Welded Construction

Welding is a joining of one or more materials by the application of heat that is localized in the region of the joint with or without the addition of filler metal. For most welding processes, the induced heat raises the temperature of the materials above the melting point, which creates a weld pool. The heat source is then removed, which allows the pool to solidify and create a metallurgical bond between the joined surfaces.

There are many welding processes, but the most widely used for joining pressure parts is fusion welding with the addition of filler metal.

All welding performed on power boilers and pressure vessels must be done by code-qualified personnel, who must follow qualified proce-dures. The personnel must be trained in using the correct welding position and the correct filler material and when to apply the neces-sary pre-heat treatment prior to welding, as well as the stress-relief heat treatment after the welding has been completed.

Fusion welding has been universally adopted in the construction of boiler drums. The construction of high-pressure boilers by use of riveted joints required an extreme thickness of plate, and increased thickness was limited because of the difficulty in obtaining a tight joint. Welded joints relieve the operators of the inconvenience caused by joint leakage and of the necessity for caulking. Welding also lessens the possibility of caustic embrittlement, which is partially attributed to boiler water entering the joint, evaporating, and thus producing a solution con-taining a large amount of impurities. For these reasons, riveted joints are not used in current-day designs.

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Rigid specifications cover the application of fusion welding to the pressure parts of boilers. This work must be done in the manufacturing plant or at the construction site with special equipment operated by a qualified welder who must pass a test that meets specific welding requirements. The completed work must be subjected to rigid inspec-tion and tests. It is not permissible to field weld the pressure parts of a boiler unless provisions are made to stress relieve the welded portion.

Pressure part welding in the field also must conform to the require-ments of the ASME code, where qualification tests and inspection requirements must be met.

Terms used to describe the welding process are defined as follows.

The throat of a weld is the joint where the filler material is of minimum thickness. The fillet weld is approximately triangular in cross section with the throat in a plane that is 45° to the surface joined. A double-welded butt joint is formed by adding welding metal from both sides of the joint and reinforcement on both sides. A single-welded butt joint is formed by adding filler metal and reinforcement on only one side.

In the fabrication of a fusion-welded drum, the carbon content of materials used must not exceed approximately 0.3 percent. Carbon contents above this are considered high carbon, and these materials cannot be used as welded pressure parts because they are not struc-turally sound. The plates to be welded may be cut to size or shape by machining, by shearing, or by flame cutting. When flame cutting is used, the edges must be uniform and free from slag.

In the design of boiler drums it is frequently necessary to join plates of unequal thickness to account for the lower ligament efficiency caused by tube holes in the portion of the drum. A two-drum boiler design is an example of this. When plates of unequal thickness are joined together, the thicker plate must be reduced by tapering until the plates are equal in thickness at the joint. The design of welded joints must be such that bending stresses are not imposed directly on the welded joint. Backing strips are used when one side of the joint is inaccessible for welding. The filler metal is then added from only one side of the joint.

After fusion welding a drum (including any connections or attach-ments), it is always necessary that the assembly be stress relieved.

Stress relieving is accomplished by heating the drum uniformly to a temperature of 1100 to 1200°F. The temperature to which the drum is heated for stress relieving is varied to meet the characteristics of the metal used in the construction. The drum to be stress relieved must be brought up slowly to the specified temperature and held there for a period of time equal to at least 1 h per inch of plate thickness. Again, code requirements must be strictly followed. After this period of heat-ing, the drum is allowed to cool slowly in a “still” atmosphere. The

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Figure 3.6 Tensile-test specimens showing that to cause failure, the area of the weld had to be reduced by 30 percent by drilling five holes. (Babcock & Wilcox, a McDermott company.)

stress-relieving procedure must be repeated if it is necessary to weld on additional outlets or if repairs are made. Welded attachments may be locally stress relieved by heating a circumferential band around the entire vessel. The entire band must be brought up to the specified stress-relieving temperature. A similar procedure is followed in stress relieving welded pipe joints.

The welding of pressure vessels is made possible not only by the improved technique of the process but also by the advancement in the method of inspecting and testing the completed joint. The tests applied to welded pressure vessels may be divided into two classifica-tions: the destructive test, in which the metal is stressed until it fails, and nondestructive testing (NDT), which does not injure the metal and may therefore be applied to the completed vessel.

Special test specimens are prepared for the application of destructive tests. Plates of the same thickness and material as those used in the actual construction are prepared for welding and attached to one end of the longitudinal seam. The same welding material and technique are used on both the joint and the test plates. Two specimens for ten-sion and one for a bend test are cut from the welded test plates. One of the tension specimens is cut transversely to the welded joint and must show a tensile strength not less than the minimum requirement for the plate material used (Fig. 3.6). The other tension specimen is taken entirely from the deposited weld metal, and the tensile strength must be at least that of the minimum range of the plate welded. When the plate thickness is less than ⁵⁄8in, an all-weld-metal tension test may be omitted. The bend-test specimen is cut across the welded joint through the full thickness of the plate with a width 1¹⁄2

times the thickness of the plate. The specimen must bend without cracking until there is an elongation of at least 30 percent of the outside fibers of the weld (Fig. 3.7). Material tests, types of test specimens,

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Figure 3.7 Bend-test specimen with the outside fibers elongated 55 percent. (Babcock & Wilcox, a McDermott company.)

and test methods are changed over time based on experience. All of these must be conducted in accordance with the procedures estab-lished by the American Society of Testing and Materials (ASTM) and other authorities.

X-rays provide a satisfactory nondestructive procedure for examining welds in boilerplate. This method of inspecting boilerplate has been developed to a high degree of perfection. The process consists of passing powerful x-rays through the weld and recording the intensity by means of a photographic film. The variation and intensity of the x-rays as recorded on the film show cracks, slag, or porosity. The exposed and processed films showing the condition of the welded joint are known as radiographs.

Both longitudinal and circumferential fusion-welded joints of boiler drums must be examined radiographically throughout their length.

Welded joints are prepared for x-ray examination by grinding, chipping, or machining the welded metal to remove irregularities from the sur-face. A small specified amount of reinforcement, or crown, on the weld is permissible. Single-welded joints can be radiographed without removing the backing strip, provided it does not interfere with the interpretation of the radiograph.

Radiography, or x-ray, has been a primary weld quality-control technique for many years. These x-ray machines have been supple-mented with the use of gamma-ray equipment, which uses a radioactive isotope. The choice of either method is usually based on the mobility of the part that has to be examined and also on the availability of equip-ment. X-ray units are best used in a manufacturing facility where fixed-position equipment is available and the parts to be examined can be readily moved into proper position. For field use or when a part cannot be moved easily, gamma-ray equipment is used. A signifi-cant advantage of radiography is that a permanent inspection record is made of each inspection.

Thickness gauges (penetrometers) are placed on the side of the plate to test the ability of x-rays to show defects. These gauges are rectangular strips of about the same metal as that being tested and

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Figure 3.8 Radiographs: (a) satisfactory weld; (b) unsatisfactory weld as a result of slag inclusions. (Babcock & Wilcox, a McDermott company.)

not more than 2 percent of the thickness of the plate. They contain identification markings and holes that must show clearly at each end of the radiograph. The entire length of the welded joint is photographed.

The purpose of these penetrometers is to ensure the correct exposure time of the x-ray. An image of the holes in the penetrometer appears on the x-ray film, and therefore any flaws of equal or larger size would be expected to appear. The ASME code specifies the acceptance standard as to the size of porosity and defects from slag inclusion.

Figure 3.8a shows a radiograph of a satisfactory welded joint, while Fig. 3.8b shows a joint that is unsatisfactory due to excessive porosity.

Welded joints are judged unsatisfactory when the slag inclusion or porosity as shown on the radiograph exceeds the amount shown by a standard radiograph reproduction that may be obtained from the ASME.

The percentage of defective welds in modern boiler shops is very small. When the radiograph shows an excessive amount of slag inclu-sion or other unsatisfactory condition within the joint, the defective portion must be chipped out. The section is then rewelded, after which it must again be x-rayed.

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In addition to radiography, there are other nondestructive tests that may be applied to determine the condition of boilers. These are ultrasonics, magnetic particle, and liquid-penetrant testing methods.

Radiography and ultrasonic testing (UT) are used for volumetric examination, while magnetic particle and liquid-penetrant testing are used for surface examination.

Ultrasonic testing (UT) is the technology that developed rapidly in the nondestructive testing (NDT) of pressure parts. The ultrasonic wave that is produced is reflected by any flaw in the material being examined. The wave is displayed on an oscilloscope.

The testing of material thickness is performed by ultrasonic test-ing. A common cause of pressure part failure is the loss of material that results from oxidation, corrosion, or erosion. Ultrasonic testing is relatively fast and is used extensively for measuring the wall thickness of tubes or piping.

During outages for planned maintenance, a UT examination of fur-nace walls (and other tubes where erosion or corrosion is expected) is often made, and the thicknesses of tubes are recorded and compared with the original thicknesses or any previous thicknesses determined by UT testing. Where thinning has occurred to a point where replace-ment is required, cladding of the tube area or actual tube replacereplace-ment is performed to reestablish tube integrity.