Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.6, pp.547-557, 2010
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547
Annealing Heat Treatment Effects on Steel Welds
*A.V. Adedayo
1,2
, S.A. Ibitoye
2
and O.A. Oyetoyan
3
1
Department of Metallurgical Engineering, Kwara State Polytechnic, Ilorin, Nigeria
2
Materials Science and Engineering Department, Obafemi Awolowo University, Ile-Ife, Nigeria
3
Department of Metallurgical Engineering, Nigerian Naval Engineering School, Sapele, Nigeria
*Corresponding author: [e-mail]
ABSTRACT
This paper reports investigations made on the annealing heat treatment effects on steel welds.
The properties of the weld investigated were hardness value and toughness. Micro examination
of the samples was also done with optical microscopy. Four (4) different grades of steel rods
(10mm) in diameter were obtained. The range of the carbon contents of the steel rods was from
0.16 wt pct C to 0.33 wt pct C. From each grade of the steel materials, grooved specimen of
about 150mm were prepared. The groves were then filled to create welds using arc welding. The
resulting welds were then subjected to annealing heat treatment. The hardness values and
toughness of the welds were determined. The microstructural analyses of the welds were carried
out as well. The results show that hardness and toughness were dependent on the carbon
content. There was also significant microstructural modification due to heat treatment.
Keywords: Welding, Annealing, Heat treatment, Hardness, Toughness, Microstructure
1. INTRODUCTION
In equipment manufacturing, there are occasions that the steel welds need to be heat treated after
welding. This is because that steel weld metallurgy differs significantly from the base metal
metallurgy in several respects: heating and cooling rates of a weld are much faster than those of a
steel base metal during its manufacturing process. The microstructure of the weld metal is
columnar and as melted and it was not subjected to any subsequent thermo-mechanical
treatment.

548 A.V. Adedayo, S.A. Ibitoye and O.A. Oyetoyan Vol.9, No.6
Generally, steel weld metal microstructure is a complex mixture of two or more constituents,
such as proeutectoid ferrite, polygonal ferrite, aligned and non-aligned side plate ferrite, ferrite
carbide aggregates and acicular ferrite [1, 2] . Sometimes, upper and lower bainites, martensites
and the A-M (austenite with martensite) microconstituents may be formed [3, 4]. This complex
microstructure mixture can lead to highly varied properties of the weld. Generally, the properties
of steels are related to its structural make-up [5]. A way to unify the structure of the welds is by
heat treatment.
Heat treatment involves heating and cooling of a metal or alloy in its solid state with the objects
of changing the characteristics of the material. Heat treatment processes include: normalizing,
hardening (age hardening, surface hardening, case hardening, flame hardening, induction
hardening etc), tempering, annealing, etc. The objects of annealing are to soften the metal,
improve machinability, refine grain size due to phase recrystalization, increase ductility of metal,
prepare steel for subsequent treatment, modify electrical and magnetic properties, relieve internal
stresses, remove gases, and produce a definite microstructure [5, 6].
2. METHODOLOGY
Materials used are 10mm steel rods supplied as-rolled from Universal Steel Rolling Mill, Ogba-
Ikeja, Lagos, Nigeria. There are four (4) different steel samples which are essentially different in
carbon content. The compositions of the steel rods are given in Table 1.

Vol.9, No.6 Annealing Heat Treatment Effects on Steel Welds 549
Table 1: Composition of steel rods used.
Element (wt pct) Sample 1 Sample 2 Sample 3 Sample 4
C 0.2529 0.1576 0.2756 0.3320
Si 0.1468 0.1821 0.1769 0.3116
S 0.0510 0.0598 0.0523 0.0520
P 0.0419 0.0288 0.0275 0.0274
Mn 0.3658 0.6440 0.6247 0.7523
Ni 0.1034 0.1030 0.1170 0.1110
Cr 0.889 0.1224 0.1306 0.1750
Mo 0.0177 0.0114 0.0141 0.0170
V 0.0003 0.001 0.0013 0.003
Cu 0.3213 0.3380 0.3949 0.2773
W 0.0023 0.0007 0.0007 0.0043
As 0.0053 0.0033 0.0033 0.0045
Sn 0.0278 0.0797 0.0797 0.0268
Co 0.0098 0.0086 0.0086 0.0094
Al 0.0049 0.0025 0.0025 0.0025
Pb 0.0012 0.0003 0.0003 0.0017
Ca 0.0004 0.0001 0.0001 0.0005
Zn 0.0033 0.0069 0.0069 0.0061
Fe 98.5550 98.2505 98.2505 97.8859
150mm long pieces were cut from all grades of the steel rods. The middle of each piece was
grooved 6mm deep and wide using a grinding wheel as illustrated in Fig. 1.

550 A.V. Adedayo, S.A. Ibitoye and O.A. Oyetoyan Vol.9, No.6
6.0mm
6.0mm
150mm
10.0mm
Fig. 1: Dimension of grooved specimen.
The grooved samples were then filled in the course of welding to create a weld. AWS E 6013
electrodes were used with a.c. arc welding process. The current used was 100A with a terminal
voltage of 80V. Eight pieces of welds were prepared in all, two from each grade of steel. A set of
four welds were untreated and kept as control, the other set was then annealed at 800
0
C for 60
minutes using Deguassa-Durferrit furnace.
Hardness values of the welds were determined using a LECO micro-hardness tester which uses a
diamond indenter. The test load was 98.07mN (10gf) and the dwell time was 10 seconds. The
LECO micro-hardness tester automatically calculates the hardness values in Vickers hardness
(VHN). The hardness values of the steel welds were evaluated at three points. The points are: (i)
the weld pool region, (ii) the weld pool and base metal junction, and (iii) the heat affected zone
(HAZ), see Fig. 2.
1
2
3
Fig. 2: Hardness test specimen (1) Weld pool, (2) Weld pool and base metal junction, (3) Heat
Affected Zone (HAZ)
The toughness values were determined by notched specimen of circular cross section prepared
from the steel welds. The specimens were notched at the required points to evaluate their
toughness at those points. Micro-examination of the steel welds was also carried out using
optical microscopy. The microstructures were captured using an Olympus metallurgical
microscope with a minisee optical viewing system connected to a computer.

Vol.9, No.6 Annealing Heat Treatment Effects on Steel Welds 551
3. RESULT AND DISCUSSION
Tables 2 to 5 were used to generate figures 3 to 6. Figures 3 and 4 show the variation of the
hardness values, while Figs. 5 and 6 show the toughness values of the specimen along the weld.
These figures show a general trend for each of the annealed samples. There is increase in
hardness values with increase in carbon content, while the toughness decreased.
Table 2: Vickers hardness values for untreated samples.
Vickers Hardness Value
Sample Carbon
content
Weld pool
zone
Weld pool and parent metal
junction
HAZ
1 0.16 230 543 805
2 0.25 243 553 827
3 0.28 281 566 874
4 0.33 296 570 892
Table 3: Vickers hardness values for annealed samples.
Vickers Hardness Value
Sample Carbon
content
Weld pool
zone
Weld pool and parent metal
junction
HAZ
1 0.16 204 493 770
2 0.25 215 503 784
3 0.28 235 520 806
4 0.33 243 534 824

552 A.V. Adedayo, S.A. Ibitoye and O.A. Oyetoyan Vol.9, No.6
Table 4: Toughness values of untreated samples.
Vickers Hardness Value
Sample Carbon
content
Weld pool
zone
Weld pool and parent metal
junction
HAZ
1 0.16 204 493 770
2 0.25 215 503 784
3 0.28 235 520 806
4 0.33 243 534 824
Table 5: Charpy toughness for annealed samples
Vickers Hardness Value
Sample Carbon
content
Weld pool
zone
Weld pool and parent metal
junction
HAZ
1 0.16 204 493 770
2 0.25 215 503 784
3 0.28 235 520 806
4 0.33 243 534 824
0
250
500
750
1000
0.050.10.150.2 0.25 0.3 0.35
Carbon content (wt pct)
Hardness Value (VHN)
Weld pool
Junction
HAZ
Figure 3: Variation of hardness with carbon content for untreated sample

Vol.9, No.6 Annealing Heat Treatment Effects on Steel Welds 553
150
250
350
450
550
650
750
850
950
0.05 0.10.15 0.20.25 0.30.35
Carbon content (wt pct)
Hardness value (VHN)
Weld pool
Junction
HAZ
Figure 4: Variation of hardness values with carbon content for annealed samples
0
5
10
15
20
25
0.10.15 0.2 0.25 0.30.35
Carbon content(wt pct)
Toughness (J)
Weld pool
Junction
HAZ
Figure 5: Variation of toughness with carbon content for untreated samples

554 A.V. Adedayo, S.A. Ibitoye and O.A. Oyetoyan Vol.9, No.6
0
5
10
15
20
25
30
35
0.10.15 0.20.25 0.30.35
Carbon content (wt pct)
Toughness (J)
Weld pool
Junction
HAZ
Figure 6: Variation of toughness with carbon content for annealed samples
This is also the trend for the untreated samples. Normally, the structure of the steels investigated,
viz: 0.16wt%C, 0.25wt%C, 0.28wt%C and 0.33wt%C are essentially ferritic. Ferritic structures
could be: proeutectoide ferrite, polygonal ferrite, aligned and non-aligned side plate ferrite,
ferrite carbide aggregates, acicular ferrite, bainitic etc [1,2]. IIW DOC IX-1533-88 gives a
detailed classification of weld metal microstructures [1]. The maximum solubility of carbon in
ferrite is 0.025wt%C [7, 8]. This suggests that the ferritic structures in the investigated steels
were supersaturated with carbon. This saturation leads to straining of the ferrite matrix and thus
consequently leading to increase in hardness values with increase in carbon content. The higher
the carbon content, the higher the straining. The straining of the ferritic structure is actually
evidenced by Figs. 7C and 7D which show bainitic structures. Bainitic structures are actually
fine dispersion of iron carbide in a strained ferrite matrix [8]. The lower toughness of the weld
metal with increase in carbon content is also a result of this straining.

Vol.9, No.6 Annealing Heat Treatment Effects on Steel Welds 555
Fig 7: Microstructure of untreated steel welds: (A) 0.16wt%C, (B) 0.25wt%C, (C) 0.28wt%C,
(D) 0.33wt%C
Fig 8: Microstructure of annealed steel welds: (A) 0.16wt%C, (B) 0.25wt%C, (C) 0.28wt%C,
(D) 0.33wt%C

556 A.V. Adedayo, S.A. Ibitoye and O.A. Oyetoyan Vol.9, No.6
By comparing the values of Figs. 3 and 4, Figs. 5 and 6, it is quite obvious that the trend is
decreasing, i.e. hardness values decrease from Fig 3 to Fig. 4 while it increased from Fig. 5 to
Fig. 6. Figures 4 and 6 are for the annealed samples, while Figs. 3 and 5 are for the untreated
samples. Generally, the values for the annealed samples are 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 super saturated ferrite during annealing as depicted in
microstructure of the annealed samples. Figure 9 clearly identifies ferrite and pearlite. The
carbon atoms are liberated by a diffusion controlled process which precipitates a second phase,
an iron – carbon compound with the formula Fe
3
C(cementite). The alternate arrangement of
cementite and ferrite gives a lamellar pearlite structure. This is evident as depicted in the
micrographs of the annealed samples.
Figure 9: Constituents of microstructure of annealed weld.
Also, apart from straining due to saturation by carbon atoms which are relieved, residual stresses
in the welds are also relieved during annealing. Normally, arc welding process can induce
residual stresses in the weld during the course of welding. Also, it is obvious that the hardness
values increased from the weld pool through to the heat affected zone (HAZ) where the hardness
values are higher. The toughness however decreased. Generally, the electrodes have low carbon
content. However, there is carbon pick-up in the weld pool due to dilution and solid state
diffusion from the base metal. The higher the carbon diluted and/or diffused into the weld pool,
the higher the hardness values.

Vol.9, No.6 Annealing Heat Treatment Effects on Steel Welds 557
4. CONCLUSION AND RECOMMENDATION
The result of the research shows that the hardness and toughness values of the specimen varies
with the carbon contents of the specimen. Generally, there was increase in hardness values with
increase in carbon content while toughness decreased. It was also found out that the hardness
values were minimal at the weld pool compared to the HAZ. It is the other way round for
toughness. Annealing significantly affects the microstructure and thus the mechanical properties
of the weld.
The range of steels investigated in this work excludes high carbon steels. Further experiment
should be carried out to investigate this range of steels.
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