Metallurgy of High Chromium-Molybdenum White Iron and ... - Mkb.be

Section I – Chapter 3
Metallurgy of High Chromium-Molybdenum White Iron and Steel Rolls
Jean-Pierre Breyer
Director Research & Development
Marichal Ketin
Rue Verte Voie, 39
4000 Liège
Belgium
Tel.: +32 4 234 72 36
rolls@mkb.be
Gisèle Walmag
Researcher
Centre for Research in Metallurgy
Rue E. Solvay, 11
4000 Liège
Belgium
Tel.: +32 4 254 64 74
walmag@rdmetal.ulg.ac.be
INTRODUCTION
The Fe-Cr-C system is the basis of a number of the most widely used wear resistant materials having
application in mining, mineral processing, cement works, agricultural sectors and rolling mill rolls.
The iron-chromium alloys were studied as early as 1892 by F. Osmond (1) who was the first to indicate the
presence of complex carbides. Other investigations have been carried out by other authors such as W.
Tofaute (2,3) and K. Bungardt (4).
In the frame of research works sponsored by the Climax Molybdenum, a very important development in the
knowledge of the metallurgical properties of those alloys was done by F. Maratray (5,6).
As concern rolling mill rolls in reviewing the history of the manufacture of rolling mill rolls, the most
important innovation was probably the introduction of the double-poured indefinite chill roll in the late
thirties. However, European experience may suggest that the introduction of the high chromium iron and
steel rolls is likely to rank as another significant innovation in the art of rollmaking.
Some Russian and American papers relate the use of high chromium iron for the manufacture of small bars
and heavy section mills rolls in the early sixties (7,8). The first attempt to use that iron for hot strip mill rolls
was made in Germany in 1965. Other experimental rolls were made in England a little later. It became
quickly clear that despite of some failure due to the lack of reliable metallurgical information of the high
chromium iron system, those rolls had an excellent resistance to both abrasion and banding. They become
quickly the standard grade used in the early finishing stands of Hot Strip Mills.

Some years later, another improvement was done with the development of the high chrome steel. Carbon
and chromium content were strongly decreased. Special heat treatment or alloying element additions gives a
material, which suits very well to the working conditions in roughing stands of Hot Strip Mills. Today, high
chrome steel rolls are used world-wide.
PRODUCTION TECHNIQUE
The first rolls have been produced by the double-pour method. The practice was very similar to that used for
double-pour indefinite chill rolls, except that a much greater volume of flush iron must be used to reduce the
chromium content of the roll core to an acceptable level. Some trials have also been done to produce those
rolls single-pour and by electroslag melting of a sleeve remelted around a premachined steel arbour(7).
Actually, all the high chromium iron and steel rolls are manufactured by centrifugal casting technique. The
process can be vertical, horizontal or even tilted. By this technique, the small volume of the high chromium
shell can solidify more rapidly than with the other casting methods thus obtaining finely dispersed carbides.
It can be told that the development of centrifugal casting as a new process for rollmaking has been strongly
connected to the implementation of high chromium rolls.
Core is made of lamellar or spheroidal graphite (S.G.) iron. The introduction of new rolling technology such
as heavy bending, shifting and crossing increases strongly the mechanical stresses in the journals and core.
For that reason most of the high chromium rolls are now cast with a S.G. graphite iron core which has higher
mechanical properties than lamellar iron ones.
METALLURGY OF HIGH CHROMIUM IRON AND STEEL ROLLS
In Fe-Cr-C alloys, chromium can substitute to iron in cementite up to a 15% content. For higher content,
cementite become unstable and is replaced by an hexagonal carbide whose composition is M7C3. Those
carbides called chromium carbide contain mainly chromium and iron, but other alloying element can be
present.
It is generally accepted that the most significant reason for the good abrasion resistance of those materials is
the presence of the chromium carbides in the microstructure. Hardness of the chromium carbide is in the
range 1500-1800 Vickers compared to the cementite whose hardness is in the 1000-1200 range.(12)
Most of the studies of the Fe-Cr-C system are based on the diagram published by Jackson (9) and given on
figure 1. The γ-M7C3 area is crossed by a eutectic which is limited by two peritectic lines. The γ-δ for low
carbon and high chromium content and M7C3-M3C for high carbon and low chromium content. The position
of the late depends strongly of the cooling speed. High cooling speeds promote the M3C formation (10).

Fig.1 : Liquidus surface of Fe-Cr-C diagram (9)
The eutectic structure depends on the amount of austenite formed at the start of solidification. When the
austenite leaves only a small volume after solidification, carbide have a tendency to form along the grain
boundaries as shown in figure 2.a. This is the carbide morphology of a chrome steel rolls. With carbide
content of 20 to 30%, the eutectic contains lamellae radiating from the centres located in the interdendritic
spaces (figure 2.b). This is the general carbide structure of a chrome iron roll as used worldwide in the early
finishing stand of Hot Strip Mill. This structure changes to a lamellar one when the austenite no longer
interferes with the eutectic formation. Finally, with 35 to 40% of carbides, the alloy becomes hypereutectic
and large hexagonal primary carbides appear. From our knowledge, no rolls are produced with such a low
toughness structure.
Fig. 2a : Typical microstructure of high Cr steel roll

Fig. 2b : Typical microstructure of high Cr iron roll
A low casting temperature and a rapid solidification will decrease the size of the eutectic cells. No
significant change in the morphology of the eutectic carbide occurs in during heat treatment operations,
which are always performed on the roll after casting.
The percentage of chromium carbides can be calculated approximately from the C and Cr content of the
alloy by the following formula:
% carbides = 12.33 (%C) + 0.55 (%Cr) – 15.2
The chromium content of the matrix increases regularly with the chromium/carbon ratio and can be
represented by the equation :
% Cr matrix = 1.95 (%Cr/%C) – 2.47
The following table gives the typical range of carbon and chromium content as well as the percentage of
carbide and chromium content of the matrix measured on the high chromium grades that are used actually in
hot rolling.
%C %Cr % carbides Cr/C % Cr matrix
Hi-Cr steel 1.0/1.5 11.0/12.0 5/15 8/10 10/14
Hi-Cr iron 2.5/3.0 16.0/18.0 25/30 5/7 6/10
The large difference in matrix composition for the two alloy families induces two distinct oxidation
behaviours in the Hot Strip Mills. High chromium irons with their low Cr content in the matrix will oxidise
three times faster than the high chromium steels.
As concern the metallic matrix, all the decomposition product of the austenite can be produced: pearlite,
bainite, martensite. A full austenitic matrix can also be completed and in many structures retained austenite
is present.
However, for most of the applications as rolling mill rolls of the high chromium alloys, a microstructure in
which the matrix has a low residual austenite level and is free of pearlite is required. In order to get such a

structure on pieces weighting more than 7 tons and which may exceed 40 tons, different process conditions
have to be put under control.
Those are :
• Tempering heat treatment
• High temperature heat treatment
• Alloying elements addition
Depending of their own facilities, most of the rollmakers use one or more of the above mentioned method in
order to get the required structure and hardness. Those manufacturing conditions interfere between them.
TEMPERING HEAT TREATMENT
During a slow cooling, in the high temperature range, austenite is saturated in carbon and a precipitation of
small secondary carbide may be observed (figure 3). Such a precipitation decreases the carbon and
chromium content of the matrix. The pearlitic transformation is delayed and a bainito-martensitic
transformation becomes possible at low temperature. Figure 4a and 4b shows the effect on the
destabilisation of the austenite on the isothermal transformation diagram and on the martensitic
transformation (5, 13).
Fig. 3 : Typical secondary carbides precipitation
10µm

Fig. 4a: Schematic isothermal transformation diagrams.
a) undestabilized austenite
b) austenite destabilized by precipitation of secondary carbides
Fig. 4b: Effect of destabilization temperature on transformation temperatures and the Vickers hardness as
air-cooled after destabilizing of a high Cr iron.
It is believed that these secondary carbides, apart from their effect on the transformation of austenite to
martensite contribute to wear resistance because of their high hardness and because of their dispersion
hardening effect on the matrix.
In the as-cast condition, the matrix contains a high percentage of austenite (30-60%) which has to be
destabilised by one or several tempering heat treatment in order to get the specified hardness with a structure
containing M23C6 carbides in an �matrix.
However there is always a stable austenite residue. If the temperature of the tempering treatment is raised in
order to decompose it, hardness will be decreased. There is therefore a limit to the efficiency of those low
temperature heat treatments. If a hardness of more than 80 shore C is required, it cannot be reached with a
minimum of residual austenite.

Figure 5 shows for a 17% Cr – 2.8% C HSM work roll the hardness and residual austenite content evolution
versus the Larson – Miller parameter P which equals:
P= (273+T)(20+log t)
where T is the temperature (°C) and t the soaking time (hours) of the tempering heat treatment.
hardness (HV50)
750
700
650
600
550
500
450
400
15,0
0
16,0
0
17,00 18,00 19,00 20,00
Larson - Miller parameter
Fig. 5: Influence of tempering temperature on hardness and residual austenite content
HIGH TEMPERATURE HEAT TREATMENT
If this procedure is standardized, slow cooling rate can be used after initial solidification of the roll. Rolls
produced in this way may have in the as-cast condition a hardness as low as 50 Shore with a pearlitic
structure and can be premachined in such condition. After that, the specified hardness can be obtained by
reaustenizing the roll at high temperature followed by a controlled cooling up to room temperature. The
controlled cooling allows a good control of the precipitation of secondary carbides in the temperature range
of 800 to 1050°c. Using that procedure, the roll metallurgist can produce and control the microstructure.
After that quenching or controlled cooling, the roll is tempered to the hardness required.
High temperature heat treatments produce a more uniform matrix homogeneity and structure than that which
can be produced by controlled cooling alone. This is due to the fact that the macrosegregation phenomena
cannot be neglected in those alloys. In case of rather fast solidification as in a spun-cast roll, we observe a
higher chromium content in the core of the austenite cells than close to the eutectic carbides as shown on
figure 6. It is clear that the diffusion connected to a high temperature heat treatment in the 1000/1050 °C
range allows to homogenize the gradient of the elements in the austenite cells.
40
35
30
25
20
15
10
5
0
-5
residual austenite content (%)

Fig. 6: Distribution of elements (Cr, C, Si) across a dendrite (15)
Such heat treatment operations are rather expensive due to the slow heating rates that are required for
massive metal sections such as rolls. They give however the best control of the hardness and of the
component of the microstructure.
ALLOYING ELEMENTS ADDITION (12, 14, 15)
There are many metallurgical considerations in the selection and amount of the alloying elements. They
interfere with each others.
• Chromium carbide composition, secondary carbide precipitation, and precipitation of special carbides
during solidification (MC, M2C, …)
• Delaying of the pearlitic transformation
• Effect on the martensitic transformation temperature
The formulation of balanced compositions for martensitic white iron casting should have the following
major objectives.
• A sufficient alloy content to suppress the formation of pearlite in a given casting section size and cooling
rate
• A sufficient quantity of carbide-stabilising elements to ensure freedom from graphite in the structure.
• A high carbon content to reach optimum abrasion resistance.
• A low silicon content to reduce the tendency for graphite formation and/or pearlite.
• Adjustment of alloy composition for controlling of the retained austenite .
• Addition of alloying elements to increase the hardness of the carbide phase.
• Addition of alloying elements to increase the toughness of carbides by modifying their composition,
shape and distribution of the carbide phase.
Molybdenum
It is usually added in the range 0.5 to 4%. It has no effect on the liquidus surface ,and only the peritectic line
δ−γ is slightly moved to the low chromium content. The solidification sequence is unaffected but it lead to
the formation of small patches of eutectic Mo2C carbides for the low Cr/C ratio alloy(Cr/C=5) and to M6C
carbides for the high Cr/C ratio alloys(Cr/C=10). Molybdenum is partitioned between Mo2C (50%), M7C3

(25%) and the matrix (25%). It however does have a significant influence neither on the total amount of
carbides nor on the hardness of the austenitic matrix. In the as-cast condition, Molybdenum stabilises the
austenite. It means that for given chromium content, pearlite or other transformation product can be
suppressed at higher carbon contents. This effect is of considerable interest, as pearlite requires a high
temperature close to solidus to be fully reaustenised during further heat treatment. On a commercial scale,
such a treatment is difficult to carry out.
During heat treatment, Mo will also inhibits the pearlitic transformation (5).
Vanadium
Vanadium is a strong carbide-forming element. In amount of 0.1 to 0.5 %, it is claimed by some authors to
refines the as-cast structure and minimises columnar grain structure. It combines with iron to form both
primary and secondary carbides during solidification thus lowering the carbon content of the matrix.
Solidification proceeds with austenite, eutectic γ−VC and finally γ−M7c3.
The amount of vanadium carbides (MC) increases with the V/C ratio, however a certain quantity of
vanadium is necessary , for a given carbon content , for the vanadium carbide to appear. Its composition is
around 62-74% V, 9-11% Cr, 1.5-4.8% Fe,0-12% Mo. Like molybdenum, vanadium does influence neither
the morphology of M7C3 nor the total amount of carbides.
The Vanadium content of the matrix increases with the V/C ratio. Vanadium improves the pearlitic
hardenability and reduces the quantity of retained austenite, but is detrimental to the hardness in the asquenched
condition, the tempering strength and the hardness corresponding to a zero austenite percentage
after tempering. This effect can be compensated by molybdenum in low Cr/C ratio but not in high Cr/C ratio
alloys where it is very pronounced.
The hardness increases with the vanadium content for a same Ms temperature.
By raising the martensite transformation temperature, it leads to fully transformed structures in the as-cast
conditions.
Copper - Nickel
Copper is frequently added to high chromium white cast irons because it is particularly effective in slowing
down the austenite transformation rate. It thus allows martensite to form even in large sections castings.
Copper is entirely located in the matrix. One disadvantage is its stabilising effect on austenite leading to
excessive amount of retained austenite limiting its amount to 1 to 1.2%. Its effect is similar but less effective
than nickel but is preferred because cheaper.
Minor elements (Mn, Si)
Manganese, like copper, nickel and molybdenum delay the appearance of pearlite in the structure. On the
opposite, silicon accelerates its appearance. Silicon like copper is entirely located in the matrix while
manganese is equally distributed between the matrix and the M7C3 carbides. Manganese favours a high
amount of retained austenite while silicon contributes to its destabilisation.
Minor elements (Mn,Si) have a very limited effect on the liquidus temperature and the solidification
sequence remains unchanged.

MECHANICAL AND THERMAL PROPERTIES
Table II gives a comparison of the main properties of indefinite iron, adamite and high chromium iron and
steel (11, 16, 17).
Table II Mechanical and Physical Properties
IC Adamite Hi Cr iron Hi Cr steel
Tensile strenght (MPa) 350 - 500 700 – 900 700 - 800 700- 800
Elongation (%) 0.2 - 0.3 0.4 - 1.0 0.4 - 0.6 0.4 - 1.0
Compression strength (MPa) 200 - 250 220 – 270 230 - 280 250 - 300
Elastic modulus ( 10 3 MPa) 180 - 190 200 – 210 220 - 230 200 - 210
Thermal expansion coefficient (10 -6 /°C) 13.0 - 13.4 12.5 - 13.0 12.7 - 13.2 11.5 - 12.0
Thermal diffusivity (cm 2 /sec) 0.030 - 0.040 0.065 - 0.070 0.020 - 0.025 0.020 - 0.025
High chromium grades have an ultimate tensile strength twice higher than indefinite chill grade. It explains
their very high performance as concern the fire crazing resistance. Fire crazing is recognised as being the
most important deterioration phenomena in those early stands of Hot Strip Mills, which undergo high
thermal solicitations.
The low thermal properties of high chromium iron and steel in comparison of indefinite and adamite
demonstrate the fact that those rolls are somewhat prone to damage in case of mill cobbles and stickers.
Steeper temperature gradients are created in the roll shell. For that reason, most of the mill adopting high
chromium grade improved their cooling practice, mainly by an increase of the cooling water amount (11).
APPLICATION AND FUTURE DEVELOPMENTS
Chromium alloyed rolls are mainly used in the roughing and early finishing stands of Hot Strip Mills and
Compact Strip Process (CSP). Many heavy plate mills have also adopted that grade in substitute or in
association with the ICDP grade.
Some attempts were done to use high chromium rolls in the last finishing stands. Due to the lack of the
formation of an oxide layer connected with the strong oxidation resistance of high chromium alloys, the rolls
were prone to sticking and led to catastrophic mill incident. In the last finishing stands, high chromium iron
rolls are presently used only for the rolling of checkered and corrugated plates.
In the late seventies and early eighties, a lot of trials have been done in tandem cold mills for sheet and tin
plate rolling. The purpose was to substitute as work rolls material a high chromium iron to forged steel. Due
to the large amount of high chrome carbide in the microstructure, the wear resistance was very high giving
output equal to twice and more of those of forged steel rolls. However, problems connected to the low
thermal conductivity of high chromium iron and to a lack of cleanliness of the rolled sheet stopped almost
all the use of that new grade in cold tandem mills.
Actually, in the cold rolling area, high chromium iron rolls are used only in :
• First stand in tandem mill for tin plate
• 2-high hot skin pass

• Back up rolls for 4-high skin pass
In Japan, high chromium iron rolls are no more used in the early stands of hot strip mills and have been
substituted by the high-speed steel rolls. But despite that that new development, most of the European and
American hot mill are still using high chromium rolls.
Presently, semi-HSS rolls are giving very high performance in roughing stand of some mills. Following
metallurgical definition, it is possible to argue that new grade belong to the chrome steel family by its
chromium content
CONCLUSIONS
The main advantages of the high chromium alloys are:
• The shape of the eutectic which favours a high toughness.
• The high hardness of the M7C3 carbides which increases abrasion resistance.
• The hardenability which allows to achieve fully martensittic structure even in large sections when
necessary.
• The stability of austenite allowing to achieve as-cast fully austenitic structure or to apply a thermal
treatment avoiding quenching.
• A high resistance to softening during tempering permitting high tempering temperature to achieve a
good toughness and temperature resistance.
• The relatively low cost of chromium.
All those advantages explain the success of the use of high chromium alloys for the manufacturing of rolling
mill rolls.
REFERENCES
1. Referenced in : A.B. Kinzel and W. Crafts, "The Alloys of Iron and Chromium", Low-Chromium
Alloys, Mc Graw-Hill (1937) Vol. I
2.W. Tofaute, A. Sponheuer and H. Bennek, "Umwandlungs-, Härtungs- unb Anlassvorgänge in Stälen mit
Gehalten bis 1% C und bis 12%Cr", Archiv für das Eisenhüttenwesen, 8, 1934-35, pp. 499-506
3.W. Tofaute, C. Küttner and A. Büttinghaus, “Das System Eisen-Chrom-Chromkarbid Cr7C3 – Zementit”,
Archiv für das Eisenhüttenwesen, 9, 1935-36, pp 607-617
5. F. Maratray and R. Usseglio-Nanot, "Factor Affecting the Structure of Chromium-Molybdenum White
Iron", Climax Molybdenum S.A. Paris publication, 1970
6. F. Maratray, "Choice of Appropriate Compositions for Chromium-Molybdenum White Iron", AFS
Transactions, 79, 1971, pp. 121-124.
7. J. Dodd, "High Chromium-Molybdenum Alloy Iron Rolls", Climax Molybdenum Company publication.
8. V.T. Zadan et al, translated from russian, Metallurg, 10, 1970, pp 30-32
9. R.S. Jackson, "The austenite liquidus surface and constitutional diagram for the Fe-Cr-C metastable
system”, Journal of the Iron and Steel Institute, 208, 1970, pp. 163-167

10. A. Sinatora, E. Albertin and Y. Matsubara, "An investigation of the transition from M7C3 to M3C
carbides in white cast iron", Int. J. Cast metals Res., 9, 1996, pp. 9-15
11. Kubota 3-layers High-Cr Rolls, Fachberichte Hüttenpraxis Metallweiter Verarbeitung, 21 (10),1983.
12. J.-C. Werquin, J.-C. Caillard, “New vertically spun cast rolls for hot and cold rolling and their ability to
satisfy the new demands of current rolling practices.”, MWSP proceedings, 1988, p. 311-336
13. C. Lecomte-Mertens, J.-P. Breyer, D. Totolidis, A. Magnée, “Study of the secondary hardening in 18%
Cr cast iron”, Bulletin du cercle d’études des métaux, n°17, 1989.
14. T.E. Norman, A. Solomon, D.V. Doane, “Martensitic white irons for abrasion-resistant castings”,
Climax Molybdenum Company publication
15. C. Barnier, P. Dupin, J. Saverna, J.-M. Schissler, “Influence d’un traitement d’une fonte blanche à haut
chrome (2,8 C – 18 Cr), Traitement thermique, n° 181, 1984, p. 30-40
16. T. Hashimoto and al, “Surface deterioration of high chromium work roll for finishing stands of hot strip
mills”, Kubokta technical report, 1982, p. 21-33
17. H. E. Muller, “High-chrome work rolls in a modern hot strip mill”, Iron and Steel Engineer, October
1975, p. 63-70