M. TER^ELJ et al.: CHARACTERISTICS OF THE THERMAL FATIGUE RESISTANCE FOR ...
515–521
CHARACTERISTICS OF THE THERMAL FATIGUE RESISTANCE
FOR 3.1C, 0.8Si, 0.9Mn, 1.7Cr, 4.5Ni AND 0.3Mo ICDP CAST IRON
ROLL AT 600 °C
ZNA^ILNOSTI ODPORNOSTI ZLITIN LITEGA @ELEZA ZA VALJE
3.1C, 0.8Si, 0.9Mn, 1.7Cr, 4.5Ni IN 0.3Mo NA ICDP TERMI^NO
UTRUJANJE PRI 600 °C
Milan Ter~elj1, Peter Fajfar1, Matja` Godec2, Goran Kugler1
1University of Ljubljana, Faculty of Natural Science and Engineering, 12 A{ker~eva street, 1000 Ljubljana, Slovenia
2Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
Goran.Kugler@omm.ntf.uni-lj.si
Prejem rokopisa – received: 2016-08-23; sprejem za objavo – accepted for publication: 2016-08-31
doi:10.17222/mit.2016.259
Using a new test rig the characteristics related to the thermal fatigue resistance for ICDP (Indefinite Chill Double Pour) cast iron
for hot working rolls was investigated. Tests were carried out at 600 °C to obtain the characteristics of cracks at (500, 1000 and
1500) cycles. For comparative purposes, additionally a test at 500 °C was carried out. The average length of all the cracks, their
density, average length of five longest cracks and relevant microstructural characteristics for the mentioned cycles were
determined. It was found that the cracks initiation and their growth are related to the cracking and spalling of the carbides, shape
of graphite, distribution of carbides and graphite particles, oxidation, etc. The obtained results contribute to a better
understanding of the thermal cracking of selected roll cast iron. Based on these results the suggestions for improving the thermal
fatigue resistance for the cast iron roll are given.
Keywords: roll cast iron, thermal fatigue, cementite, graphite, cracks
Novo razvita naprava je bila uporabljena za raziskave lastnosti ICDP litega `eleza, ki so povezane z odpornostjo na termi~no
utrujanjem. Zna~ilnosti razpok so bile dolo~ene po (500, 1000 in 1500) ciklih, pri 600 °C. Za primerjavo je bilo izvedenih nekaj
testiranj tudi pri 500 °C. Na osnovi rezultatov testiranj je bilo za vsako {tevilo ciklov dolo~ena povpre~na dol`ina vseh dobljenih
razpok, njihova gostota, povpre~na dol`ina petih najdalj{ih razpok, vklju~no z ostalimi relevantnimi mikrostrukturnimi
zna~ilnostmi. Rezultati so pokazali, da sta nukleacija in rast razpok tesno povezana s pokanjem in drobljenjem karbidov, z
geometrijo grafita, z volumsko porazdelitvijo karbidov ter grafitnih delcev, z oksidacijo, itd. Rezultati prispevajo k bolj{emu
razumevanju termi~nega pokanja izbranega materiala za valje. Na osnovi predstavljenih rezultatov so podani predlogi za
izbolj{anje odpornosti litega `eleza za valje na termi~no utrujanje.
Klju~ne besede: lito `elezo za valje, termi~no utrujanje, cementit, grafit, razpoke
1 INTRODUCTION
Dies used in hot-working applications like die
casting, rolling, forging and hot extrusion are exposed to
high thermal, mechanical, tribological and chemical
loads. The occurrence of different damage, i.e., cracks,
fracture, plastic deformation and wear, are a conse-
quence of the different magnitudes of the mentioned
loads as well as their different mutual ratios, materials
properties, etc.1–7 Thus, in the case of cyclical subjection
to high temperatures, i.e., the heating of rolls (also above
600 °C) and their water cooling, these conditions lead to
the occurrence of cracking on the die’s surface, which
appears in the shape of a crack network that conse-
quently results in spalling of the die material that further
leads to a deterioration of the surface finish of the
workpiece. In hot-working rolls the thermal stresses are
usually comparable or even larger than the mechanical
stresses.3–10
To increase the die’s service time it is important and
also the common task for both die designers as well as
for producers of die steels, to shift the occurrence of
cracks to later times and to decrease their growth rate.
Hot working rolls are usually alloyed with Cr, V, Mo, W,
and after solidification process they are heat treated
accordingly. Furthermore, with optimization of the
chemical composition and the process parameters of the
roll production improved microstructure with better ther-
mal fatigue resistance for each individual application can
be achieved. In relation to the desired microstructural
properties it is important to achieve (i) appropriate type,
size, amount, shape, distribution, etc. of carbides, (ii)
amount, size and shape of graphite particles, (iii) grain
size, etc. Thus, regarding the specific application the
appropriate combination of mechanical (ductility, yield
strength, toughness, hardness, etc.), thermal, microstruc-
tural properties as well as oxidation resistance should be
engineered.3–17
Several research works related to thermal fatigue
resistance of die materials (i.e. steels, cast irons) were
carried out where reasons for initiation and growth of
cracks at hot working dies (forging, die casting)17–30 and
hot working rolls 3–6,9,31–34 were investigated using various
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UDK 67.017:620.17:669.13 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(3)515(2017)
laboratory tests. However, detailed knowledge about the
influence of the stress state, the temperature and the
oxidation on the initiation and propagation of thermal
cracks in hot working applications is still lacking. Each
of the applied laboratory tests has some advantages and
disadvantages from the point of view of achieving of test
conditions and their controllability. Thus, the accuracy of
achieving the prescribed test temperature and subsequent
cooling rate are key factors that influence the quality of
the obtained laboratory results and consequently also on
an explanation of the crack initiation and their
growth.19–27
In this work the thermal fatigue resistance of Inde-
finite Chilled Double Poured (ICDP) cast iron for rolls at
two different temperatures was studied using a new test
rig with specially prepared test samples. The resistance
to thermal fatigue at three different numbers of heating-
cooling cycles were evaluated by measuring the average
length of cracks, their density, average length of five
largest cracks and the relevant microstructures. The
initiation and growth of the cracks were evaluated and
explained. Some suggestions for improvement of thermal
fatigue resistance for the used cast iron roll are given.
2 EXPERIMENTAL PART
2.1 ICDP cast iron
ICDP cast iron with the chemical composition given
in Table 1 was used for thermal fatigue testing. From the
table it is clear that there is an increased content of C
(above 3 %) as well as for Si, Mn, Cr, Mo in comparison
to the usual constructional steels and almost 4.5 %Ni.
Thus, due to high content of carbon as well as the pre-
sence of carbide-forming elements, i.e., Cr, Mo and V,
this indicates the presence of carbides and graphite in the
microstructure.
Table 1: Chemical composition of used cast iron in mass fraction,
(w/%)
Tabela 1: Kemijska sestava uporabljenega litega `eleza v masnih dele-
`ih, (w/%)
C Si Mn Cr Ni Mo V W Nb
3.11 0.84 0.86 1.71 4.45 0.30 0.13 0.01 0.01
2.2 Applied thermal fatigue test and conditions of
testing
The shape of the samples and the applied thermal
fatigue tests are shown in Figures 1a and 1b, respec-
tively. Hollow specimen is on both sides tightly clamped
between two copper anvils which are placed in the
working cells of a Gleeble 1500D thermo-mechanical
simulator. On the middle of 12 mm working length of
samples thermocouple wires of type K are welded, while
water and air inlet as well as their outlet from samples
are enabled by tubes fixed on its both lateral sides.
Heating of the specimen and its cooling by water and air
that flow through hollow specimen is computer guided,
whereas constant testing conditions, i.e., achieving of
constant heating to test temperature and specimen inter-
nal cooling, are ensured (Figure 2). Regarding the prog-
ram the specimens were heated to a selected temperature
in 3 s, which was followed by water cooling and empty-
ing of specimen by air flow. Thus, the temperature
gradient and consequently the stress gradient on the
internal surface of the specimen is achieved. On this area
(Figure 1a) the occurrence of cracks is expected. A
maximum selected test temperature was 600 °C and for
comparative reason also test at 500 °C was carried out.
At test temperature of 600 °C cracks were observed and
measured after 500, 1000 and 1500 thermal cooling
cycles, while at 500 °C this was done after 1000 cycles
(Table 2). After a selected number of cycles the working
lengths of samples were cut in radial as well as in the
axial directions. Characterization of the cracks was fo-
cused within area of 8 mm length of the central working
dimension of samples. The cracks were characterized by
(i) mean length of all cracks, (ii) density of cracks, i.e.,
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Figure 2: Measured time courses of temperature on outside surface of
test specimen
Slika 2: Izmerjeni ~asovni potek temperature na zunanji povr{ini
vzorca za testiranje
Figure 1: a) Used sample for thermal fatigue test and b) tight clamped
sample between two cooper anvils in working cells of the Gleeble
1500D
Slika 1: a) Vzorec za testiranje termi~nega utrujanja, b) tesno vpetje
med dvoje ~eljusti iz bakra v delovni celici naprave Gleeble 1500D
average number of cracks per length of 1mm, and (iii)
average length of the 5 longest cracks.
Table 2: Test conditions
Tabela 2: Pogoji testiranja
Test temperature No. of cycles
500 °C 1000
600 °C 500, 1000, 1500
2.3 Characterization of cracks, carbides and micro-
structure
Optical microscopy (OM, Carl Zeiss AXIO Imager.
A1m) and a field-emission scanning electron microscope
(FE SEM JEOL 6500F) in combination with the attached
Energy-Dispersive Spectroscopy (EDS, INCA x-SIGHT
LN2 with INCA ENERGY 450 software) analytical tool
were applied for observations of microstructure, i.e.,
determination of main constituents (graphite, cementite
(carbides), etc.), characterization of cracks, etc.
Furthermore, the amount of cementite (carbides) as well
as of graphite particles regarding to size, distribution and
shape were determined. Thus by means of automatic
image analysis the form, distribution, and size of
graphite nodes were classified according to the EN ISO
945 standard. The specimens used in optical microscopy
were etched with 3 % Nital reagent.
3 RESULTS
3.1 Characterization of initial microstructure of cast
iron
The obtained microstructure of the initial material at
room temperature is presented in Figure 3a (OM) with
details in Figure 3b as well as in Figure 3c (BEI) with
mapping for C in Figure 3d. As can be seen the micro-
structure consists of martensite, retained austenite,
carbides (cementite), graphite and also MnS inclusions.
The results of the EDS analyses, which served for clear
distinction between cementite and graphite, are given in
Table 3. In general, cementite contains a considerably
smaller amount of C in comparison to graphite. Thus, in
cementite (carbides) besides C and Fe also main
carbide-forming elements, i.e., Cr, Mo and V, as well as
Co and Ni were found. The relative lower obtained
values for C contents in cementite and graphite can be
attributed to the detection depth of EDS analyses, where-
as in our case this exceeded the depth of the interface
between both relevant constituents and matrix, i.e., the
matrix was also partly included in the analyses that
apparently detected lower C contents. Furthermore, it
also known that EDS analysis for light elements is not a
very accurate method. A SEI image of the initial micro-
structure is given in Figure 3c, while the distribution of
carbon is given in Figure 3d. Thus, also by this graphite
particles and cementite can be clearly distinguished.
Note that the obtained microstructure is very similar to
the microstructure found in 45 for cast iron for rolls with
similar chemical composition.
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Figure 4: Percentage of graphite particles regarding their size
Slika 4: Dele` grafitnih delcev v povezavi z njihovo velikostjo
Figure 3: a) Initial microstructure (OM) of used cast iron with b)
detail, c) BEI image showing carbides and graphite and d) correspond-
ing X-ray image (mapping) of C distribution
Slika 3: Za~etna mikrostruktura (OM) uporabljenega litega `eleza; a) z
detajlom, b) slika BEI, ki prikazuje karbide in grafit, c) pripadajo~a
XRD slika (mapiranje), d) porazdelitev ogljika
Table 3: EDS analyse of chemical elements in carbides, graphite and martensite of used cast iron in mass fractions, (w/%)
Tabela 3: EDS-analiza kemi~nih elementov v karbidih, grafitu ter martenzitu za uporabljeno lito `elezo v masnih dele`ih, (w/%)
C Si Mn Cr Mo V Co Ni Fe O Al Imp.
Carbides 5.7 0.9 3.6 0.3 0.3 1.3 1.8 91.4 rest
Graphite 93.9 - - - - - - - 6.1 - - -
Martensite 2.5 1.2 0.6 0.5 - - - 5.4 89.7 - - rest
The percentage of graphite and carbides in the micro-
structure amounted approximately 4.9 % and 29.5 %,
respectively, where the nodularity of the graphite was
0.34. Using automatic image analysis, the form, distribu-
tion, and size of graphite nodes were classified according
to the EN ISO 945 standard. The majority (almost 60 %)
of the graphite particles were of Form II (crab or spiky)
or (15 %) of Form III (vermicular) (Figure 4). More than
half of all the graphite particles were smaller than 15 μm,
equivalent to size class 8 (Figure 4).
3.2 Characterization of cracks growth
As mentioned above the obtained characteristics of
cracks refer to the length of 8 mm in central working
length of the samples. Numbers of cracks for different
class length of cracks in dependence on number of cycles
and for test temperature of 600 °C are given in Figure 5.
As can be seen, in general, the number of cracks de-
creases with increasing of class length for all numbers of
cycles. Thus, for the class length of cracks up to 100 μm
the number of cracks amounts about 70 for all number of
cycles, at class length range 100–200 μm this value
amounts slightly below 20, while at class length range
200–300 μm the number of cracks is in the range 3–6.
With further increasing of class length range the number
of cracks decreases, thus in class length range 800–900
μm only one crack for thermal cycle 1000 and 1500,
respectively, were obtained.
For the comparison at 1000 cycles and 500 °C the
number of cracks are up to class length range of 300 μm,
approximately by about 20 % lower in comparison to the
test temperature of 600 °C and the same number of
thermal cycles. Furthermore, the longest cracks at test
temperature of 500 °C and 1000 thermal cycles were
about 500 μm.
From Figure 6a it is clear that at test temperature of
600 °C the average lengths of all the cracks increases
with the number of cycles. Thus, after 500 thermal
cycles the average crack length amounts to 102 μm, at
1000 cycles, 115 μm and at 1500 cycles, 127 μm. For
comparative test conditions, i.e., 500 °C and 1000 cycles,
the average crack length amounted to 97 μm. As
expected, this value is lower in comparison to the one
obtained for a temperature of 600 °C for the same ther-
mal cycles. Similar relationships were also obtained for
the crack density, which is shown in Figure 6b. Thus at
the test temperature of 500 °C and 1000 cycles the
density of cracks was 8.4 cracks/mm, i.e., lower in com-
parison to the test temperature of 600 °C and the same
number of cycles where this value was 8.7 cracks/mm.
At 500 cycles and the same temperature this value is
slightly lower, i.e., 8.4 cracks/mm, while at 1500 cycles
it is slightly higher, i.e., 8.9 cracks/mm, regarding to
1000 cycles. Therefore, the density of the cracks in-
creases only slightly with the increasing number of
cycles and decreases with lower test temperature. The
lower density of cracks at the lower temperature can be
attributed to the lower stresses on the tested surface
layer.
Also, the average length of the five longest cracks at
test temperature of 600 °C increases with the number of
cycles (Figure 7). Thus, this value is 570 μm at 500
cycles, 700 μm at 1000 cycles and 840 μm at 1500
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Figure 6: a) Average lengths of all cracks and b) cracks density in
dependence of the number of thermal cycles and test temperature
Slika 6: a) Povpre~na velikost vseh razpok b) in gostota razpok kot
funkcija {tevila termi~nih ciklov in temperature testiranja
Figure 5: Distribution of crack number related to class length and in
dependence on number of thermal cycles and test temperature
Slika 5: Porazdelitev {tevila razpok v povezavi z razredom dol`in v
odvisnosti od {tevila termi~nih ciklov ter temperature testiranja
cycles. For comparison, this value at the temperature of
500 °C and 1000 cycles was expectedly lower and is
about 540 μm.
3.3 Microstructural characteristics of cracks
Cracks were predominately initiated at spots on the
tested surface where the carbides are present (Figures 8a
to 8b). This process, which is a consequence of tensile
stresses on surface layer, begins with cracking and
spalling of the carbides, where cracks grow along the
carbides, i.e., they use the carbides path. Figure 8b
shows an empty area as a consequence of the carbides
spalling at the crack’s origin. Namely, the carbides have
a lower temperature expansion coefficient and are also
more brittle in comparison to the matrix. Thus during
heating and cooling they cannot fully follow the dilata-
tion of the base material and consequently they represent
a notch effect, i.e., stress concentration. Furthermore,
from Figure 8b it is also clear that the crack ends at the
carbide–matrix interface. Note that crack growth within
the matrix is hindered. In Figure 8b micro-cracks are
visible that do not originate from the cooled surface but
are internal in nature and are oriented in radial as well as
in axial directions. EDS analyses revealed oxidation that
expands on both sides of the crack.
In Figure 8a it can be also seen the transition of
cracks growth from carbide to graphite and again to
carbide, where crack again finishes at the carbide–matrix
interface. This case illustrates alternating of carbide and
graphite at crack growth that is also presented in Figures
8c to 8d. It should be emphasized here that the shape of
the carbides plays an important role since cracks usually
grow in the direction of the flattened shaped graphite
where the stress intensity is increased. Thus, nodularity
(in this case 0.34) of the graphite should be increased in
order to decrease the growth rates of the cracks. Similar
results were also obtained by X. Tong et al.,18 i.e., the
thermal fatigue resistance of samples made from cast
iron were sorted as nodular graphite iron > vermicular
graphite iron > flake graphite iron. Furthermore, since
cracks grow through carbide–graphite paths, their growth
would be decreased by the interruption of these paths.
This can be done by adding rare-earth elements. Note
that in this way Wang et al.31 achieved coagulation of the
carbides and graphite and consequently also better
thermal fatigue resistance of the roll steel. It can be
assumed that a similar approach could also contribute to
improved fatigue resistance for our type of roll cast iron.
Also, slight hot deformation of the working layer of rolls
would probably act in similar way, i.e., interruption of
carbides paths by crushing of carbides.
Figure 9a shows the growth of cracks using the
carbide and graphite path, while in Figure 9b the distri-
bution of oxygen is presented. From both figures it is
clear that during crack growth oxidation has taken place.
This is also visible at the crack tip that decreases the
local strength properties of the matrix and consequently
also accelerates the growth rate of the cracks. Further-
more, it is also clear that the cracks in the graphite do not
grow through its central part but along both lateral sides.
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Figure 7: Average lengths of five longest cracks in dependence of the
number of thermal cycles and test temperature
Slika 7: Povpre~na velikost petih najdalj{ih razpok kot funkcija {te-
vila termi~nih ciklov in temperature testiranja
Figure 9: a) Crack growth in carbides (cementite) and graphite and
oxidation of cracks, b) distribution of oxygen in cracks internal and on
crack tip
Slika 9: a) Rast razpok v karbidih (cementitu) in grafitu ter oksidacija
razpok, b) porazdelitev kisika v notranjosti razpok ter v njenih koni-
cah
Figure 8: a) Initiation of cracks and cracking of carbides and b)
growth of cracks using carbides (cementite) path, (c–d) growth of
cracks using carbides and graphite particles
Slika 8: a) Nukleacija razpok ter pokanje karbidov in b) rast razpok
po karbidnih (cementitnih) poteh, (c–d) rast razpok preko karbidov in
grafitnih delcev
Further crack growth from the graphite to carbides is
characterized by crack oxidation of the main crack and
by the occurrence of new micro-cracks on both sides.
Note that improvement of oxidation resistance also leads
to an improvement of the thermal fatigue resistance.6,16,32
As already mentioned, cracks are usually initiated at
the carbides on cooled surfaces, which is shown in
Figure 10a. But in very limited cases they are also
initiated at the sharp graphite particles that are elongated
in the longitudinal direction, i.e., flattened graphite
particles. Furthermore, since the graphite is softer than
the matrix, this can also leads to a notch effect at such
spots. Otherwise, it was found that for the more
equiaxied graphite that precipitated at the cooled surface,
cracks does not occur, since only spalling of the small-
sized base material takes place, as presented in Figure
10b. From this figure it is also visible the occurrence of
microcracks in the axial direction in carbides close to the
graphite. This behaviour was very frequently observed
and can be attributed to the differences in the thermal
extensions of all the main constituents (i.e., matrix, car-
bides and graphite) leading to local bending of carbides
as a consequence of the occurrence of the temperature
gradient and the different thermal dilatation, i.e., outer
regions are subjected to higher thermal dilatation. More-
over, in this case the carbides are oriented perpendicular
to the cooled surface and the cracks appeared approxi-
mately in length area of the graphite particle.
Spalling of the material occurs also when cracks,
which originate from different initial spots on cooled
surface, unify (link) in one crack. Consequently, no
adhesion between the base and the crack limited part of
base material exists (Figure 10c). It was also observed
that not only internal micro-cracks for which extension is
limited inside the carbides, but also connected internal
cracks extend across several carbides and graphite
particles (Figure 10d). Furthermore, in some cases gra-
phite can hinder the crack growth when it is located far
enough from the cooled surface and if its shape is
approaching spherical.
4 CONCLUSIONS
Using a specially developed test rig, the resistance
against thermal fatigue for ICDP cast iron roll was
investigated. Tests were carried out at 500 °C and 600 °C
and characteristics related to cracks at 500, 1000 and
1500 cycles were obtained. The average length of all the
cracks, crack density, average length of five longest
cracks and microstructure characteristics related to
cracks were determined. From the results the following
conclusions can be derived:
• Microstructure of tested cast iron consists of carbides
(cementite), graphite particles, MnS inclusions which
are inserted in the martensite matrix. Percentage of
graphite with value for nodularity of 0.34 amounted
4.9 %, while percentage for carbides amounted 29.5
%.
• The initiation of cracks is predominately on spots of
precipitated carbides on the cooled surface.
• In a very few cases also the initiation of cracks at
graphite particles with thin (flattened) and elongated
shape, was observed. In the case of location of sphe-
rical shaped graphite particles at cooled surface this
results only in spalling of base material.
• Cracks grow using carbides (cementite) and graphite
paths. Thin (flattened) and elongated shapes of gra-
phite are more appropriate at cracks growth than
spherical.
• Average length of all cracks as well as cracks density
at test temperature of 600 °C increases almost linear-
ly with an increasing of number of cycles.
• For 1000 cycles the density of the cracks at the test
temperature of 500 °C was lower in comparison to
the test temperature of 600 °C. The occurrence of
oxidation within the cracks as well as at crack tips
was revealed, which results in acceleration of crack
growth.
• Decreasing the possibility of the connection of
carbides and graphite cracks paths will decrease the
growth rate of cracks. This could be achieved (i) by
crushing of carbides cells, i.e., by imposing of hot
plastic deformation, (ii) by refining of carbides by
additional alloying by rare-earth elements31, (iii) by
increasing nodularity of graphite and (iv) by improv-
ing the heat treatment as well as rolls production
procedures.
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Figure 10: a) Initiation of cracks at sharp shaped (formed) graphite, b)
spalling of small sized part of matrix at graphite, c) unification
(linking) of cracks originate from different initial spots at surface, d)
internal crack extending carbides and graphite particles
Slika 10: a) Nukleacija razpok na ostri obliki grafita, b) drobljenje
majhnih delov grafitne matrike, c) zdru`evanje razpok, ki izvirajo iz
razli~nih za~etnih to~k na povr{ini, d) {irjenje notranje razpoke med
karbidi in grafitnimi delci
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