Welding Joints

Welding joints are formed by welding two or more workpieces, made of metals or plastics, according to a particular geometry. The most common types are butt and lap joints; there are various lesser used welding joints including flange and corner joints.

Butt welds

Butt welds are welds where two pieces of metal are joined at surfaces that are at 90 degree angles to the surface of at least one of the other pieces.^[1] These types of welds require only some preparation and are used with thin sheet metals that can be welded with a single pass ^[2]. Common issues that can weaken a butt weld are the entrapment of slag, excessive porosity, or cracking. For strong welds, the goal is to use the least amount of welding material possible. Butt welds are prevalent in automated welding processes, such as submerged-arc welding, due to their relative ease of preparation.^[3] When metals are welded without human guidance, there is no operator to make adjustments for non-ideal joint preparation. Because of this necessity, butt welds can be utilized for their simplistic design to be fed through automated welding machines efficiently.

Types

Butt joint geometries

There are many types of butt welds, but all fall within one of these categories: single welded butt joints, double welded butt joint, and open or closed butt joints. A single welded butt joint is the name for a joint that has only been welded from one side. A double welded butt joint is created when the weld has been welded from both sides. With double welding, the depths of each weld can vary slightly. A closed weld is a type of joint in which the two pieces that will be joined are touching during the welding process. An open weld is the joint type where the two pieces have a small gap in between them during welding.

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Square butt joints

The square-groove is a butt welding joint with the two pieces being flat and parallel to each other. This joint is simple to prepare, economical to use, and provides satisfactory strength, but is limited by joint thickness. The closed square butt weld is a type of square-groove joint with no spacing in between the pieces. This joint type is common with gas and arc welding.

For thicker joints, the edge of each member of the joint must be prepared to a particular geometry to provide accessibility for welding and to ensure the desired weld soundness and strength. The opening or gap at the root of the joint and the included angle of the groove should be selected to require the least weld metal necessary to give needed access and meet strength requirements.

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Bevel butt joints

Single-bevel butt welds are welds where one piece in the joint is beveled and the other surface is perpendicular to the plane of the surface. These types of joints are used where adequate penetration cannot be achieved with a square-groove and the metals are to be welded in the horizontal position ^[4]. Double-bevel butt welds are common in arc and gas welding processes. In this type both sides of one of the edges in the joint are beveled.

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V-joints

Single-V butt welds are similar to a bevel joint, but instead of only one side having the beveled edge, both sides of the weld joint are beveled. In thick metals, and when welding can be performed from both sides of the work piece, a double-V joint is used. When welding thicker metals, a double-V joint requires less filler material because there are two narrower V-joints compared to a wider single-V joint. Also the double-V joint helps compensate for warping forces. With a single-V joint, stress tends to warp the piece in one direction when the V-joint is filled, but with a double-V-joint, there are welds on both sides of the material, having opposing stresses, straightening the material.

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J-joints

Single-J butt welds are when one piece of the weld is in the shape of a J that easily accepts filler material and the other piece is square. A J-groove is formed either with special cutting machinery or by grinding the joint edge into the form of a J. Although a J-groove is more difficult and costly to prepare than a V-groove, a single J-groove on metal between a half an inch and three quarters of an inch thick provides a stronger weld that requires less filler material. Double-J butt welds have one piece that has a J shape from both directions and the other piece is square.

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U-joints

Single-U butt welds are welds that have both edges of the weld surface shaped like a J, but once they come together, they form a U. Double-U joints have a U formation on both the top and bottom of the prepared joint. U-joints are the most expensive edge to prepare and weld. They are usually used on thick base metals where a V-groove would be at such a extreme angle, that it would cost too much to fill.

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Others

Thin sheet metals are often flanged to produce edge-flange or corner-flange welds. These welds are typically made without the addition of filler metal because the flange melts and provides all the filler needed. Pipes and tubing can be made from rolling and welding together strips, sheets, or plates of material.^[5]

Flare-groove joints are used for welding metals that, because of their shape, form a convenient groove for welding, such as a pipe against a flat surface.

The Tee Butt Weld is formed when two bars or sheets are joined perpendicular to each other in the form of a T shape. This weld is made from the resistance butt welding process.

Selection of the right weld joint depends on the thickness and process used. The square welds are the most economical for pieces thinner than 3/8”, because they don’t require the edge to be prepared.^[6] Double-groove welds are the most economical for thicker pieces because they require less weld material and time. The use of fusion welding is common for closed single-bevel, closed single J, open single J, and closed double J butt joints. The use of gas and arc welding is ideal for double-bevel, closed double-bevel, open double-bevel, single-bevel, and open single-bevel butt welds.

Below are listed ideal joint thicknesses for the various types of butt welding joints. When the thickness of a butt weld is defined it is measured at the thinner part and does not compensate for the weld reinforcement.

Workpiece thickness limits per joint type^{[citation needed]}

Joint type	Thickness
Square joint	Up to 1⁄4 in (0.64 cm)
Single-bevel joint	3⁄16–3⁄8 in (0.48–0.95 cm)
Double-bevel joint	Over 3⁄8 in (0.95 cm)
Single-V joint	Up to 3⁄4 in (1.9 cm)
Double-V joint	Over 3⁄4 in (1.9 cm)
Single-J joint	1⁄2–3⁄4 in (1.3–1.9 cm)
Double-J joint	Over 3⁄4 in (1.9 cm)
Single-U joint	Up to 3⁄4 in (1.9 cm)
Double-U joint	Over 3⁄4 in (1.9 cm)
Flange (edge of corner)	Sheet metals less than 12 gauge[clarification needed]
Flare groove	All thickness

Cruciform

Diagram of a cruciform joint between 3 plates of metal

A cruciform joint is a specific joint in which four spaces are created by the welding of three plates of metal at right angles. In the American Bureau of Shipping Rules for Steel Vessels, cruciform joints may be considered a double barrier if the two substances requiring a double barrier are in opposite corners diagonally. Double barriers are often required to separate oil and seawater,chemicals and potable water, etc.^[7]

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Plate edge preparation

In common welding practices, the welding surface needs to be prepared to ensure the strongest weld possible. Preparation is needed for all forms of welding and all types of joints. Generally, butt welds require very little preparation, but some is still needed for the best results. Plate edges can be prepared for butt joints in various ways, but the five most common techniques are oxyacetylene cutting (oxy-fuel welding and cutting), machining, chipping, grinding, and air carbon-arc cutting or gouging. Each technique has unique advantages to their use.

For steel materials, oxyacetylene cutting is the most common form of preparation. This technique is advantageous because of its speed, low cost, and adaptability. Machining is the most effective for reproducibility and mass production of parts. Preparation of J or U joints is common prepared by machining due to the need for high accuracy. The chipping method is used to prepare parts that were produced by casting. The use of grinding to prepare pieces is reserved for small sections that cannot be prepared by other methods. Air carbon arc welding is common in industries that work with stainless steels, cast iron, or ordinary carbon steel.^[8]

Welding Procedure Specification

A Welding Procedure Specification (WPS) is a formal document describing weldingprocedures. The purpose of the document is to guide welders to the accepted procedures so that repeatable and trusted welding techniques are used. A WPS is developed for each material alloy and for each welding type used. Specific codes and/or engineering societies are often the driving force behind the development of a company's WPS. A WPS is supported by a Procedure Qualification Record (PQR or WPQR). A PQR is a record of a test weld performed and tested (more rigorously) to ensure that the procedure will produce a good weld. Individual welders are certified with a qualification test documented in a Welder Qualification Test Record (WQTR) that shows they have the understanding and demonstrated ability to work within the specified WPS.

The following are definitions for WPS and PQR found in various codes and standards:

According to the American Welding Society (AWS), a WPS provides in detail the required welding variables for specific application to assure repeatability by properly trained welders. The AWS defines welding PQR as a record of welding variables used to produce an acceptable test weldment and the results of tests conducted on the weldment to qualify a Welding Procedure Specification.

The American Society of Mechanical Engineers (ASME) similarly defines a WPS as a written document that provides direction to the welder or welding operator for making production welds in accordance with Code requirements.^[1] ASME also defines welding PQR as a record of variables recorded during the welding of the test coupon. The record also contains the test results of the tested specimens.

In Europe, the European Committee for Standardization (CEN) has adopted the ISO standards on welding procedure qualification (ISO 15607 to ISO 15614) and on welder qualification (ISO 9606), with the exception of qualification for steel welders, where a new version of the old european EN 287-1 standard still applies. EN ISO 15706 defines a WPS as "A document that has been qualified by one of the methods described in clause 6 and provides the required variables of the welding procedure to ensure repeatability during production welding". The same standard defines a Welding Procedure Qualification Record (WPQR) as "Record comprising all necessary data needed for qualification of a preliminary welding procedure specification". ^[2] In addition to the standard WPS qualification procedure specified in ISO 15614, the ISO 156xx series of standards provides also for alternative WPS approval methods. These include: Tested welding consumables (ISO 15610),Previous welding experience (ISO 15611), Standard welding procedure (ISO 15612) and Preproduction welding test (ISO 15613).

In the oil and gas pipeline sector, the American Petroleum Institute API 1104 standard is used almost exclusively worldwide. API 1104 accepts the definitons of the American Welding Society code AWS A3.0.^[3]

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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 not only because it is preferred for some sorts of iron or steel welding but also because it lends itself to brazing, braze-welding, metal heating (for bending and forming), the loosening of corroded nuts and bolts, and also is the ubiquitous means for oxy-fuel cutting of ferrous metals.

The equipment is relatively inexpensive and simple, generally employing the combustion ofacetylene 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. A similar process, generally called oxyfuel cutting, is used to cut metals.^[6] Other gas welding methods, such as air acetylene welding, oxygen hydrogen welding, and pressure gas welding are quite similar, generally differing only in the type of gases used. A water torch is sometimes used for precision welding of small items such as jewelry. Gas welding is also used in plastic welding, though the heated substance is air, and the temperatures are much lower.

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Resistance

Resistance welding 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. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high.

Spot welder

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—ordinary cars can have several thousand spot welds made by industrial robots. A specialized process, called shot welding, can be used to spot weld stainless steel.

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. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance welding methods include flash welding, projection welding, and upset welding.^[25]

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Energy beam

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 (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties, and X-ray welding.^[26]

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Solid-state

Like the first welding process, 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. Other solid-state welding processes include co-extrusion welding, cold welding, diffusion welding, exothermic welding, friction welding (including friction stir welding), high frequency welding, hot pressure welding, induction welding, and roll welding.^[27]

Quality

Most often, the major metric used for judging the quality of a weld is its strength and the strength of the material around it. Many distinct factors influence this, including the welding method, the amount and concentration of energy input, the base material, the filler material, the flux material, the design of the joint, and the interactions between all these factors. To test the quality of a weld, either destructive ornondestructive testing methods are commonly used to verify that welds are defect-free, have acceptable levels of residual stresses and distortion, and have acceptable heat-affected zone (HAZ) properties. Welding codes and specifications exist to guide welders in proper welding technique and in how to judge the quality of welds.

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Heat-affected zone

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. Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input.^[31][32] To calculate the heat input for arc welding procedures, the following formula can be used:

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.^[33]

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Defects

Main article: Welding defect

There are many types of defects that can occur depending on the material and welding process. Types of defects include cracks, distortion, gas inclusions (porosity), non-metallic inclusions, lack of fusion, incomplete penetration, lamellar tearing, and undercutting.

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Weldability

Main article: Weldability

The quality of a weld is also dependent on the combination of materials used for the base material and the filler material. Not all metals are suitable for welding, and not all filler metals work well with acceptable base materials.

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Unusual conditions

Underwater welding

While many welding applications are done in controlled environments such as factories and repair shops, some welding processes are commonly used in a wide variety of conditions, such as open air, underwater, and vacuums (such as space). In open-air applications, such as construction and outdoors repair, shielded metal arc welding is the most common process. Processes that employ inert gases to protect the weld cannot be readily used in such situations, because unpredictable atmospheric movements can result in a faulty weld. Shielded metal arc welding is also often used in underwater welding in the construction and repair of ships, offshore platforms, and pipelines, but others, such as flux cored arc welding and gas tungsten arc welding, are also common. Welding in space is also possible—it was first attempted in 1969 by Russian cosmonauts, when they performed experiments to test shielded metal arc welding, plasma arc welding, and electron beam welding in a depressurized environment. Further testing of these methods was done in the following decades, and today researchers continue to develop methods for using other welding processes in space, such as laser beam welding, resistance welding, and friction welding. Advances in these areas may be useful for future endeavours similar to the construction of the International Space Station, which could rely on welding for joining in space the parts that were manufactured on Earth.^[34]