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

adyos1@yahoo.com

Amazon Energy Engineering Ltd, Plot 94, Lekki Epe Express Way,

Lagos, Nigeria. babasamo2001@yahoo.com

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

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