Download Mechanical and structural properties of AISI 1015 carbon steel

Mechanical and structural properties of AISI 1015 carbon steel nitrided
after warm rolling
C. Medrea1, G. Negrea2
1
Department of Physics, Chemistry and Materials Technology, Technological Education Institute of
Pireaus,250 , Thivon &P. Ralli Street, 12244 Aegaleo, Greece
URL: www .teipir .gr
e-mail: medrea@ internet.gr;
2
Faculty of Materials Science and Engineering, Technical University of Cluj-Napoca, Muncii Avenue 103105, 3400 Cluj-Napoca
e-mail:[email protected]
URL: www.utcluj.ro
ABSTRACT: Nitriding is usually applied to alloyed steels with the scope of increasing their surface hardness
and wear resistance. Warm working has been found to produce a fine-grained microstructure, which makes
possible further treatment of low carbon steels. In combination with a low temperature thermochemical
treatment, such as nitriding, warm working can be used to produce machine parts with a though core and with
a hard, wear resistant surface layer. This paper presents a study of mechanical and structural properties of
AISI 1015 carbon steel nitrided after warm rolling. The rolling was performed in the following conditions:
temperature 670 – 550 oC, rolling speed 1.39 s-1 and deformation ratio 36.4%. After rolling, the samples were
reheated to 550 oC for a duration varying from a few minutes to 10 hours. The microstructural changes were
assessed by light microscopy and quantitative microscopy analysis. Warm rolled samples were ion nitrided at
510-520 oC in dissociated ammonia. The microstructure was analyzed by scanning electron microscopy and
the mechanical properties were evaluated by tensile testing, surface hardness and friction coefficient
measurements. Prior application of warm rolling makes possible (in the sense that is a viable solution) the ion
nitriding of low carbon steels in order to produce machine parts with improved mechanical properties in the
core (due to warm rolling) and longer service life (due to ion nitriding).
Key words: carbon steels, warm rolling, nitriding, microstructure, mechanical properties.
1 INTRODUCTION
Generally, low carbon steels are delivered in as
rolled, annealed or normalized condition and, for a
given chemical composition, their mechanical
characteristics depend on their microstructure. At
room these steels consists of ferrite and pearlite. The
mechanical characteristics of ferrite-pearlite
microstructure are strongly influenced by the ferrite
grain size [1]. A series of methods are applied in the
industry in order to refine the ferrite-pearlite
microstructure: modification of the chemical
composition
[2],
normalizing
[3],
plastic
deformation by controlled rolling [4], rapid cooling
[5] , warm working [6,7].
Situated between the cold and hot working,
the warm-working process corresponds to a
temperature range in which, after the plastic
deformation, the material is partially strain hardened
and partially recrystallized [8]. In several works
have been studied the microstructure development
and mechanical behavior during forging [9],
stamping [10], rolling[11,12], caliber rolling and
drawing [13] at warm temperatures. For industrial
applications, warm working is very attractive
because it offers certain advantages. Thus, compared
to cold working, it requires lower deformation
forces, can be applied to a broader range of steels,
allows for higher deformation ratios, generates a
more uniform deformation across the transversal
section and leads to a less strained microstructure
[14].Compared to hot working, it leads to a finer
microstructure with superior mechanical properties,
better surface quality and better dimensional control,
lower material losses due to decarburization and
oxidation.
Attempts to apply thermochemical treatments
premise that inclusions are uniformly distributed,
have the same geometrical shape and differ only in
size. Based on Cavalieri-Aker principle [16] and by
using statistical analysis of results [17], the
frequency histograms were determined. As a result
of this study it was possible to define the nitriding
regime that can be suitable to warm worked
products.
Investigated
surface
b
b/2
to some carbon steels, previously subjected to warm
working, have resulted in superior results as
compared to those subjected to hot working [15].
However, desirable results can only be achieved if
the heat-treating regime is properly defined. In the
case of warm rolled steel products, the heating
temperature must be limited below the pearlitic
reaction temperature (Ac1) and the soaking time
needs to be established such as to preserve the fine
microstructure produced by warm working. The
present study focuses on nitriding a low carbon steel
after warm rolling and the evaluation of the final
mechanical properties and microstructure.
y
Rolling direction
x
Fig. 1. Schematic draw of a plastically deformed sample and
2 EXPERIMENTAL DETAILS
The AISI 1015 carbon steel bars were warm rolled
in the following conditions: the temperature at the
beginning and at the end of rolling - 670 oC and
550 oC, respectively, the rolling speed 1.39 s-1 and
the deformation ratio 36.4%. A number of 12
samples were cut from warm rolled steel and were
heated in an electrical laboratory furnace. Each
sample was heated separately and then cooled in still
air. The heating temperature and soaking time for
each sample are given in table 1.
Table 1. Parameters of the heat treatment applied to AISI 1015
steel after warm rolling.
Sample No.
Temperature[ o C]
Time [min.]
1
1
2
3
3
5
4
10
5
15
6
20
550
7
30
8
60
9
120
10
180
11
300
12
600
Microstructural changes that took place during the
heat treatment were assessed by light microscopy.
Quantitative microscopy analyses were also
performed by using an automated image analyzer
type Epiquand. The analyses were made in two
directions: parallel (x) and perpendicular (y) to the
rolling direction as shown in fig. 1. The field of
investigation had dimensions 4x4 mm. The
quantitative microscopy analysis was based on the
location of the specimen cut for microscopic analysis.
.
For determination of mechanical properties,
standardized samples were machined from warm
rolled steel and then subjected to ion nitriding in
dissociated ammonia for 10 h at 510-520 oC by
using a Nitrion 10 type equipment. Tensile
characteristics, surface hardness and dry friction
behavior were evaluated. Friction coefficient was
determined by using a home made ring-on-block
tribometer. The block (10x10x10 mm) was made
from warm rolled steel and the ring from gray cast
iron. The tests were performed under dry friction by
using a constant normal force (Fn=181,85N) and
recording the variation of the friction force as a
function of time. After 3 … 4 sec. the friction force
has stabilized (Ff max). The friction coefficient was
calculated by the relationship:
Ff max
(1)
μ=
Fn
The fracture surface of the samples subjected to
tensile testing was investigated by scanning electron
microscopy (SEM).
3 RESULTS AND DISCUSSIONS
After warm rolling the microstructure of the steel
consists of flattened and partially strain hardened
ferrite grains and very fine pearlite particles
distributed in lines parallel to the rolling direction
(Fig. 2 a). Figs. 2 and 3 show the microstructure and
the distribution curves, respectively, for significant
soaking times. By heating to 550 oC, the
microstructure is completely recrystallized after 10
min and displays well defined grain boundaries (fig.
2 b ). The ferrite has a fine grain size, close to the
dashed line). The stability of the microstructure to
heating
after
warm
rolling
allows
for
thermochemical treatment by nitriding of steel.
10 min
2h
10 h
100 μm
15
Frequency, %
100 μm
10
5
100 μm
Increasing of the soaking time to 2 hours does not
affect essentially the ferrite grain size (fig. 2 c). The
microstructure appears fine and homogenous. In the
rolling direction the distribution curve shifts slightly
to the right and the maximum frequency increases
from (8.0 … 11.3) μm to (11.3 … 16.0) μm (fig. 3 a,
continuous line). Large and medium size grains
grow very slightly on the expense of fine grains and
lead to a homogenous microstructure. The pearlite is
distributed in lines in the form a spheroidal
separations (fig. 2 c). The distribution curves shift
slightly to the left and the dispersion degree
decreases (fig. 3 b, continuous line).Increasing of the
soaking time to 10 hours leads only to a slight
increase of the ferrite grain size (fig. 2 d ). It can be
noticed a decrease of the frequency values in the
very fine grain classes. The distribution curves
remain in the same grain size field but the central
parts of the curves shift slightly to the right. The
dispersion degree of the grain size decreases further
(fig. 3 a, dashed line). The pearlite particles have a
spheroidal shape and the line distribution is
preserved (fig.2 d). The dispersion of the distribution
curves decreases significantly indicating a
dimensional leveling of the particles (fig. 3 b,
181.0
128.0
64.0
90.5
32.0
45.2
22.6
16.0
8.0
11.3
4.0
5.5
Grain size, μm
10 min
2h
10 h
15
Frquency, %
10
5
128.0
181.0
Grain size, μm
90.5
64.0
45.2
32.0
16.0
22.6
8.0
11.3
5.5
4.0
(b)
0
2.0
initial grain size, with uniformly distributed grains
(fig. 3 a, doted line). In the rolling direction the
dispersion is larger and the average grain size values
are shifted to the right. The maximum grain
dimension is situated in the range [45.2 – 90.5] μm.
Immediately after recrystallization, the ferrite is very
fine, with slightly elongated grains in the rolling
direction and uniformly distributed in the
microstructure. The pearlite grain size is little
affected by the soaking time (fig. 3 b doted line).
2.0
Fig. 2. The microstructure of samples warm rolled (a) and
after different soaking times at 550 oC ( b - 10 min , c - 2 h ,
d – 10 h).
2.8
(a)
0
2.8
100 μm
Fig. 3.Grain size distribution curves of the ferrite (a ) and
pearlite (b) after heating to 550 oC with different soaking times.
(Parallel to the rolling direction)
Table 2 shows the mechanical properties of the ion
nitrided samples after warm rolling.
Table2. Mechanical properties of samples after warm-rolling
and in normalized condition.
AISI1015
Warm
rolled
Nitrated
after warm
rolling
Friction
in area HV5 coefficient
[%]
μ
Yield
strength
[MPa]
Tensile
strength
[MPa]
359
493
24,7
59
188 0,342
398
538
18,6
49,5
304 0,206
Elongation
[%]
Reduction
The application of nitriding after warm rolling
determines a slight increase of the strength (10% for
yield strength and 20% for tensile strength,
respectively). The surface hardness increases
significantly (62%), while the friction coefficient
decreases much (by 40%). The significant
modifications of the hardness and friction
coefficients lead to the improvement of the wear
resistance of the parts treated in this way and to the
increase of their lifetime.
Figure 4 shows the fracture surfaces of warm rolled
and nitrided samples subjected to tensile testing.
Due to nitriding, the fracture surface losses the coneand-cup aspect. A clear delimitation appears
between the white layer (8 μm) and the diffusion
zone (0.2 mm). There is a good link between the
diffusion zone and the core (Fig. 4a). In the white
layer, the fracture surface is rather smooth indicating
a brittle fracture due the fine and very hard nitrides
(Fig. 4b). In the diffusion zone, the fracture surface
presents shallow hollows. In the core (Fig. 4c), with
fine hollows which preserves the aspect and
dimensions of those obtained by simple warm
rolling (Fig. 4d). The similarity of the two images
(Fig. 4c and 4b) confirms the stability during
nitriding of the microstructure obtained by warm
rolling.
carbon steel and implicitly to increasing their
lifetime.
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Fig. 4. SEM micrographs showing the morphology of the
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rolled (d) samples subjected to tensile testing.
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4 CONCLUSIONS
Warm rolling determines the refining of the ferritepearlite microstructure and, implicitly, an
improvement of mechanical properties. This
microstructure is stable to later heating up to 550 oC
with long soaking time. The stability of the
microstructure makes possible the thermal treatment
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Nitriding of the steel after warm rolling gives a hard
surface layer which presents a significant
improvement of wear resistance. In the same time,
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