Influence of warm-working on quality of AISI 1015 carbon steel
Carmen Medrea-Bichtas1, Gavril Negrea2
1
Technological Education Institute of Pireaus, Department of Physics, Chemistry and
Materials Technology, Aigaleo, Athens, Greece, email:medrea@internet.gr
2
Technical University of Cluj-Napoca, Faculty of Materials Science and Engineering,
Muncii Avenue 103-105, 3400 Cluj-Napoca, Romania,
Abstract
The quality of a low carbon steel depends on its chemical composition and microstructure. For a
given chemical composition the properties of the steel depend on the microstructure which, at
room temperature, consists of ferrite and pearlite. The finer the ferrite grains, the better the
mechanical properties of the steel. This paper presents the influence of warm working parameters
on microstructure and hardness of a low carbon steel (AISI 1015). Samples were subjected to
torsion tests at different temperatures from 500 oC to 700 oC in order to study their
microstructure and mechanical properties .The microstructure of the samples was investigated by
optical microscopy and quantitative microscopy and the hardness was measured by Vickers
method. The quality of warm worked steel could be compared with the normalized steel. The
results did shown that the warm working can represent an efficient method for refining the
structure of low carbon steel and can lead to the improvement of quality of this material.
Keywords: warm-working, carbon steel, mechanical properties; microstructure
1. Introduction
The low carbon steels are delivered, mainly, in normalized condition. Their properties,
including the mechanical characteristics, depend on the chemical composition and
microstructure.
Low carbon steels present, at room temperature, a microstructure consisting of ferrite and
pearlite grains [1]. The mechanical characteristics of ferrite-pearlite microstructure are
strongly influenced by the ferrite grain size. The finer the ferrite grains, the better the
mechanical properties of these steels .
A series of methods can be applied 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] and plastic deformation at warm temperature [68].
Situated between cold- and hot-working, the warm-working process becomes more and
more interesting due to the advantages that it presents. During warm-working of low
carbon steels, various microstructural changes may occur and alter the final mechanical
properties [9]. There have been published many studies on the behaviour of low carbon
steels during warm-working. Suitable conditions for warm forging and rolling have been
considered, dynamic strain aging during wire drawing and its effect on flow behaviour
was investigated, and prediction of flow behaviour have been made [10]. Flow behaviour
of low carbon steels at different temperatures has been studied in several works and
based on compression experiments [10].
Although, there are other studies dealing with warm working of steels, many aspects of
this process have not been thoroughly investigated. This paper presents the influence of
warm-working by torsion on the microstructure and hardness of a low carbon steel. The
advantages of warm-working as compared to hot-working are also assessed .
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2. Experimental Details
M 12
The experiments were conducted on AISI 1015 carbon steel. The samples, with the
geometry presented in figure 1 (deformation zone - Ø 8 x 36 mm), were cut from
normalized hot- rolled bars and subjected to torsion testing at different temperatures from
550 oC up to 700 oC. The temperature was increased in steps of 25 oC. To compare the
deformation behaviour in the warm and hot temperature ranges, several torsion tests were
also performed at 1000 oC. The samples were heated by induction to desired temperature.
All the tests were carried out on a home-build machine which allows for precise
measurements of the sample temperature, torque, rotation angle and axial force. The
microstructure of the samples was investigated by optical microscopy and quantitative
microscopy and their hardness was measured by Vickers method. For quantitative
microscopy it was used an automated image analyser type Epiquand.
8
36
136
Figure 1: Sample shape and dimensions.
The quantitative microscopy analysis was based on the premise that inclusions are
uniformly distributed, have the same geometrical shape and differ only in size. The study
is based on Cavalieri-Aker principle[11] according to which the proportion of a phase in
1 mm3 is equal to the proportion of the phase considered in 1 mm2. Thus, the volume
fraction of phases can be deduced form the statistical estimation of linear fraction of
respective phases.
The metallographic specimens were cut such as to allow for microscopic analysis in two
directions: “x”, parallel to the rolling direction, and “y” perpendicular to the rolling
direction (figure 2). The field of investigation had dimensions of 4x4 mm. The results of
the measurements were processed by a computer as to extract the linear size of ferrite and
pearlite grains [12]:
_
L P,F
l P ,F = ------(2.1)
_
N P,F
where:- l P ,F is the average linear size of pearlite and ferrite grains [μm]
-LP,F is the total length of the line that crosses pearlite and, respectively, ferrite
grains [μm]
-N P,F is the total number of pearlite and, respectively, ferrite grains.
2
Rolling direction
y
x
b/2
b
Investigated
surface
Figure 2. Schematic draw of a plastically deformed sample and
location of the specimen cut for microscopic analysis.
From the total number of grains of a phase from the field of analysis, 1000 grains are
distributed in 13 grain size classes [13]. Pearlite particles that are finer where distributed
in the 1,0 ... 90, 5 μm classes, while the ferrite particles were distributed in the classes
2,0 ... 181,0 μm. For the sake of simplicity, it was chosen a graphic representation with
equal intervals in which the particle size classes are represented in geometrical
progression with the ratio of 1.41. By cumulating the frequencies indicated in a statisticlogarithmic coordinate scale the Henry's line is obtained [14]. Henry's line demonstrates
the normality of a distribution and allows for analysis of the homogeneity of the lot of
grains subjected to analysis.
3. Results and Discussion
15
t
d
120
n
tc
v = 0.25 s-1
v = 1.39 s-1
v = 1.39 s-1
tc
v = 0.25 s-1
110
10
100
90
80
70
60
Deformability (n )
130
d
Resistance to deformation (t ) [N/mm2]
The influence of temperature and torsion speed on deformation behaviour (resistance to
deformation (τd) and deformability (ntc)) is shown in figure 3.
5
550
600
650
700
Initial temperature [oC]
Figure 3. The influence of temperature and torsion speed on the
resistance to deformation and deformability.
Note: The reference temperature reported throughout the text represents the initial
temperature of the sample. The actual average temperature of the sample during the tests
was found to be higher than initial value by 26 - 34 oC, depending on initial temperature
and rotation speed.
3
120
12
t
d
n
100
90
8
80
tc
10
Deformability (n )
tc
110
d
Resistance to deformation (t ) [N/mm2]
By increasing the temperature from 550 oC to 675 oC, the shear stress decreases by a
factor of almost 2, from 115,53 to 65 N/mm2. The reduction of the resistance to
deformation is more pronounced in the temperature interval 550 – 625 oC due to
completion of recrystallization process. If at 550 oC, the sample structure is just restored,
at 700 oC the structure is completely recrystallized. The small increase of the resistance to
deformation at 700 oC is due to the fact that the actual average temperature of the sample
exceeded A1 temperature.
By increasing the torsion speed, in other words - the strain rate, the resistance to
deformation increases significantly. However, compared to hot-working at 1000 oC, the
resistance to deformation at 675 oC is two times higher (65 and 31,2 N/mm2,
respectively), but it is incomparably lower than the resistance to cold-working[15] .
The deformability of the sample, as assessed by the number of torsions before breaking,
increases with increasing temperature and torsion speed. For a given temperature, both,
resistance to deformation and deformability, increase by increasing the strain rate as
shown in figure 4.
t = 625 oC
d
6
70
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Torsion speed (v) [s -1]
Figure 4. The influence of torsion speed on the resistance to deformation and
deformability at 625 oC.
The warm-working leads to a finer microstructure, characterized by a globular aspect
with pearlite homogeneously distributed in the ferrite matrix (Fig. 5). In the case of
warm-working at 550 oC, the resulting microstructure has very fine grains but it is strain
hardened, with less defined grain boundaries (Fig. 5 a). Increasing the working
temperature to 600 oC allows for simultaneous evolution of strain hardening and
recrystallization processes. The grains are very fine, partially recrystallyzed (Fig. 5 b).
After warm working at 650 oC, the microstructure is completely recrystallized and has a
very fine and uniform aspect (Fig. 5 c). A further increase of the initial temperature of the
sample to 700 oC resulted in a slight increase of the grains, but the microstructure
remained essentially fine and uniform.
The microstructure of the samples after warm working is much finer as compared to the
initial microstructure of the steel in the as-supplied condition (normalized, Fig. 5 d),
4
(b)
(a)
(c)
(d)
Figure 5. The microstructure of the samples after warm working at different temperatures (a – 550
o
C, b – 600 oC and c – 650 oC) and in the initial, as-supplied condition (d - normalized after hotrolling).
Average linear grain size, μm
Ferrite
Pearlite
Deformation temperature, oC
Figure 6. The average linear size of ferrite and pearlite
grains as a function of deformation temperature
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suggesting that the mechanical
properties of the steel after
warm working are also superior
to
those
obtained
after
normalizing.
Figure 6 shows the analysis of
the average linear size of
pearlite and ferrite grains as a
function of the working
temperature.
The
pearlite
presents a low sensitivity to the
deformation temperature in the
600 - 700 oC range (fig. 6 continuous
line).
With
increasing the deformation
temperature the size of ferrite
grains, that is more ductile,
presents a pronounced decrease
(fig. 6 - dashed line).
The analysis of Henry's line for
pearlite grains (Fig. 7) indicates
a normal grain size distribution
(Gaussian). The small relative
modification of the lines
orientation with the deformation
Distribution frecquency %
temperature confirms the dimensional stability of the pearlite grains in the investigated
temperature range.
600 oC
650 oC
700 oC
1000 oC
Relative size of pearlite grains, μm
Figure 7. The size distribution of pearlite grains for diffrent deformation temperatures.
Vickers Hardness [HV]
The hardness of the samples after warm working decreased with increasing the
deformation temperature as shown in Fig. 8. It can be seen that in the temperature range
550 – 675 oC, the average hardness varies from 153 to 134 HV which is about 10 to 30%
higher
150
.
140
130
550
600
650
Initial temperature
[oC]
Figure 8. Influence of the initial temperature on the hardness of the sample after
torsion tests.
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4. Conclusions
Based on the torsion tests carried out in the temperature range from 550 to 700 oC on
AISI 1015 carbon steel, the following conclusion can be drawn:
 The optimal deformation temperature is 625 to 675 oC. In this temperature range,
the warm-working requires smallest deformation energy and allows for maximum
deformation ratios to be achieved.
 The strain rate has a significant influence on plastic behavior of the material. Both,
resistance to deformation and deformability, increase with increasing the strain rate.
 Resistance to deformation and deformability, increase with increasing the strain
rate. Microstructure obtained by normalizing the material after hot rolling, with
advantageous consequences on the mechanical properties.
 The results obtained in this study show that warm-working offers a viable
alternative to cold and hot working processes.
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