K. IBRAHIM et al.: NEW CONCEPT FOR MANUFACTURING CLOSED DIE FORGINGS OF HIGH STRENGTH STEELS
623–626
NEW CONCEPT FOR MANUFACTURING CLOSED DIE
FORGINGS OF HIGH STRENGTH STEELS
NOV KONCEPT IZDELAVE ODKOVKOV IZ VISOKOTRDNOSTNIH
JEKEL V ZAPRTIH UTOPIH
Khodr Ibrahim, Ivan Vorel, Dagmar Bublíková, Bohuslav Ma{ek
University of West Bohemia, Fortech Research Centre, Univerzitní 8, 306 14 Plzeò, Czech Republic
ibrahim@kmm.zcu.cz
Prejem rokopisa – received: 2015-06-18; sprejem za objavo – accepted for publication: 2015-08-25
doi:10.17222/mit.2015.122
In the automotive industry, there is an ever growing demand for steel components with enhanced mechanical properties.
Typically, this involves steels with high strength combined with adequate ductility. With improved properties, the structural
components of these steels can be less bulky, requiring less material, energy and lower costs.
Processing a material to obtain high strength and high ductility at the same time used to be rather difficult. Today, it can be
accomplished by incorporating retained austenite in a martensitic matrix. A new heat-treatment method for closed-die forgings,
termed Q&P process (quenching and partitioning), leads to a combination of martensite and retained austenite with strengths
above 2000 MPa and an elongation of 10-15 %.
Keywords: Q&P process, retained austenite, AHSS, closed-die forgings
V avtomobilski industriji nara{~a zahteva po komponentah iz jekla z izbolj{animi mehanskimi lastnostmi. Ta zadeva tudi jekla z
visoko trdnostjo, v kombinaciji s primerno duktilnostjo. Zahvaljujo~ izbolj{anim lastnostim so te komponente iz jekel dimen-
zijsko manj{e, ker je zanje potrebnega manj materiala, manj energije in tudi stro{ki so manj{i.
Izdelava materiala, ki bi isto~asno imel visoko trdnost in dobro duktilnost je te`avna. Danes je to mogo~e dose~i in sicer z
vklju~itvijo zaostalega avstenita v martenzitno osnovo. Nova metoda toplotne obdelave odkovkov iz utopov je Q&P postopek ali
kaljenje in delitev, kar povzro~i kombinacijo martenzita in zaostalega avstenita, s trdnostjo nad 2000 MPa in raztezkom
10–15 %.
Klju~ne besede: Q&P postopek, zaostali avstenit, AHSS, odkovki iz utopov
1 INTRODUCTION
Closed-die steel forgings produced by hot forging are
made from preforms, which are converted into the de-
sired part shape using plastic deformation in impression
dies. Then, the workpiece microstructure must be altered
to obtain the desired mechanical properties. A typical
microstructure upon quenching consists of martensite. It
exhibits high strength but very poor ductility. This causes
problems in parts which may fail in service under their
operating load. Hence, parts that contain martensite are
normally tempered after quenching. Today, a handful of
modern metallurgical procedures are available for
achieving higher ductility. They include long-time
low-temperature austempering, intercritical processing of
TRIP steels, and the Q&P process. Long-time low-tem-
perature austempering can produce tensile strengths of
up to 1500 MPa and hardness levels of 600-670 HV10.1
Long-time low-temperature austempering is characte-
rised by holding times of several tens of hours at low
temperatures. The resulting microstructure consists of
very fine bainitic ferrite (Figure 1).2 Due to the long
processing times, this method has failed to find industrial
use. The concept of TRIP steels relies on a mixture of
bainite, ferrite and retained austenite formed by inter-
critical annealing and isothermal holding at the bainitic
transformation temperature during a controlled cooling
process3 (Figure 2). The third method is the Quenching
and Partitioning (Q&P) process, which allows strengths
of more than 2000 MPa to be obtained, together with an
elongation of about 10 %. An important factor in this
process is the stabilisation of austenite in the martensitic
matrix (Figure 3). One of the ways of obtaining a mar-
tensitic structure with the desired fraction of retained
austenite is a special heat treatment procedure described
Materiali in tehnologije / Materials and technology 50 (2016) 4, 623–626 623
UDK 669.017:669.14:621.73.043 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(4)623(2016)
Figure 1: Microstructure produced by long-time austempering1
Slika 1: Mikrostruktura nastala pri dolgotrajnem austempranju1
below. It is characterised by rapid cooling from the
austenite region to a temperature between the Ms and Mf.
During such cooling, martensite forms, whereas a por-
tion of austenite remains untransformed. During subse-
quent isothermal holding, the retained austenite becomes
stabilised thanks to carbon which migrates from the
super-saturated martensite to austenite. According to
current knowledge, this austenite exists primarily in the
form of thin films between martensite laths or plates.4–6
The present paper focuses predominantly on the Q&P
process, a novel metallurgical procedure for heat treating
forged parts. So far, this process has led to the best me-
chanical properties.
2 EXPERIMENTAL PROCEDURE
A major issue that affects the use of the Q&P process
in practice is the necessity to interrupt the quenching
between the Ms and Mf temperatures. With this challenge
in mind, four new experimental steels have been pro-
posed. Their particular compositions were designed to
reduce the Ms and Mf temperatures (Table 1). In all of
these experimental steels, the Ms and Mf temperatures
were depressed through the addition of manganese. To
increase the material’s strength, silicon and chromium
have been added as well. Silicon was chosen in order to
prevent carbide formation and thus to provide adequate
super-saturation of martensite with carbon. Molybdenum
was employed to both reduce the Ms and Mf temperatures
and shift the start of ferritic and pearlitic transformations
towards lower cooling rates. Nickel was added in small
amounts to stabilise austenite during cooling, to enhance
hardenability and to provide solid solution strengthening.
The carbon content was the same in all steels: between
0.42 % and 0.43 %.
In the AHSS-1 steel, the manganese level was 2.5 %
and the silicon level was 2 %. The JMatPro software was
used for calculating the approximate transformation tem-
peratures. The calculated Ms and Mf temperatures in this
steel were 218 °C and 88 °C, respectively. In order to
map the effect of molybdenum on microstructural evolu-
tion and the shift in transformation temperatures in the
AHSS-2 steel, its content was increased to 0.16 %. This
increase in molybdenum content, however, has not al-
tered the Ms and Mf temperatures in any substantial way.
The Ms temperature was 214 °C and the Mf was 83 °C. In
AHSS-3, the nickel level was set at 0.5 %, to achieve the
desired hardenability and to depress the martensitic
transformation temperatures. The Ms and Mf tempera-
tures were 209 °C and 78 °C, respectively. In the
AHSS-4 experimental steel, the molybdenum content
was increased along with the nickel content. The combi-
nation of these elements led to the lowest transformation
temperatures: Ms = 204 °C and Mf = 73 °C.
Table 1: Chemical compositions of experimental steels AHSS-1-4, in
weight percents (w/%)
Tabela 1: Kemijska sestava eksperimentalnih jekel AHSS-1-4, v
ute`nih odstotkih (w/%)
C Mn Si Cr Ni Mo
AHSS-1 0.43 2.5 2.03 1.33 0.07 0.03
AHSS-2 0.428 2.48 2.03 1.46 0.08 0.16
AHSS-3 0.419 2.45 2.09 1.34 0.56 0.04
AHSS-4 0.426 2.46 1.99 1.33 0.56 0.15
2.1 Application of Q&P process to heat treatment of
closed-die forgings
First, a heat treatment schedule for the forged parts
has been developed. The development involved testing of
various austenitising temperatures (TA), two different
quenching temperatures (QT), and various carbon parti-
tioning temperatures (PT), using small specimens in a
thermomechanical simulator MTS 810 (Material Test
System). Table 2 lists the parameters of the physical
simulation, the resulting elongation values A5mm and the
fractions of retained austenite (RA) in the matrix.
K. IBRAHIM et al.: NEW CONCEPT FOR MANUFACTURING CLOSED DIE FORGINGS OF HIGH STRENGTH STEELS
624 Materiali in tehnologije / Materials and technology 50 (2016) 4, 623–626
Figure 3: Prior austenite grain identified in martensitic matrix
Slika 3: Prvotna meja avstenitnih zrn, odkritih v martenzitni osnovi
Figure 2: Typical microstructure of TRIP steel produced by intercri-
tical annealing
Slika 2: Zna~ilna struktura TRIP jekla po interkriti~nem `arjenju
Based on the results of this modelling, two experi-
mental steels were chosen: AHSS-2 and AHSS-3. To
verify the process, two complex-shaped closed-die forg-
ings were made of these steels (Figure 4) by three
forging steps. Two different bar dimensions were used
for the forging process, with diameters of 45 mm, 31 mm
and length of 160 mm, 180 mm. The forging was done at
an automotive industry forging factory. These forgings
were then heat-treated and their microstructures and me-
chanical properties evaluated using small flat specimens
for tensile test machined from the forged parts with
dimensions of 5 mm reduced section and a cross section
2 mm × 1.2 mm.
Table 2: Processing parameters of closed-die forgings in the physical
simulation
Tabela 2: Parametri predelave odkovkov pri fizikalni simulaciji
TA(°C)/
tA (s)
Cooling
rate
(°C/s)
QT (°C) PT (°C)/tPT (s)
A5mm
(%)
RA
(%)
850/300 1 150 200/600 15 18
850/100 1 150 200/600 10 10
850/100 1 100 150/900 15 14
850/100 16 125 175/600 6 9
850/100 16 150 200/600 8 11
850/100 16 100 150/600 4 8
3 RESULTS AND DISCUSSION
The best results of the physical simulation were
obtained with the cooling rate of 1 °C/s. The ultimate
strength was in the range of 2000–2400 MPa; the elon-
gation was 15 % which corresponds to the higher volume
fraction of retained austenite in the martensitic matrix,
up to 18 % by volume.
Based on these findings, a low cooling rate has been
recommended for processing the forgings. Heating to the
austenite region was carried out in a furnace; the tem-
perature was approx. 850 °C. Subsequent quenching in
hot oil finished at 150 °C. Austenite was stabilised at
200 °C, thanks to the carbon which migrates from the
super-saturated martensite to austenite. After the forg-
ings heat treatment, the microstructure was examined
and mechanical tests were carried out. Electron micro-
scopy revealed the presence of a martensitic microstruc-
ture with a small amount of bainite (Figures 5 and 6).
Quenching with hot oil led to an ultimate strength of up
to 2300 MPa and an elongation of approximately 12 %.
The retained austenite fraction in the martensitic matrix
was 15 %.
K. IBRAHIM et al.: NEW CONCEPT FOR MANUFACTURING CLOSED DIE FORGINGS OF HIGH STRENGTH STEELS
Materiali in tehnologije / Materials and technology 50 (2016) 4, 623–626 625
Figure 6: Scanning electron micrograph of a martensitic structure
with a small proportion of bainite. The processing sequence involved
quenching in an oil bath at a temperature of 150 °C and austenite
stabilisation in a furnace at 200 °C.
Slika 6: SEM-posnetek martenzitne strukture z majhnim dele`em
bainita. Sekvenca obdelave je vklju~evala kaljenje v oljni kopeli pri
temperaturi 150 °C in stabilizacijo avstenita v pe~i na 200 °C.
Figure 4: Demonstration products: AHSS steels forgings heat-treated
using the Q&P process
Slika 4: Demonstracijski proizvodi: odkovki iz AHSS jekla, toplotno
obdelani z uporabo Q&P postopka
Figure 5: Light micrograph of martensitic structure with a small
proportion of bainite. The processing sequence involved quenching in
an oil bath at a temperature of 150 °C and austenite stabilisation in a
furnace at 200 °C
Slika 5: Svetlobni posnetek martenzitne strukture z majhnim dele`em
bainita. Sekvenca obdelave je vklju~evala kaljenje v oljni kopeli pri
temperaturi 150 °C in stabilizacijo avstenita v pe~i na 200 °C
4 CONCLUSION
Heat treatment of forgings made of newly-developed
experimental steels led to martensitic microstructures
with a fraction of stabilised retained austenite. The heat-
treating sequence was based on the quenching and
partitioning process (Q&P).
First, physical simulation was carried out on speci-
mens. The findings were then translated into the process-
ing of real-life demonstration forgings. The physical
simulation led to strengths of up to 2300 MPa and A5mm
elongation levels of approximately 12 %. As the amount,
morphology and distribution of retained austenite have
decisive impact on the resulting mechanical properties,
an X-ray diffraction examination has been conducted.
The retained austenite fraction was up to 15 %.
After the findings and parameters were translated
into a real-life process, a simple heat treatment of actual
forgings led to equivalent values of mechanical proper-
ties. The processing was carried out with hot quenching
in oil to 150 °C and a furnace at a partitioning tempe-
rature of 200 °C.
The results of both physical simulation and real-life
processing open new opportunities for closed-die forg-
ing. The key tasks for the future involve an optimisation
of the cooling rate, a correct choice of cooling media
and, where relevant, their unconventional application.
High ultimate and fatigue strengths will permit the de-
signs of forged parts to be altered towards thinner walls
and more complex shapes. As a result, the utilization of
material will improve and the weight of forgings will
decrease, while the desired specifications of the product
will still be met.
Acknowledgments
This paper includes results created within the projects
SGS-2014-022 New Martensitic Structures - Process
Parameters and Properties and CZ.1.05/2.1.00/03.0093
Regional Technological Institute. The projects are
subsidised by the Ministry of Education, Youth and
Sports from resources of the state budget of the Czech
Republic and European Regional Development Fund.
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K. IBRAHIM et al.: NEW CONCEPT FOR MANUFACTURING CLOSED DIE FORGINGS OF HIGH STRENGTH STEELS
626 Materiali in tehnologije / Materials and technology 50 (2016) 4, 623–626