A brief investigation of Hardness and Microstructure of a Thermo-Mechanically
Treated (TMT) Bar by Different Heat Treatment Process.
Golam Mostafa Nipua, Rodro Sahaa, Md Bayzid Hossaina, Tanvir Shakha, Anando Haldera,*
a
Department of Materials Science and Engineering
Khulna University of Engineering & Technology, Khulna-3206, Bangladesh
*E-mail: halder1927051@stud.kuet.ac.bd; Tel.: +880 1718111891
ABSTRACT
Steel is the most common material used for engineering materials, and these materials are heat
treated to change their physical and mechanical qualities to fit specific engineering purposes. In
this investigation, the impact of heat treatment on the microstructure and a few chosen mechanical
characteristics of TMT bar was investigated. A steel sample was collected from the local market,
and spectrometry analysis was done on it. In an electric furnace, the steel samples underwent heat
treatment at various temperatures and holding durations, followed by cooling in various media.
Standard techniques were used to measure the hardness of the treated and untreated samples, and
a metallographic microscope with a camera was used to study the materials' microstructure. The
results demonstrated that different heat treatments for a specific application can alter and enhance
the mechanical characteristics of TMT bar. It was also discovered that the annealed samples, which
primarily had ferrite structure, had the lowest tensile strength and hardness values and the highest
ductility and toughness values, whereas the hardened samples, which primarily had martensite,
had the highest tensile strength and hardness values and the lowest ductility and toughness values.
Keywords: Heat treatment; Mechanical Properties; Microstructure; Thermo-Mechanically Treated
(TMT) Bar
INTRODUCTION
In order to produce a specific microstructure and desired mechanical properties (hardness,
toughness, yield strength, ultimate tensile strength, Young's modulus, percentage elongation, and
percentage reduction), a particular metal or alloy is subjected to heat treatment, which consists of
timed heating and cooling. The most significant heat treatments which is often used to change the
microstructure and mechanical properties of engineering materials particularly steels are
annealing, normalizing, hardening and tempering. Because of the ferrite-pearlite microstructure,
annealing is the heat treatment technique most usually used to soften iron or steel materials and
refines the grain size. It is employed when engineering materials need to exhibit significant
elongations and tensile strength [1]. In normalizing, the steel or its alloy is heated to the austenitic
temperature range and then holding in this temperature for a certain time period and the cooled
with air. The major goal of this treatment is to create a pearlite-dominated matrix, which increases
strength and hardness compared to the raw material. It is also utilized to get rid of unwanted free
carbide that was in the sample when it was received [2]. A steel is frequently quenched and
tempered in order to improve its mechanical properties, particularly its strength and wear
resistance. In hardening, the steel and its alloy is heated to a elevated temperature high enough to
the austenite formation range, held at this temperature until desired amount of carbon is dissolved
and then cooled in oil or water at an appropriate rate. The steel must contain 100% martensite for
maximum performance to attain maximum yield strength, but it is very brittle in the hardened state
and, therefore, as hardened steel is used for very little engineering application. By tempering, the
properties of quenched steel can be changed to decrease hardness and gradually increase ductility
and impact strength. The resulting microstructure is bainite or carbide precipitates in the ferrite
matrix depending on the tempering temperature . Steel is an alloy of iron and carbon containing
ranges from 0.15-1.5% [3]. Steel is widely utilized for two primary reasons: (i) It is abundantly
present as Fe2O3 in the crust of the planet and converts to Fe with minimal effort. (ii) It can be
designed to have a wide range of microstructures and mechanical characteristics. Its utility can be
explained by its accessibility, ductility, and acceptable casting, working, and machining qualities.
It is also susceptible to straightforward thermal treatments to produce a variety of properties [2].
Increased operating parameters, i.e., operating at temperatures and pressures greater than 600˚C
and 27 MPa, respectively, [4–9] can be used in thermal power plants to increase efficiency and
reduce carbon dioxide emissions [10–12]. They are used in advanced high-strength steel, railways
lines, boiler tubes, and protective armor steel, among other things [13]. Even though guidelines
only recommend basing it on chemical composition, the heat treatment cycle needs to be adjusted
based on both the chemical content and thickness of the material.
The key goal of the current study is to examine how different heat treatment techniques modify
the microstructure and mechanical properties of low alloy steel. Two of the suggested heat
treatment cycles in this study include solutionizing at 1150 °C and tempering at 460 °C. A
thorough analysis of the microstructure is undertaken together with an assessment of the
mechanical properties to identify the appropriate mechanical attributes.
EXPERIMENTAL PROCEDURE
Steel reinforcing bar samples were taken from a lab. Metallographic preparation was done on the
samples. After 120 minutes of holding at three different temperatures, 890°C, 904°C, and 910°C,
the sample specimens were then normalized and quenched in a salt bath. This employed
temperature range will demonstrate how heat treatment affects metals below and above austenizing
temperatures. On the raw and heat-treated samples, a hardness test and a microstructural
examination were done. The outcomes were then contrasted with the original data. This will make
it easier to choose the optimal quenching media and achieve the desired mechanical qualities. Table
1 shows the chemical constituent of the bar as obtained from an optical emission spectrometer.
The heat treatment conditions are listed in Table 2.
Table 1: Chemical composition of the steel bar samples
Elements
Weight
%
C
Si
Mn
P
S
Cu
Ni
Cr
Al
Mo
B
0.367 0.389 1.770 0.034 0.032 0.478 0.097 0.120 0.005 0.01 0.001
Table 2: Heat Treatment Conditions
Annealing
Normalizing
Quenching
Condition
Tempering
Temperature, °C
890
904
910
460
Holding time,
min
Cooling medium
90
120
120
100
Furnace
Air
Salt Bath
Air
RESULT AND DISCUSSION
Effect of Heat Treatment on Hardness:
The hardness of the steel bar in different heat treatment techniques are also given in Table 3. The
plots of hardness at different heating temperatures and media is presented in Figures 1 while the
micrographs for the as-received samples are shown in Figure 4.
Table 3: Mechanical Properties of heat treated and untreated steel
Heat Treatment
Hardness, HRB
As Received Sample
88.1
Annealed
60.1
Normalized
80.67
Quenched
109.7
Tempered
98.5
120
100
Hardness, HRB
80
60
40
20
0
As Received
Sample
Annealed
Sample
Normalized
Sample
Hardened
Sample
Tempered
Sample
Heat Treatment
Fig 1: Hardness of treated and received samples of steel
The hardness value of the received sample was 88.1 HRB. Comparing the mechanical properties
of annealed sample with the received sample, annealed sample showed lower hardness (60.1
HRB). The decrease hardness can be associated with the formation of soft ferrite matrix in the
microstructure of the annealed sample by cooling. The hardness value of the normalized sample
was to be found 80.67 HRB. The increase in hardness as compared to annealed and received
sample was due to proper austenizing temperature at 904oC and higher cooling rate, which
resulted in decrease in elongation and toughness, which was lower than those obtained for
untreated and annealed samples due to pearlitic matrix structure obtained during normalization.
The hardness value of the quenched sample revealed that highest hardness (109.7 HRB). The
specimen was austenised at 910oC for 120 minutes and then water quenched. The hardness value
of the tempered sample was 98.5 HRB.
Effect of Heat Treatment on Microstructure:
The microstructure of received specimen showed a combination of ferrite (white) and
pearlite (black) in the core (Figure 2) and martensite in the edge (Figure 3). The microstructure of
the annealed sample is shown in Figure 4. As it can be seen in Figure 4, the grain size of Ferrite
and Pearlite is bigger than before. Because of furnace cooling recrystallization happen in the
sample and previous grains are transformed into new stress-free big grain. Figure 5 shows the
microstructure of the normalized sample. The normalized sample showed that the shape and size
of the original austenite grains were influenced to a remarkable extent. It was observed that
there was many short graphite flakes surrounded with patches of uniformly distributed pearlite
grains as seen in Figure 5. Figure 6 shows the massive martensite structure of quenched sample,
when medium carbon steels are rapidly quenched from its austenite temperature to room
temperature, the austenite will decompose into a mixture of some medium carbon martensite and
fewer pearlite as a result of this microstructure which is hard. The microstructure of tempered
specimen (Figure 7) consisted of a number of appreciable carbide particles precipitated out from
the matrix, which indicated that the precipitate carbide particles decomposed by a process of
solution in ferrite matrix.
Figure 2:Microstructure of TMT bar-Core(100X)
Figure 3: Microstructure of TMT bar
Edge(100X)
Figure 4: Microstructure of annealed
sample(100X)
Figure 5 :Microstructure of normalized
sample (100X)
Figure 6: Microstructure of quenched
sample (100X)
Figure 7 : Microstructure of tempered
sample(100X).
CONCLUSIONS
In summary, heat treatment significantly influences the properties of TMT (Thermo Mechanically
Treated) bars. Annealing enhances ductility, normalizing balances strength and toughness,
quenching boosts strength but reduces ductility, and tempering maintains strength while increasing
toughness. Choosing the right treatment depends on desired properties. These methods ensure
TMT bars meet specific performance requirements in construction, manufacturing, and
infrastructure projects.
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