IOSR Journal of Engineering (IOSRJEN) ISSN (e): 2250-3021, ISSN (p): 2278-8719 Vol. 05, Issue 05 (May. 2015), ||V2|| PP 09-13
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Comparison of residual stress in equal matching and undermatching weld joints of the high strength steel thick plate Xu Dongxia, Li Dayong, Yang Dongqing, Zhang Guangjun* State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, China Abstractďźš- In this paper, numerical simulation by finite element software MARC was used to study the residual stress in equal matching and undermatching weld joints of the high strength steel thick plate. Equal matching weld selected ferritic welding consumables and the undermatching weld consisted of austenitic welding consumables.
3D finite element model of thermal-force coupling was developed and the distribution of
residual stress in equal matching and undermatching weld was acquired. The simulation results were analyzed and compared. The results showed that the equivalent stress and longitudinal stress in undermatching weld joint are significantly smaller than those in equal matching weld joint, two transverse stresses are not very different. The stress maximum in equal matching case occurs in the weld and heat affected zone, and stress maximum in undermatching case appears only in the heat affected zone. Keywords: - equal matching weld, undermatching weld, residual stress, numerical simulation
I.
INTRODUCTION
Due to its high strength, good ductility, toughness and good weldability, low alloy high strength steel thick plate is widely used in pressure vessels, ships, construction machinery, bridges, buildings, etc. cracking is one of the main problems in welding
[1]
. Cold
[2]
. Residual stress due to local heating and strong binding of
thick plate, hardened microstructure and hydrogen content in weld are the three factors to cause cold cracking [3]
. An effective measure to avoid cold cracking for the welding of high strength steel thick plate is the use
of undermatching welding joint
[4]
. That is the strength of filler metal is lower than the base metal of high
strength steel. For undermatching welding joint, plastic deformation of the weld of good plasticity and toughness can lower its three-restraint stress, reduce cold cracking tendency [5]. In this paper, finite element software Marc was used to simulate welding and later cooling process of high strength steel thick plate with equal matching and undermatching filler metal. Calculation and analysis of the distribution of residual stress in two cases were the research focus.
II.
THREE-DIMENSIONAL FINITE ELEMENT MODEL
The model of high strength steel thick plate in the simulation is 50 mm thick, 120mm in width and 300mm in length. The mesh model is shown in Figure1. The model is established by using the simulation software MARC.MSC. In order to balance computing accuracy and efficiency, fine mesh is used around weld, but coarse mesh has to be used far from the weld. There are 30400 elements and 34542 nodes in the model.
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Comparison of residual stress in equal matching and undermatching weld joints of the high strength
Fig.1 Three-dimensional finite element model 2.1 The thermal physical and mechanical properties of used materials Carbon content of high strength steel thick plate adopted in this paper is 0.1%. At the situation of ambient temperature, the melting point is 1450℃,the Poisson’s ratio is 0.285, the density is 7.8kg/m3. The specific heat is 434J·kg-1·℃-1, yield strength 850MPa, heat conductivity 30.7k·℃·W·m-1, thermal expansion coefficient 1.06×10-5℃-1, and Young’s modulus 2.08×105MPa. Young's modulus（ ×104Mpa） Yield stress（ ×102Mpa）
20
90
Mechanical properties
Thermomechanical properties
100
80 70 60
Thermal-conductive coefficient（ W/m*℃） Specific heat（ ×10J/kg*℃）
50 40
Heat expansion coefficient（ 10-6/℃） 15
10
5
30 0
500
1000
1500
2000
2500
3000
0
3500
0
500
1000
Temperature/℃
1500
2000
2500
3000
3500
Temperature/℃
(a) Base matal 65
20
55
Mechanical properties
Thermomechanical properties
60
50
Thermal-conductive coefficient（ W/m*℃） Specific heat（ ×10J*kg/℃）
45 40 35 30 25
Young's modulus（ ×104MPa）
7.644 (13.54%)
15
Heat expansion coefficient（ 10-6/℃） Yield stress（ ×102MPa）
10
5
20 15
0
10 0
500
1000
1500
2000
2500
Temperature/℃
3000
0
500
1000
1500
2000
2500
3000
Temperature/℃
(b) Austenitic welding consumables Fig.2
Thermomechanical properties
At room temperature, the used austenitic welding consumables have 0.3 Poisson’s ratio, and its density is 7.93kg/m3. Figure 2 shows the thermodynamic properties of used materials.
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Comparison of residual stress in equal matching and undermatching weld joints of the high strength III.
RESULTS AND DISCUSSIONS
3.1 Stress nephogram It can be seen from stress nephogram in Figure 3 that the maximal equivalent stress in equal matching case occurs in weld and heat affected zone, which value is 780Mpa. The maximal equivalent stress in undermatching case exists only in heat affected zone, which value is 734Mpa, 6% less than that in equal matching joint.
(a) Equal matching
(b)
Fig.3
Undermatching
Equivalent Von Mises Stress
The transverse stress and longitudinal stress distributions are shown in Figure 4 and Figure 5 respectively. The maximal transverse stress in equal matching joint is 458MpaďźŒ434Mpa in undermatching case. The two values are roughly same.
(a) Equal matching
(b) Undermatching Fig.4
Transverse stress nephogram
(b) Equal matching
(b) Undermatching Fig.5
Longitudinal stress nephogram
For equal matching joint, maximal longitudinal stress is 922Mpa, which occurs in weld and heat-affected zone; For undermatching joint, maximal longitudinal stress is 780Mpa, which exists only in heat-affected zone. It also can be seen that stress distribution in undermatching case is more uniform than that in International organization of Scientific Research
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Comparison of residual stress in equal matching and undermatching weld joints of the high strength equal matching joint. 3.2 Stress distribution curves 900
400
800
Transverse stress/Mpa
Longitudinal stress/Mpa
300
700 600 500
the welding direction
400 300 200
Matching Undermatching
100 0 0
25
50
75
100
125
150
175
200
the welding direction
100 0 -100
Matching Undermatching
-200 0
200
25
50
75
100
125
150
175
200
Distance/mm
Distance/mm
(a) Longitudinal stress
(b) Transverse stress
Equivalent Von Mises Stress/Mpa
800
700
600
500
the welding direction
400
Matching Undermatching
300 0
25
50
75
100
125
150
175
200
Distance/mm
(c) Equivalent stress Fig.6
Distribution of residual stress on weld toe along weld direction in two cases
Residual stress distribution on weld toe along welding direction is shown in Figure 6. There exists a stable zone in the middle part of weld for the transverse stress, longitudinal stress and equivalent stress. Stresses on two ends of weld vary quickly.
The equivalent stress and longitudinal stress on weld toe of undermatching
weld joint are significantly smaller than those in equal matching weld joint. The maximal equivalent stress on weld toe in equal matching joint is 773Mpa, the maximal longitudinal stress is 801Mpa. For undermatching joint, maximal equivalent stress on weld toe is 601Mpa, 22% smaller than equal matching joint, and the maximal longitudinal stress is 653Mpa, 18% smaller than equal matching joint. The transverse stresses in tow cases are roughly same. Figure 7 shows stress distribution on top surface of the middle section with different filling materials. It can be seen that the trend of stress distribution is roughly same in equal matching and undermatching joint. The difference is that the stress in undermatching weld is significantly less than stress in equal matching joint.
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Comparison of residual stress in equal matching and undermatching weld joints of the high strength 500
Matching Undermatching
400
600
Transverse stress/Mpa
Longitudinal stress/Mpa
800
400
200
0
300 200 100 0 -100
Matching Undermatching
-200 -200 0
10
20
30
40
50
60
70
80
90
-300
100
0
10
20
30
Distance/mm
(a) Longitudinal stress
50
60
70
80
90
100
(b) Transverse stress Matching Undermatching
800
Equivalent Von Mises Stress/Mpa
40
Distance/mm
700 600 500 400 300 200 100 0 0
10
20
30
40
50
60
70
80
90
100
Distance/mm
(c) Equivalent stress Fig.7
Distribution of residual stress perpendicular to weld direction in two cases
IV.
CONCLUSIONS
(1) Temperature field and stress field of high strength low alloy steel weldment with different filling materials were successfully simulated by Marc finite element simulation software. (2)The equivalent stress and longitudinal stress in undermatching weld joint are significantly smaller than those in equal matching weld joint, two transverse stresses are not very different. (3) The stress maximum in equal matching case occurs in the weld and heat affected zone, and stress maximum in undermatching case appears only in the heat affected zone.
V.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China under grant No. 51175119.
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Deb P, Challenger K D, Therrien A E. Structure-property correlation of submerged-arc and gas-metal-arc weldments in HY-100 steel[J]. Metallurgical Transactions A, 1991, 18(6): 987-999.
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Duane K. Miller, P.E. Use Undermatching Weld Metal Where Advantageous. Welding Innovation Vol. XIV, NO.1,1997.
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