P. SKUBISZ et al.: EFFECT OF DIRECT COOLING CONDITIONS ON THE MICROSTRUCTURE ...
783–789
EFFECT OF DIRECT COOLING CONDITIONS ON THE
MICROSTRUCTURE AND PROPERTIES OF HOT-FORGED HSLA
STEELS FOR MINING APPLICATIONS
VPLIV POGOJEV OHLAJANJA NA MIKROSTRUKTURO IN
LASTNOSTI VRO^E KOVANIH HSLA JEKEL ZA UPORABO V
RUDARSTVU
Piotr Skubisz, £ukasz Lisiecki, Tadeusz Skowronek, Artur ¯ak, W³adys³aw Zalecki
1AGH University of Science and Technology, Department of Metals Engineering and Industrial Computers Science, 30A Mickiewicz Ave,
Kracow, Poland
2Ferrous Metals Institute, 12-14 K. Miarki, Gliwice, Poland
pskubisz@metal.agh.edu.pl
Prejem rokopisa – received: 2015-09-18; sprejem za objavo – accepted for publication: 2015-10-26
doi:10.17222/mit.2015.298
The article presents hot deformation and controlled direct cooling of hardened medium-carbon HSLA steel with micro-additions
of alloying elements Ti and/or V. It also contains a study of the effect of thermomechanical-processing conditions on grain
refinement and precipitation kinetics in the replacement of the conventional reheating-requiring heat treatment with a
cost-effective technology. Controlled cooling with accelerated air and mist adjusted to lower carbon and hardenability-related
alloying elements by means of employing experimentally designed heats, varying in the Mo content, was designed to meet the
mining-industry requirements for mechanical properties. Besides microstructural-property relations, the vital problem of
non-uniformity of the obtained properties is addressed with regard to within-part variances in the cooling rate of the as-forged,
undeformed and dynamically recrystallized material. The strength and plasticity versus the microstructure of the produced fine
pearlite/bainite structure with grain-boundary ferrite were evaluated. Microstructure-property relations allowed the formulation
of the conclusions on the effect of direct-cooling conditions on the microstructure and grain substructure, their mutual synergic
effect in controlling the microstructure, as well as guidelines for the transfer of the locally established process parameters into
technological conditions.
Keywords: thermomechanical processing, drop forging, accelerated cooling, grain refinement, tempered martensite
^lanek predstavlja vro~o deformacijo in kontrolirano ohlajanje srednjeoglji~nega HSLA jekla z mikrododatkom legirnih
elementov Ti in/ali V. Vsebuje tudi {tudijo vpliva pogojev termomehanske obdelave na udrobnjenje zrn in kinetiko izlo~anja pri
nadome{~anju obi~ajnega re`ima ogrevanja za toplotno obdelavo s cenej{o tehnologijo. Da bi dosegli zahteve rudarske
industrije in mikrostrukturne lastnosti, je bilo vzpostavljeno kontrolirano ohlajanje s tokom zraka in me{anice zraka ter vode,
prilagojeno ni`jemu ogljiku in drugim elementom. Ti vplivajo na prekaljivost z uporabo eksperimentalnih talin, v katerih se je
spreminjala vsebnost Mo. Poleg odvisnosti lastnosti od mikrostrukture, je glavni problem neenakost dobljenih lastnosti, kar je
delno povezano z razlikami pri ohlajanju v kosu kovanega, nedeformiranega ali dinami~no rekristaliziranega materiala.
Ocenjeni sta bili trdnost in plasti~nost v odvisnosti od nastale drobnozrnate perlitno/bainitne mikrostrukture s feritom po mejah
zrn. Odvisnost mikrostrukture od lastnosti omogo~a postavitev zaklju~kov o vplivu pogojev pri neposrednem ohlajanju na
mikrostrukturo in zrna substrukture ter njihov medsebojni vpliv pri kontroli mikrostrukture, kot tudi navodila za prenos lokalno
ugotovljenih procesnih parametrov v tehnolo{ke pogoje.
Klju~ne besede: termomehanska obdelava, kovanje s padalnim kladivom, pospe{eno ohlajanje, zmanj{anje zrn, popu{~en
martenzit
1 INTRODUCTION
Forging has long been more than a mere shaping
technique. In addition to the accuracy requirements, such
as allowances and yield, mechanical properties must be
fulfilled. As an alternative to the traditional quenching
and tempering (Q&T) heat treatment, thermomechanical
processing (TMP) in controlled conditions of forging and
direct cooling is increasingly used on an industrial scale,
offering a good combination of strength and ductility at a
lower cost.1–3
The strength levels obtained with TMP are often
lower as compared to a traditional Q&T material4, due to
an excessive surplus of indices in the case of the latter.
Furthermore, some mechanical properties of TMP mi-
croalloyed grades, such as the fatigue strength, are
second to those treated with Q&T.5 The key issue is to
control the chemistry and processing cycle so as to
achieve the sustainable strength, plasticity, cracking
and/or fatigue resistance required by the end user of a
finished product.
Thus, numerous microalloyed grades for specific
applications have been designed over the decades. So
have the technologies of forging and subsequent cool-
ing.6,7 This work is devoted to the design or modification
of a microalloyed steel grade exhibiting as-forged air-
cooled mechanical properties suitable for a mining appli-
cation. The target application of this combination is a
miners’ linking hook for coal transport, which, after a
Materiali in tehnologije / Materials and technology 50 (2016) 5, 783–789 783
UDK 621.78.08:67.017:691.714 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(5)783(2016)
controlled hot-forging and direct heat treatment, involv-
ing quenching and self-tempering, is expected to provide
a yield strength (YS) of 800 MPa, the ultimate tensile
strength (UTS) of 1050 MPa, an elongation to fracture of
A 15 % and/or an impact strength at room temperature of
KCV 60 J/cm2 to substitute C45 or SJ355 heat-treatable
structural steel grades.
2 EXPERIMENTAL METHODS
The research plan included testing the effect of forg-
ing temperature on the microstructure and mechanical
properties obtained after direct cooling in forced air and
atomizers, applied to designed microalloyed grades, hot
deformed in an inconvenient high-speed hammer-forging
process. The goal of the study was both the investigation
of the effect of material-process conditions on the final
(as-forged) properties, and its applicability to drop-forg-
ing industrial conditions with respect to the uniformity of
the properties.
Eight different sets of alloying elements were com-
posed on the basis of the computation in Thermocalc
under equilibrium thermodynamics conditions. Diagrams
were established on the basis of a volumetric-change
analysis (dilatometer DIL 805 with probe LVDT) CTT,
followed by an examination of the microstructures ob-
tained with different cooling rates of 0.17 °C/s,
0.42 °C/s, 1 °C/s, 2 °C/s, 4 °C/s, 10 °C/s, 20 °C/s,
40 °C/s, 50 °C/s and 100 °C/s, taking into consideration
the cooling rates predicted in the modeling of physical
cooling of the target geometry. Having had selected two
grades, further described as A and B, laboratory tests of
rolling and direct cooling with water and air were con-
ducted, as well as forging on a hydraulic press of 5 MN
with a ram velocity of 50 mm/s.
To estimate the amount of deformation and the actual
forge-end temperature in the bulk of the part, a nume-
rical calculation of an equivalent strain and temperature
progression in hot forging was carried out. Since the
forging temperature should be high enough to dissolve
carbides and carbonitrides so as to enable their precipi-
tation during the post-forging direct cooling, a soaking
temperature of 1190 °C was established and forging
temperatures ranging between (1100, 1000 and 900) °C
were assumed. The cooling rate – fast enough to provide
harder microstructural components during the sub-
sequent cooling8 – was realized with atomizers.
Numerical modeling was conducted with the finite-
element method (FEM) in code QForm3D. Besides
supplying the data for the prediction of transformation
products or microstructural development, and carbide
and carbonitride precipitation kinetics, the results of the
calculation formed the basis for experimental controlled
cooling tests on a laboratory continuous cooling line,
QuenchTube Duo (Figure 1).
The laboratory cooling line simulated an industrial
cooling line, providing a forced stream of mist with
intervals resulting from the transfer of a part between
consecutive cooling zones. All runs involved the same
cycle, irrespective of the forging temperature and the
alloy. Cooling was conducted at a cooling rate adapted to
the temperature changes, calculated for a drop-forged
miners’ link hook (Figure 2), which formed a case study
for an evaluation of the effectiveness of the selected pro-
cessing conditions with respect to the forging tempera-
ture and chemical composition.
3 MATERIALS
The study involved experimentally designed steel
grades, which were to provide directly cooled forgings
with the mechanical properties comparable to typical
mining-industry grades, such as C45, 41Cr, 36CrNiMo4
and 23MnNiCrMo5.
Eight grades were made, based on the assumption of
a reduction of carbon and the major alloying elements,
and micro-additions of Ti, V and Nb, with or without
Mo. The heating was performed in an induction-heating
vacuum furnace, VSG-100, with a 100 kg nominal capa-
city of the melting pot. Thermodynamic stability and
volume fractions of phase components and precipitates
were calculated in Thermocalc and confirmed with a
dilatometric analysis. Upon the investigation of the pri-
mary and transformed austenite grain size, the predicted
or calculated yield and impact strength, two grades were
P. SKUBISZ et al.: EFFECT OF DIRECT COOLING CONDITIONS ON THE MICROSTRUCTURE ...
784 Materiali in tehnologije / Materials and technology 50 (2016) 5, 783–789
Figure 2: Geometry of the target application; T1, T2 – thermocouple
locations
Slika 2: Geometrija preizku{anca; T1, T2 – lokacija termoelementovFigure 1: Laboratory simulation of the cooling line
Slika 1: Laboratorijski simulator linije ohlajanja
selected (Table 1), indicating the smallest grain size and
a potential combination of strength and ductility.
Utilizing the deformed material from plastometric tests
with formulas for characteristic times,9 continuous-cool-
ing-transformation diagrams (CCT) of non-recrystallized
materials were made, as shown in Figure 3.
4 RESULTS AND EVALUATION
4.1 Analysis of the target forging technology
Forging-machine kinematics has an enormous impact
on thermomechanical parameters for a hot-forged piece.
As could be expected, hammer forging produces an
increased amount of generated heat and temperature
gradients, which influence the flow stress and metal dis-
placement. Numerical modeling with FEM allowed for
an estimate of the level, variance and gradients of the
temperature and strain development during the forging.
This information made it possible to evaluate the ob-
tained results of direct cooling where, unlike for the
traditionally heat-treated material, the as-forged steel
inherits the aftermath of dynamic processes. Thus, to fit
the laboratory-test results to the target application, we
carried out an insightful analysis of the development of
the thermomechanical indicators of the changes influenc-
ing the final microstructure and properties.
The results of the FEM analysis are summarized in
Table 2.
Table 2: Evolution of thermo-mechanical conditions in the forged part
(in locations shown in Figure 2)
Tabela 2: Razvoj termo-mehanskih pogojev v kovanem kosu (na me-
stih prikazanih na Sliki 2)
Operation 1 2 3 4 5
Effective strain
T1 0.62 0.62 0.62 0.84 0.88
T2 0.62 1.25 2.51 2.75 2.81
Temp., °C
T1 1115 1107 1089 1105 1090
T2 1115 1130 1136 1144 1140
4.2 Effect of the cooling rate
To assess the dependence of the structural compo-
nents and the grain size on the cooling rate, the micro-
structure derived from a dilatometric investigation of
normalized material was examined. Micrographs ob-
tained for both alloys are shown in Figures 4 and 5.
4.3 Physical modeling of forging and cooling
The effect of the run-out table temperature on the ki-
netics of austenite restoration and precipitation of car-
bides and carbonitrides, and the resulting strengthening
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Materiali in tehnologije / Materials and technology 50 (2016) 5, 783–789 785
Figure 4: Microstructure of as-received (hot-rolled) alloy A after anisothermal cooling at rates: a) 1 °C/s, b) 10 °C/s, c) 50 °C/s
Slika 4: Mikrostruktura izhodne (vro~e valjane) zlitine A, po anizotermnem ohlajanju s hitrostjo: a) 1 °C/s, b) 10 °C/s, c) 50 °C/s
Figure 3: Continuous-cooling diagrams calculated with TTSteel code,
modified with theoretical equations9
Slika 3: Diagram kontinuiranega ohlajanja, izra~unan s TTSteel kodo
in spremenjen s teoreti~nimi ena~bami9
Table 1: Chemical compositions of experimental heats of microalloyed steels used in the study
Tabela 1: Kemijska sestava eksperimentalnih talin mikrolegiranih jekel, uporabljenih v {tudiji
Steel % C % Mn % Cr % Si % Mo % Ti % V % Nb % N Ac1,°C Ac3,°C
A 0.30 1.50 0.42 0.26 0 0.011 0.09 0.039 0.011 809 785
B 0.28 1.24 0.42 0.27 0.2 0.019 0.067 0.047 0.010 728 727
efficiency was investigated by varying the forging tem-
perature while employing the same cooling conditions
after the deformation.
According to the plots of temperature measured
during the experiments, the assumed temperatures
changed during the deformation and therefore the forge
end temperature was not exactly the same as expected.
Due to a non-uniform distribution of strain in the bulk,
the amount of generated deformation heat varied with
the location (Table 2). However, the assumed tempera-
tures of forging, (1100, 1000 and 900) °C, allowed an
analysis of the material response in the representative
ranges in relation to the temperatures of the dynamic-
recrystallization stop and the solution of carbonitrides/
carbides. The highest temperature, assumed for the total
content of carbon and microalloying elements, was
found in the solution, available for precipitation. The
lowest forging temperature of 900 °C dropped to 880 °C
due to the material transfer and die cooling; it was meant
to provide the alloys with a reasonable amount of accu-
mulated strain, based on the calculated recrystallization
stop temperature. The forging at 1000 °C was to show
the effect of the precipitates formed prior to forging on
the evolution of dynamically recrystallized grains. As
alloy B did not contain Mo, this test was conducted for
alloy A only.
The obtained cooling curves are shown in Figure 6
for steels A and B, respectively. For clarity, the plots
were shifted and the moment of deformation was
indicated by plotting the forging load (in grey). This was
also an opportunity to indicate unexpected dependence
of the load extreme on the temperature for steel B, which
shows a higher value at 1000 °C than at 900 °C. The
experiment conducted showed that the deformation
temperature is high enough to avoid ferrite nucleation,
which adversely affects the strength properties.10,11 The
load observed at 900 °C implies there is no occurrence of
ferrite dynamic precipitation, while incomplete softening
can be found with the aid of a metallographic analysis
(Figures 7 and 8).
5 MICROSTRUCTURE AND MECHANICAL
PROPERTIES
The microstructure obtained using accelerated cool-
ing with mist is composed of martensite and bainite,
depending on forging temperature portions of fine and
narrow-spaced pearlite, differing in the fractions of the
constituents. The cooling rate of 20 °C/s is sufficient to
omit the high-temperature onset of a diffusion-driven
transformation into recrystallized austenite. Industrial-
like conditions of direct cooling (based on the assump-
tion of quenching directly after deformation) caused an
increased amount of ferrite, with a simultaneous increase
in the fraction of martensite instead of bainite or
Widmanstätten ferrite. As indicated in the physical
simulation included in the study, 80° s–1 in a range of
P. SKUBISZ et al.: EFFECT OF DIRECT COOLING CONDITIONS ON THE MICROSTRUCTURE ...
786 Materiali in tehnologije / Materials and technology 50 (2016) 5, 783–789
Figure 5: Microstructure of as-received (hot-rolled) alloy B after anisothermal cooling at rates: a) 1 °C/s, b) 10 °C/s, c) 50 °C/s
Slika 5: Mikrostruktura izhodne (vro~e valjane) zlitine B, po anizotermnem ohlajanju s hitrostjo: a) 1 °C/s, b) 10 °C/s, c) 50 °C/s
Figure 6: Experimental cooling-curve plots for: a) steel A, and
b) steel B; load peaks (grey) indicate forge-end points
Slika 6: Diagrami eksperimentalnih krivulj ohlajanja: a) jeklo A in
b) jeklo B; konice obremenitve (siva barva) ka`ejo konec kovanja
800–500 °C suffices to produce a 50 % martensite
transformation after the post-forging operations like
trimming the flash and transfer on air. As shown in
Figure 7, by forging at 900 °C, we obtained non-
recrystallized microstructures with a high density of
crystallographic defects, such as shearing bands and
sub-cells with very fine grains at the grain boundaries,
illustrating the increment of the total strength enhance-
ment due to the reduction of the forging regime.
Based on comparable cooling conditions, the micro-
structure analysis indicated a strong effect of forging
conditions on the grain size and morphology, which is
reflected by mechanical properties. Steel A exhibits a
higher strength, with UTSs of 1688 MPa after the forging
at 1100 °C and 1486 MPa for 900 °C (Table 3). Steel B
shows a better ductility associated with the Mo addition.
Furthermore, directly cooled steel A showed a 5 %
elongation while steel B indicted an 8 % elongation,
which is a satisfactory result, taking into account that no
tempering was carried out, while auto-tempering, enabl-
ing a ductility enhancement, could be utilized in indu-
strial processes.
6 CONCLUSIONS
Deformation at a varied temperature, in connection
with direct cooling, indicates a strong influence of
hammer-forging conditions on the resultant mechanical
properties. This gave us an opportunity for investigating
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Materiali in tehnologije / Materials and technology 50 (2016) 5, 783–789 787
Figure 8: Microstructures of alloy B in as-forged direct-cooling con-
ditions, after forging at: a) 900 °C, b) 1000 °C, c) 1100 °C
Slika 8: Mikrostruktura zlitine B, ohlajene takoj po kovanju na:
a) 900 °C, b) 1000 °C, c) 1100 °C
Figure 7: Microstructures of alloy A in as-forged direct-cooling con-
ditions, after forging at: a) 1100 °C, b) 900 °C
Slika 7: Mikrostruktura zlitine A, ohlajene takoj po kovanju:
a) 1100 °C, b) 900 °C
Table 3: Mechanical properties of the microalloyed steels after accele-
rated cooling
Tabela 3: Mehanske lastnosti mikrolegiranih jekel po pospe{enem
ohlajanju
Alloy
Forging
tempera-
ture, °C
YS,
MPa
UTS,
MPa
Elongation at
fracture, A10
%
Area
reduction,
%
Steel A
1100 °C 1104 1688 3.2 6
900 °C 915 1486 5.8 20
Steel B
1100 °C 706 1208 9.4 44
1000 °C 1043 1514 7.5 38
900 °C 757 1012 11.0 59
the possibilities of controlling microstructural properties
and comparing them.
According to the prevailing studies of the TMP utili-
zation in combination with microalloyed steels, detri-
mental hammer-forging conditions and the required final
properties make big differences. Nevertheless, hammer-
forging plants, as well as those employing high-speed-
press forging, are quite numerous and the control and
design of the TMP process in such conditions are still
challenging.
Both the chemical compositions of the designed
alloys and the technology based on the concept of direct
cooling from the forge-end condition assure simplicity
and attractiveness to the forging industry, offering
reduced costs and making it possible for steelworks to
keep the imposed limits of alloying elements. Moreover,
the predefined cooling conditions are easily transferable
to forge-plant conditions. The contents of the micro-
alloying elements in alloys are kept within reasonable
limits required from the standpoint of control of the
grain-structure evolution due to the interactions of the
precipitates with microstructural defects during forging
and the subsequent cooling.12
The incremental character of forging, the thin end,
optionally allows continuous-cooling deformation
modeling. However, it may be noticed that the hook
undergoes a double reduction: during the flattening and
finishing of the impression. Hence, a simple compression
of a 25 mm flat bar ensures appropriate geometrical and,
more importantly, phenomenological similarity condi-
tions. Thus, the forging tests reflected the industrial pro-
cesses of both forging and cooling. Having passed the
800–500 °C range, the cooling rate was slowed down
producing a type of the equalizing hold (Figures 6 and 7),
reducing thermal stresses on the one hand and the time
for the Nb carbide precipitation13 on the other hand.
Precipitates greatly contribute to the hardening of an
alloy; however, the efficiency of their strengthening de-
pends on the volume fraction and size of the precipitates.
Depending on the particle relation to the dislocation
parameters, it can pile up and loop the dislocations or be
sheared by it.14 In the case of the analyzed alloys, both
instances were witnessed. The produced carbides and
carbonitrides TiC and Ti(C,N) are of the largest size
among the occurring precipitates. They are reported to
take on the form of cube-like shapes, reaching a micro-
meter in diameter, which enabled an observation with the
immersion technique using an optical microscope. On
the other hand, Nb carbides have smaller particle dia-
meters. They are found to have a spherical shape and the
size of dozens of nanometers. Their contribution to the
total strength enhancement reaches 90 MPa13,15, which
causes high effectiveness in pinning grain boundaries
during deformation and grain-restoration processes.
The volume of these precipitates allow for a signifi-
cant microstructural controllability brought about by
Nb(C,V), provided the whole Nb content dissolves at
temperatures of 1170–1190 °C. The applied Nb content
is supposed to retard recrystallization, which calls for a
lower forging temperature.
The overall level of the strength properties results
from the grain refinement, which contributes up to
250 MPa.14 It is enhanced by the presence of N and Al,
lowering the tendency for coagulation of the V(C,N)
precipitates. In addition to reducing the amount of vana-
dium dissolved in the austenite16 by decreasing the self-
diffusion of iron, N reduces the grain-growth tendency17,
as long as it is below the content necessary for the VN
formation.18
Besides the precipitates, the grain is refined due to
the grain-structure restoration during the forging, which
greatly influences the concentration of nucleation sites.
Decreasing the forging temperature from 1180 °C to
1000 °C resulted in lowering the forge-end point from
about 1210 °C to 1034 °C, producing a fine-grained
structure, which formed a base for fine colonies of pear-
lite or bainite during the cooling. The produced micro-
structure exhibits a grain size decreasing with the lower-
ing forging temperature. However, contrary to higher
temperature trials, a microstructure abundant in cry-
stallographic defects was obtained during the forging at
about 900 °C, such as shearing bands and non-recry-
stallized sub-cells with particularly fine grains at the
grain boundaries, which can be attributed to disconti-
nuous dynamic recrystallization, resulting in a selective
renovation of the grains, typical of a low-temperature
deformation.19,20 The presence of such grains confirms
that the deformation was completed below the tempera-
ture, at which the recrystallization and/or the strain-
induced precipitation inhibiting static recrystallization
can occur. 21
The mixture of fine recrystallized and strain-hard-
ened grains produced a significant strengthening, re-
sulting in an UTS of 1700 MPa, similar to the related
studies.22 On the other hand, a low plasticity, reaching at
most 11 % of A10 elongation to fracture was obtained.
However, in an industrial practice, higher plasticity indi-
ces should be expected as, in contrast to small laboratory
samples, massive parts allow the auto-tempering effect to
occur, enabling a ductility improvement. It must be noted
that the flat specimens did not have a privileged
grain-flow direction. On the contrary, the grain-flow
orientation in the gauge area of the tensile specimens
was perpendicular to the tension direction. In the
considered part, the situation was different – the metal
flow pattern produced an evident longitudinal grain flow,
allowing the ductility enhancement.
Acknowledgements
Financial assistance of NCBiR within project PBS2/
B5/29/2013 agreement 19.19.110.86730, is acknowl-
edged.
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788 Materiali in tehnologije / Materials and technology 50 (2016) 5, 783–789
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