AISI 4140

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Materials and Design 31 (2010) 1570–1575
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Materials and Design
journal homepage: www.elsevier .com/locate /matdes
Short Communication
An investigation on the microstructure and mechanical properties
of direct-quenched and tempered AISI 4140 steel
A.H. Meysami, R. Ghasemzadeh *, S.H. Seyedein, M.R. Aboutalebi
Centre of Excellent for Advanced Material and Processing, Department of Material Science and Eng., Iran University of Science and Technology, P.O. 16844, Tehran, Iran
a r t i c l e i n f o a b s t r a c t
Article history:
Received 29 July 2009
Accepted 22 September 2009
Available online 26 September 2009
0261-3069/$ - see front matter � 2009 Elsevier Ltd. A
doi:10.1016/j.matdes.2009.09.040
* Corresponding author. Address: Department of
Eng., Iran University of Science and Technology, Narm
630 2265; fax: +98 21 7745 4057.
E-mail address: rgzadeh@iust.ac.ir (R. Ghasemzade
Direct quenching (DQ) process is an appropriate method in steels heat treatment field. This method
enhances production rate, reduces energy consumption and decreases environment contamination. In
this study hot-rolled AISI 4140 steel billets with different diameters (75, 80, 85, 100, 105 and 115 mm)
and 20 m length were quenched directly in a water tank. Also some samples with similar size and com-
position were provided by conventional reheating, quenching and tempering (RQ) heat treatment pro-
cess. The quenched samples were tempered at the temperature of 630 �C for 2 h. Mechanical
properties of heat treated samples including tensile strength, yield strength, elongation, hardness and
impact toughness were measured. Also, the microstructure and harden-ability of this steel were investi-
gated under various conditions and the results were compared to RQ heat treated products. The results
showed that direct quenching and tempering processes (DQ–T) is due to enhance of mechanical proper-
ties such as tensile strength and harden-ability of AISI 4140 and it is affected by various parameters such
as steel temperature before quenching, water temperature, quenching time and also billet size.
� 2009 Elsevier Ltd. All rights reserved.
1. Introduction
AISI 4140 steel is widely used as gears, bolts, couplings, spin-
dles, tool holders, sprockets, oil industry drill collars, tools joints,
etc. This kind of steel is categorized as quenched-tempered steels
which their dominant phase after heat treatment is tempered mar-
tensite [1–3].
Numerous methods have been applied to form martensitic
microstructure during quenching in 4140 steel. In this regard, the
main applied processes are Direct Quenching (DQ) and Reheating
and Quenching (RQ) [4–8]. In the case of RQ process, the procedure
is included ingot hot rolling, self cooling in ambient temperature,
reheating to austenitizing temperature, quenching in cooling
media and tempering. The stages of the DQ process are hot rolling,
direct quenching and tempering [6–8]. The most important differ-
ence between DQ and RQ processes is in austenitizing condition. In
the case of RQ rods, reaustenization temperature and time is suffi-
cient and static recrystallization occurs completely, therefore, a
homogeneous structure is created while for DQ process, the micro-
structure is not homogenous compare to that of RQ. The reason of
this phenomenon is that in the case of DQ procedure, different
recrystallizations such as dynamic (during hot deformation) and
meta-dynamic (between stands) are occurred during the process.
ll rights reserved.
Materials and Metallurgical
ak, Tehran, Iran. Tel.: +98 912
h).
However, other than the structural differences, applying DQ meth-
od results economical advantages such as lower production cost by
deleting conventional reheating and quenching (RQ) stages. Also,
this method improves useful characteristics such as strength, hard-
en-ability, and welding-ability [4]. Moreover, it allows addition of
micro-alloying elements which can be combined with controlled
hot rolling. Therefore, DQ is a valuable and useful procedure for
thermo-mechanical control process (TMCP) [5] in order to produce
high strength steel plates and rods with relatively high toughness.
Although some investigations have been done on DQ process
and its effective parameters for carbon steel plates in laboratory
scale [4–8] but relatively few researches can be found in industrial
one. In this research, accelerated cooling of AISI 4140 steel billets
has been studied in detail in industrial scale. Also, the mechanical
properties of the billets which were quenched immediately after
hot rolling have been compared to that of RQ billets with similar
size and composition.
2. Experimental procedures
2.1. Materials
The AISI 4140 steel which was produced by continuous casting
method in the Iran Alloy Steel Co. was used as raw material. The
liquid steel was refined by various methods to reduce the level of
impurities. Also the sulfur and phosphorous levels were lowered
to 50 and 150 ppm, respectively. Table 1 shows the chemical com-
position of the used steel.
http://dx.doi.org/10.1016/j.matdes.2009.09.040
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Table 1
Chemical composition of used steel (wt.%).
Composition
C Si Mn S P Cr Mo
0.43 0.19 0.90 0.032 0.01 0.98 0.18
A.H. Meysami et al. / Materials and Design 31 (2010) 1570–1575 1571
2.2. Thermo-mechanical process
For the reheating-quenching and tempering (RQ–T) process, the
ingots with dimensions of 450 � 450 mm2 were preheated at
1200 �C and then rolled to rod shape in various diameters. The
diameter of the rods and number of the rolling passes are tabulated
in Table 2. Subsequently, the rods were reaustenitized at 840 �C for
2 h and quenched in water tank at 60 �C. After quenching, the tem-
pering procedure was carried out at 630 �C for 2 h.
In the case of the Direct Quenching and Tempering (DQ–T) pro-
cess, the hot rolling stages were similar to that of Reheating,
Quenching and Tempering (RQ–T) process (Table 2). After hot roll-
ing, the rods were cut in 20 m length sections immediately and
quenched in water tank. Temperature of the rods before direct
quenching is very important, therefore the surface temperature
of the rods were measured by thermo-vision gauge before direct
quenching. Table 3 shows the value of this parameter prior to di-
rect quenching. Temperature of the tempering heat treatment for
direct quenched rods is similar to that of RQ–T rods. Fig. 1 shows
the schematic representation of the conventional RQ–T and DQ–T
processes.
2.3. Tests method
For each rod, tensile and Charpy impact samples were prepared
from tip, navel and end of rod. The mechanical properties including
tensile strength, Charpy impact test, reduction in cross-section,
and elongation were measured using the average of three experi-
mental results for each test. The tensile samples were provided
according to DIN 50125 standards and were experimented on the
basis of DIN EN10002, also Charpy impact samples were provided
based on DIN EN-10045 [1]. Metallography of austenite grains
structure has been done according to ASTM-E112 [9].
Table 2
The hot rolling passes for production of different rods.
Rod diameter (mm) Stand 1 Stand 2 Stand 3
70 17 9 1
85 and 80 15 9 1
100 and 105 19 5 1
115 17 5 1
Table 3
Rods’ surface temperature prior to direct quenching.
Rod diameter (mm) Surface temperature prior
to direct quenching (�C)
70 829
80 836
85 850
100 850
105 863
115 871
3. Results and discussion
3.1. Effect of rod size on mechanical properties of DQ–T and RQ–T
steels
The effect of diameter size on the microstructure and mechan-
ical properties of the samples processed by DQ–T and RQ–T proce-
dures are shown in Figs. 2–6. As it is known, by increasing the
diameter size of the rods the rate of heat transfer into cooling med-
ia decreases. By considering this matter and referring to Continu-
ous Cooling Transformation diagram (i.e. CCT diagram) of this
steel[2,3], a delay time arises for diameters with higher size for
austenite transformation in rod interior areas, therefore, other than
martensite and bainite transformation, ferrite-pearlite transforma-
tion occurs within the steel microstructure. For example, according
to Fig. 2, the optical metallographic images of the surface and in-
side of the rod with 115 mm diameter show that the surface struc-
ture is martensite while in interior structure, martensite, bainite,
ferrite and pearlite can be found. But in the rod with 70 mm diam-
eter, surface and inside structure have almost similar martensite
structure.
The Effect of heat transfer decrease rate versus diameter size in-
crease could be seen in other properties such as tensile strength,
impact energy, reduction in cross-section and elongation (Figs.
3–6). According to Fig. 3, by increasing the rod diameter the tensile
strength has been decreased from 1070 and 968 to 936 and
934 MPa for DQ–T and RQ–T rods, respectively. On the contrary,
the Charpy impact energy tests versus rod diameter size (Fig. 4)
shows an increasing manner by increasing the rods’ diameter.
The reason of this phenomenon is that the austenite to ferrite-
pearlite transformation in the interior section of the rods caused
softer structures, therefore the tensile strength has been decreased
in case of DQ–T process. Moreover, due to this phenomenon the
impact energy has been increased in RQ–T process. The effects of
rods’ diameter size on the reduction in cross-section and elonga-
tion are presented on Figs. 5 and 6, respectively. In both DQ–T
and RQ–T processes, the reduction in cross-section and elongation
have not been changed significantly by increasing rods’ diameter.
The reason of this phenomenon may due to existence of relatively
high volume of tempered martensite in the microstructure of the
samples having diameter with lower sizes respect to other ones.
According to Darwish et. al. results [10], in the case of AISI 4140,
tempered martensite steel has rather similar formability and
ductility relative to ferrite – pearlite and bainite phases. Also as
mentioned, quenching of rods with small diameter produces more
volume of martensite phase than the higher ones, therefore after
tempering, diameters with small size contain more tempered
martensite. However, in diameters with higher size because of
less heat transfer during quenching, austenite to bainite and also
austenite to ferrite-pearlite phase transformations occur more
and the volumes of the bainite and ferrite-pearlite phases are more
than the other ones. So it can be concluded that near similar
ductility of these phases resulted to almost constant reduction
in cross-section and elongation of samples having different
diameters.
3.2. Comparison between properties of DQ–T and RQ–T steels with
different rods’ diameter
As mentioned, the comparison of the tensile strength, impact
energy, reduction in cross-section and elongation versus rod diam-
eter size for the samples processed by DQ–T and RQ–T are pre-
sented in Figs. 3–6. According to Fig. 3, the tensile strength for
DQ–T steel rods are higher than that of RQ–T steel rods for about
3–12%. Therefore, it is concluded that DQ process is due to higher
Fig. 1. Schematic representation of heat treatment processes: (a) RQ–T and (b) DQ–T.
Fig. 2. Microstructure of the surface and inside of the rods with 115 and 70 mm diameters in DQ–T process (etched by Nytile solution): (a) rod 115 inside, (b) rod 115 surface,
(c) rod 70 inside and (d) rod 70 surface.
1572 A.H. Meysami et al. / Materials and Design 31 (2010) 1570–1575
harden-ability respect to RQ procedure for this type of steel. How-
ever, according to Figs. 4–6, it is resulted that by applying RQ–T
process higher impact energy, reduction in cross-section and elon-
gation could be obtained (The mechanical properties in both DQ–T
and RQ–T are in the standard range based on DIN EN 10083–1 [1]).
But it is noticeable that by increasing the rods’ diameter size, the
difference between tensile strength, impact energy, reduction in
cross-section and elongation values for DQ–T and RQ–T rods have
been decreased. For example for the samples having 115 mm
diameter size, the difference between the mentioned properties
was negligible. The reason of this phenomenon is decrease of heat
transfer rate in diameter with higher size. In other words, these
samples have been cooled in a similar manner for both DQ and
RQ processes due to relatively diameter with high size of them.
Therefore, nearly similar phases have formed within their micro-
structure. Another reason is related to almost same austenite
transformation conditions. In other words, for DQ samples with
higher size diameter due to low hot deformation and strain the
Fig. 3. Variations of tensile strength versus rod diameter size in DQ–T and RQ–T
processes.
Fig. 4. The effect of rod diameter on impact energy of the samples processed by
DQ–T and RQ–T.
Fig. 5. Variations of reduction in cross-section percent versus rod diameter size for
the samples processed by DQ–T and RQ–T.
Fig. 6. Variations of elongation percent versus rod diameter size for the samples
processed by DQ–T and RQ–T.
Fig. 7. Variations of AGS versus rod diameter size (based on ASTM number – in
magnification of 100) in DQ–T and RQ–T processes.
A.H. Meysami et al. / Materials and Design 31 (2010) 1570–1575 1573
austenite recrystallization is like RQ process and occurs statically
[11–13]. So in the case of diameters with larger size the properties
of both DQ–T and RQ–T processes are almost similar.
Fig. 7 shows austenite grain size (AGS) for both DQ–T and RQ–T
processes according to ASTM standard. In RQ–T process, reausteni-
zation temperature and time was sufficient (840 �C and 2 h) and
static recrystallization occurred completely, therefore, a homoge-
neous structure has been generated. In the case of DQ–T, at the
beginning of the process, rods preheated at 1200 �C for 2 h and
then go thermo-mechanical hot rolling process. Therefore, coarse
austenite structure has been generated. However, some investiga-
tions have shown that during hot rolling, the dynamic recrystalli-
zation (due to hot deformation) and the meta-dynamic one
(between stands) occur and the new structure is finer than the
unrecrystallized one [11–13]. Fig. 8 shows the optical austenite mi-
cro structure for DQ–T and RQ–T processes. Based on Figs. 7 and 8,
the AGS in the RQ–T is finer than that of DQ–T process. This is a
reason for higher harden-ability in DQ–T than RQ–T processed
steels, because coarse austenite grains increase the harden-ability
[2].
Figs. 9 and 10 show the variations of the hardness at cross sec-
tion of as quenched rods for the specimens having 80 and 115 mm
diameters processed by RQ and DQ procedures. In the both proce-
dures, hardness has been increased from the interior to surface of
the rods, but the hardness value and its difference between 80 and
115 mm diameter for DQ rods were more than RQ rods. Referring
to Section 2.2, the reaustenization of RQ process was prolonged
for 2 h for all rods and this time was adequate for generation of a
homogeneous structure, but phase transformation in the interior
section had a delay time and based on the CCT diagram of this steel
[2,3], a softer structure has been produced. In DQ process, the rods
Fig. 10. Hardness variations of the cross section for the as-quench DQ rods.
Fig. 8. Optical austenite structure for (a) RQ–T and (b) DQ–T, according to ASTM-E112 [9].
Fig. 9. Hardness variations of the cross section for the as-quench RQ rods.
1574 A.H. Meysami et al. / Materials and Design 31 (2010) 1570–1575
after TMPC had not a uniform temperature and there was a tem-
perature gradient from the surface to center of the rods. Therefore
hardness difference of the surface and center of DQ rods were more
than RQ ones. It is obvious that heat transfer for the rod with
80 mm diameter was faster than the one having 115 mm diameter
size and as a result its hardness is higher. After temperingprocess
at 630 �C for 2 h it was found that hardness difference from surface
to inside has been decreased to lower than 30 HB for both DQ and
RQ rods. Also, hardness numbers were almost similar and about
320 to 350 HB. The reason of this phenomenon is transformation
of martensite into tempered martensite during tempering process.
The tempered martensite has less hardness than martensite.
4. Conclusions
1. The DQ process increases the harden-ability of steels.
2. Comparing to RQ process, by applying DQ procedure steels with
higher tensile strength and yield stress can be produced.
3. RQ steels have higher impact energy, elongation and reduction
in cross-section relative to DQ process in the environment
temperature.
4. Increase of thermo-mechanical process enlarges harden-ability
and strength of DQ processed rods.
5. The hardness values and their different of the DQ rods are
higher than that of RQ ones.
6. Tempering the RQ and DQ processed samples leads to similar
hardness values.
Acknowledgements
The authors wish to thank Iran University of Science and Tech-
nology (IUST) and Iran alloy steel Co. for their financial support.
References
[1] DIN handbook 404, Iron and steel, quality standards’ 4. 3rd ed. Germany:
Deutsches Institut Fur Normung E.V.; 2002.
[2] ASM Metals Handbook. Heat Treating, Metals park, vol. 4. Ohio: American
Society for Metals; 1991.
[3] Chandler H. Heat treating guides. Metals Park, Ohio: American Society for
Metals; 1996.
[4] Chang W. Microstructure and mechanical properties of 780 MPa high strength
steels produced by direct-quenching and tempering process. J Mater Sci
2002;37:1973–9.
[5] Ouchi C. Development of steel plates by intensive use of TMCP and direct
quenching processes. ISIJ 2001;41:542–53.
[6] Weiss RK. Strength difference between direct quenched and reheated and
quenched plate steels. In: Proceeding of physical metallurgy of direct-
quenched steels, TMS, vol. 1; 1992. p. 107–38.
[7] Katsumata M., et al. Relationship between microstructure and mechanical
properties in direct quenched low carbon HSLA steels used in as hot forged
conditions. In: Proceeding of physical metallurgy of direct-quenched steels,
TMS, vol. 1; 1992. p. 247–63.
A.H. Meysami et al. / Materials and Design 31 (2010) 1570–1575 1575
[8] Shikanai N, Suga M. Influence of direct quenching condition and alloying
elements on mechanical properties of HSLA steel plates. In: Proceeding of
physical metallurgy of direct-quenched steels, TMS, vol. 1; 1992. p. 93–105.
[9] Annual book of ASTM standards. ASTM E112 standard test methods for
determining average grain size, vol. 03.01. ASTM International, PA: American
Society for Testing and Materials; 2005.
[10] Darwish FA, Pereira LC, Gatts C, Graca ML. On the tempered martensite
embrittlement in AISI 4140 low alloy steel. Mater Sci Eng A 1991;132:5–9.
[11] Lee HW, Kwon HC, Ima YT, Hodgson PD. Numerical investigation of austenite
grain size distribution in square-diamond pass hot bar rolling. J Mater Process
Technol 2007;191:114–8.
[12] Phaniraj MP, Behera BB, Lahiri AK. Thermo-mechanical modeling of two phase
rolling and microstructure evolution in the hot strip mill part I: Microstructure
evolution. J Mater Process Technol 2006;178:388–94.
[13] Serajzadeh S. A study on kinetics of static and metadynamic recrystallization
during hot rolling. Mater Sci Eng A 2007;448:146–53.
	An investigation on the microstructure and mechanical properties of direct-quenched and tempered AISI 4140 steel
	Introduction
	Experimental procedures
	Materials
	Thermo-mechanical process
	Tests method
	Results and discussion
	Effect of rod size on mechanical properties of DQ–T and RQ–T steels
	Comparison between properties of DQ–T and RQ–T steels with different rods’ diameter
	Conclusions
	Acknowledgements
	References