Study of effects of heat treatment on the hardness and microstructure of welded low carbon steel pipes

Innovative Systems Design and Engineering www.iiste.org
ISSN 2222-1727 (Paper) ISSN 2222-2871 (Online)
Vol.4, No.9, 2013-Special Issue - 2nd International Conference on Engineering and Technology Research
48
Co-published with the Faculty of Engineering and Technology of Ladoke, Akintola University of Technology, Ogbomoso-Nigeria
Study of effects of Heat treatment on the Hardness and
Microstructure of Welded Low Carbon Steel Pipes
L.O. Mudashiru*1
and T.A. Adio2
1. Mechanical Engineering Department, Ladoke Akintola University of Technology, Ogbomoso, Nigeria
2. Mechanical Engineering Department, The Polytechnic Ibadan, Ibadan, Nigeria.
* E-mail of the corresponding author: lomudashiru@lautech.edu.ng
ABSTRACT
This paper reports investigations made on the effects of annealing heat treatment on the micro-structural and
mechanical property behavior of 0.078wt % C steel weld. Welding operation was carried out on the low carbon
steel of 0.078wt % carbon content in pair; followed by heat treating (annealing), the specimen was heat treated at
the weld zone to a temperature of 850°C using digital heat treatment furnace. For effective study of the effect of
annealing on the welded joint, mechanical test (hardness) was carried out and micrographs were obtained to
reveal the microstructures of the heat treated (annealed) and un-heat treated samples. From the micro-structural
analysis, the micrographs shows that the heat treated sample had a more predominant pearlite structure which is
the reason larger areas of the structure is darker and less ferrite structure due to recrystallization and carbon
diffusion; hence a significant improvement in ductility, toughness and grain refinement accompanied with
decrease in hardness and brittleness.
Keywords: Annealing, Ductility, Hardness, Heat treatment, Microstructure, Welding
1. Introduction
One of the most important methods of permanent fastening and versatile means of fabrication available to
industry is welding. This is because it is one of the most important tools available to engineers in his efforts to
reduce production, fabrication and maintenance costs (Adedayo et al., 2010).
Welding is simply an art of joining metals by heating and then pressing together which simply requires a heat
source to produce a high temperature zone to melt the material (though it is possible to weld two metal pieces
without much increase in temperature), its application includes: used in ships building, bridges, pressure vessels,
industrial machinery, automobile, rolling stock and many other fields. Hence the optimization of the welding
process is of the essence in fabrication as its importance cannot be overemphasized but it requires a good
understanding of the microstructures generated by the rapid temperature rise in the heat-affected zone (Bipin et
al., 2010; Kumaar, 2010 and Maamar, 2008).
However with its importance, problems associated with welding are common issues in these fields, as a complex
mixture of two or more constituents is formed when steel or any metal is welded altering its microstructure
(International Institute of Welding Guidelines, 1988) .This complex microstructure mixture can lead to highly
varied properties of the weld or defects (since the properties of steels or metals are related to its structural make-
up (Rajan, 1988). Weld defects may include: cracking, hardness reduction, reduction in strength, distortion, wear
properties, corrosion characteristics, internal stresses and etcetera. These defects cannot be overlooked, as a
means to control them is of essence to an effective design (Khan and Haasen, 1996).
These defects can however be reduced through various types of heat treatment to obtain the desired properties,
simply because a way to unify the structure of the welds is by heat treatment. The science of heat treatment deals
with the control of the constituents in metal by heating and cooling, it also deals with the relationship between
these constituents and the properties of the metal which can be monitored by microscopic examination known as
metallographic. Engineering materials, mostly steel, are heat treated under controlled sequence of heating and
cooling to alter their physical and mechanical properties to meet desired engineering applications (Fadare et al.,
2011).
Previous research on annealing heat treatment effects on steel welds, were carried on samples of varying carbon
content; the results show that hardness and toughness were dependent on the carbon content of the steel
(Adedayo et al., 2010). In this project, special attention was focused on the effect of annealing on the mechanical
and micro-structural properties of welded low steel pipe of 0.078%wt C in other to appropriately adapt it for
better utilization in the aforementioned area of applications of welding.
2. Materials and Method
The specimen used is a low carbon steel pipe (Φ32mm, 2mm thick and 600mm long), which was obtained from
a local market known as “Owode Onirin” market, Lagos state, Nigeria. Chemical composition analysis was
carried out at Universal Steel Rolling Mill, Ogba-Ikeja, Lagos state, Nigeria using the mass analyzer.

49
2.1 Equipment
The equipment used for this research are; mass analyzer, digital heat treatment furnace, metallurgical microscope
with in-built camera, electrical arc welding machine, Grinding and polishing machine, hack saw, TECO micro-
hardness tester and welding electrodes.
2.2 Welding Procedure
The specimen was cut into four pieces of equal sizes (150mm long) and welded together in pair. The specimen
(low carbon steel) was welded using electric arc welding at Federal Industrial Institute of Research, Oshodi,
Lagos state, with procedural steps as follows:
i. Clamping of the specimen firmly on the vice.
ii. Welding operation parameters; welding current= 140A; welding voltage=80V; Electrode
specification= 2.5mm (gauge 10)
iii. Welding was carried out at a slow speed.
2.3 Heat Treatment
The heat treatment process (annealing) was carried out at Federal Industrial Institute of Research (FIIRO),
Oshodi, Lagos state Nigeria. A Digital heat treatment furnace with a maximum temperature of 1200°C was used
for the heat treating processes. The specimen was heated at the weld centre line gradually to a temperature of
about 850°C, and then was held for about two hours at this temperature before allowed to cool in the furnace for
24hrs.
2.4 Hardness test
Hardness values of the treated and untreated weld sample was determined using LECO micro-hardness tester;
model: LM-700AT with test load of 490.3MN and a dwell time of 10secs. The LECO micro-hardness tester
automatically calculates the hardness values in Vickers hardness number (VHN). This test was carried out at
Engineering Material and Development Institute (EMDI), Akure, Ondo state Nigeria.
The hardness of the heat treated and untreated welded steel were evaluated at two points: (i) the weld pool region
(ii) the heat affected zone (HAZ).
2.5 Micro-structural Test
The micro-structural examination was carried out as follows; sectioning, grinding, polishing, etching and
microscopic viewing at the Metallurgical department of the University of Lagos, using a metallurgical
microscope.
• Preparation (Sectioning)
Sectioning was carried out by sawing at the weld bead, but care was taken to ensure that the original condition of
the material is not altered.
• Grinding
A silicon carbide paper was used. This strip of paper was laid flat on the heat-treated pipe and the specimen is
robbed to and fro on the strips. Starting with the roughest cloth (240 grit), rub continued until all traces of
cuts/roughness are removed. Turning through 90° and rubbing on the next (finer) paper (320 grit) until the
previous scratches was removed.
• Mechanical Polishing
This was done in two stages with a coarse abrasive agent and fine polishing agent respectively. The specimen
was held against a horizontal rotating wheel covered with a short - pile cloth fed with a suspension cream of the
polishing agent.
• Etching
In this, the specimen was submerged in and swabbed with a chemical reagent (mixture of acid and alcohol) that
removes the surface layer produced on polishing.
• Microscopic Viewing
The sample was illuminated by a reflected light which was mounted on the microscope; the specimen was
viewed through the eyepiece of the microscope so as to determine the micro-structure which was subsequently
by snapped by the attached camera to obtain the micrograph.
3. Results and Discussion
3.1 Spectrometry Result
The result of the chemical composition analysis carried on the low carbon steel sample is shown in table 1.
Table 1: Chemical composition of low alloy steel specimen
Element C Si Mn P S Cr Ni Cu Al Fe
Weight % 0.078 0.025 0.304 0.0049 0.021 0.017 0.031 0.008 0.015 99.4
3.2 Hardness Result
The hardness result obtained from the tests carried out on the heat treated specimen and un-treated specimen of
the low carbon steel pipe were recorded in table 2.

50
Table 2: Vickers Hardness test values for annealed and untreated samples
Weld Pool Heat Affected Zone
Annealed 319.7 191.9
Untreated 358.0 202.3
3.3 Micro-structural analysis results
Plate 1 shows the untreated specimen at the weld pool while Plate 2 shows the treated specimen at the weld pool.
Plate 1: Micrograph of untreated specimen weld pool X200
Plate 2: Micrograph of treated specimen weld pool X200
3.4 Discussion of hardness results
From the hardness test results tabulated above, it can be deducted that the annealed specimen hardness is less
than the un-treated specimen and this is due to the recrystallization of the ferrite structure thereby leading to a
reduction in carbon (due to diffusion) content hence increasing the ductility and reducing the hardness.
It is obvious from Table 2 that the hardness values have increased from the heat affected zone (HAZ) to the weld
pool where the hardness is higher; this is due to the carbon content of the electrode which is added during
welding and solid state diffusion of carbon from the base metal; the higher the carbon diluted or diffused into the
weld pool, the higher the hardness values.
3.5 Discussion of micro-structural results
(i) Untreated Specimen
The value for the untreated weld sample is maximum for hardness; this is due to a more predominant ferrite
structure which is the reason considerable area of the structure is white (the higher the carbon diluted and/or
diffused into the weld zone, the higher the hardness values.) as seen in Plate 1.
The arc welding process also induces residual stresses in the weld during the course of welding; the electrodes
have low carbon content which also adds to the original carbon content of the material thereby increasing
hardness and reducing ductility.
(ii) Annealed Specimen
Annealing the specimen, results in the transformation of austenite grains into ferrite and pearlite. The
microstructure observed, shows a coarse grain of ferrite and a coarse structure of pearlite as a result of the
furnace cooling which imparts ductility on the material.
The value for the annealed sample is minimal for hardness and maximum for toughness. This is due to the
softening effects of the ferrite matrix which arise from liberation of trapped carbon atoms in the saturated ferrite
during annealing as depicted in microstructure of the annealed samples in Plate 2. Leading to a more
predominant pearlite structure which is the reason larger areas of the structure is darker hence reducing hardness
and increasing toughness and ductility.
Also, apart from straining due to saturation by carbon atoms which are relieved, residual stresses in the welds are

51
also relieved during annealing. Normally, arc welding process can induce residual stresses in the weld during the
course of welding; the electrodes have low carbon content. The higher the carbon diluted and/or diffused into the
weld zone, the higher the hardness values. The weld structure having a low carbon content due to
recrystallization have changed from a harder to a more ability to machine the material by improving the ductility
of the material hence reducing the internal stresses.
4. Conclusion
Mechanical property of ductility and toughness in the annealed weld was found to be higher than the un-
annealed weld specimen due to carbon diffusion and re-crystallization from the weld zone during annealing
accompanied with a refined grain structure. This treatment is recommended as final treatment after welding of
low carbon steel when high ductility is required at the expense of hardness as a pointer to the material
performing creditably well in service.
REFERENCES
Adedayo A.V., Ibitoye S.A. & Oyetoyan O.A. (2010), “Annealing Heat Treatment Effects on Steel Welds”.
Journal of Minerals & Materials Characterization & Engineering, 9(6), 547-557.
Bipin, K. S., Tewari, S. P. & Jyoti, P. (2010), “A Review on Effects of Preheating and/or Post Weld Heat
Treatment (PWHT) on Mechanical Behavior of Ferrous Metals”. International Journal of Engineering Science
and Technology, 2(4), 625-626.
David, Hart (1999). Welding and Coating Metallurgy 2. Published in Austalia, 6-7, 12.
Fadare, D. A., Fadara, T. G. & Akanbi, O. Y. (2011), “Effect of Heat Treatment on Mechanical Properties and
Microstructure of NST 37-2 Steel”. Journal of Minerals & Materials Characterization & Engineering, 10(.3),
299-308.
International Institute of Welding Guidelines (1988). For the Classification of Ferrite Steel Weld Metal
Microstructure Constituents Using the Light Microscopy: International Institute of Welding, IIW DOC. IX-
1533-88.
Kahn Robert & Haasen Peter (1996). Physical Metallurgy. 4th Edition, Holland.
Kumar, Ashish B. (2010). Manufacturing Processes 2; Welding. Kanna Publishers, Unit 5, pp. 43, 49, 50, 55, 60.
Maamar, R. H., Otmani, T. R., & Fahssi, N. D. D., (2008) “Heat Treatment and Welding Effects on Mechanical
Properties and Microstructure Evolution of 2024 and 7075 Aluminium Alloys”. Centre de Recherche
Scientifique et Technique en Soudage et Contrôle. Division de Mécanique et Métallurgie, Algeria. 5, 13-15.
Rajan, T.V., Sharma C.P., & Sharma A. (1988). Heat Treatment Principles and Techniques. Prentice-Hall of
India, Private Ltd. New Delhi.

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