J. DLOUHY et al.: INFLUENCE OF THE CARBIDE-PARTICLE SPHEROIDISATION PROCESS ON THE MICROSTRUCTURE ...
159–162
INFLUENCE OF THE CARBIDE-PARTICLE SPHEROIDISATION
PROCESS ON THE MICROSTRUCTURE AFTER THE
QUENCHING AND ANNEALING OF 100CrMnSi6-4 BEARING
STEEL
VPLIV PROCESA SFEROIDIZACIJE KARBIDNIH DELCEV NA
MIKROSTRUKTURO JEKLA 100CrMnSi6-4 ZA LE@AJE PO
KALJENJU IN POPU[^ANJU
Jaromir Dlouhy, Daniela Hauserova, Zbysek Novy
COMTES FHT, Prumyslova 995, 334 41 Dobrany, Czech Republic
jdlouhy@comtesfht.cz
Prejem rokopisa – received: 2014-12-12; sprejem za objavo – accepted for publication: 2015-01-21
doi:10.17222/mit.2014.303
Bearings are used mostly in the quenched and tempered state, e.g., steel 100CrMnSi6-4 with a microstructure of low-tempered
martensite and carbide particles undissolved during the quenching austenitization. The size and density of the particles depend
on the spheroidisation annealing which is the standard operation at the beginning of the bearing manufacturing. The particles
decrease the grain growth of the austenite and determine the grain size after the quenching. The process of accelerated carbide
spheroidisation and refinement (ASR) was developed and is used as a replacement of the conventional spheroidisation soft
annealing. The ASR process produces the structure of a ferritic matrix and fine globular carbides. The carbide size is several
times smaller in comparison with the conventional soft annealing. This microstructure is better for quenching and tempering and
ensures a better bearing performance. The article compares the structures and properties of the quenched and tempered
100CrMnSi6-4 steel pre-treated with the conventional soft annealing and ASR. The smaller ASR particle size allows the use of
lower quenching temperatures, ensuring the desired final hardness. Samples in the hardened state were compared, considering
the prior-austenite grain size, the carbide-particle size and distribution as well as the hardness.
Keywords: accelerated spheroidisation, carbide-particle morphology, hardening, bearing steel
Le`aji se obi~ajno uporabljajo v kaljenem in popu{~enem stanju, na primer jeklo 100CrMnSi6-4 z mikrostrukturo nizko
popu{~enega martenzita in s karbidnimi delci, ki se ne raztopijo pri avstenitizaciji pred kaljenjem. Velikost in pogostost delcev
je odvisna od sferoidizacijskega `arjenja, ki je standardna operacija na za~etku izdelovanja le`aja. Delci zavirajo rast avstenitnih
zrn in dolo~ajo velikost zrn po kaljenju. Razvit je bil postopek pospe{ene sferoidizacije in udrobnjenja karbidov (ASR), ki je bil
uporabljen namesto obi~ajnega sferoidizacijskega mehkega `arjenja. Pri ASR procesu nastane feritna osnovna mikrostruktura in
drobni globularni karbidi. Velikost karbidov je nekajkrat manj{a v primerjavi z obi~ajnim mehkim `arjenjem. Taka
mikrostruktura je bolj{a za kaljenje in popu{~anje in zagotavlja bolj{e lastnosti le`aja. V ~lanku so primerjane mikrostrukture in
lastnosti kaljenega in popu{~anega jekla 100CrMnSi6-4 predhodno mehko `arjenega in ASR. Manj{a velikost delcev pri ASR
omogo~a uporabo ni`je temperature kaljenja za doseganje `eljene trdote. Vzorci v utrjenem stanju so bili primerjani z
upo{tevanjem velikosti prvotnih avstenitnih zrn, velikosti in razporeditve karbidnih delcev, kot tudi trdote.
Klju~ne besede: pospe{ena sferoidizacija, morfologija karbidnih zrn, kaljivost, jeklo za le`aje
1 INTRODUCTION
Bearing steels are well studied in terms of the effect
of the technological parameter on the final micro-
structural properties.1 The final properties of a hardened
product depend on the parameters of hardening, e.g., the
austenitization temperature and or the tempering tempe-
rature. Their influence on the hardness and prior-
austenite grain size was studied properly. Much less
attention was paid to the influence of the initial material
microstructure on the final material properties. The soft
annealed microstructure, used as the standard material
state for hardening, consists of globular carbide particles
with a size mostly from 0.5 μm to 1 μm dispersed in the
ferritic matrix. Two initial states were used for the
hardening – the conventional soft-annealed state and the
state after accelerated carbide spheroidisation and
refinement (ASR).2 The ASR state exhibits the same
microstructural morphology, but the globular carbides
are about 3-times smaller than after the soft annealing
and are also more densely spread in the ferritic matrix.3,4
Such a significantly finer structure provides for a faster
Materiali in tehnologije / Materials and technology 50 (2016) 1, 159–162 159
UDK 620.186:669.056:621.785 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(1)159(2016)
Table 1: Chemical composition of the 100CrMnSi6-4 bearing steel in mass fractions (w/%)
Tabela 1: Kemijska sestava 100CrMnSi6-4 jekla za le`aje v masnih dele`ih (w/%)
C Si Mn P S Cr Mo Ni Al Cu Fe
0.98 0.54 1.14 0.011 0.011 1.50 0.006 0.02 0.018 0.017 bal.
carbide-particle dissolution during the austenitization at
the quenching temperature3 and a stronger pinning of the
grain boundaries, thus leading to a smaller austenite
grain size due to the pinning effect and a martensitic
structure after the quenching. These microstructural
features can lead to a higher hardness and toughness
with the same hardening parameters. Another possibility
is to reduce the quenching temperature or austenitization
time, but still with good final mechanical properties.5,6
2 EXPERIMENTAL WORK
2.1 Material
The experimental material was the 100CrMnSi6-4
bearing steel grade with the chemical composition shown
in Table 1. The material was supplied as hot-rolled
21 mm-diameter bars, with a microstructure of pearlite
and a small amount of secondary cementite along the
austenite boundaries and a hardness of 383 HV10. The
samples were cut in the form of 30 mm long cylinders.
2.2 Spheroidisation annealing
The initial pearlitic microstructure was spheroidised
and samples with fine and coarse globular pearlite were
prepared. The coarse structure was obtained with the
conventional soft annealing in an atmospheric furnace at
a temperature of 805 °C for 11 h and air cooling. Fully
spheroidised pearlite consisted of cementite globules
with diameters of 0.3–1 μm in the ferritic matrix. The
fine structure was obtained with the ASR heat treatment
consisting of three temperature cycles. Each cycle
consisted of the induction heating to 780 °C, a 15-second
dwell period at this temperature and cooling in air to a
temperature of 650 °C. The overall annealing duration
was 5 min. The structure after the ASR process consisted
of globular carbides with a size of 0.1–0.3 μm dispersed
in the ferritic matrix.
2.3 Quenching and tempering
The heating to the quenching temperature was
performed in an electric atmospheric furnace. The
samples were held at the austenitization temperature for
30 min. Austenitization temperatures of (800, 820, 840
and 860) °C were used. The quenching was performed in
an oil bath, immediately followed by tempering. All the
samples were tempered for 4 h at a temperature of
240 °C.
2.4 Sample analyses
All the samples were cut longitudinally and
metallographic specimens were prepared by mechanical
grinding, polishing and Nital etching. The microstructure
was examined with SEM JEOL 7400F. The prior-auste-
nite grain size (PAGS) was assessed on the samples
etched in a picric-acid-saturated aqueous solution at a
temperature of 80 °C and the average grain diameter was
determined with a linear intercept procedure. The
hardness was measured with the Vickers method at a
load of HV10.
3 RESULTS AND DISCUSSION
3.1 Microstructure analyses
The microstructures after the soft annealing and ASR
treatment are shown in Figures 1 and 2. Much finer
carbide globules were formed during the ASR treatment.
Figures 3 and 4 show quenched-sample microstructures
(quenched from 800 °C). There is a clear difference in
the carbide density between the samples. This difference
is pronounced with higher austenitizing temperatures.
The ASR-treated samples retained a much denser
carbide distribution even after the quenching from 860 °C
(Figures 5 and 6). A higher dispersion strengthening
could cause a hardness increase in comparison with the
conventionally soft-annealed samples. Apparently, a
denser carbide distribution causes a pronounced pinning
effect on the prior-austenite grain boundaries. There
were coarse carbides retained in the soft-annealed
samples after the quenching from higher temperatures
J. DLOUHY et al.: INFLUENCE OF THE CARBIDE-PARTICLE SPHEROIDISATION PROCESS ON THE MICROSTRUCTURE ...
160 Materiali in tehnologije / Materials and technology 50 (2016) 1, 159–162
Figure 2: ASR-treated sample with finer carbides
Slika 2: ASR-obdelan vzorec z drobnimi karbidi
Figure 1: Conventionally soft-annealed sample
Slika 1: Obi~ajno mehko `arjen vzorec
(840 and 860 °C). Smaller carbides with a size of up to
0.5 μm were almost completely dissolved.
3.2 Hardness and PAGS
Different austenitization temperatures resulted in
different hardness values found after the quenching and
tempering and also in different PAGS values. There were
significant differences between the samples with coarse
and fine structures as shown in Figure 7.
There is a clear trend in the hardness value in the
case of the conventionally soft-annealed samples with
coarse carbides in the microstructure. A higher austeni-
tization temperature caused a hardness increase due to
the dissolution of a larger amount of carbon. On the
other hand, there is no clear trend in the case of the hard-
ness of the quenched ASR samples. The hardness after
the tempering increased with the quenching temperature
and was significantly higher with all the quenching
temperatures in comparison with the conventionally
soft-annealed samples.
J. DLOUHY et al.: INFLUENCE OF THE CARBIDE-PARTICLE SPHEROIDISATION PROCESS ON THE MICROSTRUCTURE ...
Materiali in tehnologije / Materials and technology 50 (2016) 1, 159–162 161
Figure 7: Hardness and PAGS of samples after quenching and tem-
pering
Slika 7: Trdota in PAGS vzorcev po kaljenju in po popu{~anju
Figure 5: Soft-annealed sample quenched from 800 °C
Slika 5: Mehko `arjen vzorec po kaljenju iz 800 °C
Figure 6: ASR-treated sample quenched from 800 °C
Slika 6: ASR-obdelan vzorec po kaljenju iz 800 °C
Figure 3: Soft-annealed sample quenched from 800 °C
Slika 3: Mehko `arjen vzorec po kaljenju iz 800 °C
Figure 4: ASR-treated sample quenched from 800 °C
Slika 4: ASR-obdelan vzorec po kaljenju iz 800 °C
The PAGS is much smaller in the case of the
ASR-treated samples. Its values grew with the increasing
quenching temperature for both sample types – the
ASR-treated and conventionally soft-annealed samples.
The finer-structured ASR-treated samples exhibited a
more pronounced PAGS increase. Finer carbides
dissolved at higher quenching temperatures and thus
their pinning effect on the austenite grain boundaries was
diminished. The difference between the PAGS values of
the ASR and soft-annealed samples decreased with the
rising quenching temperatures.
4 CONCLUSION
The microstructures and hardness values of hardened
samples with different initial microstructures were
compared. The influence of the carbide-particle density
is clearly visible in terms of the prior-austenite grain size
and hardness. The samples with fine carbides after the
ASR process had higher hardness values after the
quenching and tempering for the whole range of
examined quenching temperatures from 800 to 860 °C.
The hardness increase was about 40 HV10 in
comparison with the conventionally soft-annealed
samples with coarser carbide particles and was probably
caused by the dispersion strengthening and a possible
higher dissolution of the fine carbides during the
austenitization.
Acknowledgment
This paper was created by project Development of
West-Bohemian Centre of Materials and Metallurgy No.:
LO1412, financed by the MEYS of the Czech Republic.
5 REFERENCES
1 H. K. D. H. Bhadeshia, Steels for bearings, Progress in Materials
Science, 57 (2012), 268–435, doi:10.1016/j.pmatsci.2011.06.002
2 D. Hauserova, J. Dlouhy, Z. Novy, Microstructure Development of
Bearing Steel during Accelerated Carbide Spheroidisation, Materials
Science Forum, 782 (2014), 123–128, doi:10.4028/
www.scientific.net/MSF.782.123
3 J. H. Kang, P. E. J. Rivera-Díaz-del-Castillo, Carbide dissolution in
bearing steels, Computational Materials Science, 67 (2012),
364–372, doi:10.1016/j.commatsci.2012.09.022
4 D. Hauserova, J. Dlouhy, Z. Novy, J. Zrnik, Accelerated carbide
spheroidization and refinement (ASR) of the C45 steel during
induction heating, Mater. Tehnol., 47 (2013) 6, 701–705
5 H. Jirkova, D. Hauserova, L. Kucerova, B. Masek, Energy- and
time-saving low-temperature thermomechanical treatment of low-
carbon plain steel, Mater. Tehnol., 47 (2013) 3, 335–339
6 A. I. Katsamas, A computational study of austenite formation
kinetics in rapidly heated steels, Surface & Coatings Technology,
201 (2007), 6414–6422, doi:10.1016/j.surfcoat.2006.12.014
J. DLOUHY et al.: INFLUENCE OF THE CARBIDE-PARTICLE SPHEROIDISATION PROCESS ON THE MICROSTRUCTURE ...
162 Materiali in tehnologije / Materials and technology 50 (2016) 1, 159–162