Y. GE, K. WANG: EFFECT OF TEMPERING TEMPERATURE ON PRECIPITATE EVOLUTION AND ...
527–534
EFFECT OF TEMPERING TEMPERATURE ON PRECIPITATE
EVOLUTION AND MECHANICAL PROPERTIES OF 34CrNiMo6
STEEL
VPLIV TEMPERATURE POPU[^ANJA NA POTEK IZLO^ANJA IN
MEHANSKE LASTNOSTI JEKLA 34CrNiMo6
Yanming Ge, Kehong Wang
*
School of Materials Science and Engineering, Nanjing University of Science and Technology, No. 200 Xiaolingwei Street, Nanjing,
Jiangsu 210094, China
Prejem rokopisa – received: 2018-11-20; sprejem za objavo – accepted for publication: 2019-01-30
doi: 10.17222/mit.2018.248
The effect of the tempering temperature on mechanical properties are studied by investigating the precipitate evolution during
the heat treatment of the 34CrNiMo6 steel. Five different tempering temperatures are used at the same holding time of 3 h. Then
the mechanical properties of the 34CrNiMo6 steel are studied under steady-state conditions and impact conditions. The strength
decreases while the elongation after breakage increases with the increasing tempering temperature. The impact-absorbing
energy at 20 °C and –40 °C increases with the increasing tempering temperature. Based on the experimental results, the
recommended tempering temperature is 660 °C. The differences in mechanical properties related to different tempering
temperatures may be caused by the nucleation rate and growth of precipitates under different conditions. The spherification
degree of cementite is more complete with the recommended tempering temperature, resulting in a decrease in mechanical
properties and improvement of ductility.
Keywords: 34CrNiMo6 steel, tempering temperature, mechanical properties, precipitates
Avtorji so raziskovali vpliv temperature popu{~anja na mehanske lastnosti jekla 34CrNiMo6 s preu~evanjem poteka izlo~anja
med njegovo toplotno obdelavo. Izvedli so preizkuse popu{~anja pri petih (5) razli~nih temperaturah in v vseh primerih enakem
(3-urnem) ~asu zadr`evanja preizku{ancev na teh temperaturah. Nato so dolo~ili mehanske lastnosti toplotno obdelanega jekla
pri stacionarnih in udarnih (dinami~nih) pogojih preizku{anja. Trdnost preizku{ancev se je zmanj{evala in raztezek je nara{~al z
nara{~ajo~o temperaturo popu{~anja. Udarna `ilavost pri 20 °C in –40 °C je nara{~ala z nara{~ajo~o temperaturo popu{~anja. Na
osnovi eksperimentalnih rezultatov avtorji priporo~ajo temperaturo popu{~anja 660 °C. Ugotavljajo, da so razlike v mehanskih
lastnostih posledica razli~nih pogojev toplotne obdelave, ker sta hitrost nukleacije in potek rasti izlo~kov pri razli~nih
temperaturah popu{~anja razli~na. Stopnja sferifikacije (okroglenja) cementita se stopnjuje z nara{~ajo~o temperaturo
popu{~anja, kar posledi~no vodi do zni`evanja trdnosti in pove~evanja duktilnosti jekla.
Klju~ne besede: jeklo 34CrNiMo6, temperatura popu{~anja, mehanske lastnosti, izlo~ki
1 INTRODUCTION
The modern development of the commercial wind-
power industry started in the 1970s in the United States
before spreading to Europe where Denmark and Ger-
many became significant champions of the technology.
1
Later, the focus of its growth was shifted to Asia and
particularly to China where there was a massive deve-
lopment of wind power during the past decade. So, the
wind energy is one of the fastest growing sources of
electricity and it is nowadays capable of providing power
in most parts of the world.
2
According to the estimation
of the International Energy Agency (IEA), the annual
wind-generated electricity of the world will reach 1282
TW h by 2020. By 2030, the annual wind-generated
electricity will reach 2182 TW h.
3
That requires the wind
turbines to exhibit a higher safety and reliability, with the
power of wind turbines increasing from kilo-watt class to
million-watt class.
4
That is to say, the mechanical
properties of the material used for making the main
bearings should meet the related demands.
5
The
34CrNiMo6 steel is one kind of heat-treated low-alloy
steel with a high hardenability and strength, which
contains nickel, chromium, and molybdenum.
6
More-
over, the 34CrNiMo6 steel exhibits very good toughness
properties with a Charpy V-notch at a very low tem-
perature. Hence, the 34CrNiMo6 steel is widely used for
making the wind-turbine main shaft.
7
In the actual
production process, the typical heat treatment of this
steel involves two stages, quenching and tempering.
8
The
34CrNiMo6 steel can obtain high strength after
quenching to a full martensitic microstructure, while the
ductility and toughness can be improved with tempering.
N. Popescu et al.
9
studied the effects of bulk tem-
pering on the hardenability and temperability of the
34CrNiMo6 steel. The experiment investigated the corre-
lation between the hardness achieved after high tem-
pering of products and their equivalent diameter and the
heat and time parameters of tempering. J. Cochet et al.
10
investigated the heat-treatment parameters of the
Materiali in tehnologije / Materials and technology 53 (2019) 4, 527–534 527
UDK 620.1:67.017:669.14.018:298:621.785.796 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(4)527(2019)
*Corresponding author's e-mail:
wkh1602@126.com

34CrNiMo6 steel used for shackles. The results firstly
provided a validation of the input data and the prediction
of the phase volume fractions and the resulting hardness,
which showed that the proposed approach could yield a
very good representation of the material properties. The
results showed that the heat-treatment method could
significantly improve the mechanical properties via
changing the nucleation rate and growth rate of austenite.
However, there are very few researches about the speci-
fic effect of the tempering temperature on the micro-
structure and mechanical properties of the 34CrNiMo6
steel during a tempering process.
One of the most important things for a production
company is to provide high-quality products, which
should meet the certification standard because any fail-
ure during the actual service time is costly and dange-
rous. The fault statistics for a wind turbine in Figure 1
shows that the outages caused by the main-shaft failure
account for less than 0.1 % (marked in blue), but its
downtime accounts for a very large share, about 24.8 %
(marked in orange).
11
So it is necessary to investigate the
mechanical properties of the 34CrNiMo6 steel for the
wind-turbine main shaft obtained after the heat treat-
ment.
The objective of this paper is to investigate the
microstructure evolution and mechanical properties of
the 34CrNiMo6 steel by adopting different tempering
temperatures after quenching. That could help us under-
stand and optimize the heat-treatment process parameters
that affect the microstructures, as well as determining the
final mechanical properties of the 34CrNiMo6 steel for
the wind-turbine main shaft. Firstly, we used five diffe-
rent tempering temperatures during the heat treatment.
Then the mechanical-property tests involving tensile
strength, yield strength, elongation after breakage and
impact-absorbing energy under different conditions were
carried out. To get a better understanding of the influ-
encing mechanism of the tempering temperature on the
microstructures evolution of the 34CrNiMo6 steel, the
metallographic results obtained with the optical micro-
scope (OM), scanning electron microscope (SEM) and
transmission electron microscope (TEM) were presented.
2 EXPERIMENTAL PART
2.1 Sample preparation
The material used for the present study is the heat-
treated low-alloy 34CrNiMo6 steel. The chemical com-
position of the material is shown in Table 1.
The heat treatment of the 34CrNiMo6 steel involves
two stages, quenching and high-temperature tempering
(QT). Again, the steel could obtain high strength by
quenching and improve its toughness due to tempering.
Tables 2 illustrates the mechanical properties required in
accordance with the certification of the 34CrNiMo6 steel
for the wind-turbine main shaft, and BS EN
10083-3:2006.
According to the actual measurement, a fracture
position occurred in the variable-diameter section of the
inner diameter of the main shaft. A schematic diagram of
Y. GE, K. WANG: EFFECT OF TEMPERING TEMPERATURE ON PRECIPITATE EVOLUTION AND ...
528 Materiali in tehnologije / Materials and technology 53 (2019) 4, 527–534
Figure 1: Outage and downtime weights of wind-turbine faults
Table 1: Chemical composition of 34CrNiMo6 steel
Element C Si Mn P S Cr Mo Ni
Standard value
BS EN 10083-3:2006
0.30–0.38 0.40 0.50–0.80 0.025 0.035 1.30–1.70 0.15-0.30 1.30–1.70
Measured value 0.35 0.23 0.76 0.011 0.001 1.58 0.23 1.54
Table 2: Mechanical properties of 34CrNiMo6 steel for the wind-turbine main shaft
Test items
Tensile strength
R
m
/MPa
Yield strength
R
e
/MPa
Elongation A /%
Impact absorbing energy
AKV /J
Standard value
BS EN 10083-3:2006
1100–1300  900  10
 100 (+20 °C)
 90 (-40 °C)

the wind-turbine main shaft is shown in Figure 2. The
image of the fracture surface is shown on the right.
According to the finite-element-method simulation, we
can tell that the positions of the maximum internal stress
of the main shaft are always located at the center of the
variable section under both steady-state conditions and
impact conditions.
12
So, the specimens used in this ex-
periment were taken from the variable-diameter section.
2.2 Tempering treatment and performance tests
The heat treatments of the 34CrNiMo6 steel consist
of several steps, each of them having a specific purpose.
Specifically, the tempering process is to reduce the
brittleness, relieve the internal stress, and obtain relati-
vely good microstructures. The tempering process
includes heating the martensitic steel to the value below
the low transformation temperature (A1), holding it at
this temperature for a specified period and then cooling it
down to room temperature, which makes the 34CrNiMo6
steel achieve the combination of desired mechanical
properties.
13
The most important process parameters
during tempering are the tempering temperature, the
holding time and the steel chemical composition.
14
The
tempering temperature can be identified according to the
superficial color during the heating and the holding time
can be determined by the product size. Generally, the
larger the product size, the more time is needed for
heating the material to the tempering temperature. On
the other hand, the temperature and holding time are
interdependent parameters in the tempering process. So,
a range of mechanical properties can be obtained with
the right combination of the tempering temperature and
holding time, as shown in Figure 3.
In this experiment, we designed five different temper-
ing temperatures to investigate the effect of the temper-
ing temperature on the microstructure and mechanical
properties of the 34CrNiMo6 steel. As shown in Table 3,
five different tempering temperatures were adopted
while the holding time remained the same.
Table 3: Heat-treatment conditions
Sample
no.
Austenitizing
temperature /°C
Tempering
temperature /°C
Holding time
/h
1# 870 570 3
2# 870 600 3
3# 870 630 3
4# 870 660 3
5# 870 690 3
Y. GE, K. WANG: EFFECT OF TEMPERING TEMPERATURE ON PRECIPITATE EVOLUTION AND ...
Materiali in tehnologije / Materials and technology 53 (2019) 4, 527–534 529
Figure 2: Outage and downtime weights of wind-turbine faults
Figure 3: Flow chart of the main-shaft heat treatment

This wind-turbine main shaft needs to exhibit high
levels of strength and toughness over an extended period.
Accordingly, the related tests were carried out to verify
the mechanical properties according to standard specifi-
cations. The mechanics-performance testing contains
tensile strength, yield strength, elongation after breakage
and impact-absorbing energy test. Strength is one of the
main mechanical properties of metals and alloys, defined
as the amount of stress that a material can withstand
while being stressed or compressed before failing.
Toughness is known as the capability of the metal to
resist the fracture by absorbing impact energy and be-
coming plastically deformed. Fracture toughness is used
to characterize toughness. An impact test is used to
obtain an indication of fracture toughness through
measuring the resistance of a material to impact load
without fracture,
15
which is a tool for evaluating the
ductile-brittle transition temperature. In addition, hard-
ness is also a very important parameter of a material as it
is a measure of the resistance to localized plastic defor-
mation induced by either mechanical indentation or
abrasion.
A tensile test is based on the ASTM E8 standard: a
specimen is taken from the 1/4 position of the circular
cross-section of the 34CrNoMo6 steel, whose original
diameter is 10 mm, original standard distance is 50 mm
and total length is 120 mm. Our tensile test was carried
out via a WAW-300B hydraulic universal tester. The
samples were tested 3 times, and the test results were
averaged for each group of the samples.
According to the ASTM A370-2013a standard,
impact tests are carried out to measure impact-absorbing
energy. The specimen is shaped as the V-type, taken
from the 1/4 position of the circular cross-section of the
34CrNoMo6 steel, whose total length is 50 mm. Both the
height and width of the section are 10 mm, the notch
angle is 45±2° and the height of the notch at the bottom
is 8 mm. The impact tests were carried out with a
ZBC2302 impact-testing machine. The samples were
tested 3 times, and for each group of samples the test
results were averaged.
The hardness test is based on the ASTM E18-2015
standard. All the sample surfaces were ground and
polished before the hardness tests. After placing the sam-
ples in the hardness-testing machine, a load of 150 kg
was applied on the samples. After the load was removed,
the Rockwell hardness C (HRC) could be calculated.
The metallographic specimens exposed to all the
heat-treatment conditions were suitably sectioned,
mounted, mechanically polished and etched; the etchant
was 4 w/% nital. The microstructural observations of
these specimens treated with standard metallographic
procedures and the analyses of the fracture surfaces
removed from the fractured tensile bars were carried out
using a ZEISS  IGMA field-emission scanning electron
microscope (SEM), operated at 2.5 kV.
A transmission electron microscope (TEM) was used
for a detailed examination of the precipitate evolution.
TEM samples were cut by a spark-cutting machine and
prepared by utilizing the conventional jet-polishing
technique in a mixture of 5 % perchloric acid and 95 %
glacial acetic acid (by volume). The process was carried
out at 0 °C and a voltage of 20–30 V, until the discs were
perforated.
3 RESULTS AND DISCUSSION
3.1 Effects on the mechanical properties
The evolutions of strength and elongation with
different tempering temperatures are shown in Figure 4,
respectively.
As shown in Figure 4a, both the tensile strength and
yield strength decrease with the increasing tempering
temperature. In contrast, the elongation after breakage
increases with the tempering temperature increasing up
to 660 °C, while remaining almost stable from 660 °C to
690 °C. In accordance with the certification standard
from Table 2, we could conclude that the integrated
mechanical properties of the 34CrNiMo6 steel meet the
demands when the tempering temperature is below
660 °C. The tensile strength is lower than the required
value when the tempering temperature is 690 °C. From
the above, the tempering temperature is recommended to
be no more than 660 °C.
Y. GE, K. WANG: EFFECT OF TEMPERING TEMPERATURE ON PRECIPITATE EVOLUTION AND ...
530 Materiali in tehnologije / Materials and technology 53 (2019) 4, 527–534
Figure 4: Evolutions of strength and elongation with the tempering
temperature: a) tensile strength and yield strength, b) elongation after
breakage

The evolution of the surface hardness with the tem-
pering temperature is shown in Figure 5.
The Rockwell hardness C decreases with the increas-
ing tempering temperature up to 660 °C, while remain-
ing almost stable from 660 °C to 690 °C. According to
the specification design, the surface hardness always
meets the related requirements.
3.2 Effect on the impact energy absorbed
The evolutions of the impact-absorbing energy with
the tempering temperature at different holding times are
shown in Figure 6.
As shown in Figure 6, the impact-absorbing energy
increases with the increasing tempering temperature, and
reaches the peak value when the testing temperature is
20 °C. Then the impact-absorbing energy decreases from
660 °C to 690 °C. According to the specification design,
the impact-absorbing energy meets the requirements
when the tempering temperature is over 630 °C. In con-
trast, the impact-absorbing energy increases monoton-
ously with the increasing tempering temperature when
the testing temperature is –40 °C, as shown in Figure 6.
According to the specification design, the impact-absorb-
ing energy meets the requirements when the tempering
temperature is over 600 °C.
According to the previous mechanical-performance
test results and related certification standard, it is highly
recommended that the tempering temperature should be
between 630 °C and 660 °C.
3.3 Effects on the microstructures and fracture mor-
phologies
The metallographs of the 34CrNiMo6 steel after
quenching and QT obtained with the OM are shown in
Figure 7, respectively.
As previously mentioned, the 34CrNiMo6steel can
obtain a high strength by being quenched to a full
martensitic microstructure. According to the major
morphologies, martensites can be divided into two main
types, martensite lath and plate martensite. In this paper,
the morphology of martensites consists mostly of dis-
located martensite laths, as shown in Figure 7a.
After comparing the metallographs of the
34CrNiMo6 steel obtained with the OM under different
tempering temperatures, the investigations of the sam-
ples point out that there are few differences among their
microstructures. The microstructure recorded after
tempering is shown in Figure 7b. During the tempering
process, the martensite decomposes and then transforms
Y. GE, K. WANG: EFFECT OF TEMPERING TEMPERATURE ON PRECIPITATE EVOLUTION AND ...
Materiali in tehnologije / Materials and technology 53 (2019) 4, 527–534 531
Figure 7: Metallographs of 34CrNiMo6 specimens quenched in oil
(870 °C/30 min) and then tempered at 660 °C: a) quenched in oil
(870 °C /30 min), b) quenched and tempered at 660 °C
Figure 5: Evolutions of hardness with the tempering temperature
Figure 6: Evolutions of impact-absorbing energy with the tempering
temperature under different temperatures

into ferrite and the metastable phase of  -carbide. Ce-
mentite is formed later and eventually the ferrite matrix
recrystallizes.
16
Most of the recrystallized ferrite has a
chaotic ordering and interlocking structure that contri-
butes to improving strength and toughness. So, the
34CrNiMo6 steel can obtain good integrated mechanical
properties after the quenching and tempering processes.
To get a better understanding of the influencing me-
chanism of the tempering temperature on the micro-
structure of the 34CrNiMo6 steel, micrographs of the
34CrNiMo6 steel after quenching and QT at different
temperatures obtained with TEM are shown in Figure 8.
Lath martensite is usually massive, having crystals
shaped like interconnected plates and with nearly the
same orientation,
17
which can be found in Figure 8a.
Moreover, the plate dimensions are limited by the
dimensions of the austenite grains, which means that
small martensite grains create a fine needle structure.
18
This kind of structure has a relatively desirable micro-
structure. Micrographs of the 34CrNiMo6 steel recorded
at different tempering temperatures are shown in Figures
8b and 8c. The microstructure always includes sorbite
tempered at the tempering temperatures used in this
experiment. Generally, tempered sorbite is a mixture of
polygonal ferrite and relatively large cementite forma-
tions.
18
Compared with the tempering temperature of
570 °C, the spherification degree of cementite is more
complete (Figure 8c) due to the higher temperature.
19
As
the grain boundary can inhibit the movement of dis-
location, a reduction in the grain size can reduce the
movement of dislocation, resulting in a significant effect
on the mechanical properties.
15
Hence, the tensile
strength of the 34CrNiMo6 steel decreases at 660 °C
compared with that at 570 °C. In contrast, the toughness
of the 34CrNiMo6 steel increases with the increasing
tempering temperature due to the brittleness of cemen-
tite. The size of cementite has an effect on the mecha-
nical behavior. Generally, a microstructure that consists
of fine cementite is harder as well as stronger than that
consisting of coarse cementite, whereas coarse cementite
is more ductile.
TEM micrographs of the 34CrNiMo6 specimens
quenched in oil (870 °C/30 min) and tempered at diffe-
rent tempering temperatures are shown in Figure 9.
As mentioned before, the microstructure is sorbite
tempered under the heat-treatment conditions of this
experiment, consisting of polygonal ferrite and cemen-
tite. The spherification degree of cementite is more
Y. GE, K. WANG: EFFECT OF TEMPERING TEMPERATURE ON PRECIPITATE EVOLUTION AND ...
532 Materiali in tehnologije / Materials and technology 53 (2019) 4, 527–534
Figure 9: TEM micrographs of 34CrNiMo6 specimens quenched in
oil (870 °C/30 min) and tempered at different tempering temperatures:
a) as quenched in oil (870 °C/30 min), b) quenched and tempered at
630 °C for 3 h, c) quenched and tempered at 660 °C for 3 h, d)
quenched and tempered at 690 °C for 3 h
Figure 8: TEM micrographs of 34CrNiMo6 specimens quenched in
oil (870 °C/30 min) and tempered at different tempering temperatures:
a) as quenched in oil (870 °C/30 min), b) quenched and tempered at
570 °C, c) quenched and tempered at 660 °C

complete with the recommended tempering temperature,
resulting in a decrease in mechanical properties and
improvement of ductility. This conclusion is consistent
with the previous one.
Fracture morphologies of the specimens quenched in
oil and tempered at different tempering temperatures are
shown in Figure 10.
As shown in Figure 10, fracture characteristics are
always dominated by the ductile mechanism for all the
tempered specimens. The features observed from the
micrographs are typical of most of the fracture surfaces
at all the tempering temperatures. Moreover, the voids in
these micrographs exhibit a wide variation in the size
and shape, indicating that localized shear stresses may
occur in addition to tensile stresses during the defor-
mation process. It should be pointed out that although
some voids existed in the fracture initiation area, there is
no certain evidence proving that these voids were caused
by carbides.
4 CONCLUSIONS
The paper investigates the microstructure evolution
during the heat treatment of the 34CrNiMo6 steel by
varying the tempering temperatures. In the experiments,
five different tempering temperatures are adopted, with
the same holding time. The following conclusions can be
drawn based on the experimental results:
(1) Both the tensile strength and yield strength
decrease with the increasing tempering temperature. In
contrast, the elongation after breakage increases with the
increasing tempering temperature up to 660 °C, while
remaining almost stable from 660 °C to 690 °C. The
Rockwell hardness C decreases with the increasing tem-
pering temperature up to 660 °C, while remaining almost
stable from 660 °C to 690 °C.
(2) The impact-absorbing energy increases with the
increasing tempering temperature and then decreases
between 660 °C and 690 °C when the testing tempera-
ture is 20 °C. The impact-absorbing energy increases
monotonously with the increasing tempering temperature
when the testing temperature is –40 °C.
(3) Based on the mechanical properties determined
during the testing under steady-state conditions and
impact conditions, and the related certification standard,
it is highly recommended that the tempering temperature
should be 630–660°C.
(4) The differences in the mechanical properties
determined at different tempering temperatures may be
caused by the nucleation rate and growth of precipitates
under different conditions. The spherification degree of
cementite is more complete with the recommended tem-
pering temperature, resulting in a decrease in mechanical
properties and improvement in ductility.
(5) The fracture characteristic is always dominated
by the ductile mechanism for all the tempered speci-
mens. The voids in the micrographs exhibit a wide vari-
ation in the size and shape, indicating that localized
shear stresses may occur during the deformation process.
Although some voids existed in the fracture initiation
area, there is no certain evidence proving that these voids
were caused by carbides.
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