EFFECT OF MULTI-PASS ON QUALITY OF A BUTT ARC WELDED MILD STEEL PLATE.
by
S . M. Adedayo, Ph.D and S. Babatunde, M.Eng
*Mechanical Engineering Department, University of Ilorin, Ilorin, Nigeria
Amazon Energy Engineering Ltd, Plot 94, Lekki Epe Express Way,
Lagos, Nigeria. [email protected]
ABSTRACT
Welding is one of the most versatile material joining processes widely used in
industry. Mild, Low carbon steel plates and AISI type 304 stainless steel plates
with 6,8 and 12mm thickness are widely used in the fabrication of pipeline and
pressure vessels. Multi-pass weld is justified under circumstances in which a
single pass weld cannot effectively fill the weld groove. An overlapping heat input
occurs under multipass welding conditions when minimal time between passes is
allowed. The temperature distribution that occurs during multipass welding
affects the material microstructure, hardness, mechanical properties and residual
stresses. This paper attempts to look at what effect multi-run passes has on the
mechanical properties. A 90 X 55 X 12mm thick mild steel plate with a 30o Vee
weld groove was subjected to single, double and four pass weld under
controlled welding conditions.
ABSTRACT CONTD
Toughness, hardness and tensile tests under conditions of single and multiple passes were carried
out. It was observed that toughness values of welds made under multi-pass welds is higher than
both single pass and No weld conditions. Under multi-pass weld, maximum toughness value of
2310KN/m as compared with 2061KN/m under No welding at a distance 9mm from the weld line
was observed. Hardness values of welds made under multi-pass was lower than that made under
single pass welding but both are less than that of an unwelded metal. Maximum hardness value
under 4-pass and double pass was 38.4RB and 40.5RB as compared with 43.2RB under single pass
welding and 48.5RB under No-welding conditions at a distance 12mm from weld-line. Tensile
strength values under multi-pass are less than those made under a No-welding condition. Yield and
ultimate tensile stress under multi-pass and No-weld are 323KN/m2 (347KN/m2 ) and 424(453 KN/m2
) respectively at a distance 9mm from weld line. The overlapping heat inputs resulting from the
multi-run passes had significant effects on the mechanical properties of weldment. Knowledge of
mechanical properties after multiple passes of weld may justify the need for inter-pass annealing or
otherwise.
Keywords: Multi-run weld, weld groove , mechanical properties , microstructure , weld-line.
INTRODUCTION / LITERATURE SURVEY
Multi-run weld entails repetitive welding along the same joint with the aim of achieving
a strong joint and complete filling of the weld groove. This heating cycle however has
the consequent effect of changing the microstructure within the vicinity of the heat
affected zone and by extension the mechanical properties. An uncontrolled welding
heat circle could result into metal cracks and metal toughness reduction in ferrous
materials with relatively high carbon content [ Ibhole et al, 1983 ]. Conventional post
weld heat treatment such as stress relief annealing often causes a reduction in
hardness and residual stresses of the welded plates [ Alexander et al, 1963 ]. Multi-run
weld may affect metal properties just as pre-heat or post weld heat treatment does
affect metal properties depending on time intervals between runs. Post weld heat
treatment ( PWHT ) causes stabilization and reformation of structure of the welded
metal [ Palmar, 1997 ].
The increased volume of grains refined and possible removal of segregation such as
columnar grain boundary carbides do result in higher notch toughness and lower
hardness. [ Smallman, 1985 ]. More recent researches on multi run welds examined
mechanical properties using digital image correlation [ Acar et al, 2009 ]. They
investigated the structural integrity of multi- run welded pipeline using digital image
correlation ( DIC ) technique. Proof stress values from computed local stress strain
variation in mechanical properties within the weld and between the passes was
examined. This work examines the effect of multi-run welds on mechanical
properties of an arc welded steel plate largely at locations around the heat affected
zone (HAZ) and Parent metal zone. Work piece dimension was a pair of mild steel
plate of specification 90 X 55 X 10mm with a weld groove of 30o along the 90mm
length. Standard test pieces were cut from the welded workpieces and results
appropriately presented.
3.0 EXPERIMENTAL PROCEDURES AND MATERIALS
3.1 Work Materials and Machining
Work piece material is mild steel of composition
0.25%C, 0.05%S, 0.08%Si, 0.75%Mn and 0.06%P
and the rest Fe. Each unwelded testpiece material
was machined to specification 110 X 55 X 10mm as
shown in Fig.1( a & b ) for single and multi run
weld respectively.
Figure 1(a,b)- Welding workpiece for single and multi-run passes
Indicated point A was for temperature monitoring. The welded edge of
length 110mm was milled to an angle 15o on one side for single pass and
both sides for multi-run weld passes. Thermocouple probe hole of
diameter 3mm is drilled and located 6mm from the weld centre line.
3.2 Annealing and Alignment
The two halves of the workpieces were tack welded together and set in
proper alignment. Subsequently all work-pieces were annealed at a
temperature 800oC and soaked for one hour. The stress free specimens
were thereafter welded under the following conditions :-
Welding current = 140A
Welding voltage = 80V
Electrodes specification = 2.5mm ( Gauge 10 )
Average welding speed = 2mm/sec.
3.3 Welding Procedures and Temperature Monitor
Three different modes of welding that was done are :-
(i) Single pass welding (ii) Double pass welding
(iii) Four run weld
Each weld mode was done repetitively three times. Welding speed was also
monitored. At the end of the welding operations, the materials were allowed to cool
to normal room temperature and subsequently cut into various test sizes.
Temperature monitoring was done using Ni Cr. thermocouple with the test point as
the hot junction and an insulated bowl of ice blocks at 0o serving as the cold junction.
Necessary precautions was taken to ensure that thermocouple wires remains stable
throughout the duration of welding. Temperature history was monitored in respect
of single pass and double pass weld with the second pass commencing immediately
after the first pass weld.
4.0 MECHANICAL PROPERTY TESTS
4.1 Toughness Test
The machine used was the Avery Denison Izod impact tester with impact velocity of
3.65m/sec. and a capacity of 150joules. Fig.2 shows the test-piece specimen
specifications
Figure 2- Izod Impact Test Specimen
The pendulum of the testing machine was raised and anchored. With no specimen
clamped on the vice the scale on the machine was set to maximum and the pendulum
released
to swing past the vice. Initial value readings was taken and subtracted from all subsequent
experimental readings.. Indicated Izod values were divided by the cross sectional area at
the 60o notch .
4.2 Hardness Tests
Hardness tests were carried out on each zone of the welded plate at equidistance
spacings of 12mm. The hardness scale adapted was the Rockwell Hardness ( RB ) scale
with a 15KgF ball indenter. See Fig.3 for specific test points on welded workpieces.
Three repetitive tests were carried out at each location and mean values reported.
Figure 3- Test pieces layout on welded plate
4.3 Tensile Strength Test
Cylindrical rods of diameter 5mm and 60mm gauge length with enlarged diameter at ends for tensile
machine grip were prepared. The test pieces were cut along parallel lines to the centre line at
equidistance points of 10mm. See Fig.3 for detailed arrangement. Tensile tests were carried out using
Monsanto Tensometer. Three repetitive experimental tests were done at each location and mean
values reported.
5.0 RESULTS AND DISCUSSION
5.1 Effect of Double Pass on Temperature History
Fig.4 shows the effect of multi run weld pass on thermal history at a distance 6mm from the weld
line. Peak temperatures of 460oC was attained under single pass and 670oC under double pass weld.
Higher peak temperature was observed under double pass weld conditions due to a pre-heating effect
caused by the first-run weld. Maximum cooling rate values of 8oC and 7.3oC were observed for the
double and single pass weld respectively. Temperature differential between peak values and adjacent
heat transfer channels accounted for the different cooling rates.
0100
200
300
400
500
600
700
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102108114120126132138144150
Tem
pe
ratu
re (
oC
)
Time ( Sec. )
Fig.4 Effect of Double Pass Weld on Thermal History at Distance 6mm.
Single Double
5.2 Effect of Double Pass on Hardness
Fig.5 shows the effect of multi-pass weld on metal hardness property
between the weld centre-line and unaffected base metal region. A
consistent decrease in hardness value is observed with increasing
number of passes for all weld regions. This is explainable in terms of an
annealing effect caused by each weld run. At 12mm from weld centre
line hardness reduction of 10.9%, 16.5% and 20.8% were observed
under single pass, double pass and four passes weld run respectively
with respect to a main parent metal hardness of 48.5RB
010
20
30
40
50
60
12 24 36 48 60
Ro
ckw
ell
Har
dn
ess
Nu
mb
er
( R
)
Distance From Weld Centre Line (mm)
Fig.5 Effect of Multi-run Weld on Metal Hardness
Single
Double
4 - Pass
No-Weld
5.3 Effect of Multi run Weld on Toughness
Fig.6 shows the effect of double pass run on metal toughness. At a distance 12mm
from weld centre-line, toughness values was 2464KN/m , 2341KN/m and 2170KN/m
for a double , single-weld and unwelded metal conditions. This indicates an 11.93 and
7.9% increases in toughness under double and single pass welds respectively.
0
500
1000
1500
2000
2500
3000
12 24 36 48 60
Tou
ghn
ess
(K
N/m
)
Distance From Weld Centre (mm)
Fig.6 Effect of Double - Pass Weld on Toughness.
Single
Double
4-Pass
No-Weld
5.4 Effect of Multirun Weld on Yield and Ultimate Tensile Strength
Fig.7 and 8 respectively show the effect of multi run weld passes on
Yield and Ultimate tensile stresses. Yield and ultimate tensile stresses is
slightly increased under multi run weld as compared with single pass
weld. Higher stress values were obtained near the heataffected zone
due to microstructural transformations that are likely to take place.
Single and multi-run weld specimens however experienced a reduction
in both stresses in comparison with an unwelded workpiece. Yield
stress reduction of 21.3% and 30.3% were observed for multi-run and
single pass weld at 9mm from weld line.
050
100
150
200
250
300
350
400
450
9 18 27 36 45
Yie
ld S
tre
ss (
KN
/m^2
)
Distance from Weld Centre - Line ( mm )
Fig.7 Effect of Multi-Pass on Yield Stress.
4-Pass
Single
No-Weld
050
100
150
200
250
300
350
400
450
500
9 18 27 36 45
Ult
imat
e T
en
sile
Str
ess
( K
N/m
^2 )
Distance from Weld Centre-Line ( mm )
Fig.8 Effect of Multi-Pass on Ultimate Tensile Stress.
4 Pass
Single
No-Weld
CONCLUSION
Mechanical properties of hardness, tensile strength and toughness were examined for
a single and multi-run weld situations. The following conclusions are drawn :
Weld plate peak temperature is increased under multi-run welding conditions
Metal hardness reduces with increasing number of passes. Hardness values in
welded metal are generally lower than that of the unwelded metal.
Weld metal toughness increase with a double pass weld. Toughness values in
welded metals are above that of an unwelded metal at distances close to the weld
line only.
Tensile strength of weld metal under multi-pass exceeds that of single pass. Tensile
strength values in welded metals are lower than that of unwelded metal
THANKS FOR LISTENING