Journal of Nondestructive Evaluation

Journal of Nondestructive Evaluation ULTRASONIC INVESTIGATION OF THE EFFECT OF CARBON CONTENT IN CARBON STEELS ON BULK RESIDUAL STRESS --Manuscript Draft-Manuscript Number:

JONE-D-15-00024R1

Full Title:

ULTRASONIC INVESTIGATION OF THE EFFECT OF CARBON CONTENT IN CARBON STEELS ON BULK RESIDUAL STRESS

Article Type:

Original Research

Keywords:

residual stress, carbon content, ultrasonic, finite element

Corresponding Author:

Fatih Uzun Yeditepe University İstanbul, TURKEY

Corresponding Author Secondary Information: Corresponding Author's Institution:

Yeditepe University

Corresponding Author's Secondary Institution: First Author:

Fatih Uzun

First Author Secondary Information: Order of Authors:

Fatih Uzun Ali Nezihi Bilge

Order of Authors Secondary Information: Abstract:

Formation of residual stress as a result of welding process is a familiar fact, but its relation with material composition is unknown. This study aims to investigate the effect of carbon content on welding residual stress in carbon steels. For this purpose, samples of ultra-low carbon interstitial free, low carbon and medium carbon steels are selected. Welding is performed as a beam on plate in spite of a joining process. Weld grooves are prepared at center of the rectangular samples. Automated submerged arc technique is preferred for welding process in order to ensure same welding parameters at each sample. Ultrasonic sound waves are used to determine residual stress. This non-destructive technique provides bulk residual stress which is average of shear and normal stresses through thickness of a material. A new approach is practiced to verify experimentally determined bulk residual stress with finite element simulation results. The model geometry of numerical analysis is divided into equivalent parts and average of shear and normal stresses is calculated. Non-destructive ultrasonic technique seems to be in good with finite element analysis.

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Response to Reviewer Comments

Journal of Journal of Nondestructive Evaluation Dear Editor, Please find attached for your kind review our manuscript entitled “The Effect of Carbon Content in Carbon Steels on Formation of Welding Residual Stress”. Previously submitted manuscript is revised according to the reviewer comments. Incorrect statement is corrected. Details about the full annealing process are added. Investigations on the effects of machining process are explained. Required details about shear waves are discussed. Details of corrections are listed below point by point:  Page 2: Response to first comment (you mean travel and oscillates) Incorrect statement is replaced with a new statement, "oscillate and propagate".  Pages 3 and 4: Respond to second and third comments (How do you checked this?, see my question above.) Heat treatment process is emphasized and details that prove reliability of full annealing process are explained in page 4.  Page 5: Respond to fourth comment (machining the weld groove has put -in again res. stress?) Formation of compressive residual stress is expected around the weld groove but investigations showed that they cannot be observed with macro scale measurements. In addition, these micro scale compressions are remained under the weld beam far from residual stress measurement points after the welding process and they do not have effect on the scope of this study, effect of carbon content. These points are explained with details in the revised manuscript.  Page 11: Respond to comment about shear waves. The use of shear waves for measurement of stress induced acoustical birefringence in solids is discussed in the beginning of third section (Results). Reasons for selection of longitudinal waves in spite of shear waves are explained. The manuscript has been resubmitted to your journal. Look forward to your favorable consideration. Most sincerely, Dr Fatih Uzun, Yeditepe University

Manuscript Click here to download Manuscript: Manuscript.docx Click here to view linked References 1

ULTRASONIC INVESTIGATION OF THE EFFECT OF CARBON CONTENT IN CARBON STEELS

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ON BULK RESIDUAL STRESS

Dr. Fatih Uzun1, *, Department of Chemical Engineering, Yeditepe University, Istanbul (Turkey) Prof. Dr. Ali Nezihi Bilge2, Department of Energy Systems Engineering, Bilgi University, Istanbul (Turkey)

1

[email protected], *corresponding author

2

[email protected]

ABSTRACT

Formation of residual stress as a result of welding process is a familiar fact, but its relation with material composition is unknown. This study aims to investigate the effect of carbon content on welding residual stress in carbon steels. For this purpose, samples of ultra-low carbon interstitial free, low carbon and medium carbon steels are selected. Welding is performed as a beam on plate in spite of a joining process. Weld grooves are prepared at center of the rectangular samples. Automated submerged arc technique is preferred for welding process in order to ensure same welding parameters at each sample. Ultrasonic sound waves are used to determine residual stress. This non-destructive technique provides bulk residual stress which is average of shear and normal stresses through thickness of a material. A new approach is practiced to verify experimentally determined bulk residual stress with finite element simulation results. The model geometry of numerical analysis is divided into equivalent parts and average of shear and normal stresses is calculated. Non-destructive ultrasonic technique seems to be in good with finite element analysis.

Keywords: residual stress, carbon content, ultrasonic, finite element

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1. INTRODUCTION

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Stress that remain after removal of the source of stress are called residual stress. Manufacturing processes introduce stresses into the materials. One of these manufacturing processes is welding which is used for joining of materials. Strains, formed during welding process, may not be fully recovered and remain in materials as residual stresses. Residual stresses within joining parts of materials may have catastrophic effects and should be controlled. For this purpose, many studies were performed that deal with destructive and non-destructive measurement of residual stresses.

Non-destructive evaluation of residual stress can be performed using high frequency ultrasonic sound waves. Longitudinal wave is a type of ultrasonic waves which propagate and oscillate in the same direction. This type of waves were used for measurement of bulk residual stress. Scott determined residual stress in extruded billets with longitudinal acoustic waves [1]. Sanderson used laser generated ultrasound to measure residual stress [2]. Doxbeck used laser generated creeping longitudinal waves to figure residual stress [3]. Immersion ultrasonic technique was used to investigate total welding residual stress by Uzun [4].

Many elements are composed to form different type of engineering materials according to engineering requirements. Composition of these elements effect material properties. Carbon is one of the main constituents of steels. Effect of carbon on properties of steels have been studied by various authors. Murata investigated the effect of carbon content on the mechanical properties of 10 Cr - 5 W ferritic steels [5]. Yang simulated the stress field of carburized specimens with different carbon content during quenching process [6]. The carbon content and microstructure influence on corrosion behavior of low alloy steels in a Cl- containing environment was investigated by Yang [7]. Author also investigated the effect of carbon content on stress field simulation of carburized specimens. The influence of prior austenite deformation and nonmetallic inclusions on ferrite formation in two kind of low carbon steels was studied by Zhang [8]. The effect of carbon content (0.45, 0.79, and 1.26 wt % ) in warm deformation behavior of steels with martensitic structure was investigated by Li [9]. Esling investigated the carbon content dependent effect of magnetic field on austenitic decomposition of steels [10]. Narayan proposed that the carbon content have influence on strain hardening behavior of sintered plain carbon steel preforms [11]. Serajzadeh studied the effect of carbon on kinetics of dynamic restoration and flow behavior of carbon steels [12]. Itabashi investigated the effect of carbon content on high strain rate tensile

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properties for carbon steels [13]. Guo observed that carbon content affects Mechanical properties and weather

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resistance of high performance bridge steels [14].

Residual stresses, formed as a result of welding process, was studied by various researchers. These studies usually focused on welding conditions and welding type. Different residual stress measurement techniques were developed. Most of these studies were supported with numerical simulations. However, material composition can have an effect on formation of welding residual stress as well, which has not been well investigated thus far.

Carbon is one of the main elements of steels. Its composition effects properties of steels. Depending on carbon content, steels show different behaviors under load. Accordingly, strain formation during welding processes and remaining stresses after welding processes are expected to be effected by carbon content. In this study, relation between carbon content of steels and residual stress is investigated. For this purpose, three type of carbon steels with different carbon compositions are selected. Non-destructive ultrasonic wave velocity measurements are carried out in order to calculate residual stress. Numerical analyses are performed for each type of steel in order to verify experimental results.

2. METHODOLOGY

This study is composed of three stages. Initially, stress free samples of steel plates with different carbon content are prepared by performing heat treatment and then ultrasonic wave velocity measurement of these samples are performed before machining of weld grooves. After the machining process, welding of sample plates are accomplished and ultrasonic wave velocity measurements are repeated. Finally, finite element method is used to simulate submerged arc welding process.

2.1. Sample Preparation

In order to investigate the effect of carbon content, samples with similar composition but different carbon content should be used. For this purpose, three different type of carbon steels are selected. Ultra low-carbon interstitial free steel (IF steel) is issued as carbon free steel sample. Low carbon and medium carbon steels with

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carbon contents of 0.092 wt % and 0.478 wt % are other samples with higher carbon content. Material properties

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of samples and weld wire are given in Table 1.

Table 1. Room temperature material properties of samples and weld wire EM12K

IF

LOW

MID

Specific Heat (J/kgK)

535.85

475.85

470.12

509.41

Conductivity (W/mK)

76.432

78.426

55.778

-1

-6

12.65×10

-6

11.99×10

55.643 -6

11.99×10-6

Thermal Exp. Coef. (K )

11.99×10

Young's Modulus (GPa)

227.11

227.11

227.11

227.11

Yield Strength (MPa)

371.87

189.47

311.71

397.69

Poisson's Ratio

0.30903

0.30903

0.30903

0.30903

Three samples of each type of steel with dimensions of 280 × 200 × 10 mm are prepared as illustrated in Figure 1. In order to observe the effect of welding process on residual stress, it is necessary to use steel samples with no initial stresses remained during the manufacturing process. For this purpose, full annealing heat treatment process is applied to materials at a specific temperature for each steel type, slightly above the upper critical temperature [15]. Samples are heated to and held at these specific temperatures for 2 hours and then slowly cooled in the furnace. During this process old grains are removed and new stress free grains are formed. Time and temperature parameters are determined according to standards determined by ASM.

Figure 1. Illustration of sample geometry and ultrasonic wave velocity measurement sections along start (S), middle (M) and end (E) lines

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After removal of remaining stresses of manufacturing process, weld grooves are prepared according to the

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geometric properties which are illustrated in Figure 2. Depth of the weld groove is 3 mm with an angle of 60 o. Width of the bottom part of the weld groove is 2 mm. The weld groove with a length of 93.3 mm, one-third of the length of base plates, is located in the center of sample plates. Non-destructive measurements are carried out after the preparation of samples. It is expected to form compressive residual stresses within the weld grove after machining process, however they could not been examined after careful ultrasonic macro scale residual stress measurements. Micro scale measurements can prove formation of compressive residual stresses in that zone, but this have no influence on the scope of this study. In addition, it is assumed that these micro scale compressions does not have effect on macro scale residual stress formations.

Figure 2. Dimensions of the weld groove

In order to investigate the effect of carbon content on residual stress formation, it is aimed to minimize magnitude of remaining stresses. For this purpose, a beam on plate type of welding is performed in spite of joining two parts. This lowered volume of weld beam and heat input during the welding process. Welding process is required to be performed using a fully automated system with common parameters. Submerged arc welding technique is a proper technique to this end which minimizes possible operator errors. Parameters of submerged arc welding process are given in Table 2. Dupont and Marder evaluated the effect of welding parameters and process type on arc and melting efficiency [16]. Authors compared various arc welding processes and determined efficiencies of those processes including submerged arc welding. That efficiency parameter is preferred to be used in the simulation of welding process.

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Table 2. Parameters for submerged arc welding process

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Electric potential energy (Volt)

30

Electric current (Amp)

500

Efficiency (%)

84

Torch travel length (mm)

90

Time (s)

6.75

In this study, welding process is performed in TEKFEN Manufacturing and Engineering Co. lnc. Facility. AWS EM12K submerged arc wire is selected as filler material. Single pass welding is performed to fill each weld groove and then samples are leaved to cool within stagnant air. Non-destructive measurements are repeated at the welded samples after 24 hours following the welding process. Another point related with low magnitude of residual stress is welding type and welding wire. Submerged arc welding process is an automated technique that minimizes effect of technician. Flux injected during this process covers the weld beam and retards cooling speed. Weld wire used in this process is also an important factor for this purpose. A weld wire with similar yield strength with investigated samples are used. This reduced magnitude of stress remained after welding process.

2.2. Non-Destructive Evaluation of Residual stress

The medium in which waves propagate is defined as a continuum. In continuum approach, structural parameters are related to properties of an equivalent continuous medium. By using continuum mechanics it is possible to postulate fields of density, stress, velocity etc., since these fields satisfy basic conservation laws of equations related with balance of mass, momentum, angular momentum and energy. For an infinitely large solid medium physical acoustic theory indicates that sound wave velocities are functions of Young’s modulus and shear modulus [17]. In addition, a particular medium can be characterized by constitutive relations which relate stress to other variables [18]. Acoustoelastic theory determines the relation between changes in elastic structure of continuum and velocity of waves in the medium. This theory states that the velocity of elastic wave propagation in solids is dependent on the mechanical stress [19].

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Hughes and Kelly [20] derived expressions for the speed of elastic waves in stressed solids using Murnaghan’s

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theory of finite deformations [21] and third-order terms for energy. In the case of longitudinal waves that propagate and oscillate in the same direction, relation between ultrasonic wave velocity and stress can be defined with Equation 1 [22] where (

),

is stress change from a reference level,

is longitudinal wave velocity, and

is acoustoelastic constant

is longitudinal wave velocity at axial zero stress. D. Vangi [23]

also correlated time of flight of ultrasonic waves with mono-axial stress and obtained a linear relation.

(1)

In this study it is required to develop a correlation between ultrasonic wave velocity and stress. During the welding process, thermal strains form within the material and some of them remain as residual stress. Consequently, the correlation should be developed using thermal strains. In this study, stress free samples of each type of steel are heated up with increasing temperature step. Effect of thermal strain on ultrasonic wave velocity is investigated at each temperature step. Equation 2 is used to calculate stress related to temperature variations where σ is stress, E is Young’s modulus,

is thermal expansion coefficient, and ΔT is temperature

change. For this purpose, temperature dependent properties of modulus of elasticity and thermal-expansion coefficient are used.

(2)

Immersion ultrasonic systems transmit waves into a material through water. Ultrasonic waves reflect back from upper surface and back wall of materials. Computer integrated systems measure time difference between these two echoes. If thickness of a material is known, wave velocity in this material can be calculated. In this study, thickness of steel plates are accepted to be constant throughout the samples. Ultrasonic wave velocities of stress free samples are measured at predetermined measurement points. After the welding process ultrasonic wave velocity measurements are repeated at the same measurement points. Relation between stress and ultrasonic wave velocity and measured ultrasonic wave velocity variations are used to calculate residual stress.

Ultrasonic wave velocity measurements are performed using immersion probe with a diameter of 10 mm. Measurement points determined according to center of probes. Measurements cover average, bulk, residual

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stress within the cylindrical volume travelled by ultrasonic waves. Results are neither transversal nor

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longitudinal stresses. They are average of normal and shear stresses. This average is not obtained by arithmetic calculations. Ultrasonic waves are affected by all six type of stresses during their movement through thickness of the materials. In addition, it is aimed to perform measurements as close as possible to weld beam center, but texture of weld beam prevents ultrasonic wave velocity measurements at weld zone.

2.3. Finite Element Analysis

Several heat source models were examined through the decades for modeling of welding process. Hibbitt used a surface heat model to develop a numerical model for welding process [24]. Andersson applied a surface heat input to investigate stresses in submerged-arc welded joints [25]. Effect of ramped heat model was investigated by Shim [26]. Gaussian surface heat distribution was used by Nickell [27] and Friedman [28] in their studies. In order to obtain a more detailed model of heat source, studies on Gaussian heat source were continued by various authors [29-32]. Goldak proposed a double ellipsoidal model for heat distribution in an ellipsoidal geometry [33] which is illustrated in Figure 3.

Figure 3. Double ellipsoid heat source configuration [33]

Simulation of submerged arc welding is a complicated process. Dust particles are injected during application of weld beam which surround weld arc and weld beam. This mechanism should be considered for accurate prediction of residual stress. Goldak’s model is capable of modeling of heat input. However, double ellipsoidal shape is not able to cover heat source of submerged arc welding. Dust particles around the weld arc prevents

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formation of double ellipsoidal shape. Therefore, an ellipsoidal heat source model with symmetrical sides is

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preferred to be used. Details of this model is given in Equation 3. In this model is the weld heat input, power density point, and

,

, and

are central coordinates of heat source,

represents the depth of ellipsoidal heat source and

is the power density,

, , and and

are coordinates of

are dimensions along the

surface of the source. Length of the heat source along x axis in Goldak’s model is represented by which are equal to

and

in the ellipsoidal heat source. According to weld bead profile measurements, semi axes a, b,

and c are determined as given in Table 3.

(3)

Table 3. Heat source parameters

Semi-axe

Length (mm)

Width of heat source (a)

7.7

Height of heat source (b)

3.0

Length of ellipsoidal (c)

7.7

Thermal structural coupled analysis of welding process is a combination of thermal and structural analyses. Specific elements should be used for thermal and structural analyses. SOLID70 element is the selected for thermal analysis [34]. This element is capable of performing three dimensional transient thermal analysis. It has eight nodes with a single degree of freedom which is temperature. Movement of heat source and cooling period is simulated with transient thermal analysis. Results of each time strep is recorded and used as thermal loads in the following structural analysis. Structural analysis is performed with SOLID185 element which is structural equivalent of SOLID70 element. It has eight nodes with three degree of freedom which are translations in the nodal x, y, and z axes. SOLID185 has the same geometry with SOLID70 which is illustrated in Figure 4.

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Figure 4. Illustration of SOLID70 and SOLID185 elements and its optional geometries [34]

The finite element model is meshed with brick and tetrahedral geometries of selected elements. Brick geometry is used to mesh the base metal. Curved structure of the weld beam prevents using brick geometry, therefore the weld beam and the base metal around weld beam is meshed with tetrahedral geometry. Finite element mesh density of the model is higher inside and around the weld beam and it is being reduced in the outer parts of the specimen as illustrated in Figure 5. Weld beam is meshed with 6531 tetrahedral elements.

Figure 5. Illustration of the finite element mesh

Pulse-echo ultrasonic technique for non-destructive investigation of residual stress provides bulk stress through thickness of a material. Ultrasonic waves travel through different sections of a material and they are exposed to different stress zones during their movement. These different stress zones affect velocity of ultrasonic waves.

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Accordingly, an average stress value through thickness of the material is obtained at each measurement point.

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Finite element simulations do not provide a similar average stress data. For this purpose, the model geometry is divided into 11 sections through the thickness. Shear and normal stress values within these sections at each experimental measurement point are identified. This data is used to determine equivalent stress in all sections. Finally, averages of these 11 data at each measurement point are calculated.

3. RESULTS

There are two bulk ultrasonic waves that travel through the thickness of materials [35], longitudinal and shear waves, even though longitudinal waves have some benefits when compared to shear waves. In spite of the fact that shear wave transducers can be used to measure stress induced acoustical birefringence in solids [36], shear waves are weak, require elastically solid materials and cannot propagate effectively in liquid materials [37]. Immersion ultrasonic technique is not convenient for this type of waves. In addition, shear waves have higher wavelength. Ultrasonic waves with low frequency cannot penetrate into grains with lower diameter than wavelength. This factor can reduce accuracy of stress measurement. Accordingly, shear waves are applicable for residual stress measurement in materials with large grain size, by using a contact material denser than water, but in this study, high frequency longitudinal ultrasonic waves are preferred. Acoustoelastic constants are determined using longitudinal wave velocity variations and same kind of measurements are repeated for residual stress calculations. Results show that longitudinal ultrasonic waves are capable of determination of deformations in steel structures.

3.1. Acoustoelastic Constant

Relation between ultrasonic wave velocity variations and stress at each type of steel is given in Figure 6. It is observed that carbon content has an effect on formation of stress. Results state that variation of ultrasonic wave velocity is inversely proportional to stress. Acoustoelastic constants of IF, low carbon and medium carbon steels are -0.00017260 MPa-1, -0.00011164 MPa-1 and -0.00007098 MPa-1 respectively. Acoustoelastic constant of IF steel has the lowest value among others. With increasing carbon content, that constant gets higher value.

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Figure 6. Relation between ultrasonic wave velocity variations and stress in the three samples from top to bottom. First one is IF steel, second one is low carbon steel and last one is medium carbon steel

Investigation of relation between ultrasonic wave velocity and stress showed that ultrasonic wave velocity decreases linearly with increasing stress. This linear relation is given in Equation 4 where ultrasonic wave velocity,

is initial ultrasonic wave velocity and

(4)

is stress,

is acoustoelastic constant.

is

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3.2. Residual Stress

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In this part of the study, results of the experimental measurements and the finite element simulations are depicted. Results of three samples of each steel type are plotted and averages are obtained with fifth order polynomial trend lines. The finite element results are also given with the experimental results within the same plots. As it is stated before, measurement sections are defined as start, middle (mid), and end as illustrated in Figure 1. Results are plotted in accordance with these measurement sections.

Residual stress variations in each type of steel are given at separate plots. The experimental results are compared with the finite element simulation results which are observed to be consistent. The aim of this study is to demonstrate the effect of carbon content on formation of residual stress. Accordingly, residual stress distribution in welded structures, at the same measurement section of all steel types, are proposed in terms of experimental and finite element results.

The results of non-destructive measurements and finite element simulations of each steel type are given in Figures 7-9. Residual stress is maximum at the start section and minimum in the middle section of IF steel. Residual stress peak value increases up to a value around 30 MPa at the end section of IF steel. These results are consistent with the finite element simulation results. Low and medium carbon steels show similar behaviors. According to nondestructive experiments, residual stress peak values of the start and the end sections are similar and higher than the middle section peak value in these steel types. Results of non-destructive measurements are consistent with finite element analyses, but peak residual stress values can be different.

Residual stress at the start and the end regions of the weld beam is significantly higher than the central region in all steel types. This is caused by thermal effects and mechanism of the welding process. Tips of the weld beam are in direct contact with cold steel plate. Temperature difference during rapid heating and cooling is higher at these points. Accordingly, residual stress is higher at the start and the end regions of the weld beam. Another reason is time lag without movement of the weld arc which causes addition of more heat. Similarly, time lag occurs until termination of the weld arc at the end of weld beam.

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Residual Stress (MPa) (FEM)

40

Residual Stress (MPa) (Non-Destructive Exp.)

35 30 25 20 15

45

start

40

mid end

35

start 30

mid

25

end

20 15 10 5 0

10

0

10

20

30

40

50

60

70

80

90

100

Distance from Weld Center (mm) 5 0 -5

-5

5

15

25

35 45 55 Distance from Weld Center (mm)

65

75

85

95

Figure 7. Experimental and FEM equivalent residual stress distributions in IF steel

35

start


40

30 Residual Stress (MPa) (Non-Destructive Exp.)

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25 20 15

mid

35

end 30

start

25

mid

20

end

15 10 5 0

10

0

10

20

30 40 50 60 70 Distance from Weld Center (mm)

80

90

100

5 0 -5 0

10

20

30

40

50

60

70

80

Distance from Weld Center (mm)

Figure 8. Experimental and FEM equivalent residual stress distributions in low carbon steel

90

100

15

start

40

35

mid


35 30 Residual Stress (MPa) (Non-Destructive Exp.)

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25 20 15

end

30

start

25

mid end

20 15 10 5

10 0 0

10

20

5


80

90

100

0 -5 0

10

20

30

40

50

60

70

80

90

100


Figure 9. Experimental and FEM equivalent residual stress distributions in medium carbon steel

The effect of carbon content on residual stress formation is investigated at each measurement section separately. Residual stress distributions at each measurement section are illustrated in Figures 10-12. The nondestructive measurements and the finite element analysis results show similar relations between carbon content and residual stress. IF steel has higher peak of residual stress than other two type of steels. Low and medium carbon steels have almost similar magnitude at start section. Medium carbon steel has a residual stress peak lower than low carbon steel along middle section. IF steel reaches to a maximum residual stress value higher than other two steels along this section. Residual stress peak of low and medium carbon steels are similar while they are lower than IF steel at the end section of the weld beam. The experimental results show that residual stress peak values of all type of steels are similar at this region, but IF steel have slightly lower value according to finite element results.

16


40


35 30 25 20

45

IF

40

low med.

35

IF

30

low

25

med.

20 15 10

15

5 0

10

0

10

20

30

40

50

60

70

80

90

100

Distance from Weld Center (mm) 5 0 -5 0

10

20

30

40

50

60

70

80

90

100


Figure 10. Experimental and FEM equivalent residual stresses in IF, low carbon and medium carbon steels along S line (start section of weld beam)


25


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20

15

10

20

IF

18

low

16

med.

14

IF

12

low

10

med.

8 6 4 2 0 0

10

20

5


80

90

100

0

-5 0

10

20

30

40

50

60

70

80

90

100


Figure 11. Experimental and FEM equivalent residual stresses in IF, low carbon and medium carbon steels along M line (middle section of weld beam)

17



IF

40

35

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25 20 15

low

35

med. 30

IF

25

low

20

med

15 10 5 0

10

0

10

20


80

90

100

5 0 -5 0

10

20

30

40

50

60

70

80

90

100


Figure 12. Experimental and FEM equivalent residual stresses in IF, low carbon and medium carbon steels along E line (end section of weld beam)

4. CONCLUSION

Non-destructive experiments and finite element simulations showed that carbon content of steels have effect on formation of welding residual stress, but this effect is very low when compared to magnitude of remaining stresses. Low and medium carbon steels show similar distributions, while IF steel have slightly higher peaks. Residual stress peak values are lower at middle section of all the weld beams, but an increasing or decreasing order is not observed within a single sample. Another point observed during this study is that steel carbon content affects acoustoelastic constant which increases with increasing carbon content. As a summary, carbon content have a slight effect on welding residual stress, have a strong effect on acoustoelastic constant, relation between stress and ultrasonic wave velocity, and does not have an effect on residual stress distribution.

Magnitude of residual stress values are aimed to be low as it is stated before. Similar yield strength of weld wire and samples, automated submerged arc welding process without operator errors and volume of weld arc satisfied this. Another point is that, both weld beam and sample plates have the same crystal structure. BCC type of crystal structure remains as it is after welding process. This is also a positive effect on low magnitude of

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residual stress. In addition, bulk residual stress is average in the measurement volume. It provides residual stress

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close to equilibrium condition, which is balance of tension and compression stresses. Further studies should be accomplished with micro-scale residual stress measurements to determined normal stresses in transversal and longitudinal directions separately.

Ultrasonic technique is an alternative non-destructive residual stress evaluation method, but it has some restrictions. This cost effective method can only be applied to materials with flat surface. Indentations cover rough surface of weld beams as a result of irregular pouring of melted filler metal. They prevent penetration of ultrasonic waves into materials. Another limitation of this method is the requirement of exact thickness data. Measurement of ultrasonic wave velocity before and after the welding process eliminated this requirement.

REFERENCES

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8. Zhang C, Xia Z-X, Yang Z-G, Liu Z-H (2010) Influence of prior austenite deformation and non-metallic

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inclusions on ferrite formation in low-carbon steels. Journal of Iron and Steel Research, International 17 (6):3642 9. Li Q, Wang T-S, Li H-B, Gao Y-W, Li N, Jing T-F (2010) Warm Deformation Behavior of Steels Containing Carbon of 0. 45% to 1. 26% With Martensite Starting Structure. Journal of Iron and Steel Research, International 17 (5):34-37 10. Zhang X, Wang S, Zhang Y, Esling C, Zhao X, Zuo L (2012) Carbon-content dependent effect of magnetic field on austenitic decomposition of steels. Journal of Magnetism and Magnetic Materials 324 (7):1385-1390 11. Narayan S, Rajeshkannan A (2011) Influence of Carbon Content on Strain Hardening Behaviour of Sintered Plain Carbon Steel Preforms. Journal of Iron and Steel Research, International 18 (9):33-40 12. Serajzadeh S, Taheri AK (2003) An investigation into the effect of carbon on the kinetics of dynamic restoration and flow behavior of carbon steels. Mechanics of materials 35 (7):653-660 13. Itabashi M, Kawata K (2000) Carbon content effect on high-strain-rate tensile properties for carbon steels. International journal of impact engineering 24 (2):117-131 14. Guo J, Shang C-J, Yang S-W, Wang Y, Wang L-W, He X-L (2009) Effect of carbon content on mechanical properties and weather resistance of high performance bridge steels. Journal of Iron and Steel Research, International 16 (6):63-69 15. Committee AIH (1991) ASM handbook: Heat treating, vol 4. Asm Intl, 16. DuPont J, Marder A (1995) Thermal efficiency of arc welding processes. Welding Journal-Including Welding Research Supplement 74 (12):406s 17. Phani KK, Sanyal D, Sengupta AK (2007) Estimation of elastic properties of nuclear fuel material using longitudinal ultrasonic velocity – A new approach. Journal of Nuclear Materials 366 (1–2):129-136. doi:http://dx.doi.org/10.1016/j.jnucmat.2006.12.045 18. Thurston R (1964) Wave propagation in fluids and normal solids. Physical acoustics 1 (Part A):1-110 19. Lu J, James M, Roy G (1996) Handbook of measurement of residual stresses. Fairmont Press, 20. Hughes DS, Kelly J (1953) Second-order elastic deformation of solids. Physical Review 92 (5):1145 21. Murnaghan FD (1967) Finite deformation of an elastic solid. 22. Egle D, Bray D (1976) Measurement of acoustoelastic and third‐order elastic constants for rail steel. The journal of the Acoustical Society of America 60 (3):741-744

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23. Vangi D (2001) Stress evaluation by pulse-echo ultrasonic longitudinal wave. Experimental mechanics 41

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(3):277-281 24. Hibbitt HD, Marcal PV (1973) A numerical, thermo-mechanical model for the welding and subsequent loading of a fabricated structure. Computers & Structures 3 (5):1145-1174 25. Andersson B (1978) Thermal stresses in a submerged-arc welded joint considering phase transformations. Journal of Engineering Materials and Technology 100 (4):356-362 26. Shim Y, Feng Z, Lee S, Kim D, Jaeger J, Papritan J, Tsai C (1992) Determination of residual stresses in thick-section weldments. Welding Journal 71 (9):305-312 27. Nickell RE, Hibbitt HD (1975) Thermal and mechanical analysis of welded structures. Nuclear Engineering and Design 32 (1):110-120 28. Friedman E (1978) Analysis of weld puddle distortion and its effect on penetration. Welding journal 57 (6):161 29. Mahin K, MacEwen S, Winters W (1988) Evaluation of residual stress distributions in a traveling GTA weld using finite element and experimental techniques. Modeling and Control of Casting and Welding Processes IV:339-350 30. Friedman E (1975) Thermomechanical analysis of the welding process using the finite element method. Journal of Pressure Vessel Technology 97 (3):206-213 31. Mahin K, Winters W, Holden T, Hosbons R, MacEwen S (1991) Prediction and measurement of residual elastic strain distributions in gas tungsten arc welds. Welding Journal 70 (9):245s-260s 32. Bae K, Na S, Park D (1994) A study of mechanical stress relief (MSR) treatment of residual stresses for onepass submerged arc welding of V-grooved mild steel plate. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 208 (3):217-227 33. Goldak J, Chakravarti A, Bibby M (1984) A new finite element model for welding heat sources. MTB 15 (2):299-305. doi:10.1007/BF02667333 34. ANSYS Release 13, Finite Element Analysis Software Help Document, 2010. 35. Kundu T, Placko D (2013) Advanced Ultrasonic Methods for Material and Structure Inspection. Wiley, 36. Hsu N (1974) Acoustical birefringence and the use of ultrasonic waves for experimental stress analysis. Experimental Mechanics 14 (5):169-176. doi:10.1007/BF02323061 37. Burrascano P, Callegari S, Montisci A, Ricci M, Versaci M (2014) Ultrasonic Nondestructive Evaluation Systems: Industrial Application Issues. Springer International Publishing

Figure Click here to download Figure: figure_1.docx

Figure 1


Figure 2


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Figure 5

Figure Click here to download Figure: figure_6_edt.docx

y = -0.00017260x R² = 0.99695620

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Figure 6 (image)



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Figure Click here to download Figure: figure_captions.docx

FIGURE CAPTIONS Figure 1. Illustration of sample geometry and ultrasonic wave velocity measurement sections along start (S), middle (M) and end (E) lines

Figure 2. Dimensions of the weld groove

Figure 3. Double ellipsoid heat source configuration [33]

Figure 4. Illustration of SOLID70 and SOLID185 elements and its optional geometries [34]

Figure 5. Illustration of the finite element mesh

Figure 6. Relation between ultrasonic wave velocity variations and stress in the three samples from top to bottom. First one is IF steel, second one is low carbon steel and last one is medium carbon steel

Figure 7. Experimental and FEM equivalent residual stress distributions in IF steel

Figure 8. Experimental and FEM equivalent residual stress distributions in low carbon steel

Figure 9. Experimental and FEM equivalent residual stress distributions in medium carbon steel

Figure 10. Experimental and FEM equivalent residual stresses in IF, low carbon and medium carbon steels along S line (start section of weld beam)

Figure 11. Experimental and FEM equivalent residual stresses in IF, low carbon and medium carbon steels along M line (middle section of weld beam)

Figure 12. Experimental and FEM equivalent residual stresses in IF, low carbon and medium carbon steels along E line (end section of weld beam)

Table Click here to download Table: table_1.docx

Table 1. Room temperature material properties of samples and weld wire

Specific Heat (J/kgK) Conductivity (W/mK) -1

Thermal Exp. Coef. (K )

EM12K

IF

LOW

MID

535.85

475.85

470.12

509.41

76.432

78.426

55.778

55.643

0.000011989

0.000012653

0.000011989

0.000011989

227.11

227.11

227.11

227.11

Yield Strength (MPa)

371.8700

189.474

311.7100

397.6900

Poisson's Ratio

0.30903

0.30903

0.30903

0.30903

Young's Modulus (GPa)


Table 2. Parameters for submerged arc welding process

Electric potential energy (Volt)

30

Electric current (Amp)

500

Efficiency (%)

84

Torch travel length (mm)

90

Time (s)

6.75


Table 2. Heat source parameters

Semi-axe

Length (mm)

Width of heat source (a)

7.7

Height of heat source (b)

3.0

Length of ellipsoidal (c)

7.7