a certain depth, which enables the rolls to attain a high wear resistance property.
Depending upon the requirement, the shell and core iron composition are adjusted to produce Double Poured Rolls with varied combination of hardness and toughness.
The shell material for such rolls, may be alloyed clear chill with pearlitic matrix or highly alloyed indefinite chilled cast iron with bainitic to martensitic matrix depending upon its use. The core material on the other hand is usually grey iron having pearlitic matrix with flake graphites or nodular iron with nodular graphite.
NOMINAL CHEMICAL COMPOSITION & MECHANICAL PROPERTIES
|
% Chemical
Composition |
Mechanical Properties
|
|
C
|
Si
|
Mn
|
P
|
S
|
Cr
|
Ni
|
Mo
|
Hardness
Sh0C |
Tensile
Strength of core (kg/mm2) |
Grade
|
|
3.20
3.50
|
0.80
1.20
|
0.60
0.90
|
0.15
Max
|
0.10
Max
|
1.30
2.00
|
3.40
4.40
|
0.30
0.40
|
65-85
-
|
15-22
-
|
04 (ICDP)
-
|
|
3.10
3.50
|
0.80
1.40
|
0.40
0.80
|
0.15
Max
|
0.02
Max
|
1.30
2.00
|
3.40
4.40
|
0.30
0.70
|
65-85
-
|
15-45
-
|
04 (ICDP
Special)
- |
|
3.20
3.40
|
0.80
1.40
|
0.60
1.00
|
0.14
Max
|
0.06
Max
|
1.40
1.80
|
4.30
4.50
|
0.30
0.70
|
62-85
-
|
15-45
-
|
08 (ICDP
Centrifugally
Cast)
- |
|
|
|
|
MICRO STRUCTURE
 |
|
|
|
|
|
| |
Spheroidal
Graphite Iron Rolls
Spheroidal Graphite Cast Iron (SG Iron) or Ductile Iron
(DCI) or Nodular Iron is considered one of the most versatile roll material now-a-days. In SGCI 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.
The SG Rolls are generally chill cast with the working surface having eutectic cementite and spheroids of free graphite. By adding suitable alloying elements like Ni, Mo, the matrix can be controlled from fine Pearlite to Bainite or even to Martensite. The proportion of eutectic carbide decreases from the barrel surface towards core with increasing amount of graphite nodules.
SG Irons are the strongest amongst cast iron varieties and have strength comparable to steel rolls.
NOMINAL CHEMICAL COMPOSITION & MECHANICAL PROPERTIES
|
% Chemical Composition |
Mechanical Properties |
|
C
|
Si
|
Mn
|
P
|
S
|
Cr
|
Ni
|
Mo
|
Tensile
Strength Kgf/mm2 |
Bending
Strength Kgf/cm2 |
Hardness Sh0C |
Grade
|
|
2.90
3.50
|
1.20
2.40
|
0.50
0.80
|
0.15
Max
|
0.016
Max
|
0.20
0.80
|
1.50
2.90
|
0.30
0.90
|
40-60
-
|
80-100
-
|
45-75
-
|
Pearlitic
(05)
|
|
3.00
3.50
|
1.20
2.40
|
0.50
0.80
|
0.15
Max
|
0.016
Max
|
0.20
0.60
|
2.80
4.20
|
0.80
1.20
|
45-65
-
|
90-110
-
|
45-75
-
|
Accicular/
Bainitic
(05)
|
|
|
|
|
MICRO STRUCTURE
 |
|
|
|
|
|
| |
Cast Steel /
Graphitic Steel / Cast Steel Backup Rolls
Such rolls are available with varied chemical composition starting from Hypo Eutectoid Steel to Hyper Eutectoid Steel. Cast Steel undergoes stringent heat treatment cycle to arrive at the most optimum microstructural mix suitable for different application for which it is meant. Different alloying elements like Cr, Ni and Mo etc. are normally added to attain the desired physical and mechanical properties in such rolls. In conventional Cast Steel Work Rolls, carbides / ferrite are evenly distributed in the Matrix of Pearlite / Bainite. In case of Graphitic Steel Rolls, special heat treatment is given to have graphite distributed in the Matrix of Pearlite / Bainite.
For Cast Steel Backup Rolls, where the hardness and toughness requirements are high, rolls undergoes suitable heat treatment to ensure globular carbides distributed over the matrix of predominantly
Bainite.
NOMINAL CHEMICAL COMPOSITION & MECHANICAL PROPERTIES
|
% Chemical
Composition |
Mechanical
Properties |
|
C
|
Si
|
Mn
|
P
|
S
|
Cr
|
Ni
|
Mo
|
Tensile
Strength Kgf/mm2 |
Bending
Strength Kgf/mm2 |
% Elongation
|
Hardness Sh0C |
Grade
|
|
For Cast
Steel Work Roll
|
|
0.5
2.10
|
0.30
1.40
|
0.70
1.40
|
0.40
Max
|
0.04
Max
|
0.80
1.60
|
180
Max
|
0.50
Max
|
50-120
-
|
80-110
-
|
1-10
-
|
30-55
-
|
06
-
|
|
For Cast
Steel Backup Rolls
|
|
0.4
0.8
|
0.30
0.60
|
0.50
0.80
|
0.04
Max
|
0.04
Max
|
1.5
3.5
|
1.0
Max
|
0.40
0.70
|
100-150
-
|
-
-
|
-
-
|
50-70
-
|
06
-
|
|
|
|
|
MICRO STRUCTURE
 |
|
|
|
|
|
| |
Centrifugally
Cast High Chromium Rolls.
The essential characteristic of high chromium iron is the presence in structure of a chromium carbide of the M7C3 type whose hardness in 1400 HV as against hardness of 900 HV of M3C type iron carbide (Cementite). These very hard
M7C3 type carbides do not form a continuous system as in Chilled Iron but are isolated in a matrix composed mainly of bainite and martensite or fully martensite. The continuous cementite system (
M3C type ) is one cause of brittleness of Chilled Iron. Furthermore, the high chromium iron matrix contains secondary carbides, whose dimensions are between 1 to 10 microns. High chromium rolls are basically composite rolls, manufactured by centrifugal casting technique. The inverse wear rate of such rolls are 2 to 3 times higher than that of ICDP rolls.
The high wear-resistance permits high performance per operation period with very low reductions in diameter.
The surface hardness of the roll barrel (up to approximately 850 Shore C ) is sufficient to avoid almost completely marking by hard material particles, thereby
also avoiding premature roll changings.
The high cold working resistance in the micro range, ensure extremely good polishing properties.
The high micro-hardness of the carbide constituents and their degree of distribution in the basic structure diminish the wear of the roughness when working with shot blasted barrel surface.
The gripping capacity of the rolls permit a fast acceleration of the mill to full rolling speed irrespective of the barrel diameter and, thus, a better rate of utilization of the mill.
Due to the uniform hardness throughout the whole range of usability down to scrap diameter, these rolls can, regardless of their rate of utilization, be used in all stands of the
pre finishing block of HSM and CSP mills, for all cold strip grades, for reducing as well as skin-passing, which means a considerable reduction of the necessary roll stock in the mill. The journal strength is increased by using high-grade cast iron and by the effect of spin casting. Here also the choice of core metal is open.
NOMINAL CHEMICAL COMPOSITION & MECHANICAL PROPERTIES
|
% Chemical Composition |
Mechanical Properties
|
|
C
|
Si
|
Mn
|
P
|
S
|
Cr
|
Ni
|
Mo
|
Hardness
Sh0C |
Grade
|
|
2.3
3.0
|
0.4
0.8
|
0.80
1.20
|
0.06
Max |
0.02
Max |
13.0
18.0
|
0.8
1.5
|
0.90
1.60
|
65-85
|
09
|
|
|
|
|
MICRO STRUCTURE
 |
|
