Comparative analysis of microstructure and quality of gas metal arc welded and shielded metal arc welded joints.
Bendikiene, R. ; Janusas, G. ; Zizys, D. 等
1. Introduction
Welding, the fusing of the surfaces of two work pieces to form one
is a precise, reliable, cost-effective, and convenient method for
joining metal alloys. No other technique used by manufacturers to join
metals and alloys in such a big extent, because welding is a fast and
economical process to compose joint of two different materials.
Actually, many products such as building constructions, pipelines,
automobiles, and others could not be made without the use of welding
[1-3].
Every year, a lot of rejects appear due to poor techniques of
welders, lack of control or choice of poor materials in order to save a
fraction of expenses.
The welding processes found their own niche in metal production and
manufacturing industry. Numerable welding techniques used in practise
include submerged arc welding [4], tungsten inert gas welding [1, 5],
metal inert gas welding [2], plasma arc welding [6] and etc.
In fusion welding processes, a metal alloy undergoes large local
structural changes. The thermal expansion of the weld metal and nearby
areas is restricted by the surrounding cold metal. This initiates the
formation of residual plastic strains in the weld metal and the nearby
area. These plastic strains are referred to as characteristic strains
and are considered to be responsible for causing welding deformations
and further defects of the weldment. Once the relation between the
welding heat input and the characteristic strain distribution is
established, the residual stress and deformation can be calculated by
elastic analysis using characteristic strain as initial strain [5].
The main target of this study is to compare two of the most
commonly used types of welding to make a joint and to evaluate quality
and microstructure [7] of welds.
Gas metal arc welding (GMAW) is widespread for plastically deformed
or closed parts of the automobile body and is frequently used where the
part geometry restricts the application of resistance spot welding (RSW)
or when the design requires supplementary joint strength and stiffness.
The application of arc welding and flash butt welding processes for
steel welding has also been reported [8]. The joined parts using GMAW
typically undergoes a higher heat input and lower heating and cooling
rates than other welding techniques in automotive applications. During
GMAW welding, the microstructure is effected by metal arc heat different
from that used for its production. Local heat input of the welding heat
source that induces a large temperature gradient on the work piece
changes the microstructure, and hence the mechanical properties.
GMAW is very useful due to its flexibility, possibility to weld
metals of different thickness, high production capability, and
possibility of automatic implementation. As in many other types of
welding, the weld geometry and molten pool thermal properties are
controlled in order to increase the mechanical strength of weld
connections and reduce the presence of weld defects and in general,
increase the quality of weld [9].
Shielded metal arc welding (SMAW) can be performed on different
materials of different thickness as well. This explains why repair
welding has conventionally been carried out by manual SMAW operations
[10]. The metal coalescence of SMAW is heated by an electric arc between
the covered metallic electrode and the base metal. The electrode
consumes itself during the SMAW process. Shielding is obtained from the
decomposition of the electrode covering. Filler metal is obtained from
the electrode. The arc is initiated by momentarily touching of the
electrode to the base metal. The heat of the arc melts the surface of
the base metal to form a molten pool at the end of the electrode.
The aim of this work is to quantify the microstructure in different
parts of the GMAW and SMAW welded joints and to make a qualitative
correlation between the microstructure, quality and the tensile strength
of the welded joint.
2. Materials and experimental procedures
The material used in this research as base metal was non-alloy
structural steel S235JR. It was chosen for the analysis and quality
evaluation of welded joints using GMAW and SMAW technologies; steel
characteristics are given in Table 1.
Presented steel grade is suitable for cold forming such as bending,
folding, bordering, flanging, etc. It possesses good weldability with
conventional welding processes. In most cases, pre- or post-heat
treatment is not necessary when it comes to welding. Steel S235JR is
applied for building components, containers, and storage tanks and for
rolled profiles. With distinctly closer chemical composition values and
mechanical properties, the steel grades of the S235-S355 series are used
as material for wheels of passenger cars, Lorries and other vehicles.
Maximum carbon equivalent value:
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15 = 0.35.
Welding samples of the non-alloy structural steel (dimensions
specified in Fig. 1) were machined according to Lithuanian standards.
The welding was done in two passes. Six specimens were welded using SMAW
method and six using GMAW technique. The welding scheme is shown in Fig.
2.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
The composition of shielding gas used was: 82% argon, 18%
C[O.sub.2] and < 0.03% NO. OK Autrod 12.50 coated non-copper filler
metal was used as filler metal. The microstructural analysis of ground,
polished and etched specimen was performed using optical microscopy. The
test pieces for light optical microscopy (LOM) examination were prepared
in longitudinal direction towards the welding seam in order to see all
specific zones of the weld (heat affected zone). Also test pieces were
polished up to fine diamond (~1 [micro]m) finish and etched chemically
for 10-30 sec. using solution: 2 volume parts of nitric acid, 98 volume
parts of alcohol (nital reagent) and saturated solution of picric acid
in alcohol (picral). Afterward, the screening of microstructures was
done using a light microscope LMA 10 equipped with the YCH 15 camera,
magnification of images 100x. To analyse the microstructure of the
welded sample, the following areas were selected for microstructural
analysis (Fig. 3).
It can be seen from the figures, that the welding was done in two
passes. The distinctive areas from 1 to 5 show the microstructure inside
the weld, while areas from 6 to 13 show the heat affected zone. There,
the melting of base metal and filler metal was expected to be seen.
Areas from 15 to 18 show the base metal; it was expected to be
intact and not affected by the heat in the welding seam. The highest
temperature, as well as the biggest microstructure change, was expected
in the welding pool. The temperature drop was inversely proportional to
the distance from the welding pool. The other important zone is HAZ.
Micro structural changes may affect the mechanical properties of the
weld (possible brittle martensite formation).
[FIGURE 3 OMITTED]
Ferrite and ferrite with tertiary cementite was expected to form in
the boundary zones. Three main zones were expected at HAZ: sub-critical,
heated to less than 723[degrees]C, free of austenite, with some stress
relief; intercritical, partial austenite formation on heating which
reverts to ferrite pearlite on cooling, and super-critical, complete
transformation to austenite, grain refinement or the possibility of
growth depending on maximum temperature.
3. Results
Two types of welding methods were used. For the evaluation of
mechanical properties tensile specimens were made. The theoretical
models characterise the intended model of grain structure formation due
to melting and solidification of the weld. In this section, the obtained
microstructure images will be compared to the intended theoretical
models of HAZ and fusion zone micro-structure formation. The
micro-structure investigation results will be compared to the tensile
strength results [11] for a validation.
The grain size of test pieces was investigated, because it has a
great effect on the ductility of material. It will be considered that
the microstructure for all the test pieces that share similar welding
types is comparatively the same and conclusions will be drawn on this
basis.
3.1. Analysis of GMAW test pieces microstructure
The analysis of the specimen was carried out with the 18 images
taken in distinctive areas of HAZ of GMAW welded specimen.
The analysis was done according to the theoretical model of heat
distribution and grain structure formation in relation to carbon
percentage and temperature shown. The grain evaluation method, also
known as Jeffries' Method was used for counting the average number
of grains per known microstructure. This method helped to evaluate the
effect of grain size on ductility of the test pieces.
Analysis of the obtained samples (areas from 1 to 5 shown in
mapping of Fig. 3, a) indicated that the microstructure of given
location mainly contained of ferrite (sample 1), and the structure was
changing into pearlite + ferrite (sample 3 in Fig. 4) and then it
revealed a considerable amount of pearlite in ferrite matrix (sample 5
as shown in Fig. 4).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The investigation of areas further from welding bead confirmed the
predictions that the structure should contain some pure ferrite with
aligned carbides and pearlite + ferrite structures. The ferrite grains
in the outer regions of the welding seam gets coarser, indicating the
decreased ductility of the welding seam. The finer grains are
characterised as having better mechanical properties than the metal of
the same chemical composition though having more coarse grains: better
tensile properties, higher yield points and strength, fatigue
resistance. The grain diameter varies from 0.5 pm to 4 pm, which mostly
depends on temperatures reached and on the cooling rate.
The higher temperature, the coarser ferrite grains --the lower
material ductility. Fig. 5 shows other distinctive areas of the welding
seam and its microstructure. Judging on grain diameters and no presence
of transformation products, the conclusion may be drawn that the
specimens were not overheated and the cooling rate was enough, thus
preserving material's ductility and strengthening the welding seam.
[FIGURE 6 OMITTED]
The comparison of the grain size of GMAW welded specimens was done
to find out how the grain size changes along the weld seam. The visual
results, shown in Fig. 6, indicate that the grain sizes in the welding
seam are quite coarse but change slightly depends on the location. The
coarsest structure was found at the bottom of the weld (Fig. 3, a). The
finest grains were found outside the welding seam (No. 17) and furthest
from the welding surface with ASTM grain size number around 1. ASTM
grain size number in the upper layers of HAZ is 0 and negative.
The comparison was done by inscribing a circle of a known area, A =
314 [cm.sup.2] on an image of [100.sup.x] magnification. The number of
grains completely falling into the area was counted, and then the number
of grains partially falling into the area was counted and divided by 2.
Both results are summed up and divided by the area of the circle. The
results are the number of the grains per 1 [cm.sup.2] at [100.sup.x].
3.2. Analysis of SWAW test pieces microstructure
The SMAW welded test pieces were found to be quite different from
the GMAW ones.
Fig. 7 indicates that the central part of the weld has no fine
ferrite; it remains similar through the whole welding seam and has
assumed a needle-shaped form with coarse structure.
[FIGURE 7 OMITTED]
This clearly indicates that the structure has been overheated and
probably slightly hardened since the welding procedure took place in
room temperature and this can have contributed towards poor weld
condition and appearance of inner stresses. The mapping of the SMAW
microstructure investigation specimen is seen in Fig. 3, b.
[FIGURE 8 OMITTED]
Investigation of further locations of HAZ revealed that it is very
similar to the central part of the welding seam. The microstructure
images of test pieces' are seen in Fig. 8.
The dendrite and needle -shaped ferrite structure with coarse
grains is seen everywhere. These features clearly indicate the
overheating of the whole welding seam and possible hardening.
The comparison of SMAW welded test pieces' grain size was done
to find out how the grain size changes along the weld seam. The visual
results, shown in Fig. 9, indicate that the grain sizes in the welding
seam are extremely coarse, differing significantly from the grain size
out of weld. The exact results of grains per 1 [cm.sup.2] at 100x are
given in Table 2.
[FIGURE 9 OMITTED]
3.3. Comparison of the results of microstructure investigation with
the results of tensile strength test Test results of the tensile
strength performed in the work [11] and microstructure results partially
coincide. The microstructure investigation of GMAW specimens parallel to
the results of tensile strength test; microstructure of GMAW specimen
does not show any significant anomalies or faults done due to
overheating of the welding seam or other characteristics of HAZ. On the
other hand, the microstructure of SMAW specimens' show clear signs
of overheating and signs of hardening that may have caused inner
stresses of the structure and weakening of the welding seam. The coarse
grain structure also confirms the conclusion of overheating, and the
dendrite, needle-shaped ferrite structure speaks of partial hardening
due to cold temperatures of the surroundings. Comparison of grain sizes
of SMAW and GMAW specimens shown in Table 2.
[FIGURE 10 OMITTED]
The comparison clearly shows that the microstructure of GMAW
specimen weld seam contains 4 to 5 times more grains per [cm.sup.2] than
SMAW test pieces.
The one problem with the GMAW is that the weld surface contains
significantly coarser structure than the rest of GMAW weld. This may
also indicate effect of cold air or local overheating of material.
The table and diagram (Fig. 10) were presented for the comparison
of GMAW and SMAW test pieces according to location of failure, number of
grains per cm2 at 100x and comparative elongation of specimens. This was
done to demonstrate how specimens' ductility depends on the grain
size.
Comparing results of microstructure investigation with results of
tensile strength test, given in the Table 3 [11], correlation between
both is clearly seen.
The coarse structure of SMAW test pieces welding seam resulted in
loss of ductility and mechanical properties and thus in weakening of the
welding seam as a whole. Most of SMAW test pieces have failed tensile
strength test due to weakness of welding seam, thus failing demonstrates
mechanical properties of base metal. The weakness of welding seam in
SMAW test pieces was caused by two factors: macro structural faults of
the welding seam (such as undercutting, lack of fusion, insufficient
penetration, etc.) and micro structural faults, such as overheating or
wrong temperature treatment chosen.
Failure of SMAW specimen was more of a rule than an exception in
this particular analysed case.
GMAW specimens have sustained their strength and ductility, passing
the test with one exception. Fig. 10 clearly indicates that GMAW test
pieces are spread in the upper zone of the diagram, sustaining ductility
and finer grain structure. The welding seam withstood the test; the
fractures took place in the base metal instead of welding seams thus the
standard indicates that the welding seam is required to hold 90% of
maximum stress required for base metal. Micro structural investigation
confirmed the results. No significant overheating was detected, thus
indicating good quality of the welding seam. Failure of a GMAW specimen
was more of an exception than a rule in this particular case.
4. Conclusions
1. The grain size in GMAW test pieces varies from 0.5 pm to 4 pm,
which is mostly dependent on temperatures reached and the cooling rate.
The higher the temperature - the coarser the ferrite grains, on other
hand the coarser the grains--the lower the material ductility. Finer
grains are characterised as having better mechanical properties. The
GMAW specimens were not overheated during welding, therefore giving it a
strengthening effect.
2. The grains in SMAW welding seam are extremely coarse, differing
significantly from the grain size out of welding seam. The
microstructures of SMAW test pieces' show the clear signs of
overheating and signs of annealing that cause inner stresses of the
structure and weakening of the welding seam.
3. The comparison clearly shows that the microstructure of GMAW
test piece weld seam contains 4 to 5 times more grains per [cm.sup.2]
than SMAW test pieces.
4. The coarse structure of SMAW test pieces welding seam resulted
in the loss of both ductility and mechanical properties thus in
weakening of the welding seam as a whole.
http://dx.doi.org/ 10.5755/j01.mech.21.3.9861
Received February 17, 2015
Accepted April 02, 2015
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R. Bendikiene *, G. Janusas **, D. Zizys ***
* Kaunas University of Technology, Studenty 56, 51424 Kaunas,
Lithuania, E-mail: regita.bendikiene@ktu.lt
** Kaunas University of Technology, Studenty 56, 51424 Kaunas,
Lithuania, E-mail: giedrius.janusas@ktu.lt
*** Kaunas University of Technology, Studenty 56, 51424 Kaunas,
Lithuania, E-mail: kat0715@ktu.lt
Table 1
Characteristics of steel S235JR
Steel C % for nominal thickness of metal
grade
S235JR * [less than or >16 [less than or > 40
equal to] 16 equal to] 40
[less than or [greater than or [less than or
equal to] 0.17 equal to] 0.17 equal to] 0.20
Chemical composition, %
Mn P S N CEV
1.4 0.035 0.035 0.012 0.35-0.40
Characteristic yield strength [[sigma].sub.y] = 235 MPa
Characteristic ultimate strength [[sigma].sub.u] = 360 MPa
Countable yield strength [[sigma].sub.yd] = 215 MPa
Countable ultimate strength [[sigma].sub.u,d] = 325 MPa
Nominal thickness of base metal- [less than or equal to] 16 mm
* Steel grade according to LST EN 10027-1
Table 2
Comparison of SMAW and GMAW welded test pieces
grain size per [cm.sup.2] at [100.sup.x]
Left joint of Center of Right joint
base metal welding seam of base metal
and weld and weld
SMAW GMAW SMAW GMAW SMAW GMAW
Grains in 0.02 0.04 0.04 0.06 0.03 0.03
[cm.sup.2] at 0.02 0.15 0.01 0.06 0.01 0.11
[100.sup.x] 0.02 0.11 0.01 0.14 0.02 0.15
Average 0.02 0.10 0.02 0.09 0.02 0.10
grain in
[cm.sup.2] at
[100.sup.x]
Table 3
Comparison of SMAW and GMAW welded test pieces [11]
No. Location of Min. Min.
fracture allowed allowed
[[sigma].sub.y] [[sigma].sub.u]
2 Weld seam 213 324
3 Base metal 235 360
4 Base metal 235 360
5 Base metal 235 360
6 Base metal 235 360
8 Weld seam 213 324
9 Weld seam 213 324
10 Weld seam 213 324
11 Base metal 235 360
12 Weld seam 213 324
No. Actual Actual Status Extent
[[sigma].sub.y] [[sigma].sub.u] [delta], %
2 143.9 395.7 Failed 7.2
3 243.0 427.0 Passed 10.3
4 275.5 420.1 Passed 11.8
5 243.0 427.0 Passed 9.7
6 238.8 432.0 Passed 12.7
8 190.5 213.5 Failed 1.7
9 155.4 258.4 Failed 2.6
10 162.0 260.3 Failed 4.9
11 236.8 432.0 Passed 10.9
12 141.7 181.8 Failed 3.2
No. Comparative Faults *
contraction of
cross-section [psi], %
2 16.2 4;
3 43.6 0;
4 48.5 0;
5 44.9 0;
6 59.5 0;
8 23.1 1; 2; 3; 4;
9 35.1 1; 5; 3;
10 45.2 1; 3; 4;
11 51.4 1;
12 29.6 1; 4; 5;
