Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.8, pp.755-7 64, 2011
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755
Tempering Heat Treatment Effects on Steel Welds
A.V. Adedayo1,3, S.A. Ibitoye1 and O.O. Oluwole2
1 Materials Science and Engineering Department, Obafemi Awolowo University, Ile-Ife,
Nigeria
2Mechanical Engineering Department, University of Ibadan, Ibadan, Nigeria
3 Metallurgical Engineering Department, Kwara State P olytechnic, PMB 1375, Ilorin,
Nigeria
*Corresponding author: [e-mail]
ABSTRACT
This paper reports investigations made on the tempering 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 in
10mm diameter were obtained. The range of the carbon contents of the steel rods was from 0.16
wt% C to 0.33 wt % C. From each grade of the steel materials, grooved specimen of about 150
mm were prepared. The grooves were then filled to create welds using arc welding. A set of the
resulting welds were then subjected to tempering 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, Tempering, Heat treatment, Hardness, Toughness, Microstructure
1. INTRODUCTION
Normally, during the arc welding process, the peak temperature varies from near the melting
point to the lower critical temperature. The duration of the cycle is very short relative to normal
soaking times in heat treating cycles. At high peak temperatures near the fusion line, diffusion is
more rapid and the solute atoms, especially carbon, disperse uniformly in the austenite [1]. Also,
austenite grain growth occurs. At lower peak temperatures, slightly above the austenite

756 A.V. Adedayo, S.A. Ibitoye and O.O. Oluwole Vol.10, No.8
transformation temperature, carbides may not be completely dissolved in the austenite.
Furthermore, the solute atoms that do dissolve because of the relatively low temperature may not
diffuse far from the original site of the carbide. Thus, the austenite at these lower peak
temperatures contains areas of high alloy content and low alloy content. In addition, the austenite
microstructure is fine grained. At intermediate peak temperatures, the homogeneity and the grain
size of austenite are between these extremes. On cooling, the austenite decomposition
temperature and decomposition products depend on the local chemistry and the grain size as well
as the cooling rate. Non-uniform volumetric changes caused by non-uniform heating of the base
and deposit metal, shrinkage of molten metal after welding also lead to internal residual welding
stress.
In the whole, a complex steel weld microstructure which consists of two or more micro
constituents is formed. Most often micro constituents such as: proeutectoid ferrite, polygonal
ferrite, aligned and non-aligned side plate ferrite, ferrite carbide aggregates and acicular ferrite
are formed [2 – 4]. Sometimes, upper and lower bainites, martensites and the A-M (austenite
with martensite) micro constituents may be formed [5, 6]. This complex microstructure mixture
can lead to highly varied properties of the weld [3, 7]. A way to unify the structure of the welds
is by heat treatment. When the presence of martensite may cause embrittlement, tempering is
employed to improve the strength and tough n e s s o f t h e material.
In this study, the effects of tempering heat treatment on mechanical properties and microstructure
of plain carbon steel weld is investigated.
2. EXPERIMENTAL PROCEDURE
Materials used are 10mm steel rods supplied as-rolled from Universal Steel Rolling Mill, Ogba-
Ikeja, Lagos, Nigeria. These are four (4) different steel which are essentially different in carbon
content. The compositions of the steel rods are given in Table 1. 150 mm long pieces were cut
from all grades of the steel rods. The middle of each piece was grooved 6 mm deep and wide
using a grinding wheel as illustrated in Figure 1.
Figure 1: Dimension of grooved specimen.

Vol.10, No.8 Tempering Heat Treatment Effects on Steel Welds 757
Table 1: Composition of steel rods used.
Element (wt %) 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
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 100 A with a terminal
voltage of 80 V. Eight pieces of welds were prepared in all, two from each grade of steel. A set

758 A.V. Adedayo, S.A. Ibitoye and O.O. Oluwole Vol.10, No.8
of four welds were untreated and kept as control, the other set was then tempered at 640 ˚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: (i) the weld pool
region (ii) the weld pool and base metal junction (iii) the heat affected zone (HAZ) see fig. 2.
Figure 2: Hardness test specimen (1) Weld pool, (2) Weld pool and base metal junction, (3) Heat
Affected Zone (HAZ)
The toughness values were determined from 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 carried out with optical
microscopy. The microstructures were captured with an Olympus metallurgical microscope
which has a minisee optical viewing sy s t e m connected to a computer
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 tempered samples. There is increase in
hardness values with increase in carbon content, while the toughness decreased.
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 [2]. IIW DOC IX-1533-88 [2] gives a
detailed classification of weld metal microstructures. The maximum solubility of carbon in
ferrite is 0.025wt%C [8, 9]. This suggests that the ferritic structures in the investigated steels

Vol.10, No.8 Tempering Heat Treatment Effects on Steel Welds 759
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. 7 A2 and 7 A3 which show bainitic structures. Bainitic structures are actually
fine dispersion of iron carbide in a strained ferrite matrix [9]. The lower toughness of the weld
metal with increase in carbon content is also a result of this straining.
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 tempered samples
Vickers Hardness Value
Sample Carbon
content Weld pool zone Weld pool and parent metal junction HAZ
1 0.16 213 521 796
2 0.25 230 546 813
3 0.28 273 553 859
4 0.33 285 560 872

760 A.V. Adedayo, S.A. Ibitoye and O.O. Oluwole Vol.10, No.8
Table 4: Toughness values of untreated samples
Toughness values (J)
Sample Carbon
content Weld pool zone Weld pool and parent metal junction HAZ
1 0.16 20.16 15.6 8.32
2 0.25 18.32 12.6 7.78
3 0.28 16.6 11.64 6.02
4 0.33 15.84 9.8 4.84
Table 5: Charpy toughness for tempered samples
Toughness values (J)
Sample Carbon
content Weld pool zone Weld pool and parent metal junction HAZ
1 0.16 23.59 16.4 8.82
2 0.25 22.48 13.52 8.29
3 0.28 20.56 11.90 6.52
4 0.33 17.71 10.2 5.56
0
250
500
750
1000
0.05 0.10.15 0.20.250.30.35
Carbon content (wt %)
Hard ne ss Val ue (VHN)
Weld pool
Junction
HAZ
Figure 3: Variation of hardness with carbon content for untreated sample

Vol.10, No.8 Tempering Heat Treatment Effects on Steel Welds 761
0
100
200
300
400
500
600
700
800
900
1000
0.05 0.1 0.15 0.2 0.25 0.3 0.35
Carbon c ontent (wt %)
Hardn ess (V HN)
Weld pool
Junction
HAZ
Figure 4: Variation of hardness values with carbon content for tempered samples
0
5
10
15
20
25
0.10.15 0.2 0.25 0.30.35
Carbon c on t ent(wt %)
Toughness (J)
Weld pool
Junc tion
HAZ
Figure 5: Variation of toughness with carbon content for untreated samples
0
5
10
15
20
25
0.05 0.1 0.15 0.2 0.25 0.3 0.35
Carbon c ont ent (wt % )
Toughness (J)
Weld pool
Junc tion
HAZ
Figure 6: Variation of toughness with carbon content for tempered samples

762 A.V. Adedayo, S.A. Ibitoye and O.O. Oluwole Vol.10, No.8
Figure 7: Microstructure of untreated steel welds: (A) 0.16wt%C, (A1) 0.25wt%C, (A2)
0.28wt%C, (A3) 0.33wt%C
Figure 8: Microstructure of tempered steel welds: (A) 0.16wt%C, (A1) 0.25wt%C, (A2)
0.28wt%C, (A3) 0.33wt%C

Vol.10, No.8 Tempering Heat Treatment Effects on Steel Welds 763
By comparing the values of Figs. 3 and 4, Figs. 5 and 6, it is 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 tempered samples, while Figs. 3 and 5 are for the untreated samples.
Generally, the values for the tempered samples are minimal for hardness and maximum for
toughness. During tempering heat treatment, there is ejection of carbon from the super saturated
ferrite matrix [7,10]. This leads to the softening of the ferrite matrix. Also any A-M (Austenite
with Martensite) phases present are converted to ferrite by losing carbon through diffusion
process. Furthermore, the tempering temperature employed (640 ˚C) is equivalent to that used in
recrystallization annealing [11-13], as a result, there is nucleation and growth of new crystals
due to recrystallization. Figure 8 (B, B1, B2, B3) which reveals micrographs with finer crystals
compared to Fig. 7 (A, A1, A2, A3) which are more coarse confirms that recrystallization
actually took place during the tempering heat treatment. The structural modification from coarse
to fine structure after tempering heat treatment is also responsible for the increased toughness
observed.
Also, apart from straining due to saturation by carbon atoms which are relieved, residual stresses
in the welds are also relieved during tempering. 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.
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. Tempering 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.
ACKNOWLEDGEMENT
The authors wish to thank Mr Adepoju O. Elijah of the Universal Steels Rolling Mill, Ogba-
Ikeja, Lagos, Nigeria, for the help in providing the metallurgy steel rods. The authors further

764 A.V. Adedayo, S.A. Ibitoye and O.O. Oluwole Vol.10, No.8
wish to convey their gratitude to the Engineering Materials Development Institute (EMDI)
Akure, Ondo State Nigeria for offering the distinguished opportunity to use the institute’s LECO
micro-hardness tester.
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