American Journal of Applied Science and Technology
8
https://theusajournals.com/index.php/ajast
VOLUME
Vol.05 Issue 07 2025
PAGE NO.
8-11
10.37547/ajast/Volume05Issue07-02
Low-Temperature Treatment of Malleable Cast Iron
Bobur Ibrokhimov
PhD student of Tashkent State Technical University, Uzbekistan
Sarvar Tursunbaev
Associate professor of Tashkent State Technical University, Uzbekistan
Received:
10 May 2025;
Accepted:
06 June 2025;
Published:
08 July 2025
Abstract:
This study explores the effect of low-temperature treatment (LTT) on the graphitization behavior of
malleable cast iron during annealing. The results show that a properly selected LTT regime significantly promotes
the formation of graphite inclusions, thereby accelerating cementite decomposition. The efficiency of LTT is
determined by several key factors, including treatment temperature, duration, number of cycles, and the pre-
treatment cooling conditions. Single-stage LTT is most effective at 300
–
400°C, while double and multi-stage
treatments demonstrate enhanced performance, especially when the first cycle is conducted within 100
–
400°C.
Longer exposure times further improve graphite inclusion formation, and the resulting structural changes remain
stable even after prolonged storage at room temperature. Additionally, slow heating from 20°C to 700°C is
identified as a viable alternative to traditional LTT, offering similar structural benefits. The most effective annealing
approach involves holding the material at 300
–
400°C followed by a gradual rise to the pearlite transformation
range, a method well-suited to the capabilities of standard industrial equipment. These findings provide valuable
guidance for optimizing heat treatment practices in malleable cast iron manufacturing.
Keywords:
Pearlite, malleable cast iron, annealing, graphite, cooling.
Introduction:
Low-temperature treatment (LTT) of white cast iron,
also known as artificial aging, is increasingly
recognized as the most convenient way to intensify
the graphitization annealing process. In this regard, it
is undoubtedly interesting to compare the possible
options for low-temperature treatment, which can
produce the largest number of graphite inclusions in
the structure of malleable cast iron [1]. The chemical
composition of the experimental castings in the
experiments conducted was within the following
ranges: 2.49
–
2.57% C; 1.29
–
1.42% Si; 0.43
–
0.49% Mn
and 0.129
–
0.14% S. Modification was carried out with
nickel (0.1%). The samples were rectangular in shape
with a thickness of 15 mm. The first part of the study
investigated the effect of temperature and duration
of LTT on the amount of graphite inclusions in cast
irons. Normal cooling (in the casting mold) was
interrupted at certain temperatures.
Experimental and results
The experimental technique involved quickly
transferring the samples removed from the mold to
preheated furnaces (Table 1).
Table 1
The effect of cooling temperature and LTT mode on the number of graphite inclusions
Cooling
temperature of
samples in the
casting form,
℃
LTT
temperature,
℃
Number of graphite inclusions per 1 mm
2
, with LTT
duration, hours
4
10
20
500
500
17
19
21
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American Journal of Applied Science and Technology (ISSN: 2771-2745)
400
400
30
35
38
300
300
42
50
56
200
200
58
98
145
The results obtained show that the pre-cooling
temperature has a decisive influence on the
effectiveness of LTT, with lower temperatures causing
an increase in the number of graphite inclusions.
Under these conditions, the effect of the holding
time, which also contributes to the quantitative
growth of inclusions, is more pronounced [2]. The
following LTT variants were preceded by normal
cooling of the samples to room temperature. The
accepted modes provided for the first LTT at a
relatively low temperature (up to 400°C) and the
second at 600°C (Table 2). It can be seen that after the
second LTT, the number of graphite inclusions in the
structure of malleable cast iron increases sharply. The
higher the temperature of the first LTT, the higher the
efficiency of the second LTT [3-7]. Other variants of
double treatment were also studied (Table 3): the
first LTT was carried out at a higher temperature (500-
600°C), and the second at a lower temperature.
Table 2
The effect of temperature and duration of LTT on its effectiveness
LTT
temperature,
℃
Number of graphite inclusions per 1 mm
2
, with LTT duration, hours
1
3
5
7
10
400
62
80
101
285
205
530
315
750
390
980
300
60
72
150
355
265
560
372
743
420
1062
200
35
54
80
194
155
342
236
568
268
750
100
28
80
56
118
80
178
102
210
115
296
Table 3
The influence of LTT on its effectiveness
LTT
LTT temperature,
℃
Number of graphite inclusions
per 1 mm
2
, with LTT duration,
hours
First
500
165
400
406
300
410
200
267
100
115
Second (temperature of first
500
℃
)
400
136
300
170
200
130
Second (temperature of first
600
℃
)
400
90
200
93
100
95
The results obtained indicate that with the selected
LTT option, the second treatment is completely
ineffective. The effectiveness of multiple LTT,
including holding castings at temperatures ranging
from 100 to 600°C, was investigated. The duration of
the holding times and the results of the experiments
are shown in Table 4. Multiple treatment leads to a
significant increase in the number of graphite
inclusions in malleable cast iron.
As expected, the same results were obtained during
continuous heating at different rates in the range of
20
–
700°C (Fig. 1). In this case, too, a slower rate of
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American Journal of Applied Science and Technology (ISSN: 2771-2745)
temperature increase corresponds to an increased
number of grains.
Table 4
The impact of multiple LTT on its performance
Continuation of
LTT, hours
Number of graphite inclusions per 1 mm
2
during multiple LTT (first LTT at
100
℃
, duration 1, 2, and 4 hours)
Second LTT,
200
℃
Third LTT,
300
℃
Fourth LTT,
500
℃
Fifth LTT,
600
℃
1
95
145
250
330
2
120
190
320
400
3
265
509
755
882
Figure 1. Effect of average heating rate in the range of 20-700 °C/h on the number of graphite
inclusions in malleable cast iron
Finally, in the last series of experiments, we
attempted to determine whether cooling the samples
to room temperature after LTT had any effect on the
efficiency of the treatment. The samples under
investigation were cooled in the casting mold to room
temperature. The only difference between the
subsequent heat treatments of the individual samples
was that in one case they were cooled after LTT in air
to room temperature, and in the other case they were
immediately transferred for graphitization to an
electric furnace preheated to 950°C. The results
obtained reveal another feature of LTT, namely the
stability of the changes it introduces. As can be seen
from the data presented below, these changes are
preserved not only after cooling, but also during
prolonged exposure of the samples to room
temperature:
Duration of holding at room temperature,
hours, after LTT 300 degrees, 10 hours
Right after the LTT
1
500
Number of graphite grains per 1 mm
2
182
180
156
The data from these experiments are a logical
consequence of the mechanism of graphite grain
nucleation discussed above. It is evident that with an
increase in the duration of LTT in the temperature
range up to 400°C, the most favorable conditions for
the formation of graphite inclusions are created. This
is explained by the relatively low rate of diffusion
processes, which determines the positive effect of
double and multiple LTT. At the temperatures about
300°C (at which LTT is usually carried out), the
American Journal of Applied Science and Technology
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American Journal of Applied Science and Technology (ISSN: 2771-2745)
complete redistribution of silicon requires much
more time than the practice of graphitizing annealing.
Under normal LTT conditions, when the holding time
in the 300-400°C range does not exceed 5-10 hours,
the formation of a high-silicon ferrite barrier is only
just
beginning.
Further
holding
at
higher
temperatures accelerates this redistribution. At the
same time, the high-silicon ferrite barrier, by shifting
the PG lines to the right, protects carbide inclusions
from dissolving in ferrite.
The situation is completely different when LTT is
carried out at temperatures between 500 and 700°C.
In this case, the supersaturation of the alpha solution
is minimized. The effectiveness of such treatment in
terms of the number of graphite nuclei is also
minimal. It does not increase with subsequent LTT
treatments, regardless of the temperatures at which
they are carried out.
CONCLUSION
All of the above facts testify to the high potential of a
correctly selected LTT to change the amount of
graphite inclusions, and hence the rate of cementite
decomposition during the annealing of malleable cast
iron. A comparison of the obtained dependencies
reveals some general patterns that are important for
the theory and practice of this process. The most
significant of these can be summarized as follows:
1. The effectiveness of low-temperature treatment is
directly related to the preliminary cooling of castings.
The lower the temperature at which it is carried out,
the greater the ability of LTT to increase the number
of graphite inclusions.
2. Single-use LTT is most effective in the temperature
range of 300
–
400°C.
3. Double and multiple LTT treatments give good
results when the first treatment is carried out at a
temperature of 100-400°C. The higher the
temperature (up to 300°C), the more graphite
inclusions appear during the graphitization annealing
process.
4. The effectiveness of all the above-mentioned LTTs
is directly related to the duration of exposure. As the
latter increases, so does the number of inclusions in
the cast iron structure.
5. The most effective are double and multiple LTT
variants, which involve prolonged exposure at
temperatures between 300 and 400°C.
6. Single and multiple LTTs can be successfully
replaced by slow heating at temperatures ranging
from 20 to 700°C.
7. Changes caused by LTT in relation to the ability of
castings to graphitize with an increased number of
graphite grains are highly stable. They remain after
cooling and prolonged exposure of cast iron at room
temperature.
The most effective annealing methods are those that
involve holding the material at a temperature of 300-
400°C and then slowly heating it to the pearlite
transformation range, as modern equipment used for
annealing malleable cast iron is not designed to
handle sudden temperature increases.
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