Solid State Phenomena Vol. 118 (2006) pp. 91-96
online at http://www.scientific.net
© (2006) Trans Tech Publications, Switzerland
Vacuum Carburizing of AISI S7 Tool Steel
Liu-Ho Chiu1, a, Yu-Jen Chen1, b, Chang-Hui Wu1, c and Heng Chang2, d
Department of Materials Engineering, Tatung University, 40 Chunshan North Road, 3rd Sec., Taipei
10451, Taiwan
1
2
Department of Mechanical Engineering, Chinese Culture University, 55 Hwa Kang Road, Yang
Ming Sang, Taipei 11192, Taiwan
a
lhchiu@ttu.edu.tw, bg9107015@mail.ttu.edu.tw, cwuch1234@ms52.hinet.net,
d
hchang@faculty.pccu.edu.tw
Keywords: shock-resistant tool steel, vacuum carburizing, tempering, fracture toughness
Abstract. The effects of vacuum carburizing under an acetylene atmosphere at 950k and 1000k,
followed by gas quenching and tempering at various temperatures on the properties of AISI S7
shock-resistant tool steel were studied. As carburized specimens undergo low temperature
tempering, the surface hardness of the quenched specimens carburized at 1000k is lower than those
of the specimens carburized at 950k, due to the large amount of retained austenite in specimens
carburized at 1000k. Under high temperature tempering, specimens carburized at 1000k have
higher surface hardness than specimens carburized at 950k. As specimens are tempered in the
range between 450k to 550k, the surface hardness of carburized specimens show a modest
increase due to the secondary hardening effects. According to the fracture toughness data, the
toughness of carburized specimens peaked at tempering at 600k.
Introduction
Generally, shock-resistant tool steels are used where repetitive impact stresses are encountered,
such as rivet sets, punches, driver bits, and other applications requiring high toughness, high surface
hardness and resistance to shock loads. S7 tool steel has good fracture toughness performance but
with low-to-medium wear resistance [1,2]. To improve the surface hardness of tempered S7 steel,
the vacuum carburizing surface hardening method is adopted to obtain high surface carbon content
and hardness.
An increased interest in the vacuum carburizing concept has developed in the last few years as a
result of the progress in designing modern furnaces for thermo-chemical treatment under low
pressure and better physico-chemistry knowledge of these processes [3,4,5,6]. In 2003, Chen et al.
[5] conducted a forced-convection gas flow vacuum carburizing experiment that using acetylene,
and reported that a component with a deep hole could be carburized with a high uniformity
regardless of the aspect ratio.
The purpose of this work is to investigate the hardness variation and fracture toughness of S7
steel specimens vacuum carburized using acetylene (C2H2) at 950k and 1000k, followed by gas
quenching with nitrogen and tempering at various temperatures.
Experimental Procedure
An AISI S7 shock-resistant tool steel bar, produced by Gloria Material Technology Corporation
(GMTC), was received. A glow discharge optical emission spectrometer (GDOES) was used to
analyze the chemical composition of the bar. The result is listed in Table 1. Round steel bars were
machined into 25×60×63 mm blocks to produce compact tension (CT) specimens 25 mm thick
following the ASTM E399-90 specification. A two-chamber vacuum furnace was employed for
studying the vacuum carburizing. The CT specimens were preheated at 650~700k for 30 minutes,
carburized at 950k and 1000k under a low pressure acetylene atmosphere for 0.5, 1 and 2 hours
respectively, followed by a rapid quench in nitrogen gas. A subgroup of the carburized and
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92
Heat Treatment of Materials
quenched specimens were treated in subzero conditions using liquid nitrogen before tempering. The
carburized and quenched specimens were tempered at 200, 300, 400, 450, 500, 550 and 600k for 2
hours. The sequences of the furnace temperature and corresponding pressure of vacuum carburizing
process are shown in reference [3].
/
Table 1. The chemical composition of AISI S7 [wt.%]
C
Si
Mn
S7 specimen 0.472 0.237 0.716
P
S
Cr
Ni
Mo
V
0.017
0.003
3.012
0.174
1.515
0.290
Metallographic samples were taken from the CT specimens, which were ground, polished and
etched in the 3% Nital solution to examine their microstructures. A Jeol 2000CX II microscope was
used for the TEM investigation.
Three hardness measurements were taken from each specimen using the Rockwell C scale (HRC)
using a Matsuzawa Seiki MARK-M2 Rockwell hardness tester to examine the heat treatment on the
specimens. Microhardness profiles of the carburized specimens were conducted in a Future-Tech
FM-7 microhardness tester to obtain the Vickers microhardness values. The load used for testing
was 300 g.
An MTS 810.13 testing machine was used for the fracture toughness test. Based on the ASTM E
399-90, the plain strain fracture toughness test was conducted. Deformation mode I was used in the
study to calculate the final fracture toughness.
Results and Discussion
Microstructure (OM). Figure 1 shows the microstructures of S7 tool steel specimens carburized
at 1000, quenched with nitrogen gas and tempered at 600k. In the as-quenched specimen, the
network formation in the surface area was clearly different from the core area as shown in Figure
1(a). After tempering, the tempered martensite was revealed as in the needle structure inside the
cemetite network formation.
(a)
(b)
Fig. 1 Microstructures of the S7 tool steel specimens carburized at 1000k for 1 hour (a) quenched,
and (b) quenched and tempered at 600k.
Hardness (HRC). Figure 2 shows the surface hardness of the S7 tool steel carburized at 1000k
as a function of the tempering temperature with (a) nonsubzero and (b) subzero treatments. The
time duration of carburizing at 1000k was 0.5, 1, and 2 hours respectively. As carburizing in high
austenitizing temperature, alloying elements are easily dissolved in the solid solution. Accordingly,
the Ms temperature drops and a large amount of austenite is retained in the case as the specimen
quenched to room temperature. As a result, the surface hardness of the specimens carburized at
1000k and gas quenched, drop off as shown in Figure 2(a). As can also be seen in Figure 2(a), the
hardness of the specimens carburized, quenched and tempered at 200k were lower than those
tempered at higher temperatures. When quenched specimens were tempered at temperatures above
Solid State Phenomena Vol. 118
93
300k, the retained austenite was gradually transformed to the tempered martensite and the hardness
of the tempered specimens was increased. The secondary hardening phenomenon was observed as
the specimens were tempered in the range between 450 and 550k. Thelning [7] studied the A2 tool
steel and showed that tempering process, in a combination of retained austenite transformation and
secondary hardening, can raise the hardness of the as-quenched steel with large amounts of retained
austenite.
70
70
1000 carburized for 0.5 h (nonsubzero)
1000 carburized for 1 h (nonsubzero)
1000 carburized for 2 h (nonsubzero)
60
65
Hardness (HRC)
Hardness (HRC)
65
55
50
60
55
50
1000 carburized for 0.5 h (subzero)
1000 carburized for 1 h (subzero)
1000 carburized for 2 h (subzero)
45
45
40
40
200
200
300
400
500
Tempering Temperature ( )
(a)
600
300
400
500
600
Tempering Temperature ( )
(b)
Fig. 2 The surface hardness of the S7 tool steel carburized at 1000℃ as a function of the tempering
temperature with (a) nonsubzero and (b) subzero treatment.
Figure 2(b) shows the surface hardness of the S7 tool steel carburized at 1000k as a function of
the tempering temperature with subzero treatment. The carburized specimens were gas quenched,
subzero treated in liquid nitrogen and tempered at various temperatures. The subzero cooling had a
great effect in allowing a large volume fraction of the retained austenite to transform into martensite
under subzero temperature. Therefore, the hardness of the specimens carburized, quenched and
tempered at 200k was higher than those tempered at higher temperatures. When quenched
specimens were tempered at temperatures above 300k, the surface hardness of the tempered
specimens decreased from 62 HRC to 52 HRC as the tempering temperature increased. The
hardness also exhibited a modest increase when specimens were tempered in the range between
450k to 550k. This results from the high carbon content and carbide forming elements exiting in
the carburized case.
To recognize the influence of carburizing temperature together with subzero treatment, the
surface hardness of the specimens carburized at 950 and 1000k under subzero treatment was also
studied in this work. Under low temperature tempering, the surface hardness of the specimens
carburized at 950k was higher than that of specimens carburized at 1000k, until the tempering
temperature reached 500k. Generally, subzero treated specimens produced higher hardness values
than nonsubzero treated specimens.
Microhardness (HV0.3kg). Figure 3 shows the microhardness profiles of S7 tool steel specimens
at given conditions and tempered at 200k and 600k. As can be seen in Figure 3(a), the case
hardness of the specimen carburized at 950k for 1 hour was about 850 HV and the core hardness
was about 690 HV. The case hardness of specimens carburized at 950k for 1 hour with subzero
treatment was about 910 HV and the core hardness was about 690 HV. The case hardness of
specimens carburized at 1000k for 1 hour was about 740 HV and dropped rapidly to about 400 HV
at a depth of 0.2 mm. Again, the hardness increases from 400 HV to 700 HV at the depths of 0.2
and 0.7 mm, and the core hardness was about 690 HV. The case hardness of the specimen applied
subzero treatment was about 850 HV and the core hardness was about 740 HV.
Heat Treatment of Materials
1300
950 carburized for 1 h (nonsubzero)
950 carburized for 1 h (subzero)
1000 carburized for 1 h (nonsubzero)
1000 carburized for 1 h (subzero)
1200
Hardness (HV0.3kg)
1100
1000
900
800
700
600
1100
950 carburized for 1 h (nonsubzero)
950 carburized for 1 h (subzero)
1000 carburized for 1 h (nonsubzero)
1000 carburized for 1 h (subzero)
1000
900
Hardness (HV0.3kg)
94
800
700
600
500
500
400
400
300
0.0
0.2
0.4
0.6
0.8
Depth (mm)
1.0
1.2
300
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Depth (mm)
(a)
(b)
Fig. 3 Microhardness profiles of the carburized S7 specimens at given conditions and tempered at (a)
200k and (b) 600k.
Figure 3(b) shows the microhardness profiles of the S7 tool steel specimens carburized at given
conditions and tempered at 600k. The case hardness of the specimen carburized at 950k for 1 hour
was about 690 HV and the core hardness was about 510 HV. The case hardness of the specimen
carburized at 950k with subzero treatment was about 600 HV and the core hardness was about 500
HV. The case hardness of the specimen carburized at 1000k for 1 hour was about 620 HV and
increased to about 740 HV at a depth of 0.2 mm. The core hardness was about 550 HV. The case
hardness of the specimen carburized at 1000kwith subzero treatment was about 750 HV and the
core hardness was approximately 550 HV. These data indicate that the carburized S7 specimens
tempered at 600k possessed high surface hardness values. Those specimens tempered at 500 and
550k also exhibited high microhardness in the case.
(a)
(b)
(c)
Fig. 4 (a) TEM bright field image of S7 specimen carburized at 1000 then quenched with
nitrogen gas, (b) the twin martensite structure, (c) the diffraction pattern of twin martensite.
Microscopic Structure. Figure 4 shows the TEM microstructure of specimen carburized at
1000k for 1 hour, followed by gas quenching. The region sampled is taken from a depth of 0.2 mm
beneath the specimen surface. In Figure 4(a), the twin martensitic structure and the retained
austenite within the matrix are clearly observed. The twin martensitic structure and its diffraction
pattern are shown in Figures 4(b) and 4(c) respectively. The large amount of the retained austenite
exists in the surface region supporting the low surface hardness of the carburized specimens.
Figure 5 shows the TEM microstructures of the specimens carburized at 1000k, followed by gas
quenching and tempering at 300 and 600k, respectively. In Figure 5(a), the matrix consists of
martensite entirely, and the retained austenite is reduced distinctly. Figure 5(b) shows the TEM
microstructure of the carburized case in which carbide precipitation is obviously observed. In some
Solid State Phenomena Vol. 118
95
regions, the secondary carbides appear as regularly distributed dark spots of 3-10 nanometers in
diameter. This result is in agreement with Pippel's [8] observation of carbide precipitation in the
hardened and tempered high-speed steel specimens. During tempering, alloying elements,
especially Cr, Mo and V, combined with the high carbon content supplied by the carburizing to form
a number of carbides of different sizes and compositions, which strongly influenced the toughness
and strength of the steel. Therefore, the microstructure of Figure 5(b) confirms that the carburized
S7 specimens exhibit high surface hardness, even when the specimens were tempered at the high
temperature of 600k.
(a)
(b)
Fig. 5 TEM case microstructure of S7 specimen carburized at 1000 then gas quenched and
tempered at (a) 300, (b) 600.
/
Fracture Toughness (KIC). Figure 6 shows the fracture toughness of S7 tool steel specimen
carburized at 950k and tempered at 300k and 600k as a function of the carburizing time. For the
S7 specimen austenitized at 950k, gas quenched and tempered at 300k, the KIC value is measured
at 55 MPa·m1/2. While the carburizing treatment is applied to the specimens for 0.5, 1 and 2 hours
respectively, the KIC values drop down to 47.2±0.8, 45.5±0.8 and 50.8±1.5 MPa·m1/2. As the
specimen is tempered at 600k, the KIC value is 61±1.9 MPa·m1/2. When the specimens are
carburized for 0.5, 1 and 2 hours respectively, the KIC values increase to 80.2, 72.4 and 73.3
MPa·m1/2. Figure 6 shows a trend which is similar to that reported by Lee et al. [9] for specimens
tempered at 600k. The KIC values clearly increase as a function of the carburizing time. Because
the case structure is primarily high carbon tempered martensite dispersed some carbides, it can be
postulated that the static fracture toughness of the high carbon tempered martensitic structure in the
case is tougher than that in the core.
90
90
950 carburized and tempered 300
950 carburized and tempered 600
950 carburzed for 1 h
950 carburzed for 2 h
80
1/2
KIC (MPa m )
1/2
KIC (MPa m )
80
70
60
50
70
60
50
40
40
0
1
2
Carburizing Time (h)
Fig. 6 The fracture toughness of S7 specimens
at given conditions as a function of the
carburizing time.
200
300
400
500
600
Tempering Temperature ( )
Fig. 7 The fracture toughness of S7 specimen
carburized at 950 and tempered
at different temperatures.
Figure 7 shows the fracture toughness of S7 tool steel specimens carburized at 950k for 1 and 2
96
Heat Treatment of Materials
hours as a function of the tempering temperature. For the specimen carburized at 950k for 1 hour
and quenched by gas, the fracture toughness increases along with the increasing tempering
temperatures. However, the fracture toughness of the specimen tempered at 500k shows a slight
drop. Furthermore, the KIC value increases rapidly from 45.5±1.1 MPa·m1/2 at 500k to 72.4
MPa·m1/2 at 600k. For the specimen carburized at 950k for 2 hours and quenched by gas, a similar
trend is obtained.
Conclusions
From the TEM image, a large amount of retained austenite is observed in the case of the
quenched specimen carburized at 1000k. Followed by low temperature tempering, the surface
hardness of the quenched specimens carburized at 1000k was lower than those of the specimens
carburized at 950k. As the specimen was tempered at temperatures above 300k, the retained
austenite in the case was distinctly reduced. When the carburized specimens were tempered in
between the range 450k to 550k, the secondary hardening phenomenon occurred. Thus, the
surface hardness of carburized specimens tempered under such conditions show a modest increase.
From the KIC data, the fracture toughness peaks as the carburized specimen treated at a high
tempering temperature of 600k. Specimen, carburized at 950k for 0.5 hour followed by quenching
in gas and tempering at 600k, has the highest fracture toughness value of 80.2 MPa∙m1/2.
Acknowledgements
This research was financially supported by the National Science Council, Republic of China,
under grant NSC91-2622-E-036-004-CC3. That support gratefully acknowledged.
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