PROJECT REPORT

ON

DISTORTION IN WELDING

Submitted by:PREM DIWAKAR

BIRLA INSTITUTE OF TECHNOLOGY, MESRARANCHI

Under the guidance of:

Mr. SOMANATH NAYAKMANAGER

QUALITY ASSURANCE, TELCON

ACKNOWLEDGEMENT

The project that I undertook was very helpful in my better understanding of the

various welding processes and the difficulties in the industrial world during various

welding operations. I am greatly indebted to MR. A. K. TRIVEDI, AGM, QA and

MR. RAJAT BISWAS, Sr. Div. Manager, QA for offering this wonderful opportunity to

acquire first hand knowledge of production activities in TELCON.

I offer my sincere gratitude to MR. SOMANATH NAYAK, Manager, QA for his

guidance throughout the course of this project. I would also like to thank all the persons

in the fabrication shop for helping me to understand the various welding processes for

their helpful attitude. Lastly I would also like to thank MR. D SUNDARESAN, Manager,

QA and MR. RAJEEV RANJAN from International Auto Limited (IAL) for rendering

their kind help without which it would have been difficult to complete the project.

I found quality assurance department to be highly disciplined where endeavors are

respected and achievements rewarded. I appreciate all the employees of TELCON for

their devotion to their jobs, which is the reason behind their success story.

Thanking you,

Certificate of Approval

This is to certify that Mr. Prem Diwakar, BE/220/05 of Birla Institute of Technology,

Ranchi has successfully completed his training in the Quality Analysis Department from

05.05.2008 to 14.06.2008

During this period he has worked on to the project:

DISTORTION IN WELDING

The above project was successfully completed in the stipulated time period.

Mr. SOMANATH NAYAK

Manager

Quality Assurance

Telcon

CONTENTS ABSTRACT

CHAPTER 1: About TELCON 1.1 A saga of vision, commitment and fortitude

1.2 About us1.3 Vision1.4 Mission1.5 Value and purpose1.6 Five core values

1.7 Tata Engineering and Hitachi in joint venture for earth moving equipment 1.8 Euclid-Hitachi to make dumpers for Telcon

CHAPTER 2: Welding overview2.1 An introduction2.2 Welding processes2.3 Types of welding joints2.4 Heat affected zone2.5 Welding defects

CHAPTER 3: Distortion in welding3.1 An introduction3.2 What is weld distortion?3.3 Understanding distortion 3.4 Main types of distortion3.5 Factors affecting distortion 3.6 Ways to reduce distortion3.7 Correcting distortion

CHAPTER 4: Case study.4.1 Arm (ZX 210) 4.2 Centre frame (ZX 210)4.3 Tail frame (ZX 210)

CHAPTER 5: Conclusions and references.

ABSTRACT

Welding distortion is a phenomenon that appears after welding and causes sometimes part failure and hence has to be observed seriously. High fitting and welding costs are the consequence of this large welding distortion. This problem is exacerbated as the fairness requirements are tightened. Distortion like buckling can cause plate failures or distortion like various shrinkages can really cause some real problems like change in dimension. There are various ways to check distortion knowing the fact that it can’t be removed completely and hence required steps has to be taken to minimize it as much as possible. One of the ways to reduce distortion is to make a good welding design i.e. to reduce number of passes or like balanced welding one can control distortion to a large extent. Further, there are lots of other ways like tack welding or restrained welding to minimize distortion.

The case study of three parts of excavator 210(arm, centre frame and tail frame) at TELCON shows the effect of distortion on these part’s dimensions or plates. Distortion causes buckling of bottom plates of both tail and centre frame or change in dimension like decrease in the distance between side plates of centre frame thus increasing fitting cost and decreasing production rate.

CHAPTER 1: ABOUT TELCON

1.1 A saga of vision, commitment and fortitude As much an institution as it is a business conglomerate, the Tata Group is unique in more ways than one. Established by Jamsetji Tata in the second half of the 19th century, the Group has grown into one of India's biggest and most respected business organizations, thanks in no small part to its entrepreneurial vision, its commitment to ideals that put people before profits, and its fortitude in the face of adversity.

1.2 ABOUT TELCON: Telco Construction Equipment Co. Ltd. is a joint venture company between Indian automobile giant Tata Engineering Ltd and world leaders in hydraulic technology, Hitachi Construction Machinery Co. Ltd., Japan. Telco started the Earthmoving Equipment Division in 1961, in collaboration with P&H, USA for rope shovels and cranes. The manufacturing of hydraulic excavators began in 1984, under technical collaboration with Hitachi, Japan. 1987 marked the manufacture of Tata-Hitachi KH500, the country's first hydraulic crane crawler. The product range includes hydraulic excavators, crawler cranes, wheel loaders, backhoe loaders, off-highway dumpers, motor graders, skid steer loaders, cane loaders and truck loader cranes.

Telcon's credentials lay together in its Design capabilities, in its associations with world leaders such as Hitachi, John Deere, Euclid, Tadano, ZF and the like, state-of-the-art manufacturing facilities is Jamshedpur and Dharwad and excellent and wide-spread customer support, the key to smooth running of capital equipment is well taken care of by its network of more than 30 offices and many more service associates.

1.3 Vision: “Global Top 25 in CE Industry By 2012”.

1.4 Mission: To be The Most Trusted Partner for providing Full Line of Constructive Solutions for Mining, Construction, Infrastructure & Agriculture Sectors. While Dominating the Indian Market, we shall make concerted efforts to meet our Global Ambitions. Our Hallmark shall be Our Motivated People, Strong Partnerships at all levels, Superior Technologies and Our Widespread Customer Support, all working in Unison with Environment and Society.

1.5 Values and purpose Leadership with trust

Purpose At the Tata Group our purpose is to improve the quality of life of the communities we serve. We do this through leadership in sectors of national economic significance, to which the Group brings a unique set of capabilities. This requires us to grow aggressively in focused areas of business. Our heritage of returning to society what we earn evokes trust among consumers, employees, shareholders and the community. This heritage is being continuously enriched by the formalization of the high standards of behavior expected from employees and companies. The Tata name is a unique asset representing leadership with trust. Leveraging this asset to enhance Group synergy and becoming globally competitive is the route to sustained growth and long-term success

1.6 Five core values

The Tata Group has always sought to be a value-driven organization. These values continue to direct the Group's growth and businesses. The five core Tata values underpinning the way we do business are:

Integrity: We must conduct our business fairly, with honesty and transparency. Everything we do must stand the test of public scrutiny.

Understanding: We must be caring, show respect, compassion and humanity for our colleagues and customers around the world, and always work for the benefit of the communities we serve.

Excellence: We must constantly strive to achieve the highest possible standards in our day-to-day work and in the quality of the goods and services we provide.

Unity: We must work cohesively with our colleagues across the Group and with our customers and partners around the world, building strong relationships based on tolerance, understanding and mutual cooperation.

Responsibility: We must continue to be responsible, sensitive to the countries, communities and environments in which we work, always ensuring that what comes from the people goes back to the people many times over.

1.7 Tata Engineering and Hitachi in joint venture for earth moving equipment (January 31, 2000)

Tata Engineering and Hitachi Construction Machinery Company, Japan (Hitachi) today announced a joint venture to manufacture technologically advanced earth moving equipment in India. Hitachi will pay Tata Engineering US$ 21.5 million for a 20 per cent stake in Telco Construction Equipment Company Limited (Telcon), the Tata Engineering earth moving equipment subsidiary. Consequently, Tata Engineering will now hold 80 per cent of the equity. Telcon is the market leader in India with a 67 per cent market share in the hydraulic excavator sector. Hitachi Construction Machinery Company Limited was established in 1970. Its main products are hydraulic excavators, crawler cranes, foundation machines and shield machines, all of which have earned an excellent reputation world-wide for performance, reliability and safety. Hitachi and its affiliates have branched out into the fields of mini-excavators, wheel loaders, bulldozers and off-road dump trucks.

1.8 Euclid-Hitachi to make dumpers for Telcon Telco Construction Equipment Co, formerly the construction equipment business unit of Telco, has chosen Euclid-Hitachi, a US-Japanese joint venture, for manufacturing dumpers at its works in Jamshedpur.The company, which became a 80:20 joint venture between Telco and Hitachi Construction Machinery Ltd on January 31, 2000, has been manufacturing the EX series of excavators with knowhow from Hitachi during the last few years.

It also makes its own TMK models of excavators, an 11-tonne wheel loader (the TWL 3036) which is self-developed and a seven-tonne version of backhoe loader in technical collaboration with John Deere of the US.Telcon's small capacity high volume models (up to 20 tonnes) include the EX 40, EX 60, EX 100, EX 200 and EX 210-V range of excavators. The country's requirement of nearly 150-200 motor graders used specially in road making is being met through imports from Fiat-Hitachi or Mitsubishi or those manufactured by BEML (Bharat Earth Movers Ltd).

The company has 68 per cent marketshare in the excavators segment, a whopping 95 per cent in the cranes market and only 10 per cent in the wheel loader and backhoe loader segments. Telcon faces competition from L&T-Komatsu, BEML, Hindustan Motors and JCB-Escorts for its excavators, wheel loaders ad backhoe loaders. The network has expanded at a fast pace and today there are offices in 30 locations, 5 distribution centers and two plants at Jamshedpur and Dharwad. This is supported in parallel with network of 30 Authorized Customer Support Centres (Dealerships) for sales and after sales service of Telcon machines. Warehouses are in 11 locations, though essentially the warehousing activity has been given away to a clearing and forwarding agency. Telcon is now making forays into international markets and has set up dealerships and service centers in Iraq, Bangladesh, Nepal & Sri Lanka.

CHAPTER 2: WELDING OVERVIEW

2.1 An Introduction Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld puddle) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld.

Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join metals by heating and pounding them. Arc welding and oxyfuel welding were among the first processes to develop late in the century, and resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding; now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding and flux-cored arc welding. Developments continued with the invention of laser beam welding and electron beam welding in the latter half of the century. Today, the science continues to advance. Robot welding is becoming more commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality and properties.

2.2 Welding processes:2.2.1: Arc welding These processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well. One of the most common types of arc welding is shielded metal arc welding (SMAW), which is also known as manual metal arc welding (MMA) or stick welding.

2.2.2. Gas metal arc welding (GMAW): It is also known as metal inert gas or MIG welding, is a semi-automatic or automatic process that uses a continuous wire feed as an electrode and an inert or semi-inert gas mixture to protect the weld from contamination. Welding speeds are greater for GMAW than for SMAW. Also, the smaller arc size compared to the shielded metal arc welding process makes it easier to make out-of-position welds (e.g., overhead joints, as would be welded underneath a structure). The equipment required to perform the GMAW process is more complex and expensive than that required for SMAW, and requires a more complex setup procedure. Therefore, GMAW is less portable and versatile, and due to the use of a separate shielding gas, is not particularly suitable for outdoor work. However, owing to the higher average rate at which welds can be completed, GMAW is

well suited to production welding. The process can be applied to a wide variety of metals, both ferrous and non-ferrous.2.2.3. Gas tungsten arc welding (GTAW) or tungsten inert gas (TIG) welding: It is a manual welding process that uses a non-consumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds. GTAW can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications.

A related process, plasma arc welding, also uses a tungsten electrode but uses plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process, and furthermore, it is much faster.

2.2.4. Submerged arc welding (SAW): It is a high-productivity welding method in which the arc is struck beneath a covering layer of flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself, and combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes, since the flux hides the arc and almost no smoke is produced. The process is commonly used in industry, especially for large products and in the manufacture of welded pressure vessels.2.2.5. Gas welding: The most common gas welding process is oxyfuel welding, also known as oxyacetylene welding. It is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes, as well as repair work. It is also frequently well-suited, and favored, for fabricating some types of metal-based artwork. Oxyfuel equipment is versatile, lending itself not only to some sorts of iron or steel welding but also to brazing, braze-welding, metal heating (for bending and forming), and also oxyfuel cutting. The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C. The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels.

2.2.6. Resistance welding:

It involves the generation of heat by passing current through the resistance caused by the contact between two or more metal surfaces. Small pools of molten metal are formed at the weld area as high current (1000–100,000 A) is passed through the metal.

Spot welding is a popular resistance welding method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry.Like spot welding, seam welding relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds.

Energy beam welding: Energy beam welding methods, namely laser beam welding and electron beam welding, are relatively new processes that have become quite popular in high production applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both processes are extremely fast, and are easily automated, making them highly productive. The primary disadvantages are their very high equipment costs and a susceptibility to thermal cracking.

2.2.7. Solid-state welding: Like forge welding, some modern welding methods do not involve the melting of the materials being joined. One of the most popular, ultrasonic welding is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. The equipment and methods involved are similar to that of resistance welding, but instead of electric current, vibration provides energy input. Welding metals with this process does not involve melting the materials; instead, the weld is formed by introducing mechanical vibrations horizontally under pressure. When welding plastics, the materials should have similar melting temperatures, and the vibrations are introduced vertically. Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper, and it is also a very common polymer welding process.

Another common process, explosion welding, involves the joining of materials by pushing them together under extremely high pressure. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated. The process is commonly used for welding dissimilar materials, such as the welding of aluminum with steel in ship hulls or compound plates.2.3 Types of welding joints:

Welds can be geometrically prepared in many different ways. The five basic types of weld joints are the butt joint, lap joint, corner joint, edge joint, and T-joint. Other variations exist as well—for example, double-V preparation joints are characterized by the two pieces of material each tapering to a single center point at one-half their height. Single-U and double-U preparation joints are also fairly common—instead of having straight edges like the single-V and double-V preparation joints; they are curved, forming the shape of a U.

Fig 2.1: Common welding joint types – (1) Square butt joint, (2) Single-V preparation joint, (3) Lap joint, (4) T-joint.

Often, particular joint designs are used exclusively or almost exclusively by certain welding processes. For example, resistance spot welding, laser beam welding, and electron beam welding are most frequently performed on lap joints. However, some welding methods, like shielded metal arc welding, are extremely versatile and can weld virtually any type of joint.

Fig 2.2: Different zones in weldpool The cross-section of a welded butt joint, with the darkest gray representing the weld or fusion zone, the medium gray the heat-affected zone, and the lightest gray the base material. The weld itself is called the fusion zone—more specifically, it is where the filler metal was laid during the welding process. The properties of the fusion zone depend primarily on the filler metal used, and its compatibility with the base materials.

2.5 Heat-affected zone (HAZ):

The blue area results from oxidation at a corresponding temperature of 600 °F (316 °C). This is an accurate way to identify temperature, but does not represent the HAZ width. The HAZ is the narrow area that immediately surrounds the welded base metal. The effects of welding on the material surrounding the weld can be detrimental—depending on the materials used and the heat input of the welding process used, the HAZ can be of varying size and strength. The thermal diffusivity of the base material plays a large role—if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Conversely, a low diffusivity leads to slower cooling and a larger HAZ. The amount of heat injected by the welding process plays an important role as well, as processes like oxyacetylene welding have an unconcentrated heat input and increase the size of the HAZ. Processes like laser beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ.

To calculate the heat input for arc welding procedures, the following formula can be used Q = (V * I * 60) / (S * 1000)

where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min). The efficiency is dependent on the welding process used, with shielded metal arc welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas tungsten arc welding, 0.8.

2.7 Welding Defects:

2.7.1. Introduction: There are excessive conditions, outside the acceptance limits, which risks compromising the stability or the functionality of the welded structure and are known as welding defects. These cause rejection of the items and hence, have to be avoided. To produce good quality welds, you must learn to recognize and correct possible welding defects.

Common weld defects include: i. Lack of fusion ii. Lack of penetration or excess penetration iii. Porosity iv. Inclusions v. Cracking vi. Undercut vii. Lamellar tearing viii Distortion and cracking

2.7.2. Types of Defects

i. Lack of fusion results from too little heat input and / or too rapid traverse of the welding torch (gas or electric). It is weld discontinuity in which fusion did not occur between weld metal and fusion faces or adjoining weld beads.

Fig 2.3: Lack of fusion at joints

ii. Excess penetration arises from to high a heat input and / or too slows transverse of the welding torch (gas or electric). Excess penetration - burning through - is more of a problem with thin sheet as a higher level of skill is needed to balance heat input and torch traverse when welding thin metal.

iii. Porosity - This occurs when gases are trapped in the solidifying weld metal. These may arise from damp consumables or metal or, from dirt, particularly oil or grease, on the metal in the vicinity of the weld. This can be avoided by ensuring all consumables are stored in dry conditions and work is carefully cleaned and degreased prior to welding.

iv. Inclusions - These can occur when several runs are made along a V join when joining thick plate using flux cored or flux coated rods and the slag covering a run is not totally removed after every run before the following run.

v. Cracking - This can occur due just to thermal shrinkage or due to a combination of strain accompanying phase change and thermal shrinkage. In the case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high residual stresses and cracking. Where alloy steels or steels with carbon content greater than about 0.2% are being welded, self cooling may be rapid enough to cause some (brittle) martensite to form. This will easily develop cracks. To prevent these problems a process of pre-heating in stages may be needed and after welding a slow controlled post cooling in stages will be required. This can greatly increase the cost of welded joins, but for high strength steels, such as those used in petrochemical plant and piping, there may well be no alternative.

vi Undercutting - In this case the thickness of one (or both) of the sheets is reduced at the toe of the weld. This is due to incorrect settings / procedure. There is already a stress concentration at the toe of the weld and any undercut will reduce the strength of the join.

Fig 2.6: Undercut at welded joint

vii Lamellar tearing - This is mainly a problem with low quality steels. It occurs in plate that has a low ductility in the through thickness direction, which is caused by non metallic inclusions, such as suphides and oxides that have been elongated during the rolling process. These inclusions mean that the plate can not tolerate the contraction stresses in the short transverse direction. Lamellar tearing can occur in both fillet and butt welds, but the most vulnerable joints are 'T' and corner joints, where the fusion boundary is parallel to the rolling plane. These problems can be overcome by using better quality steel, 'buttering' the weld area with a ductile material and possibly by redesigning the joint.

viii Distortion and cracking:

Welding methods that involve the melting of metal at the site of the joint necessarily are prone to shrinkage as the heated metal cools. Shrinkage, in turn, can introduce residual stresses and both longitudinal and rotational distortion. These stresses can reduce the strength of the base material, and can lead to catastrophic failure through cold cracking. Thus it is given much of importance during welding that is explained as.

CHAPTER 3: DISTORTION IN WELDING

3.1 Introduction:- Welding involves highly localized heating of the metal being joined together. The temperature distribution in the weldment is therefore non-uniform. Normally, the weld metal and the heat affected zone (HAZ) are at temperatures substantially above that of the unaffected base metal. Upon cooling, the weld pool solidifies and shrinks, exerting stresses on the surrounding weld metal and HAZ. If the stresses produced from thermal expansion and contraction exceed the yield strength of the parent metal, localized plastic deformation of the metal occurs.

Plastic deformation results in lasting change in the component dimensions and distorts the structure. This causes distortion of weldments.

3.2 What is Weld Distortion? Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process. Doing all welding on one side of a part will cause much more distortion than if the welds are alternated from one side to the other. During this heating and cooling cycle, many factors affect shrinkage of the metal and lead to distortion, such as physical and mechanical properties that change as heat is applied. For example, as the temperature of the weld area increases, yield strength, elasticity, and thermal conductivity of the steel plate decrease, while thermal expansion and specific heat increase (Fig. 3-1). These changes, in turn, affect heat flow and uniformity of heat distribution.

Fig 3.1: Variation of co-efficient of expansion with temperature.

3.3 Understanding Distortion-:

Fig 3.2: Heating and cooling process.

To understand how and why distortion occurs during heating and cooling of a metal, consider the bar of steel shown in Fig. 3-2. As the bar is uniformly heated, it expands in all directions, as shown in Fig. 3-2(a). As the metal cools to room temperature it contracts uniformly to its original dimensions. But if the steel bar is restrained -as in a vise - while it is heated, as shown in Fig. 3-2(b), lateral expansion cannot take place. But, since volume expansion must occur during the heating, the bar expands in a vertical direction

(in thickness) and becomes thicker. As the deformed bar returns to room temperature, it will still tend to contract uniformly in all directions, as in Fig. 3-2 (c). The bar is now shorter, but thicker. It has been permanently deformed, or distorted. (For simplification, the sketches show this distortion occurring in thickness only. But in actuality, length is similarly affected.)In a welded joint, these same expansion and contraction forces act on the weld metal and on the base metal. As the weld metal solidifies and fuses with the base metal, it is in its maximum expanded from. On cooling, it attempts to contract to the volume it would normally occupy at the lower temperature, but it is restrained from doing so by the adjacent base metal. Because of this, stresses develop within the weld and the adjacent base metal. At this point, the weld stretches (or yields) and thins out, thus adjusting to the volume requirements of the lower temperature. But only those stresses that exceed the yield strength of the weld metal are relieved by this straining. By the time the weld reaches room temperature - assuming complete restraint of the base metal so that it cannot move - the weld will contain locked-in tensile stresses approximately equal to the yield strength of the metal. If the restraints (clamps that hold the workpiece or an opposing shrinkage force) are removed, the residual stresses are partially relieved as they cause the base metal to move, thus distorting the weldment.

Why stress relieve...? It is good practice to relieve the residual stresses of a constrained welded assembly. The reason is that these stresses can sum up with external stresses in service and exceed the material strength, producing failure, or further deformations.

3.4 The Main Types of Distortion:- On heating and cooling metals expand and contract according to their expansion coefficient with expansion and contraction occurring in all three principal axes. Usually expansion in the through thickness direction is unconstrained, but there is constraint in the transverse and longitudinal directions. Distortion can be categorized into six main types, i.e. longitudinal shrinkage, transverse shrinkage, angular distortion, bowing and dishing, buckling, and twisting. 1) Transverse shrinkage Shrinkage stresses leading to a shortening of the member across the toes of the welded joint 2) Longitudinal shrinkage Shrinkage stresses leading to a shortening of the member along the principal axis of the welded joint. 3) Angular distortion

Weld zone transverse shrinkage stresses not in the plane of the neutral axis leading to rotation of one member with respect to an adjacent member.

4) Bowing and dishing Shrinkage of edges or surfaces where the distortion is not coincident with the neutral axis of the member leading to bowing or dishing due to asymmetric shortening 5) Buckling Similar to bowing and dishing but more pronounced localized deformations as seen on larger structures or thinner or less restrained sections. 6) Twisting Seen in slender structures with shear deformation at welded joints. Box sections and fabricated beams and columns can twist.

Fig 3.3: Types of distortion

3.5 Factors affecting distortion:- If a component were uniformly heated and cooled distortion would be minimized. However, welding locally heats a component and the adjacent cold metal restrains the heated material. This generates stresses greater than yield stress causing permanent distortion of the component. Some of the factors affecting the distortion are listed below:

• Amount of restraint. • Welding procedure. • Parent metal properties. • Weld joint design. • Part fit up.

• Heat and temperature.

Restraint can be used to minimize distortion. Components welded without any external restraint are free to move or distort in response to stresses from welding. It is not unusual for many shops to clamp or restrain components to be welded in some manner to prevent movement and distortion. This restraint does result in higher residual stresses in the components.

• High restraint ⇒ lower distortion, higher residual stress• Low restraint ⇒ higher distortion, Lower residual stress

Welding procedure impacts the amount of distortion primarily due to the amount of the heat input produced. Following proper welding procedures ensures that weld metal shrinkage is consistent.

• Balanced welding sequence lower ⇒distortion• Unbalanced welding sequence ⇒ higher distortion

Parent metal properties, which have an effect on distortion, are coefficient of thermal expansion and specific heat of the material. The coefficient of thermal expansion of the metal affects the degree of thermal expansion and contraction and the associated stresses that result from the welding process. This in turn determines the amount of distortion in a component. 1. Thermal expansion coefficient This is the amount of expansion (or contraction) in a material as it is heated (or cooled).The symbol for linear thermal expansion coefficient is α, and is expressed as [mm/(mm·K)] – strain/Kelvin Carbon steel (α Steel = 12x10 –6/°K) expands less than stainless steel (α Stainless steel = 17.3x10 –6/°K), which expands less than aluminum (α Aluminium = 23x10 –6/°K).

• Lower thermal expansion ⇒ lower distortion• Higher thermal expansion ⇒ higher distortion

2. Specific heat per unit volume Specific heat, c, is the amount of energy required to raise the temperature of one kilogram of the substance by one Kelvin and is expressed as expressed as [J/(kg·K)] – Joule per kilogram Kelvin.

• Lower specific heat ⇒ lower distortion• Higher specific heat ⇒ higher distortion

3. Thermal conductivity coefficient The coefficient of thermal conductivity, k, is a measure of the rate Q (W) at which heat flows through a material and is expressed as [W/(m·K)] – W = Watts, m = metres, K = degrees Kelvin.

• Lower conductivity ⇒ lower distortion• Higher conductivity ⇒ higher distortion

4. Yield point and yield point at elevated temperatures The yield point is the limit of elastic behavior where any increase in strain (or stress) causes permanent deformation. The yield point is expressed as a stress, σ, expressed as MPa.

• Higher yield ⇒ lower distortion, higher residual stress• Lower yield ⇒ higher distortion, lower residual stress

5. Melting point• Lower melting point ⇒ lower distortion• Higher melting point ⇒ higher distortion

Weld joint design will affect the amount of distortion in a weldment. Both butt and fillet joints may experience distortion. However, distortion is easier to minimize in butt joints.

• Minimal weld volume and symmetrical shrinkage forces ⇒ lower distortion• Larger weld volume and asymmetrical shrinkage forces ⇒ higher distortion

Part fit up should be consistent to fabricate foreseeable and uniform shrinkage. Weld joints should be adequately and consistently tacked to minimize movement between the parts being joined by welding. Good fit-up of components reduces the potential for movement as gaps close during welding and also minimizes weld volumes.

• Precise fit-up ⇒ lower distortion• Poor fit-up ⇒ higher distortion

Heat and temperature Heat input

• Lower heat input ⇒ lower distortion• Higher heat input ⇒ higher distortion

Preheat temperature• Higher preheat ⇒ lower distortion• Lower preheat ⇒ higher distortion

3.6 Ways to Reduce Distortion:- Distortion reduction can be done by following techniques:- 3.6.1. By good design. 3.6.2. By fabrication technique 3.6.3. Welding processes

3.6.1. Preventing Distortion by Good Design Good design incorporates principles that reduce the detrimental effects of weld zone shrinkage and underpins good workshop practices. Design principles Implementation of the following design principles should be considered to minimize distortion in welded structures. 3.6.1.1. Elimination of Welding Welding can often be eliminated by: • Utilizing plates and profiles in the largest sizes available thus reducing the frequency of joining. • Forming plates rather than cutting and welding. • Using rolled or extruded sections rather than welded sections. • Using stiffeners, thus allowing reductions in weld sizes.

3.6.1.2. Weld Placement The location of welds as close as possible to neutral axes is important in minimizing distortion. The closer a weld is to the neutral axis of a member, the lower the leverage effect of the shrinkage forces and hence the final distortion.

Fig 3.4: Welding at neutral axis 3.6.1.3. Reducing Volume of Weld Metal Since weld shrinkage is proportional to the volume of weld metal it follows that the smaller the total volume of weld metal deposited the smaller will be the overall contraction during cooling and hence reduced distortion. Details of weld preparations and welding process selection should aim for the minimum weld volume consistent with satisfying the design strength and weld quality requirements. A large fillet weld may be replaced by an incomplete penetration groove weld and smaller fillet to achieve the same effective throat thickness with a significant reduction in weld volume. Increased cost in preparation of the bevel has to be considered. A double V plate butt weld has approximately half of the weld volume of a single sided V plate butt weld. The trade off is the additional cost in preparation of a double V preparation, the need to access both sides for welding and the possibility that rotating the part may be impractical, costly or time consuming. It is worthwhile when distortion is an issue to

review weld detail designs and ensure that specified weld sizes are not greater than necessary. 3.6.1.4. Reducing the Number of Weld Runs Where possible use intermittent rather than continuous welds. Stagger intermittent fillet welds. For complete penetration joints that require multiple weld passes to fill the groove, the larger the volume of each weld deposit the lower the distortion in the lateral direction. Paradoxically the use of a higher number of smaller passes can lead to a reduction in longitudinal distortion. This is because of the substantially greater longitudinal rigidity especially of thicker plates and hence the greater tendency of a smaller bead to yield longitudinally compared to a large bead. The implementation of modern mechanized high-energy processes for single sided complete penetration welding of plates offers major advantages in reducing distortion as well as the obvious productivity gains.

Fig 3.5: Minimum no. of weld pass 3.6.1.5. Use of Balanced Welding Wherever practicable and particularly on thicker sections, use double side joints and a balanced welding sequence. This approach can be applied where components are small and rotation is practical.

3.6.2. Preventing Distortion by Fabrication Techniques Workshop personnel have control over a number of activities as follows: 3.6.2.1. Precision in Marking Out and Cutting Modern shipyards are utilizing CAD/CAM in their laser and plasma cutting operations. The same equipment is now increasingly used for component identification and marking out. Marking out for subsequent assembly as part of the CAD/CAM cutting process minimizes subsequent requirements for marking out, greatly reduces errors in marking out and improves assembly times.This enables:

• High accuracy in cutting leading to good fit up in the fabrication shop, leading to less minor corrections and accompanying distortion.

• Identification of parts and marking out of cut pieces using dot matrix, laser or plasma systems.

This leads to greatly enhanced traceability of parts, enhanced precision of assembly, minimizing errors and rework.

3.6.2.2. Precision in Weld Preparation Preparation of bevels for plate butt welds is now commonly by machining. While machining is more expensive than thermal cutting it enables compound bevels to be produced with precision not achievable by thermal cutting processes. Extremely accurate fitment of parts to be joined can be achieved. This is particularly important for larger welds such as main plate butt welds where major gains can be made in controlling overall distortion.

3.6.2.3. Precision in Assembly This is where it all comes together and precision in assembly is dependent on accuracy of design, accuracy of cut parts, accuracy of marked assembly lines and last but not least the skills of the people doing the assembly.

3.6.2.4. Tack welding Tack welding plays a critical role in firstly holding the assembled structure together ready for welding and secondly in maintaining correct root gaps in butt welds and preventing movement in the structure as welding progresses. The number of tack welds, the length tack welds and the distance between them will depend on the length and thickness of the weld, the degree of rigidity needed, the details of the weld preparation and the welding process being used. The tacking sequence can also have an effect and may need to be controlled to ensure correct root gaps are maintained along the length of a joint. Procedures used for tack welding to prevent transverse shrinkage: a) Tack weld straight through to end of joint. b) Tack weld one end, then use back-step technique for tacking the rest of the joint.c) Tack weld the centre, then complete the tack welding by the back-step technique.

Fig 3.6: Tack welding.

3.6.2.5. Back-To-Back Assembly Back to back assembly of identical asymmetrical structures provides a method of counteracting the shrinkage forces of one component with the shrinkage forces of another. Additional presetting may be required so that when the two components are

freed from each other there is no residual distortion due to spring back from locked up residual stresses.

Fig 3.7: Back to back assembly 3.6.2.6. Stiffening Stiffening of a structure can be achieved in a number of ways. Use of larger tack welds, partially welding, provision of temporary bracing, use of assembly jigs with preset camber can be used to minimize distortion of a weldment. Longitudinal stiffeners welded along each side of a long seam can be used to prevent bowing of long members. Stiffener location is important. If stiffeners are too far from the joint they are stiffening they may be ineffective, whereas if stiffeners are too close they may interfere with welding of the joint.

Fig 3.8: Stiffening

3.6.2.7. Pre-setting

Where a known amount of angular distortion will occur, presetting the joint by the amount of angular distortion expected ensures the alignment of the finished weld. This method can be very effective if consistent shrinkage rates are achieved through close control of welding procedures.

3.6.2.8. Jigs and Fixtures Jigs and fixtures can be used for assembly and welding of subassemblies where the components are held rigidly until welded. This approach works well for production of multiple smaller sub-assembles.

3.6.3. Welding 3.6.3.1. Welding Process Higher energy processes that allow higher welding speeds generally lead to lowering of shrinkage and distortion rates with the advantage of increased welding productivity. Implementation of processes enabling higher welding speeds may be difficult to justify solely on the basis of reduced welding time, but overall savings can be significant when the downstream costs of distortion correction are considered.

3.6.3.2. Controlled Welding Procedures Ensuring all operators are following welding procedures ensures that weld metal shrinkage is consistent. Maintaining consistency in shrinkage outcomes requires good welding management systems. Welding procedures should be developed to ensure that minimal weld metal is deposited while maintaining the specified weld quality level. When carrying out the fabrication it is important that the weld sizes are produced within the specified size range and weld shape is correct. Over-welding of thin structural sections is common although there is no advantage to the fabricator or customer in over-welding. On the other hand, undersize welds can lead to costly re-work with inevitable increased distortion. 3.6.3.3. Welding Technique General rules for minimizing distortion are:

• Keep weld volumes/size to the minimum specified• Balance welds about neutral axes• Keep the time between runs to a minimum• Maintain preheat temperatures

3.6.3.4. Welding Sequence The direction and sequence of welding is important in distortion control. Generally welds are made in the direction of free ends. For longer welds, back-step welding or skip welding is used.

a) For back-step welding short weld lengths are placed with welding in the opposite direction to the general progression

b) For skip welding a sequence is worked out to minimize and balance out shrinkage stresses.

Fig 3.9: Sequencing in welding

3.7 Correcting Distortion:- Whilst the aim of this guidance note is to provide information to minimize distortion, the following is to assist a fabricator in the techniques of distortion correction.

3.7.1. Mechanical Techniques The following techniques rely on applying a force to change the shape of a component to correct the distortion produced by welding.

(a) Hammering and Peening This is a simple, cheap and sometimes effective method of correcting minor distortions. Hammering has limited application because it can lead to local surface damage and work hardening. Peening of welds is an effective means of countering distortion due to weld metal shrinkage. Peening is carried out progressively as each weld or layer of weld is deposited in a multi-layer welds. The surface of the weld is spread out to reduce the tensile shrinkage stresses across and along the joint. Peening must be done carefully to avoid introduction of undesirable features on the peened surface and is not allowed by some fabrication codes.

(b) Dogging and Wedging This method may be effective to correct minor distortions where hammering alone is ineffective, enabling greater forces to be applied by using wedges. Care must be taken in the attachment and removal of dogs. Attachment points can provide sites or defects and may be restricted by some fabrication codes.

(c) Pressing Hydraulic presses can be used to correct distortion in the form of bowing and angular distortion. This approach is limited by the size of press available and the size and complexity of the component. Distortion can be corrected progressively, and with care there will be minimal damage to component surfaces

Fig 3.10

(d) Hydraulic Jacking Hydraulic jacking is a variation on the dogging and wedging approach but offers more control and higher forces.

3.7.2. Thermal Techniques Thermal techniques are based on creating compressive yielding at locally heated sites, which then provide a tensile stress to “shorten” the heated zone. The part to be shortened is rapidly heated to generate a temperature gradient with thermal strain sufficient to cause compressive yielding as it expands against the surrounding cold, higher yield strength metal. When the heated area cools, the part that underwent compressive yielding contracts to a smaller size than before it was heated.

(a) Localized heating to correct distortion.

Fig 3.11: Localized heating.

(b) Spot heating for correcting buckling.

Fig 3.12: Heating at buckled area.

(c) Static thermal technique. “Static Thermal Tensioning” (STT) is one method of reducing the weld residual stress. STT involves pre-stretching the weld area by means of an applied thermal gradient. The gradient is achieved by raising the temperature on either side of the weld by resistive heating bands, while simultaneously quenching the weld zone (e.g., with a water-spray on the bottom of the plate). A schematic is shown in Fig. The stress field produced by the temperature gradient opposes the normal weld stress state, and reduces the peak tensile stresses in the weld. While the technique is effective, quenching is difficult to implement in practice on existing shipyard equipment. Finite element analysis has shown that using heating bands without the quenching necessitates impractically large heating bands.

Figure 3.13 – Thermal tensioning uses temperature gradients to produce stresses which oppose the development of welding stresses that cause buckling.

(d) Transient thermal technique. “Transient Thermal Tensioning” (TTT) is another method of reducing the weld residual stress. TTT uses local heat sources (Fig. 3.14) that move along the plate to induce zones of local plate tension where the weld compressive zones would normally exist. Burners (or other heat sources of sufficient intensity) are carried along with the welding head. The heat source locally yields and shrinks an area in the plate, in a similar manner as is done with flame straightening. Bands of residual tension stress are produced in the panel in opposition to the residual compressive buckling stress caused by welding, as Fig. 3.15 illustrates.

Fig. 3.14: TTT Fig.3.15: Illustration of stress pattern

CHAPTER 4: CASE STUDY

1. ARM LUGS

Arm faces distortion at their lugs causing angular distortion of 0.5mm to 1.0mm which comes under tolerance. It can be further minimized by skip welding as it doesn’t cause stress to concentrate at any particular area but spreads it all around near welding zone in discrete manner.

Fig 4.1: Line diagram of arm

Standard distance between plates of lugs 1= 216mmAfter tack welding

After complete welding

1 216 2152 216 215.53 215.5 215.54 215.5 215

Standard distance between plates of lugs 2= 188mmAfter tack welding

After complete welding

1 188 1872 188 1873 188 1874 188 187.5

Existing distortion problems:1. Angular distortion at lugs.

Lugs1Lugs 2

Preventing actions used:1. Use of manipulators and clamping devices.

2. Use of stiffeners.

Fig 4.2: Use of manipulators and stiffeners during arm welding.

Suggestions: Use of skip welding procedure.

.

Fig 4.3: Actual and suggested welding sequence.

Use of stiffeners

Use of manipulators and clamping devices

1

4 2

3

8

7

6

5

4

3

2

1

2. CENTRE FRAME

Centre frame faces distortion at their side plates, foot lugs and bottom plate. Buckling at bottom plate is reduced by the help of spot heating and other is reduced by using manipulators and clamping devices as shown above. Distortion is maximum at other end w.r.t. lugs and a clamping device should be use their to reduce distortion at that place. And with help of step welding it can be further reduced.

Fig 4.4: Centre frame.

Standard distance between side plates=770mmAfter tack welding

After complete welding

1 769.5 767

1

2

3

4 5

6

7

2 769 7663 769 7684 769 7685 769 7686 769 769.57 769.5 771

Standard distance between plates of foot lug=145mm

Left foot lug:After tack welding After complete

welding1 144.5 1442 144.5 1443 144.5 144

Right foot lug:After tack welding After complete

welding1 145 1442 143 1443 144 143.5

Buckling in the bottom plate after complete welding:After complete welding

1 12 1.53 2.54 15 1

Existing distortion problems:

1. Angular distortion at side plates and foot lugs.

2. Buckling of bottom plate.

Preventing actions used: 1. By spot heating.

Fig 4.5: Spot heating to reduce buckling in bottom plate.

2. Use of manipulators and clamping devices.

3. Use of stiffeners for foot lugs.

Fig 4.6: Use of stiffeners and clamping devices during welding.

Suggestions: Use of another clamping device at opposite end from lugs.

Use of step welding procedure.

Fig 4.7: Actual and suggested welding sequence.

3. TAIL-FRAME

Tail frame faces distortion at their side plates and bottom plate. In order to reduce distortion, there should be a manipulator and clamping devices that can be used during welding. Further, there should be a stiffener to hold the bottom plate in order to reduce the buckling distortion present in it.

1

4

2

3

2

6

4

8

7 5

3 1

Fig 4.8: Tail frame

Standard distance between side plate= 771mm.After tack welding After complete

welding1 771 771.52 771 771.53 771 771.54 771 772

Buckling in the bottom plate after complete welding:After complete welding

1 32 1.53 14 1.55 2

Existing distortion problems:1. Buckling of bottom plate.

Preventing actions used:1. Use of clamping devices.

BOTTOM

Fig 4.9: Use of clamping devices during welding of tail frame.

Suggestions: Use of manipulators during final welding. Use of stiffener at cut out of bottom plate to control its buckling. Use of step welding procedure.

Fig 4.10: Actual and suggested welding sequence.

CHAPTER 5: CONCLUSION: Distortion is one of the most important factor affecting welding procedures everywhere. It has to be understood carefully and suitable measures have to taken in order to minimize it. It can’t be eliminated but can be reduced to minimize loses as it can cause even failure of parts. Some of the factors controlling distortion of welded fabrications have been identified. Adopting best practice principles can have significant cost benefits.

Reduce the effective shrinkage force• Use welding processes that deposit the weld metal as quickly as possible, i.e.

minimizing overall heat input• Minimize the amount of weld metal• Avoid over welding• Use correct edge preparation and ensure good fit-up

6

5

4

3

2 1

4

3

2 1

• Minimize weld passes• Place welds near the neutral axis• Use intermittent welds• Use “back-step” welding sequence

Balance shrinkage forces with other forces• Use tack welds to set up and maintain the root gap• Balance welds symmetrically about a neutral axis, e.g. double vee butt welds

welded alternately from each side. Components set up back-to-back• Use of longitudinal stiffeners to prevent longitudinal bowing in butt welds of

thin plate structures• Use jigs and fixtures, flexible clamps, strong-backs and tack welds to apply

restraint during welding. Consider the risk of cracking which can be quite significant, especially for fully welded strong-backs. Welds for temporary attachments for restraint should be made using an approved procedure and may require preheat to avoid forming imperfections in the component surface.

REFERENCE:1. Practical welding techniques to minimize distortion.

By C. Conrardy, T. D. Huang, D. Harwig2. Distortion Control in Shipbuilding

Article by Welding technology institute of Australia.3. Fundamentals of professional welding

4. Prediction of Welding Distortion By Panagiotis Michaleris and Andrew DeBiccari Edison Welding Institute, Columbus, Ohio