M. K. KULEKCI et al.: ANALYZING THE HEAT-TREATMENT EFFECT ON THE MECHANICAL PROPERTIES ...
337–341
ANALYZING THE HEAT-TREATMENT EFFECT ON THE
MECHANICAL PROPERTIES OF FREE-CUTTING STEELS
ANALIZA VPLIVA TOPLOTNE OBDELAVE NA MEHANSKE
LASTNOSTI AVTOMATNIH JEKEL
Mustafa Kemal Kulekci1,2, Ugur Esme1,2, Funda Kahraman1,2, Rahim Ozgun3,
Adnan Akkurt4
1Mersin University, Institute of Applied and Natural Sciences, Manufacturing Engineering Department, 33480 Tarsus, Mersin, Turkey
2Mersin University, Faculty of Tarsus Technology, 33480 Tarsus, Mersin, Turkey
3Mersin University, Faculty of Tarsus Technical Education, 33480 Tarsus, Mersin, Turkey
4Gazi University, Faculty of Technology, 06500 Ankara, Turkey
mkkulekci@yahoo.com
Prejem rokopisa – received: 2015-01-14; sprejem za objavo – accepted for publication: 2015-06-02
doi:10.17222/mit.2015.011
In this research, the heat-treatment effect on the mechanical properties of free-cutting steels was investigated. Free-cutting steels
(FCSs) are used where high degrees of machining are required as they increase the machining speed and reduce the tool wear.
The effect of heat treatment on mechanical properties was identified using tensile and fatigue tests, and microstructure images
taken with a scanning electron microscope (SEM). The studied material included commercially available AISI 12L14
cylindrical bars of free-cutting steel. FCS was heated to 900 °C and held at this temperature for different time spans. The
degradation of the mechanical properties of free-cutting steel due to the elevated temperature was assessed. At a microscopic
level, more mechanical damage was observed between the steel matrix and the second phase of the heat-affected specimens.
Keywords: fatigue, tensile strength, free-cutting steel, heat treatment
V prispevku je bil raziskan vpliv toplotne obdelave na mehanske lastnosti avtomatnih jekel. Avtomatna jekla (FCS) se
uporabljajo v primerih, ko so zahtevane velike stopnje obdelave, saj dopu{~ajo ve~je hitrosti rezanja ob manj{i obrabi orodja.
Vpliv toplotne obdelave na mehanske lastnosti je bil dolo~en z rezultati nateznega preizkusa in preizkusa utrujenosti. Z
vrsti~nim elektronskim mikroskopom (SEM) so bili narejeni posnetki mikrostrukture. Preiskovane so bile komercialno
dosegljive palice iz avtomatnega jekla AISI 12L14. FCS je bil segret na 900 °C in razli~no dolgo zadr`an na tej temperaturi.
Dolo~eno je bilo zmanj{anje mehanskih lastnosti avtomatnega jekla zaradi povi{ane temperature. V toplotno obdelanih vzorcih
je bilo z mikroskopom opa`enih ve~ mehanskih po{kodb.
Klju~ne besede: utrujenost, nateg, avtomatna jekla, toplotna obdelava
1 INTRODUCTION
A significant increase in mechanical-machining costs
has led to a reappraisal of the importance of steel ma-
chinability.1 In order to achieve higher automation and
cost competitiveness, a series of steels commonly known
as free-cutting steels (an S minimum of 0.10 %) are
being increasingly used. Special lead-containing steels
differ from the normal structural, quenched-and-
tempered and case-hardened steels because of the
presence of Pb (approximately 0.15–0.35 %) in order to
improve their machinability. These steels allow an
excellent chip removal and they are particularly suitable
for large productions.2,3 Lead has a very good lubricating
effect and, combined with the heating produced by the
tools, breaks the chips, thus allowing for a higher
productivity resulting in more advantageous production
runs. It also guarantees a lower tool wear.
Free-cutting steels are used where high degrees of
machining are required as they increase the machining
speed and reduce the tool wear. These steels contain
manganese, lead, sulphur and phosphorous, which im-
prove the machining performance. The additives reduce
the coefficient of friction between the tool and chip,
thereby reducing the cutting force, the temperature and
the built-up edge formation, allowing higher feeds and/or
speeds.4–6 Free-cutting steels are preferred in manu-
facturing mechanical components for their improved
machinability.6–8
AISI 12L14 is a Pb-added low-carbon resulphurised
free-cutting steel containing 0.3 % Pb and 0.3 % S. It is
used in large quantities in automotive applications such
as transmission oil hydraulic control valves and oil
hydraulic hose connectors, in printer shafts and other
parts of office automation equipment. Lead is insoluble
in free-cutting steel and during the cutting process lead
particles are sheared and smeared over the tool-chip
interface.9,10 Lead improves the machinability with little
effect on mechanical properties. Due to its low shear
strength, lead acts as a solid lubricant. Manganese and
sulphides assist the chip formation and reduce the fric-
tion and wear of a cutting tool. Free-cutting steels are
used where high degrees of machining are required as
they increase the machining speed and reduce the tool
wear.
Materiali in tehnologije / Materials and technology 50 (2016) 3, 337–341 337
UDK 621.78:67.017:691.714 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(3)337(2016)
In this study, the heat-treatment effect on the me-
chanical properties of FCS was investigated. FCS was
heated to 900 °C and held at this temperature for diffe-
rent time spans. Degradation of the mechanical proper-
ties of the free-cutting steel due to the elevated tempera-
ture was assessed. The effect of heat treatment on
mechanical properties was investigated using tensile and
fatigue test results, and microstructure images taken with
a scanning electron microscope (SEM).
2 EXPERIMENTAL PROCEDURE
The studied material included commercially available
AISI 12L14 cylindrical bars of free-cutting steel with a
diameter of 12 mm. The bars were machined into tensile
and fatigue specimens with dimensions as shown in Fig-
ure 1. After the machining, the specimens were grinded
with 4000 emery paper. Two samples were used for each
stress level and the results were averaged for the as-
received specimens and the specimens exposed to 900 °C
for (3, 6, 9, 12 and 15) h. The percent chemical com-
position (% of mass fractions) and mechanical properties
of the FCS are given in Tables 1 and 2, respectively.
Tensile-test specimens were machined according to EN
ISO 6892-1 standard. Fatigue tests were conducted on an
R. R. Moore type rotating-beam fatigue-testing machine
with a frequency of 25 Hz. Microstructural changes in
the FCS exposed to high temperature were investigated
using SEM images. Tensile tests were conducted to inve-
stigate the heat-treatment effect on the ultimate tensile
strength (Rm), the yield strength (Re), the ductility and the
toughness. FCS specimens were heated to 900 °C and
held at this temperature for different time spans. Degra-
dation of the mechanical properties of free-cutting steel
due to elevated temperature was investigated. More
mechanical damage was observed between the steel
matrix and the phase in the heat-affected specimens at a
microscopic level. The effect of heat treatment on the
mechanical properties was investigated using tensile and
fatigue tests, and microstructure images taken with SEM.
The Rm and Re values of the FCS decreased as the
toughness and ductility increased due to the effect of
elevated heat.
Table 2: Mechanical properties of free-cutting steel (AISI 12L14)
Tabela 2: Mehanske lastnosti avtomatnega jekla (AISI 12L14)
Yield
strength
(MPa)
Tensile
strength
(MPa)
Elongation
(%)
Brinell hardness (HB)
(10 mm steel ball and
500 kg load)
465 587 12 150
3 RESULTS AND DISCUSSION
Fatigue test results for the FCS specimens with
different dwell times and the as-received specimens are
given in Figure 2. From this figure, it is seen that the
fatigue strength of the FCS exposed to elevated tempe-
rature decreased. There was a limited effect of the time
span affecting the fatigue strength of the tested speci-
mens. Under the cyclic load of 300 MPa, the heat-
affected and as-received specimens fractured at 50,000
and 300,000 cycles, respectively. The fatigue strength of
the FCS reduced in the ratio of 83 % because of the
effect of high temperature. The regression lines of the
fatigue strength given in Figure 2 are expressed as:
y = –a ln(x) + b (1)
M. K. KULEKCI et al.: ANALYZING THE HEAT-TREATMENT EFFECT ON THE MECHANICAL PROPERTIES ...
338 Materiali in tehnologije / Materials and technology 50 (2016) 3, 337–341
Figure 1: Dimensions and images of test specimens: a) fatigue, b) tensile test
Slika 1: Mere in posnetki preizku{ancev: a) utrujanje, b) natezni preizkus
Table 1: Chemical composition of free-cutting steel (AISI 12L14) in mass fractions, (w/%)
Tabela 1: Kemijska sestava avtomatnega jekla (AISI 12L14) v masnih dele`ih, (w/%)
C Si Mn P S Cr Mo Ni Cu Pb Fe
0.074 0.005 1.203 0.045 0.294 0.04 0.02 0.08 0.12 0.30 balance
where y is the loaded stress range, x is the number of
cycles to failure, a and b are the fitting constants. The
constants of the equations of regression lines, the
R-square, which indicates how well data points fit a
statistical model (the coefficient of determination: R2)
and the residual variance (Rv), which is a measure of the
variation of the y values about the regression line (Rv =
1 – R2) of the fatigue tests are given in Table 3.
Table 3: R-square, residual variance and fitting constants of the tests
Tabela 3: R-kvadrat, preostale variance in ustrezne konstante preizku-
sov
Experimental
variables
Fitting constants of
regression lines R-square
(R2)
Residual
variance
(Rv = 1 – R2)a b
Base material –89.19 1433.9 0.9841 0.0159
Heat treated:
15h –83.61 1202.9 0.9824 0.0176
Heat treated:
12h –78.819 1176.1 0.9885 0.0115
Heat treated:
9h –79.957 1178.6 0.9805 0.0195
Heat treated:
6h –81.643 1188.4 0.9816 0.0184
Heat treated:
3h –71.404 1093.6 0.9367 0.0633
The reduction in the fatigue strength of the specimens
exposed to high temperature can be explained with
microstructural changes. The grain size of the base
material and the heat-affected specimens were measured
as 10 μm and 25 μm, respectively, as seen from Figure 3.
From this figure, it is seen that the grain size of the FCS
increased by 250 % with the heat effect and 3–15 h time
spans. The grain growth reduced the load-carrying
capacity of the specimens. At the elevated temperature of
900 °C and under a long dwell time, the precipitated
phase coalesced and grew in the microstructure as seen
in Figure 3. Kalpakjian states that the grain growth
reduces the grain boundaries per volume unit of a
grain.11,12 Smaller grains have a higher strength and a
higher contact surface area.
The increase in the size of the grains reduced the
fatigue strength of the FCS. The ultimate tensile strength
(Rm) and the yield strength (Re) of the heat-effected FCS
were reduced as seen in Figure 4. The toughness of the
heat-affected FCS specimens was calculated with the
following Equation (2):
Toughness
e m=
+
⋅
R R
e
2
(2)
where Re and Rm are the yield and tensile strengths,
respectively, and e is the engineering strain. The Re and
Rm values of the initial FCS specimens were 616 MPa
and 512 MPa, respectively. For the specimens heat-
treated at 900 °C for 3 h, these values were reduced to
441 MPa and 318 MPa. The reduction in Rm and Re was
28.4 and 37.8 %, respectively. The amounts of energy
per volume (toughness) that the FCS absorbed before
rupturing were 100.51 MPa and 143.67 MPa for the
specimens in the as-received state and heat treated at
900 °C for 15 h, respectively, as seen in Figure 4. The
toughness of the FCS increased in the ratio of 42.94 %
due to the effects of heat and time. This increase in the
toughness of FCS can be explained with the disposal of
the residual stress induced during manufacturing stages.
The ductility of the FCS specimens was determined
M. K. KULEKCI et al.: ANALYZING THE HEAT-TREATMENT EFFECT ON THE MECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 50 (2016) 3, 337–341 339
Figure 3: SEM images of grains: a) as received and b) heat treated at
900 °C for 15 h
Slika 3: SEM-posnetek zrn: a) stanje ob dobavi in b) `arjena 15 h na
900 °C
Figure 2: Effect of the heat exposure and time on the fatigue test
results for FCS
Slika 2: Vpliv ~asa ogrevanja na rezultate utrujenostnega preizkusa
FCS
with the elongation calculated with the following
Equation (3):
Elongation
f 0
0
=
+
⋅
l l
l
100 (3)
where l0 and lf are the gage lengths of the original and
fractured sample, respectively. The ductility of the FCS
changed with the effect of heat treatment, as shown in
Figure 5. The ductility was 16.78 % and 39.06 % for
the as-received specimens and the specimens heat-
treated at 900 °C for 15 h, respectively. SEM images of
the fatigue-fractured surfaces of the as-received and
heat-affected specimens are given in Figure 6. The heat
effect increased the voids between the matrix and the
second phase of the FCS under cyclic load during the
fatigue test, as seen in Figure 6. From this figure, it is
clearly seen that the voids between the matrix and the
M. K. KULEKCI et al.: ANALYZING THE HEAT-TREATMENT EFFECT ON THE MECHANICAL PROPERTIES ...
340 Materiali in tehnologije / Materials and technology 50 (2016) 3, 337–341
Figure 6: SEM images of fatigue-fractured surfaces of FCS: a) as received, b) heat treated at 900 °C for 15 h (x and y are voids)
Slika 6: SEM-posnetek povr{ine utrujenostnega loma FCS: a) stanje ob dobavi, b) `arjeno 15 h na temperaturi 900 °C (x in y so praznine)
Figure 5: Heat-treatment effect on the elongation of free-cutting steel
Slika 5: Vpliv `arjenja na raztezek avtomatnega jekla
Figure 4: Effect of heat treatment on ultimate tensile strength (Rm),
yield strength (Re) and toughness of FCS
Slika 4: Vpliv `arjenja na natezno trdnost (Rm), mejo plasti~nosti (Re)
in `ilavost FCS
Figure 7: Macrostructures (49X) and microstructures (1000X) of
fractured specimens: a) as received and b) heat treated at 900 °C for
15 h
Slika 7: Makrostruktura in mikrostruktura prelomljenega vzorca: a)
dobavljeno in b) `arjeno 15 ur na 900 °C
second phase are larger in the structure of the heat-
affected specimens (Figure 6b). The heat-treatment
effect made the material more ductile and, as a result,
the bonding between the matrix and the second phase
was weakened, while the fatigue and tensile strengths of
the FCS decreased as well.
Macro- and microstructure images of the fractured
specimens in the as-received and heat-affected states are
given in Figures 7a and 7b, respectively. The base
material exhibited brittle fracture, while the heat-treated
specimens exhibited ductile fracture. The fatigue values,
Rm and Re, of the base material were higher than those of
the heat-treated FCS, as seen in Figures 2 and 4. This
situation can be explained with the residual stress
included in the as-received specimens. The grain growth
due to the heat treatment of the FCS material also
reduced these values.
4 CONCLUSION
From the above results, the following conclusions
can be drawn:
• The heat effect made the material more ductile and
weakened the bonding between the matrix and the
second phase.
• The microstructure of the FCS changed at 900 °C.
The elevated temperature and a long dwell time
resulted in the grain growth and precipitation, which
caused a decrease in the fatigue and tensile strengths.
• The grain size of the FCS increased by up to 250 %
with the heat treatment at 900 °C and a dwell time of
15 h. The grain growth reduced the load-carrying
capacity of the specimens.
• The fatigue strength of the FCS was reduced in the
ratio of 83 % due to the effect of the high temperature
and long dwell time.
• The heat treatment at 900 °C, for a period of less than
3 h reduced Rm and Re in the ratio of 28.4 and 37.8 %,
respectively, when compared to the base material.
• The toughness of the FCS increased in the ratio of
42.94 % due to the effect of heat treatment and dwell
time.
• The heat effect led to increased ductility and larger
voids between the matrix and the second phase of the
FCS under a cyclic load. At a microscopic level,
more mechanical damage was observed between the
steel matrix and the second phase of the heat-affected
specimens.
• The effect of time on the fatigue strength of the tested
specimens is less significant.
Acknowledgement
This work was financially supported by the Scientific
Research Foundation of the Mersin University under
contract MAE-1-2010. This support is gratefully ackno-
wledged.
5 REFERENCES
1 M. Toshiyuki, T. Kunikazu, S. Tetsuo, Development of free cutting
steel without lead addition to replace AISI12L14, JFE Technical
Report, 15 (2010), 10–16
2 L. D. Chiffre, of metal cutting and cutting fluid action, International
Journal of Machine Tool Design and Research, 17 (1977) 4,
225–234, doi:10.1016/0020-7357(77)90016-6
3 A. J. Deardo, C. I. Garcia, U. S. Patent Nr. 6200395, 2001
4 P. L. B. Oxley, Mechanics of Machining: An Analytical Approach to
Assessing Machinability, John Wiley & Sons, New York 1989
5 P. S. Sreejith, B. K. A. Ngoi, Dry machining: Machining of the
future, Journal of Materials Processing Technology, 101 (2000) 1–3,
287–291, doi:10.1016/S0924-0136(00) 00445-3
6 H. Roelofs, A. M. G. Boeira, R. Margot, J. T. Gomes, M. Eglin,
Machinability of Inclusion Engineered Free Cutting Steel Under
Built Up Edge Conditions, 8th Int. Conf. on Advanced Manu-
facturing Systems and Technology, Udine 2008, 12–13
7 Z. Li, D. Wu, Effect of Free-cutting Additives on Machining Charac-
teristics of Austenitic Stainless Steels, Journal of Materials Science
& Technology, 26 (2010) 9, 839–844, doi:10.1016/S1005-0302(10)
60134-X
8 Y. Y. Wei, Z. Q. Liu, Q. L. An, M. Chen, Study on the Machinability
of Free-Cutting Steels 1214 and 12L15 with Coated Tool, Advanced
Materials Research, 426 (2012), 151–154, doi:10.4028/www.scienti-
fic.net/AMR.426.151
9 A. D. Foster, J. Lin, D. C. J. Farrugia, T. A. Dean, An Investigation
into Damage Nucleation and Growth for a Free-Cutting Steel at Hot
Rolling Conditions, The Journal of Strain Analysis for Engineering
Design, 42 (2007) 4, 227–235, doi:10.1243/03093247JSA230
10 J. Polak, M. Petrenec, J. Man, Cyclic plasticity, cyclic creep and
fatigue life of duplex stainless steel in cyclic loading with positive
mean stress, Metallic Materials, 49 (2011) 5, 347–354, doi:10.4149/
km_2011_5_347
11 R. Özgün, Investigation of the Effect of High Temperature and Boric
Acid Environment on the Mechanical Properties of Free Cutting
Steels, University of Mersin, 2014
12 S. Kalpakjian, S. Schimid, Manufacturing Engineering and Tech-
nology, 6th Ed., Pearson Hall, New York 2010
M. K. KULEKCI et al.: ANALYZING THE HEAT-TREATMENT EFFECT ON THE MECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 50 (2016) 3, 337–341 341