welding project report.pdf

September 5, 2017 | Author: Ezadin Baban | Category: Welding, Finite Element Method, Electric Arc, Steel, Electric Current

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CHAPTER 1 INTRODUCTION Among the welding typical problems and most important are the residual stress/strain and the induced distortions in structures of the components. In order to get better understand the welding process and its effects on structures, engineers and researchers around the world, covering a large number of industries, have been trying to create algorithms and methodologies to simulate the complete welding process or just individual phases (e.g. the cooling phase). In recent years, due to the high expansion of computer software, computations possibilities, many researchers identified the Finite Element Analysis (FEA) as a reliable method for this purpose. Hence ANSYS is used for the welding simulation. 1.1 Need of the Project The need of the project arises due to the problem of inaccuracies that take place during the heating or welding process of the production and hence increase partly from the thermal distortions and partly in the form of dimensional variations due to human factors. Furthermore, with the increasing use of the automation, it is necessary to quantify and calculate the thermal distortions by means of mathematical models by using FEA, so that the required tolerances of the automation process can be achieved as efficient as possible. In order to improve planning and work scheduling by reducing the rework, to reduce the production cost significantly by reducing the measurements and rework, and to improve the quality of the weldment the significance of FEA is studied. 1.2 Scope of the Project The problem of creation of residual stresses during welding

dimensional inaccuracies and misalignments of structural members, which can result in complex tasks or rework when tolerance limits are crossed. Hence cause increase in the cost of production and leads to loss of time. In fabrication sector, for example, expenses required for rework such as straightening could have a heavycost. Therefore, the problems of distortion and residual stresses arealways of great concern in welding sector. In order to deal with this problem, it is necessary to interpret the extent of distortion resulting from the 1

welding operations. One way to interpret the distortion and shrinkage of steel welding is through numerical analysis such as finite element analysis (FEA). Once the techniques to predict the distortion and shrinkage are recognised, then the problems can be controlled accordingly. During the welding process, there are so many factors such as welding process type, welding process parameters, preheat patterns, welding sequence, level of constraint and joint details that contribute to the creation of the residual stresses in the welded structure. By determining which parameters have an effect on the quality of the weld and which factorswill give the most significant effect on the weld quality are the main aspects in welding industry. 1.3 Problem Statement In this project work the direct coupled field transient thermal structural analysis will be carried out using finite element analysis for arc welding for a butt joint. To carry the FEA two plates of ASTM 36 were considered to be welded. The dimensions of the plate were taken as 200 x 100 x 3 mm for each plate. Welding was assumed to be done along 100 mm length of the plate. The groove angle for welding was taken as 60º. 1.4Methodology of the Project •

The adapted experimental methodology is as follows, – Collection of data or information for the arc welding simulation using Finite element analysis software. – 3D modeling of the weld and ASTM 36 steel plates to be welded using CAD software – Finite element model creation by using finite element software – Carry out thermal analysis to find temperature distribution due to welding process in the steel plate. – Structural analysis of weld and welded plate to find residual stresses. – To find out temperature distribution across the center line of the weld at 100 seconds after completion of welding process 2

residual stress calculation due to welding by using FEA

– To find out strain across the center line of the weld after cooling of the welded steel plate. – Comparison of results from experiments with FEA results of residual stresses observed due to welding

1.5 Objectives of the Project This report introduces finite element analysis for the modeling of welds and it explains a brief history of the simulation of welds. Welding is one of the reliable and highly effective metal fabrication process which is widely used in industries. Localized and excessive heating during the welding process, followed by rapid cooling causes to generate residual stress and distortion in the weld and base metal. In the last few years various research efforts have been directed towards the control of welding process parameters in order to reduce the residual stress and distortion. Objectives of the project are to find out 1. Residual stresses in the welded parts due to phase change due to solidification

of

weld with respect to distance from weld. 2. Temperature change with respect to time due to phase change to solidification of weld with respect to distance from weld. 3. Strain in the welded parts due to phase change to solidification of weld with respect to distance from weld. 4. Suitability of software to interpret residual stresses.

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1.6 Publications A paper has been published related to the project work entitled as “Finite Element Analysis for Residual Stress, Strain and Temperature Characteristics of Butt Welded Steel Plate” in International Journal of Scientific Research, Volume:2,Issue:6, June 2013, ISSN no. 2277 - 8179.

1.7 Use of Finite Element Analysis Finite element analysis makes it possible to evaluate a detailed and complex structure with a very short period of time. The adequate strength of the structure and the possibility of improving the design during planning could justify the cost included in this analysis work. FEA has also been known to analyze the structure that were significantly over designed and built many decades ago.In absence of finite element analysis (or other numerical analysis) development of structures must be based on manual calculations only. In case of complex structures, the simplifying assumptions necessary to make any calculation possibly can lead to a conservative and heavy design. An important factor of ignorance can remain as to whether the structure will be adequate for all design loads. Considerable changes in designs involve risk. Design will require prototypes to be built and field tested completely. The field tests may include expensive strain gauging to evaluate strength and deformation of the components.

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CHAPTER 2 LITERATURE REVIEW The research work in welding simulation was initializedmany years ago.Understanding of the theory of heat flow is essential in order to study the welding processanalytically, numerically or experimentally. Rosenthal (1946) was the first researcher who succeeded in developing an analytical solution of heat flow during welding based on conduction heat transfer for determining the shape of the weld pool for two and three-dimensional welds. By using the Fourier partial differential equation (PDE) of heat conduction, he initialized the moving coordinate system to develop solutions for the point and line heat sources and also applied this successfully to apply to a wide range of welding problems. His analytical solutions for the heat flow made possible for the first time the analysis of the process from the point of view of the welding parameters namely the voltage, current, welding speed, and weld geometry. Due to the pioneering work done by Rosenthal,significant interest in the thermal aspects of welding was developed by many researcherssuch as Goldak (1984) The most critical and important input data required for welding thermal analysis are the parametersnecessary to determine the heat input to the weldment by the arc. Goldak et al. (1984) developeda mathematical model for welding heat sources based on a Gaussian distribution of powerdensity. They proposed a double ellipsoidal distribution in order to capture the size and shapeof the heat source of shallow and deeper penetrations. Some of the researchers have alsodeveloped the thermal finite element simulation to determine the temperature distribution of ametal.Over the past forty years, finite element techniques have been used extensively in order topredict distortion and residual stresses due to welding operations such as the studies byFriedman (1975), Michaleris and Debiccari (1997) and Taylor et al. (1999). The finiteelement method has already been proven to be a successful tool for the simulation of the complexwelding process as performed by Friedman (1975). His 2-D finite element analysis work wasthen used by Taylor et al. (1999) for the verification of their 3-D computational modeling of weldingphenomena. The final results of finite element analysis done by Taylor et al. (1999) were inreasonable agreement with the result obtained by Friedman (1975). Most of the welding research work in the past was conducted to investigate the distribution and effect ofresidual stress and distortion of welded metal component. It is possible to estimates the welding shrinkage in a 5

welded butt joint by applying amathematical model approach. Michaleris and Debiccari conducted thermo-elasto-plastic finite element analysis for welding simulation to determine thewelding distortion. It has been claimed by them that their approaches have been similar toexperimental and empirical data. Not only the welding residual stress and distortion have been studied by welding researchers,but also the effects of welding sequence, welding joint geometry,welding parameter and rootopening has also been studied by several researchers in the last years.Harwig et al. (1999) for example, studied the effect due to welding parameters and electrodeclassification on the diffusible hydrogen content of gas shielded flux cored arc welds. In 1999,Tsai et al. studied the influence of welding sequence on warping and buckling behavior of athin-plate panel structure. Tsai et al. (2004) have also investigated the effects of weldingparameters and joint geometry on the distribution and magnitude of residual stresses onthick-section butt joints. As proven by the researchers, residual stress distribution and distortion in a weldedplate are strongly affected by their interaction and by many parameters. Yet, certain aspects ofthe welding phenomenon are still subjected to further research specifically the effects of welding speed, restraint, heatinput, gap on arc welding responses and plate curvature as appliedto curved steel plate welding.In the recent ten years, there is growing need to concern about the numerical research on line heatingand welding process. Murakawa et al. (2005) interpreted the hot cracking of a weldment usingtemperature-dependent interface elements. Mahapatra et al. (2007) modeled the influence of theposition of tack weld constraints on the angular distortions that were created in one-sided fillet weldingcreated by SAW. It is cleared from this literature review that the thermal reactions and the resultingdistortions in a welded joint are strongly affected by various parameters and their interactions.A number of finite-element (FE) models used for illustrating the effect of using differentmodeling strategies for the simulation of the thermo-elasto-plastic stages of the weldingprocess are applied by Mollicone et al. (2006)Adak and Mandal (2010) studied the heat sinking as a method of distortion mitigation and usedthe pseudo-linear equivalent constant rigidity concept for thermo-mechanical analysis of platesundergoing welding with simultaneous heat sinking. The proposed concept was found to becomputationally more efficient and simpler to model as compared to FEM for solving similarthermo-elasto-plastic 6

nonlinear problems.In 2011, Heinze et al. investigated a single-layer gas metal arc (GMA) weld of 5 mm thickstructural steel is experimentally and numerically. The numerical modeling begun with a meshanalysis based on modal analyses. The sensitivity of weldinginduced distortion is examinedrelated to different continuous cooling transformation (CCT) diagrams. Fusion welding includes heating by an intense heat source, melting and solidification of parent metals, and often, addition of filler material in the localized fusion zone during welding. The heat source causes nonuniform temperature distributions across the joint and the parent metals. The computational weld pool models have became a significant route for a priori estimation of weld pool dimensions, peak temperature, cooling rate, and many other associated aspects of the weld pool and the surrounding heat-affected zone. Particularly, finite element method-based weld pool models are found very useful to interpret weld thermal cycle and fluid flow pattern at an early stage of welding process development and product design. However, the complex geometry of real engineering components and the complexity of welding processes itself have made the prediction of weld thermal cycle and corresponding flow field to be a very difficult task. It is further noticed that the accuracies of these quantitative calculations are largely dependent on the accuracy of several input parameters in welding. Some of the input parameters are also not certain in nature, and optimization algorithms are integrated with the numerical process model to determine the suitable values of these uncertain input parameters for modeling calculations. Welding results in very complex thermal cycles which cause irreversible elastic plastic strain and residual stresses in and around fusion zone and heat affected zone (HAZ). Residual stresses may be an advantage and disadvantage in structural components depending on their magnitude and nature. The beneficial effects of compressive stresses have been widely used in industry as these effects are believed to reduce stress corrosion cracking and brittle fracture and increase fatigue strength of the component. In large steel fabrication industries such as marine structures, aero-space industry, shipbuilding, high speed train guide ways and pressure vessels and piping in chemical and petrochemical industry; the problem of residual stresses and overall distortion has been observed and continue to be a major issue. It is well known fact that material response of structural components is strongly affected by the residual stresses when subjected to structural and 7

thermal loads. Due to the presence of these residual stresses produced in and around the weld zone the strength and life of the component is definitely reduced. Welds are always an essential part of engineering components. Residual stresses produced in the welded regions of the metal components, due to the nonlinear thermal processes during welding, can have detrimental effects, such as hydrogen-induced cracking ,stress corrosion cracking, and reduced fatigue strength. Due to this reason it is pertinent to simulate the welding process to predict the behavior of welded structures from finite element residual stress and strain results.

CHAPTER 3 8

WELDING TECHNOLOGY 3.1 Introduction to Welding In general, welding is defined as any process in which two or more pieces of metal are joined together by the application of heat, pressure, or a combination of both. Almostall the processes may be grouped into two main categories: heat welding, in which the welding process is achieved by heat; and pressure welding, where the welding process is achieved by pressure. Heat welding is the welding which is used today. The most important welding parameters are the welding speed and the arc energy per unit length of the weld. [Jonsson,M., Karlsson, Lindgren L.E;1985] During welding process, the weldment is locally heated due to the welding heat source. Because of the non-uniform temperature distribution during the thermal cycle, incompatible strains cause to develop thermal stresses in the components. These incompatible strains due to dimensional changes related with solidification of the welded metal, plastic deformation and metallurgical transformations are the major sources of residual stresses and distortion.[M Sundar1, G Nandi, A Bandyopadhyay And S C Roy; 2005] The parameters of the line heating process have major effect on thermal distribution and the resulting residual deformation of the heated plate. The thermal transients are generally dependent on various factors like torch height, gas pressure, torch speed, and nozzle size, which then controls the residual deformation of the plate. [Biswas, P., Mandal, N.R. & Sha, O.P. 2007] Arc

welding,

which

is

a

heat-type

welding,

is

the

most

important

manufacturingoperation for the joining of structural components for a wide range of applications, including ships, bridges, building structures,

guide way for trains,

automobiles, and nuclear reactors, to give an example. It is necessary to provide a continuous supply of either direct or alternating electric current, which is normally used to develop an electric arc to generate enough heat to melt the metal and form a weld. The arc welding process is a complex operation involving extremely high temperatures, which produce high levels of residual stresses and severe distortions . Theseextreme phenomena results in reduction of the strength of a structure, which becomes vulnerabletocorrosion, buckling, fracture and other type of failures. 9

3.2 Types of Arc Welding The most widely used types of arc welding processes are gas tungsten arc, gas metal arc, shielded metal arc and submerged metal arc. 3.2.1 Shielded Metal Arc Welding (SMAW) In shielded metal-arc welding, a metallic electrode which is used to conductelectricity, is coated with flux and is connected to a source of electric current. The metal which is to be welded isconnected to the other end of the same source used to supply current. By having touch of the tip of theelectrode to the metal and then drawing it away, an electric arc is developed. The intenseamount of heat of the arc causes to melt both parts to be welded and alsothe point of the metal electrode. Thispoint of the metal electrode supplies filler metal for the weld. 3.2.2 Gas Tungsten Arc Welding (GTAW) In gas tungsten arc welding, a tungsten electrode is used instead of the metal electrode as used in shielded metal arc welding. A chemically inert gas, such as helium, orhydrogen ,argon, is used to shield the metal from oxidation. The heat produced from the arc formedbetween the electrode and metal melts the edges of the metal component. Metal used for the weld may beadded by placing a bare wire in the arc or at the point of the weld. The GTAW process can be used with almost all metals and produces a high-quality weld between two joints. Instead of this, therate of welding is considerably slower as compared to other types of welding processes. 3.2.3 Gas Metal Arc Welding (GMAW) In this type of arc welding, a bare electrode is shielded from the air by surrounding it with argon or carbon dioxide gas or by coating the electrode with some type of flux for welding. The electrode is required tofeedinto the electric arc, and melts off in droplets so as to enter the liquid metal that forms the weld.

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3.2.4 Submerged Arc Welding (SAW) This type of arc welding is similar to gas metal arc welding, but in this process no any gas is used to shield the weld. Instead of gas the arc and tip of the wire are submerged beneath a layer offusible, granular material formulated to produce a proper weld. This process is very effective but is only used with steel. 3.3 Types of Welding Joints Welds are made at the junction of the various pieces that cause to make up the weldment. The junctionsof parts, or joints, are termed as the location where two or more members are to be joined. Metal parts being joined to produce the weldment may be in the form of castings, forgings, rolled plate, sheet, pipes, or billets. The five basic types of welding joints for plates are explained below.[ Bai-Qiao Chen,2011] 3.3.1 Butt Joint It is a joint between two members lying approximately in the same plane as shown in Figure 3.1

Figure 3.1 -Butt Joint Arrangement.

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3.3.2

Corner Joint

It isa joint between two members located nearly at right angles to each other in the form of an angle as shown in Figure 3.2.

Figure 3.2 -Corner Joint Arrangement.

Figure 3.3 - Edge Joint Arrangement.

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3.3.3

Edge Joint

It is a joint between the edges of two or more parallel or mainly parallel members or components as shown in Figure 3.3 3.3.4

Lap Joint

It isajoint between two overlapping members as shown in Figure 3.4.

Figure3.4 - Lap Joint Arrangement.

3.3.5

T Joint

It is a joint between two members located nearly at right angles with respect to each other in the form of a T as shown in Figure 3.5.

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Figure 3.5 - T Joint Arrangement.

3.4 Technique of Arc Welding Thetechnique of arc welding is related with complex physical phenomena associated with electricity, magnetism, welding including heat, light, and sound. By the application of intense heat, the metal at the joint between two parts is melted and is allowed to intermix. In general, itisdescribed by an electric field between the positive anode and the negative cathode which is surrounded by an ionization gas. In arc welding, the intense heat required to melt metal is produced by an electric arc. On the surface of metal, there is a thin layer of surface electrons andwhichare being accelerated in the field towards the anode.

These electrons get collide with the atoms in the gas, resulting in

impact ionization where these atoms are decomposed into electrons and positive ions, which tend to cause further ionization. The amount of current of electrically charged particles in the arc and the temperature are related with each other as high temperatures increases ionization. The temperature rise is due to the released energy.

To obtain

suitable welding conditions the temperature or the current must initially be brought up to a certain level, which is achieved by igniting the arc. Arcignition is achieved by the short circuit current which is produced as the anode and the cathode are brought into direct contact. Due to direct contact, the short-circuit current shortly increases the temperature and the current and subsequently the arc can be maintained constant in the electric field existing under normal welding conditions when 14

applied.

The arc is being surrounded by a magnetic field which directs the charged

particles towards the center of the arc, hence causing the arc to localize in the spots on the anode and the cathode.During the period, when the electrically charged particles get impacted on the anode and the cathode, the anode and the cathode spots are being heated to high temperatures. This high temperature of approximately 3000 to 5000 C causes both theelectrode and the welded metal, both to melt. Because ofthe suction force of the plasma flow, the droplets of the electrode material are deposited on the metal.

Heat source

Heat affected zone Melt-pool zone

Basemetal

Figure 3.6 - Illustration of Melt-Pool Zone and Heat Affected Zone (HAZ)

3.5

Heat Affected Zone (HAZ) 15

A welded joint normally consists of melt-pool zone (MPZ), heat affected zone (HAZ), and unaffected base metal portion. The HAZ is generally defined as the portion of the base metal which has not been melted and whose mechanical properties or microstructures have been changed by the heat of the welding. The HAZ is crucial and important to the strength of the weldment since the fracture and cracking occur inside the HAZ region. During welding process, the temperature at this region is very much severe. As a result, it contains a different types of microstructures, some of which may have weak toughness and strength properties.Figure 3.6 shows a melt-pool zone (MPZ) and heat affected zone (HAZ) in the weldment during welding. The effects of temperature distribution on microstructure of material in the HAZ have been studied by Weisman (1976).

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3.6

Theory of Welding Strains

Due to its nature, the welding process is one of the complicated transient procedure which typically occurs in a 3-D (3-dimensional) structure. A typical welding simulation is considered to be consists of two phases one is a transient heat flow analysis phase and another is quasistatic plastic structural analysis phase. A reasonably good welding simulation has requred to have a fine enough mesh in order to accommodate the high thermal gradients that are developed (in time and in space) during the transient heat flow analysis phase. At the same time, the mesh is required to be able to solve the plasticstructural phase of the problem. [Cristian Simion,Corneliu Manu, Saleh Baset and Julian Millard] During the period ofthe heating and cooling cycles of a welding process, there are many factors which affect shrinkage of the metal and therebymaking accurate predictions of distortion complex and difficult. The mechanical and physical properties of the metal that affect the degree of distortion change with respect tothe application of the heat. When the temperature of the weld gets increasedthe modulus of elasticity, the yield strength and the thermal conductivity of the steel decrease, whereas, the coefficient of thermal expansion and the specific heat increase. Thechanges in temperature and stresses during welding process have been studied by Weisman (1976).

To explain the temperature changes during welding, various cross-

sections are required to be analyzed as shown in Figure 3.7(a).In some distance apart of the welding torch i.e. section along A-A, the temperature gradient, ΔT due to the welding is nearly zero. Along section B-B, which crosses the welding arc, at this section, the temperature change is very high and the distribution is very uneven. Along section C-C, having some distance behind the welding arc, the temperature change becomesmore even and less steep. Finally, along section D-D, which is very much far from the welding arc, at that section, the temperature change due to welding has reached to nearly zero.

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(a) Temperature distribution during welding

(b)) Stress σx distributionduring welding. Figure 3.7 – Schematic Representation of Changes of Temperature and Stresses during Welding (Weisman, 1976).

The effective distribution of stresses x,in the X-direction at cross sections A-A, B-B, C-C, and D- D are shown in Figure 3.7(b).

Nor Normal stresses in y-direction, direction,y,shearing

stresses,xy,also exist, but they are usually much smaller thanx.Along section A-A, A the thermal al stresses due to welding are nearly zero. Stresses in region below the weld pool at section B-B B are also nearly zero because molten metal etal cannot support a load. Stresses in the heat-affected affected zone (HAZ) are normally co compressive, because thee expansion of these areas is restrained by surrounding metal etal where the te temperature perature is generally lower. As the metal temperature perature in these regions is high and the yield strength of material aterial is low, the stresses are as high due to the yield strength of the material at the corresponding temperature. perature. The magnitude of compressive pressive stress gets to a maximum value with increasing distance from the 18

weld or with decreasing temperature. Stresses in regions which are away from the weld line tend to tensile to balance with the compressive stresses in areas near the weld. At section C-C, where the welded metal and heat-affected zone have cooled, the result is creation of tensile stresses in regions near the weld as they tend to shrink and compressive stresses at greater distance. Finally, section D-D which represents a region which is cooled-down and where high- tensile stresses are present in the HAZ zone and compressive stresses exist in base plate normally away from the weld.

3.7 Types of Welding Strains Duringwelding process, there are non-uniformheating and cooling cycles in the weld and adjacent base metal, which produces complex thermal strains. The stresses formed due to the strains produce internal forces resulting in shrinkage of the material. The stresses that would exist in weldment after all external loads are removedare called residual stresses. Depending on the shape of the structure welded and the shrinkage pattern , various strains such as buckling, bending and rotation take place. When steel structure parts are connected by welding, they are subjected to not only welding residual stresses but also distortion. This distortion found in the welded structures results due to four fundamental dimension changes that occur during welding process. (Refer fig. 3.8) 1. Transverse shrinkage of butt joints which occurs due to dimensional reduction in the directionwhich is perpendicular to the welding line. 2. Longitudinal shrinkage of butt joints which occurs due to dimensional reduction in a direction parallel to the weld

line. Themagnitude of the longitudinal shrinkage is

small as compared to the transverse shrinkage, which is about 1/1000 of the weld length as reported by Weisman (1976).

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(a)

(b)

(c)

(d)

Figure3.8 -Various Types of Welding Distortion - (a) Transverse se Shrinkage in a Butt-Joint; Butt (b) Angular Change in a Butt Butt-Joint; (c) Angular Change in a T-Joint; (d) Longitudinal Distortion in a Fillet Joint.

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3. Angular distortion of butt joints which is caused due to an angular change that occurs due to a non-uniform thermal contraction through the thickness of the plate. The thermal contraction which is non-uniform originates from the uneven heating through the thickness during welding.

4. Similar to the angular distortion of welds in butt joints, there is non-uniform thermal contraction through the thickness of the flanges creates a moment M about the flange neutral axes and cause angular distortion of fillet welds.

5. Longitudinal bending distortion is produced by bending stresses induced by the longitudinal shrinkage forces of the welds which do not coinciding with the neutral axis of the weldment.

3.8 Residual Stress Theresidual stress is produced in the weldment primarily because the weld material that has been melted contracts on cooling down from melting point to roomtemperature. Residual stress getsincreases with increase in plate thickness. Arc voltage and welding current, both have direct effect on the residual stress in the welding process. An increase in the voltage and current increases heat input rate thus increasing residual stresses. Also, increase in welding speed cause reduction in residual stresses but at the same time reduces quality of weld as well.[Gurinder Singh Birar; 2013]Figure3.9shows the deflection of a welded plate at the moment when a longitudinal edge is heated by amoving welding arc. The metal which is near the heat sourceisheatedtohighertemperaturesthan the metal which is away fromthe heat source. The hotter metal gets expands, and the plate firstdeforms as shown by curve AB.

Due to the presence ofplastic strains, when the plate cools to

roomtemperature, the final strain δ remains as shown by curve ABCD.

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Figure3.9- Strain of Welded lded Plate u under nder the Influence of a LongitudinalMovingHeat Source.

To understand this phenomenon, enon, let’s consider a butt joint welding with two cases with a gap and without a gap. When hen the weld material aterial first starts solidifying, the heated material expands but the base metal etal prevents it to get expand. As can be seen in the Figure 3.10 (a), element ent 1 is in tension whereas ele element 2 is under compression. At the time, when the weld cools down, the opposite pheno phenomena can be observed; the weld material aterial contracts but the base metal stops it to contract.. Consequently, ele element 1 is in compression pression and element ele 2 is in tension as shown in Figure 3.10 (b). As a result of which themetal etal plates will deform in such a way that the top surface is in tension and bottom surface is under com mpression. In case of the butt joint welding without gap, less strain is expected due to the bottom surface is free to expand as it cools down.

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Figure 3.10 3.10- Contraction and Expansion Phenomena.

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3.9 WeldingApplications Welding application for the followingmaterials are explained below. A) Steel:1. Structural carbon steel welded tostructuralcarbon steel. 2. Concrete reinforcing steel 3. Carbon or low-alloy and high-strength, steels for all types of piping systems. 4. Rails. 5. Steel castings, carbon or high-strength and low-alloy. 6. Structural carbon steel welded tohigh- strength, low-alloy steel 7. High-strength and low-alloy steel weldedto high-strength andlow-alloy steel. B) Stainless steels. 1. Cryogenic vessels and piping materialsusedfor storage and transport of extremelylow temperature liquids 2. Vacuumchambers. 3. All otheruses. C) Nickel Steels andnickelalloysare used forcryogenicvessels andpipingsystems. D) Alloys of Aluminum used for cryogenic vessels, piping systems, andotheruses. E)Carbon

and

high-strength and low-alloy steels welded tostainlesssteels. An

exampleofthis useiswhen steel supports or stiffeners areattached to stainless steel vessels. 3.10 Introduction to ASTM 36 As the material used for welding simulation is ASTM 36, itsvarious properties and applications are necessary to know.ASTM 36 is the most commonly used mild steel and hot-rolled steel. This hot rolled steel has excellent welding properties and also it is suitable for tapping, drilling, grinding, punching and machining processes. The yield strength of ASTM 36 is less compared to cold roll C1018, hence enablingASTM 36 to bend more easily than C1018. Normally, larger diameters in ASTM 36 are not manufactured since C1018 hot roll rounds are used. Steel shapes like channels, angles, 24

H-beams and I-beams can also be produced with ASTM 36. [30] ASTM 36 is usually available in the following three forms: 1. Rectangle bar 2. Square bar 3. Circular rod Table 3.1shows the Chemical composition, Table 3.2 shows Physical Properties and Table 3.3 showsMechanical properties of ASTM 36 respectively. Table 3.1 - Chemical Composition of ASTM 36 Element Carbon, C Copper, Cu Iron, Fe Manganese, Mn Phosphorous, P Silicon, Si Sulfur, S

Content 0.25 - 0.290 % 0.20 % 98.0 % 1.03 % £ 0.040 % 0.280 % £ 0.050 %

Table 3.2 - Physical Properties of ASTM 36 Physical Properties Density

Metric 7.85 g/cc

Imperial 0.284 lb/in3

Table 3.3 - Mechanical Properties of ASTM 36 Mechanical Properties Tensile Strength, Ultimate Tensile Strength, Yield Elongation at Break (in 200 mm) Elongation at Break (in 50 mm) Modulus of Elasticity Bulk Modulus (typical for steel) Poissons Ratio Shear Modulus

Metric 400 - 550 MPa 250 MPa 20.0 % 23.0 % 200 GPa 140 GPa 0.260 79.3 GPa

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Imperial 58000 - 79800 psi 36300 psi 20.0 % 23.0 % 29000 ksi 20300 ksi 0.260 11500 ksi

Machining The machinability rate of ASTM 36 is estimated to be 72%, and the average surface cutting feed of ASTM 36 is 120 ft/min. Machining process of ASTM 36 steel is not as easy as AISI 1018 steel. Welding ASTM 36 steel is easy to weld by using any type of the welding, and the welds and joints so formed are of excellent quality. Heat Treatment Any standard carburizing and hardening types of AISI 1018 steel is suitable for ASTM 36. Heat treatment processes for ASIM A36 ASTM 36 is subjected to the following heat treatment processes: Normalizing at 899°C – 954°C (1650°F-1750°F) Annealing at 843°C – 871°C(1550°F-1600°F) Stress relieving at 677°C – 927°C(1250°F-1700°F) Carburizing at 899°C – 927°C (1650°F-1700°F) Hardening at 788°C – 816°C(1450°F-1500°F)

Applications of ASTM 36 ASTM 36 steel has the following applications: It is used for various parts obtained by flame cutting such as in walkways, boat landing ramps, parking garages and trenches. It is used in bolted, riveted or welded construction of buildings, bridges and oil rigs. It is also used in forming bins, sprockets, cams, gears, stakes, bearing plates, base plates, forgings, ornamental works, fixtures, tanks, rings, templates, jigs, brackets, automotive and agricultural equipment, frames, machinery parts etc.

3.11. TemperatureEffects of Butt Welding on Steel Plate 26

1. The heat affected zone of double-layer weld pass remains togreater extentthan single-layer weld pass. 2. The changes in temperature gradient are quite small at a certain distance from weld pass due to heat convection effect. 3. The temperature gradient of vertical weld pass is normally greater than the direction of weld heat source. 4. With the same number of weld passes, heat affected zones get decreased with the increase of weld speed. 5. When the heat source has shifted half the size of the parent material, there exists still high temperature gradient where the heat source has initially caused effects. 3.12 Important Factors to Determine the Welding Quality A) Strict control over welding proceduresandoperations is necessary in five cases 1. Distress in onemembercan

causeatleast partial collapse or

failurewith

somehazardto property and life ; applicationofthedesignload mayapproach 10,000 cycles overmanyyears. 2. Some

of

the

welds

necessary

forstructural

integrity

are

highly

stressed;applicationof thedesign loadmayexceed10,000 cyclesovermanyyears. 3. Empirical design requirementscompensate for overloads, mishandling, “acts of God,” abuse

and similar hazards; applicationof the designload

maybein the range ofnearly about 100,000cycles. 4. Failure of welds or components couldbecatastrophic, as in structures such as bridges orhigh--pressure gas piping systems; fatigue ofmaterials must be carefully considered in to account or

order of 2 million cycles. 5. Applications require the highest qualityofmaterial and workmanship throughout, such asfor nuclear, space,

and ballistic applications

andforsystems subjected to extreme pressures,hazardous chemicals, or temperatures. B)Less control overweldingproceduresandoperations is needed where: 27

1. Stress levelsare toolow. 2. Welds are subjected occasionallyto design loads. 3. The structure is composed ofmanycomponents, and distress in one memberwill

result

ininconvenience

rather

orcatastrophicfailure.

CHAPTER 4 FINITE ELEMENT ANALYSIS 4.1

Introduction 28

than

collapse

At present the finite element method (FEM) is the most widely used tool for solving this kind of thermal problems When the aim of the analysis is to determine the mechanical effects of welding(residual stress and distortions) the simple approach is to consider the thermal and mechanical relation only, because there is a weak connectionfrom mechanics to heat flow. Heat generated bydeformation can be neglected) and hence the most used approach is to carry a sequentially coupled thermal and mechanical analysis, if the structure deformation during weldingdoes not change significantly. This aspect gives also thepossibility for the use of general purpose finite element computer codes. [Viorel Deaconu;2007]

The results obtained by FEA emphasize the ability of this method to give quality results, in agreement with experimental results and also offer the possibility to a better understanding of residual stress field characteristics. Despite the limitations related to the need for information related to the welding process and for complex material data and numerical modeling remains actually the sole method which is able to fully characterize the residual stress field through the whole structure without any limitations related to his geometry or shape and size. Once having residual stress distributions information, subsequent simulations related to stress relief carried by mechanical loading or by post weld heat treatments can be performed. [Viorel Deaconu;2007]Following are the important aspects of FEA 4.1.1

Simulationof Butt WeldingProcess

During fabrication of welded components residual stresses are produced as a result of nonuniform temperature distribution during the welding process and particularly the cooling processes. The residual stresses have a significant effect on the overall performance of the components in service, [Pornwasa Wongpanya;2009]To simplify the welding simulation,

it

is bettertoperform thermaland mechanicalanalyses separately. At

first,

thecomputationof

thetemperaturehistory

duringweldingand

subsequentcoolingiscompletedand thistemperaturefield isapplied tothemechanicalmodel asabodyforcetomaketheresidualstressanalysis. ismodeledin

Theheatinputrequired

commerciallyavailablesoftwareby

using

duringwelding theequivalentheat

inputwhichincludesbodyheat flux. Theamount ofheatinput,QRhas been calculated by usingempiricalrelationshowninEq.1. [M.Jeyakumar; 2011]Arcefficiency isdenotedby η,arcvoltagebyV,arccurrent byI. 29

Heat input = efficiency x voltage xcurrent QR = η x V x I(1) By usingequation (1)ofheatinputthe amount of heat input is determined.Bythermal analysisthetemperature

at

different

pointsarenoted.

This

simulatedtemperaturefieldisthenused in analysis step forcalculatingthe residualstresses andafterthat thevaluesof residual stressesarecalculated by doingstressanalysis. 4.1.2 Use of Finite Element Analysis Finite element analysis is helpful to make it possible to evaluate a detailed and complex structure, in a computer, during the planning of that structure. The possibility of improving the design during planning and the adequate strength of the structure can justify the cost of the analysis work. FEA has been known to improve the design ofthe structure that were built many decades ago and significantly over designed. In absence of finite element analysis (or other numerical analysis) development of structures depends onlyupon hand calculations only. For complex structures, the simplified assumptions which are required to make any calculation possible can lead to a conservative and heavy design. Animportant factor of ignorance can remain as to whether the structure will be adequate for all design loads. Significant changes in designs involve a lot of risk. Design will require prototypes to be built and they should be field tested. These field tests may involve expensive strain gauging to evaluate characteristics like strength and strain. With the help of finite element analysis, the weight of a design can be optimized and there could be a reduction in the number of prototypes built. Field-testing is required to establish loading on structures, which can be used to do future design improvement by using finite element analysis. 4.1.3 Advantages of FEA The power of the finite element technique resides principally in its versatility. The FEA can be applied to various physical problems. The body to be analyzed can be having arbitrary size and shape, loads and various support conditions. The command mesh can mix elements of differentphysical properties,types and shapes. 30

Another important feature of finite element method lies in the close physical resemblance between the actual structure and its finite element model. 4.1.4 Limitations of FEA 1. A computer, a reliable program and intelligent use and software skills are essential. 2. Deep experience and good engineering judgment are needed in order to define a good finite element model. 3. Different input data are required and voluminous output must be sorted and understood. 4. Specific numerical results are required to be found for specific problem. 5. The finite element analysis does not provide any close form of solution that allows analytical study of the effects of changing parameters.

4.2

Geometric Modeling

The geometry and the element type are required to be considered together. Shellelement is typically used for structure where the thickness is almost negligiblecompared to its length and width. Nevertheless, a plate modeled with solid element would provide similar results. The disadvantage can be found in the computation time. FEA provides large no. of choices of elements. Following are the basic aspects of geometric modeling. 4.2.1

Mesh Generation

Mesh generation can be defined as the process of dividing the analysis continuum into a number of discrete parts or finite elements. The finer the mesh, the better will be the result, but the longer will be the analysis time. Hence, a compromise between accuracy & solution speed is usually done. The mesh can be created manually. In the manually created mesh, it will be noticed that the elements which are at the joint are smaller. This is termed as mesh refinement, and it helps in allowing the stresses to be captured at the geometric discontinuity (the junction). Manual meshing is somewhat a long & tedious process for models with any degree of geometric complication, but providedwith useful tools emerging in pre-processors, the work is becoming easy. Automatic mesh generators are found to be very useful & popular. The mesh is being created automatically by a mesh engine; the only requirement is 31

to define the mesh density along the path of the model's edges. Automatic meshing has limitations as related to mesh quality & solution accuracy. Automatic brick element (hex) meshers are limited in function, but are getting steadily improving. Any mesh can be usually applied to the model by simply selecting the mesh command on the preprocessor list provided in the GUI. 4.2.2

Properties Assignment

Material properties (coefficients of expansion, friction, thermal conductivity, Young’s modulus, Poisson’s ratio, the density, & if applicable, damping effect, specific heat etc.) are required

to be defined. In addition element properties may be needed to be set. If 2D

elements are going to be used, the thickness property is required. 1D-beam elements require area, Ixx, Iyy, Ixy, J, & a direction cosine property, that defines the direction of the beam axis in 3D space. Shell elements like 2D elements in 3D space which are 2½D in nature, require neutral surface offset parameters to be defined & orientation also. Special elements likemass, contact, spring, gap, coupling, damper etc. require properties to be defined for their use and should be specific to the element type. Following properties of ASTM 36 were considered during analysis by finite element method. Table no 4.1 shows thermal properties while table no.4.2 shows mechanical properties of ASTM 36.

Table no.- 4.1 Thermal Material Properties Temp (k) 273

Density (kg/m³) 8038.7

Enthalpy (J/m³) 9.76E+08

Thermal Expansion(1/k) 2.03E-05 32

Conductivity (W/mk) 13.29

Specific Heat(J/kgk) 456.28

293 373 473 573 673 773 873 973 1073 1173 1273 1373 1473 1573 1673

8030.47 7997.02 7954.03 7909.76 7864.18 7817.31 7769.13 7719.66 7668.9 7616.83 7563.47 7508.81 7452.85 7395.6 7354.75

1.05E+09 1.35E+09 1.75E+09 2.16E+09 2.59E+09 3.03E+09 3.48E+09 3.94E+09 4.41E+09 4.90E+09 5.40E+09 5.93E+09 6.49E+09 7.09E+09 7.54E+09

1.99E-05 1.50E-05 1.96E-05 1.93E-05 2.03E-05 2.06E-05 2.08E-05 2.12E-05 2.15E-05 2.21E-05 2.26E-05 2.29E-05 2.33E-05 2.37E-05 2.40E-05

13.63 14.99 16.62 18.19 19.72 21.26 22.81 24.42 26.09 27.86 29.76 31.81 34.03 36.46 38.29

464.73 494.23 522.74 543.93 559.87 572.69 584.49 597.38 613.45 634.82 663.58 701.85 751.72 815.30 869.09

Table no. - 4.2 Mechanical Material Properties Temp (k) 273 293 373 473 573 673 773 873 973 1073 1173 1273 1373 1473 1573 1673

Yield Strength(N/m²) 3.47E+08 3.20E+08 2.11E+08 1.67E+08 1.45E+08 1.35E+08 1.29E+08 1.23E+08 1.17E+08 1.11E+08 1.05E+08 9.90E+07 6.60E+07 2.40E+07 1.05E+07 1.00E+07

Tensile Strength(N/m²) 6.45E+08 6.20E+08 5.20E+08 4.60E+08 4.40E+08 4.20E+08 4.00E+08 3.80E+08 3.60E+08 3.40E+08 3.20E+08 3.00E+08 2.54E+08 9.80E+07 3.90E+07 2.70E+07

33

Elastic Modulus(N/m²) 2.00E+11 1.96E+11 1.92E+11 1.84E+11 1.76E+11 1.68E+11 1.60E+11 1.52E+11 1.44E+11 1.35E+11 1.27E+11 1.19E+11 1.05E+11 2.00E+10 7.00E+09 5.56E+08

Conductivity Conductivity (W/mK)

45 40 35 30 25 20 Conductivity

15 10 5 1673

1573

1473

1373

1273

1173

1073

973

873

773

673

573

473

373

293

273

0

Temp (K)

Figure4.1 Graph for Thermal Conductivity V/S Temperature 4.2.3 Graphs of Material Properties For transient thermal structural coupled filed analysis temperature dependent material properties as shown in fig 4.1, fig.4.2, fig.4.3, fig.4.4 &fig.4.5 areconsidered. The specific heat of the material goes on increasing with the increase of temperature as shown in figure 4.2.

34

Specific Heat Specific Heat (J/Kg K)

1000.00 900.00 800.00 700.00 600.00 500.00 400.00

Specific Heat

300.00 200.00 100.00 1673

1573

1473

1373

1273

1173

1073

973

873

773

673

573

473

373

293

273

0.00

Temp (K) Figure 4.2 Graph for Specific Heat Inverse to the specific heat property the density of ASTM 36 decreases with increase in temperature as shown in fig.4.3

Density Density Kg/m3

8200 8000 7800 7600 7400

Density

7200 7000

Temp (K) Figure 4.3 Graph for Density 35

Elastic Modulus

Elastic Modulus (N/m2)

2.50E+11

2.00E+11

1.50E+11

1.00E+11

Elastic Modulus

5.00E+10

0.00E+00

Temp (K)

Figure 4.4 Graph for Elastic Modulus The elastic modulus of the material goes on decreasing with temperature as shown in fig.4.4.When the material attains a temperature which is equal to 1373 K, the value of elastic modulus begins to decrease drastically. Fig 4.5 shows Graph for Thermal Expansion against various values of temperatures.

36

Thermal Expansion Thermal Expansion (m/m K)

3.00E-05 2.50E-05 2.00E-05 1.50E-05 Thermal Expansion

1.00E-05 5.00E-06

1673

1573

1473

1373

1273

1173

1073

973

873

773

673

573

473

373

293

273

0.00E+00

Temp (K)

Figure 4.5 - Graph for Thermal Expansion 4.3

Some specific type of load is usually applied to the analysis model. The loading may be in the form of a pressure, a point load or a displacement in a stress (displacement) analysis, a heat flux or a temperature in a thermal analysis & a fluid pressure or velocity in a fluid analysis. The loads may be applied to an edge, a surface, a point, or even a complete body. The various loads should be in the same units as used in the model geometry & material properties are to be specified. In the cases of buckling analyses & modal (vibration) , a load is not required to have to be specified for the analysis to run. In this project analysis, load due to increase in temperature i.e. load due to heat flux was considered.At

first,

thecomputationof

thetemperaturehistory

duringweldingand

subsequentcoolingiscompletedand thistemperaturefield isapplied tothemechanicalmodel asabodyforcetomaketheresidualstressanalysis.

37

Theheatinput

required

duringwelding

ismodeledin

commerciallyavailablesoftwareby

using

theequivalentheat

inputwhichincludesbodyheat flux. Theamount

ofheatinput,QRhas

been

calculated

by

usingstandardrelationshowninEq.1.Arcefficiency isdenotedby η,arcvoltagebyV,arccurrent byI.Typicalweldingparameterstaken in thisstudy are,arcvoltage is considered30 volts, arccurrent to be as 200 amp and arc efficiency70%. Heat input = efficiency x voltage xcurrent QR = η x V x I

(1)

By usingequation (1)ofheatinputthe amount of heat input is determined. Bythermal analysisthetemperature

at

simulatedtemperaturefieldisthenused

different in

analysis

pointsarenoted. step

forcalculatingthe

This residual

stressesandafterthat thevaluesof residual stressesarecalculated bymeans of stressanalysis. 4.4

BoundaryConditions

If a load is applied to the model, in that case in order to stop it accelerating infinitely through the computer's virtual ether (which is mathematically alsoknown as a zero pivot),it is necessary that at least one constraint or boundary condition must be applied. The structural boundary conditions are usually taken in the form of zero displacements, thermal boundary conditions such as temperatures ranges are usually specified, fluid BCs are usually specified by pressures. A boundary condition may be specified in order to act in all directions (x, y, z), or in some cases to certain directions only. They can be placed or applied on key points, nodes, and areas or on lines. Boundry conditions's on lines can be represented in the form of symmetric or anti-symmetric type boundary conditions, one allowing in plane rotations &out of plane translations, whereas the other allowing in plane translations & out of plane rotations for a given line. The applications of correct boundary conditions are critical related to the accurate solution of the design problem. At least one boundary condition is required to be applied to every model, in case when even buckling & modal analyses with no loads applied.

The welding process simulation was carried out in three different steps. As the welding process is time dependent the transient thermal structural coupled field analysis is carried out by using following boundary conditions, 38

Stage I - Welding a) Weld temperature of 1200º C applied on weld volume b) Room temperature 20º C applied on plate volume c) Weld time defined as 10 sec Stage II – Phase Change a) Convection applied on surfaces subjected to air b) Phase change time defined as 100 sec Stage III – Solidification a) Convection applied on surfaces subjected to air

4.5

Finite Element Analysis The Finite Element solver may be logically divided into three main parts i.e. the pre-

solver, the post-solver and the mathematical-engine. The pre-solver is able to read in the model created by the pre-processor and also formulates the mathematical representation of the model. All important parameters defined in the pre-processing stage are going to be used to do this, so if something is left out, chances are the pre-solver will complain & cancel the call to the mathematical-engine. If the model is correct then the solver proceeds to form the element-stiffness matrix for the problem & calls the mathematical-engine which calculates the result (temperatures, pressures, displacement etc.) The results are then returned to the solver & the post-solver is used to calculate heat fluxes, velocities, strains and stresses etc.) for each and every node within the component or continuum. Finally all these results are sent to a result file, which may be read by the post-processor. During finite element analysis of welding process, a transient heat transfer analysis is carried out by determining the temperature distribution in all nodes. After that a static mechanical analysis is performed. Each step of the mechanical analysis represents a time step in the thermal analysis. Finally, at the last step, when temperatures attain their initial values and the residual stress field is obtained as result of all intermediary analysis steps.[Viorel Deaconu; 2007]

39

4.6 Simulation During simulation of welding process, the results of the analysis are read & interpreted. They can be represented in the form of a contour plot, a table, deformed shape of the component or the mode shapes and natural frequencies in case if frequency analysis is involved. Other results are also available for thermal, fluids and electrical analysis types. Most of the post-processors provide an animation service, which is able to produce animation & hence brings your model to life. Contour plots are generally the most effective way of viewing results for structural type problems. Slices maybe made through 3D models to facilitate the viewing of internal stress patterns. All post-processors nowadays include the calculation of stress & strains in any of the x, y or z directions, or if requred in a direction at an angle to the coordinate axes. The principal stresses and strains can also be plotted and if required the yield stresses and strains can also be plotted according to the main theories of failure (Von mises, St. Venant, Tresca etc.). Other necessary information such as plastic strain, the strain energy, and creep strain may be obtained for certain types of analyses.

40

CHAPTER- 5 COUPLED FIELD ANALYSIS 5.1 Coupled Field Analysis When the input of one of the field analysis depends on the results from another analysis, the analyses are said to be coupled. A coupled-field analysis is defined as a combination of analyses from different engineering disciplines (physics fields) that interact to solve a global engineering problem; due to this, we often refer to a coupled-field analysis as a multiphysics analysis. The Coupled field analysis may be carried out using following two methods, 1. Direct Method 2. Sequential Method

5.1.1 Introduction to Direct Coupled Field Analysis The direct method for doing a coupled-field analysis includes a single analysis that uses a coupled-field element. There are different types of coupled filed analyses which are available according to different engineering streams. For this dissertation work thermal structural direct coupled field analysis is used for simulation of the welding process.

5.1.2 Objective of Direct Coupled Field Analysis The aim of thermal structural direct coupled filed analysis is to simulate welding process using finite element analysis and thereby find the residual stresses due to phase change.

41

5.2 Steps for Coupled Field Analysis Coupled field analysis consists of the following steps 1. Preprocessing:Create the model geometry. Define Material Properties Mesh generation. 2. Solution:Boundary conditions (loads & constraints) Solve. 3. Post Processing:Review results 5.3 Modeling Geometry The ultimate purpose of finite element analysis is to re-produce mathematically the behavior of an actual engineering system. In other words, modeling geometry is used to create an accurate mathematical model of a physical prototype. In the broadest sense, this model can comprises material properties, real constants, all the nodes, elements, boundary conditions and other features that may be used to represent the physical system. In terminology, the term model generation usually takes on the narrower meaning which includes generating nodes & elements that can represent the spatial volumes & connectivity of the actual system. In thepresentstudy, thebutt-weldjointof twoASTM 36stainless using

a

commerciallyavailable

finite

steel

elementsoftware.

plates is modeled Thetwo

semi-

infiniteplatesofthejointare3mm thickand100mmwide(alongthewelding direction).Theweldgrooveangleis 60°.3Dview oftheASTM 36steel plateisshowninFigure 5.1.

42

Figure5.1.-3D Model of the Plate to be Welded

43

Figure5.2 Geometry of the Plate to be Welded

Fig.5.2

showstheGeometry

of

the

platetobebuttwelded.Meshcontrolisappliedtothe

weldmentarea.Plates arefixedattheends andheat convectionwasallowed atthetop surface oftwo plates. ASTM

A

36steelmaterial

can

beweldedby

using

andinductionweldingorby

Fromallthesetypestheanalysisofstainlesssteelinthis

project

is

beingperformed bymanual metalarc welding,or also known as MMAW.

5.4 Material Properties Thetemperaturedependantthermalmaterialproperties forthe plates,thefillerweld material and heataffectedzone (HAZ) were assumedto be thesame,see Table 1.Forthe mechanicalmaterial properties,

samematerial

models 44

were

usedforthe

weldbeadsandthebasematerialsaccordingto theyieldstrength. The plasticitymaterialmodel usedwas vonMisses rate-independentisotropicbilinearhardening. Table 5.1.MaterialProperties [Dragi Stamenkovic; 2009]

Specific Temperature

heat

Yield Conductivity

Density

(J/kg°C) (°C)

stress (MPa)

(W/m°C)

(kgm-3)

Thermal expansion coefficient (10-5/°C)

Young΄s modulus (GPa)

Poisson΄s ratio

0

480

60

7880

380

1.15

210

0.3

100

500

50

7880

340

1.2

200

0.3

200

520

45

7800

315

1.3

200

0.3

400

650

38

7760

230

1.42

170

0.3

600

750

30

7600

110

1.45

80

0.3

800

1000

25

7520

30

1.45

35

0.3

1000

1200

26

7390

25

1.45

20

0.3

1200

1400

28

7300

20

1.45

15

0.3

1400

1600

37

7250

18

1.45

10

0.3

1550

1700

37

7180

15

1.45

10

0.3

5.5 Meshing Geometry Meshing the geometry is creation of finite element model, which consists of nodes and elements. Fig 5.3 shows the meshed model of the plate that is to be welded. Meshing is done through following three important steps as a) Set element attributes. b) Set meshing controls. c) Generate mesh

45

a) Element attributes: Before starting generation of a mesh appropriate elements must be defined. The Solid70 element type is being used for meshing the plate using ET command. The material properties for analysis are appliedwith thehelp of MP command.

Type of elements : Solid 45 No. of Nodes

: 12393

No. of elements

: 8250

Figure5.3 - Meshed Model

b) Meshing controls: After setting the element attributes, the element density is required to be defined for meshing the model. The element or mesh density is set by using ESIZE, ESHAPE, KESIZE and LESIZE command.

46

c) Mesh generation: There are two main types of meshing as free & mapped mesh. Meshing can be made by using AMESH, VMESH commands. But 3-D model must be meshed with only VMESH command only. The plate is meshed by using VMESH (mapped) command. Then the solid model is meshed by using solid 70 element by creating the FEA model having no. of nodes 12393 and no. of elements 8250 as shown in fig.5.3 and is used for further processing then in FEA solution the FEM model created is analysed for coupled field analysis (also termed as coupled field analysis).After completion of simulation the results are read and tabulated by using post-processing stage. 5.6 Boundary Conditions The welding process simulation was carried out in three different steps. As the welding process is time dependent the transient thermal structural coupled field analysis is carried out by using boundary conditions as mentioned earlier [Cristian Simion, Corneliu Manu, Saleh Baset and Julian Millard],

5.7 Solution This is the one of the important stage in FE analysis. With the help of ANTYPE command the type of the solution is specified as transient. Then by using SOLVE command program solves the analysis using numerical methods for three different stages.

5.8 Post Processing In post processing section, the results of stress analysis were reviewed. There are two types of post processing methods viz. /POST1 & /POST26. With /POST1 only static analysis results can be viewed and with /POST26 mainly time dependent analysis results are seen. /POST26 is also termed as time history post processing.

47

CHAPTER 6 RESULTS AND DISCUSSION Depending upon the simulation results, shrinkage or distortion

of the weldment can be

predicted. In this way, the experimental analysis, which is costly, can be avoided. The temperature near the region of weld bead and the HAZ decreases rapidly proportional to the distance from the centre of the heat source. The different results obtained by using FEA are studied as follows. 6.1. Temperatureat Different Distances from Center Line of Weld

with FEA.

During study of welding process, it has been found that the magnitude and distribution of

Figure 6.1. Temperature Distribution in Steel Plate after Welding at T = 100 Sec

48

residual stresses is strongly influenced by temperature distribution through the various crossections of the plates and mechanical properties of material at elevated temperatures. As the welding process progresses, both the weld metal and the base metal experience an increase in temperature. At the solidus temperature, the material within the fusion zone begins to liquefy. Upon cooling, a reverse process occurs and the fusion zone solidifies to complete the joining process A 3 D model showing the relation between temperature and distance from weld at 100 seconds after the completion of welding process is shown in fig. 6.1. The relation between temperature and distance from weld can be predicted by getting tabular data from the graph as shown in table no.6.1.From graph as shown in fig. 6.2, we can see that temperature of

FEA 500 450 400 350 300 250

FEA

200 150 100 50 0 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Figure6.2 - Graphbetween Temperature and Distance from Center Line of the Weld at 100 Sec welded plate goes on decreasing drastically from weld center line from 0 to 85 mm and after 85 mm it becomes almost constant up to 100 mm. The temperature distribution as a consequence of thermal load was employed to calculate equivalent shrinkage forces. 3D finite element elastic structural analysis was conducted to estimate the deformations and 49

residual stresses.The variation in temperature profile and the distribution of thermal stresses was simulated with different welding parameters, e.g. the number of welding layer during the butt welding and the welding speed.[Chun-Ho Yin , Chao-Ming Hsu and Jao-Hwa Kuang; 2013] The distributions and the peak magnitude of residual stress with different welding parameter were also simulated.

Table no.6.1-Temp.(k)(results from FEA) v/s Distance From Weld(mm)

Sr. no.

Distance From Weld(mm)

Temp.(k)(results from FEA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

450 446 446 442 441 433 421 418 415 408 380 370 368 366 362 346 344 332 332 331 331

50

6.2 Strain Calculation with FEA Anundesirable result of the induced nonlinear thermal cycles associated with the welding processisthedevelopmentofresidualstresses. Thefluidmaterial in the fusion zone is Table no.6.2- strain (results from FEA) v/s Distance fromWeld (mm)

Sr. no.

Distance From Weld(mm)

Strain x10-3 (from FEA)

0

67.2

1 2 3 4 5 6 8 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

46.08 43.2 40.32 19.2 6.72 4.8 4.8 2.88 1.92 1.92 1.728 1.536 1.248 0.96 0.864 0.768 0.672 0.576 0.48 0.192 0.1056 0.1056 0.1056 0.0864 0.0864 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

51

bounded by the solid HAZ material & unaffected base metal. The solid material adjacent to the fusion zone acts as a mechanical constraint which results in inelastic strains being produced as the fusion zone material gets solidified. As the fusion zone gets solidified, the grain growth progresses from the heat affected zone toward the center of the weld. The elastic strains tend to result in tensile residual thermal stresses in the fusion zone. From the graph as shown in fig.6.3, the values of strains at different distances from center line of the

Strain (from FEA) 80 70 60 50 40

Strain (from FEA)

30 20 10 0 0

2

4

6

10

20

30

40

50

60

70

80

90 100

Distance From Weld(mm)

Figure6.3 -Graph between Strain at Mid Section Perpendicular to the Weld of the ASTM36 Steel Plate along its Length Direction weld can be obtained. The value of strain for different distances from center line of the weld is shown in table 6.2. It can be seen from the table that the value of strain goes on decreasing

52

as the distance from the center of weld gets increased. As stress is directly proportional to strain the value of stress gets varied according to the value of stress. The maximum strain observed in the parts due to welding process is 67.3 x10 -3 mm.

6.3 StressCalculation with FEA Fig. 6.4shows hows the von moises plot in FEA FEASoftware. Fig.6.5 showsthe graph of residualstress in axial direction formanualmetala etalarcwelding obtained by FEA. The highest residual stress level in welded plate of single V-butt butt joints has been observed in the weld metal etal adjacent to the fusion zone.Table no.6.3givesthevalu ues of residualstresses distributionagainstthediistance

weldcentre whenplatethicknesssis 3 mm.Itis clearfromthe plotthatheat

Figure 6.4 - Von Mises Plots 53

from

affectedzone extendsupto 38mm onbothsidesoftheweldment.Itisseenthatthe residual stress in the transverse

direction for

butt weldingof3mmthick platescomesoutto bemaximum

(320Mpa)in thecentre ofweldment. Simulated results are in a good agreement with experimental results, which shows the reliability of FEA or equivalent load method. Von mises residual stress goes on decreasing rapidly in transverse direction in areas near the weld region. In contrast to this, welding deformation shows a smooth continuous increasing behavior. The residual stress distribution is ununiform through the thickness of plate with maximum value at the top surface of plate and decreases gradually to minimum

at the

bottom. Every mode of deformation has greater value in case when thickness is very small.

FEA Stress 400 300 200 100

FEA Stress

-100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

0

-200 -300

Figure 6.5 - Graph between residual stress at mid section perpendicular to weld of the ASTM36 steel plate along its length direction An increase in plate thickness causes reduction of residual stresses in areas adjacent to fusion zone. Weld induced stresses are the one which are major contributors to the overall stress states in welded structural components. Residual stresses may be determinable to the performance of component assemblies and can be induced, in addition to distortions failures through stress corrosion cracking, brittle fracture and can cause to the detoriation of fatigue life.

54

Table No.6.3- Stress (in Mpa)Results from FEA V/S Distance from Weld (mm)

Sr. no.

Distance From Weld(mm)

FEA Stress(Mpa)

1

0

270

2

5

320

3

10

290

4

15

150

5

20

-110

6

25

-180

7

30

-220

8

35

-80

9

40

-55

10

45

-30

11

50

-10

12

55

-5

13

60

10

14

65

20

15

70

20

16

75

30

17

80

30

18

85

35

19

90

40

20

95

40

21

100

40

6.4 ComparisonoftheFEMwithExperimentalResultsforAxial ResidualStresses The values ofaxialresidualstresses calculatedbythe finite element method are compared withthe experimental results[M.Jeyakumar; 2011]with the help of tabular data as shown in table no.6.4. The results from these tables are obtained from the graph as shown in figure6.6.

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Table no.6.4-comparison of experimental stress v/s FEA stress

Sr. no.

Distance From Weld(mm)

Experimental Stress(Mpa)

FEA Stress

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

350 390 300 100 -120 -200 -125 -100 -50 -25 -5 0 5 10 15 18 20 22 25 28 30

270 320 290 150 -110 -180 -220 -80 -55 -30 -10 -5 10 20 20 30 30 35 40 40 40

These results are obtained by reading the results at various nodes and elements at various distances of the plate along x axis and across y-z plane. These results show that thecomputatedfinite element resultsarevery closeto the experimentalresults.There is a little variation between the two results because in actual practice there are different types of heat losses and error in manual welding process which causes ununiform distribution of heat through welding path. The results emphasize the ability of this method to create quality results which are in agreement with experimental data and to offer the possibility to a better calculation of residual stress field characteristics as well as to reduce the conservatism its components quantification according to the current integrity assessment procedures. Despite the shortcomings in relation to the need for information related to the welding process and

56

500 400 300 200 Experimental Stress(Mpa)

100

FEA Stress

0 -100

0

10

20

30

40

50

60 70

80

90 100

-200 -300

Figure 6.6 -ComparisonoftheFEA ResultswithExperimentalResultsforAxial ResidualStresses

For complex material data, numerical modeling is practically the only method able to fully characterize the residual stress field over the whole structure without imposing any limitations related to his geometry or size. After having residual stress distributions data, subsequent simulations of stress relief by mechanical loading or by post weld heat treatments can be performed. Validated by experiments, FE modeling techniques can be an effective support for integrity assessments of structures containing welds.

CHAPTER 7 57

CONCLUSIONS From the present study the following major conclusions are drawn 1) In case of experimental stress analysis it will require the prototype of the structure to be analyzed while analysis by FEA software eliminates that and so the cost & time of product design also gets reduced. With the help of FEA software the behavior of the structure can be studied for any number of welds. By using different mesh densities the results can be obtained with great accuracy. Thus FEA is one of the important tool for thermal and structural analysis which gives results very fast. Thetime and cost required to find residual stress in the parts due to welding process using FEA software is very less as compared to the experimental method. Calculation of Residual stress for any complicated welding process as well any complicated structure is possible by using finite element methods without altering the physics of the problem. 2) Themaximum strain observed in the butt welded steel plate is found to be 67.2 x10-3 at the region of weld.The maximum temperature at 100 seconds after completion of welding process, observed in the butt welded steel plate is found to be 450 K at the region of weld and it goes on decreasing along the length of plate. Finally the temperature at 100 seconds at a distance of 100 mm from the center line of the weld after completion of welding process is found to be 331 K which is the minimum temperature along the length of the butt welded steel plate. Distribution of temperature, strain and residual stresses with respect to distance from weld are showed in tabulated form as well as by using graph for better understanding of the welding process. The maximum residual stress observed in the butt welded steel plate is found to be 320 Mpa at a distance of 5mm from the center line of the weld.

References 58

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