Welding, Welding Procedure, Personnel Qualification

In the past post, I already mention about welding, but now Piping & Fabrication will gave to you the detail of welding process and procedure or qualification, and hope that would be useful to all of us.

Welding. Welding constitutes the bulk of the work involved in fabrication of modern piping systems, so it is essential for all involved to have a good working knowledge of this subject.

Procedure and Personnel Qualifications. All of the ASME Boiler and Pressure Codes and most of the ASME B31 Pressure Piping Codes reference ASME Section IX for the requirements for qualifying welding procedures and welding personnel. The ASME B31.4,23 B31.8,24 and B31.1125 Codes also permit qualification to API- 1104,26 published by the American Petroleum Institute. ASME B31.527 permits qualification to AWS D10.9.28 The purpose of procedure qualification is to assure that the particular combination of welding process, base metal, filler material, shielding fluxes or gases, electrical characteristics, and subsequent heat treatment is capable of producing a joint with the required chemical and physical characteristics.

The purpose of personnel qualification is to assure that the welder or welding machine operator is capable of performing the operation in accordance with a qualified procedure in the required position.

Procedure Qualification. ASME Section IX requires the preparation of a Welding Procedure Specification (WPS), which lists the various parameters to be used during welding. When each WPS is qualified, the parameters used in the qualification are recorded in a Procedure Qualification Record (PQR). For each type of welding process, ASME Section IX has established a series of variables. These are base metal, filler metal, position, preheat, post weld heat treatment, shielding gases, joint configuration, electrical characteristics, and technique. Base metal must not only be considered from a chemical and physical properties point of view, but in piping, the diameter and thickness of the test coupon limits the qualification to certain sizes. Differing fluxes, use of solid or gaseous backing, and single- or multipass techniques are some of the other variables which must be considered. Careful study of Section IX, AWS D10.9, or of API 1104 as may be applicable is in order.

The variables for welding are classed as essential, supplementary essential, and nonessential. The manner in which the variables are classed can vary depending. on the welding process. That is, what may be classed as an essential variable for one may be a nonessential variable for one another. For a given process, each combination of essential variables must be qualified separately. A change in any one of them requires a new qualification. When welds must meet certain fracture toughness requirements, the supplementary essential variables become essential and the procedure must be requalified for the particular combination of essential and supplementary essential variables. Nonessential variables do not require requalification but should be referenced in the WPS.

Personnel Qualification. The fabricator and/or installer must qualify each welder or welding operator for the welding processes to be used during production welding. The performance qualification must be in accordance with a qualified WPS. Each performance qualification is also governed by a series of essential variables which are a function of the welding process for which the welder is being qualified. The welder or welding operator may be qualified by mechanical tests or in some cases by radiographic examination of the test coupon. The record of each performance qualification is kept on a Welder/Welding Operator Performance Qualification (WPQ). Under ASME Section IX rules, a qualified welder who has not welded in a specific process within a specified period of time must be requalified for that process. API 1104 and AWS D10.9 have similar requalification provisions.

Other Forming Operations and Layout, Assembly, and Preparation for Welding

Still continuing from the last post of Piping & Fabrication, but now we will enter the other topics and here we go.

Other Forming Operations. Some additional forming operations which can be performed in a pipe shop are extrusion, swaging, and lapping. Extrusions involve forming outlets in pipe by pulling or pushing a hemispherical or conical die from the inside of the pipe through an opening in the wall. The work may be done hot or cold depending on the characteristics of the material. Ferritic steels, austenitic steels, and nickel alloys are usually formed hot; aluminum and copper are usually formed cold. In order to assure that the outlet will have sufficient reinforcement, it is necessary to increase the wall thickness of the header as a function of the outlet size desired. An increase of 30 percent may be needed for large outlet-toheader ratios.

Swaging involves the size reduction of pipe ends by forging, pressing, or rolling operations. The operation is usually used to produce reductions of one to two pipe sizes. Ferritic steels, austenitic steels, and nickel alloys are usually formed hot. Aluminum and copper are formed cold. In lapped joints, a loose flange is slipped over the end of the pipe which is then heated to forging temperature, upset, and flared at right angles to the pipe axis. After heat treatment and cooling, the lapped section is machined on the face to attain a good gasket surface and on the back for good contact with the flange. The finished thickness of the lapped flange should be equal to or exceed the thickness of the pipe.

Layout, Assembly, and Preparation for Welding. In fabrication shops, piping subassemblies are often assembled on layout tables. Aprojection of the subassembly is laid out on the table in chalk. This establishes the baseline for locating the components and terminal dimensions of the subassembly, and the components are assembled relative to the layout. Prior to fit-up, it is essential that all weld surfaces be properly cleaned of rust, scale, grease, paint, and other foreign substances which might contaminate the weld. If moisture is present, the weld joint should be preheated. For alloy steels the heat-affected zone (HAZ) which results from thermal cutting should be removed by grinding or machining.

Depending on the configuration of the subassembly and root opening required by the welding procedure, some allowance may be required for weld shrinkage in the longitudinal direction. Actual shrinkage is difficult to predict and can vary considerably because of the many variables involved. For most open butt and backing ring joints, one-half the root opening is a reasonable allowance. For joints with other root configurations it may be as little as 1⁄₁₆ in (2.0 mm) for the lighter walls, increasing to as much as ⁵⁄₃₂ in (8 mm) for walls 4 to 5 in (100 to 127 mm) thick. Each weld joint should be carefully aligned within required tolerances using alignment fixtures, spacers, or jigs if necessary. Poor alignment may result in a poor weld. Once alignment is attained, the joint is usually tack-welded to maintain the alignment. The process used for tacking is usually that being used for the root-pass weld. Numbers and size of tacks should be kept to a minimum, but if the subassembly is to be moved elsewhere for weld out, their size must be sufficiently large so as not to crack during the moving operation. Temporary lugs or spacer bars may also be used for this purpose provided they are of a compatible material, the temporary welds are removed, and the surface examined after removal to assure sound metal.

Tack welds made by the shielded metal are welding (SMAW) or gas metal arc welding (GMAW) processes at the root of a weld should be removed or ground smooth since they can become a source of lack of fusion. For gas tungsten arc (GTAW) root welds, tacks usually fuse the adjacent lands to each other or to the insert, and filler metal is often not used. Tack welds are then fused into the weld during the root pass without further preparation. After tacking, the recommended practice is to complete the root pass and one or more weld out passes before starting to complete the weld by other processes to avoid burning through the relatively thin root.

Nonferrous Pipe and Tubes

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Induction Bender

Nonferrous pipe and tubes: Although most of the equipment used to bend ferrous materials is also used for bending nonferrous materials, the details of bending do differ from those for ferrous materials and also vary between the several nonferrous materials themselves. Accordingly, it is wise to obtain specific procedural information from the materials’ producers or from other reliable sources such as the latest edition of The Metals Handbook.20 Certain nonferrous materials can be hot bent.

Aluminum and aluminum alloys can be bent cold using the same types of bending equipment used for ferrous materials. Alloys in the annealed condition are easiest to bend, but care is required in selecting tooling because of the low tensile strength and high ductility of these materials. Alloys with higher tempers and heat-treatable alloys require larger bending radii for satisfactory results. It is seldom necessary to heat aluminum for bending: however, non-heat-treated materials can be heated to 375°F (190°C) with minimal loss of properties, Heat-treated alloys require specific time-temperature control. More detailed information is available from the manufacturers of aluminum products.

Copper and copper alloy pipe and tube can be readily bent to relatively small radii. Although copper can be bent hot, the vast majority is done cold. For draw bending an internal mandrel is required and for other methods internal support is recommended. For very tight radii a snug-fitting forming block and shoe which practically surround the pipe at the point of bending are needed to preclude buckling. Hot bending of copper and copper alloys particularly in larger diameters and walls is common. Pipes are usually sand-filled, and contoured bending dies are recommended. See Table A6.1. More information can be obtained from the Copper Development Association.

Nickel and nickel-alloy pipe can be cold bent with the same type of bending equipment used for ferrous materials. Use of material in the annealed condition is preferred. For bends with radii 6 diameters and less, filler material or internal mandrels are required. Draw bending with internal mandrels is the preferred method for close-radius bending. Galling can become a problem, and chromium-plated or hard bronze-alloy mandrels should be used. Nickel and nickel alloys can be hot bent using the same practices as for ferrous steels. Sand filling is appropriate. Care should be taken to assure that the sand and heating fuel are low in sulfur and that any marking paints or crayons or lubricants have been removed. These materials can be bent over a wide temperature range. The best bending is usually between 1850 and 2100°F (1010 to 1149°C). Other nickel alloys may exhibit carbide precipitation and should not be worked in the sensitization range. Postbending heat treatment may be required. For more information contact nickel product manufacturers such as Huntington Alloys.

Titanium can be bent using draw bending equipment. However, those parts of the equipment which will wipe against the inner and outer surfaces of the pipe should be of aluminum bronze to minimize galling. For better formability, the pipe, the pressure die, and the mandrel should be heated to a temperature between 350 and 400°F (177 and 204°C). Unalloyed titanium can be hot worked in the temperature range of 1000 to 1400°F (538 to 760°C). Titanium alloy grade 12 requires a temperature range of 1400 to 1450°F (760 to 788°C). Heat treatment of titanium is recommended after forming. This is usually a furnace treatment at 1000 to 1100°F (538 to 593°C) for a minimum of 1/2 h for the unalloyed grades and 1 h for the alloy (grade 12). Prolonged exposure to temperatures in excess of 1100°F (593°C) will result in heavy scaling and require some type of descaling treatment.

Continuing of Bending Methods

Hot bending: In those cases where suitable cold bending equipment is unavailable, hot bending may be employed. For hot bending of ferrous materials the pipe to be bent is usually heated to temperatures in the range of 1750 to 2050°F (954 to 1121°C). For austenitic materials these temperatures may introduce sensitization, and for ferritic materials they will exceed the critical temperature where metallurgical phase changes occur. See the section ‘‘Heat Treatment’’ for a discussion of these subjects.

FIGURE A6.9 Tooling for a draw bend application

The traditional method of hot bending is performed on a bending table. Depending on the diameter-to-thickness ratio, the pipe to be bent may be packed with sand to provide more rigidity and thus reduce the tendency for buckling. A rule of thumb is to sand fill if the diameter-to-thickness ratio is 10 to 1 or greater for 5-diameter bends. However, when the diameter-to-thickness ratio approaches 30 to 1, sand begins to lose its effectiveness, and buckles will appear. As the diameter of the pipe increases, the probability of buckling will increase since the sand fill will not expand in proportion to the pipe, leaving a void between the pipe and packing. It becomes pronounced around NPS 24 (DN 600).
After the pipe has been packed with sand, it is placed in a specially designed bending furnace. The furnace is usually gas fired through ports along its length, placed to direct the flames around the pipe and avoid direct flame impingement. The furnace is controlled by thermocouples or pyrometers to assure that the required bending temperature is attained but not exceeded. Depending on the length of arc to be bent, it may be necessary to make the bend in more than one heat. After the segment to be bent has attained the required temperature throughout its thickness, the pipe is placed on the bending table. One end is restrained by holding pins and the other is pulled around by block and tackle powered by a winch. As bending progresses, the arc is checked against a bending template. Repositioning of the holding pins may be necessary. See Fig. A6.12. For ferritic steels, it is recommended that the bending be completed above the upper critical temperature of the metal, usually about 1600 to 1725°F (870 to 940°C).

FIGURE A6.10 Cold bending ranges

FIGURE A6.11 Operating essentials

There are certain limits as to the combination of diameters, thicknesses, and bending radii which can be accommodated by the hot table bend method. PFI Standard ES-24 contains a chart of suggested limits for bend radius versus diameter to wall thickness ratios.
To fulfill the need for a bending process beyond the capabilities of hot table bending, the M. W. Kellogg Co. developed the increment bending process, which was further refined by Pullman Power Products Corp. In this process, one end of the pipe is fixed in an anchor box while a clamp connected to a hydraulic piston is attached to the other. A gas torch ring burner assembly is positioned at one end of the arc to be bent. The burner assembly is sized to heat a length of arc (increment) about 1 to 2 times the pipe wall thickness. The increment length is selected to be less than the buckling wave length of the pipe. The increment is then heated to bending temperature. Optical pyrometers are used to control the heating to assure that proper temperature is attained but not exceeded. At bending temperature the hydraulic piston pulls the clamped end a fixed amount to bend the heated increment.
The increment is then water cooled, the torch ring moved to the next increment, and the process is repeated. As many as 350 increments may be required for a typical NPS 24 x 3/8-in (DN 600 x 9.5 mm), 90°, 5-diameter bend.
The process can produce bends in sizes from NPS 8 to 48 (DN 200 to 1200) with bending radii of 3 pipe diameters and larger in ferrous and nickel-alloy materials. Because the heat is applied from one side only, thicknesses are limited to 2 in (50 mm) and less. In more recent years a more sophisticated piece of bending equipment has entered the pipe-bending field, notably the Induction Bender. In this process the increment to be bent is heated by an induction coil, and the bending operation is continuous. The pipe to be bent is inserted in the machine, and the start of the arc is positioned under the induction coil. The portion of the pipe upstream of the coil is clamped to a rotating arm fixed to the required bend radius. The downstream portion of the pipe is pushed hydraulically through the coil, where it attains bending temperature. Since it is clamped to the rotating arm, a bending moment is imposed on the pipe and it bends as it moves through the coil. As soon as it has been bent, the heated section is cooled to restore its prior rigidity. The permissible rate of cooling is a function of material composition. Low-carbon steels and some low Cr molys may be water quenched. It is recommended that the 9Cr-1Mo-V material be cooled in still air.
The Induction Bender is manufactured in several sizes depending on the expected combinations of pipe size and bending radius. These range from NPS 31⁄₂ to 64 (DN 80 to 1600) and from 8 to 400 in (DN 200 to 10,000 mm) in radius. Since induction is used as the heating method, wall thicknesses as heavy as 4 in (100 mm) can be bent.

BENDING METHODS

Now Piping & Fabrication entering chapter of Bending Methods, and in this chapter we will discuss all about bending methods.

Bending Methods. Pipe is bent by a variety of methods, using bending tables or bending machines, with and without the application of heat. The selection of one method over another is a function of economics, materials properties, pipe size, bending radius, and equipment availability. The arc length of the bend may be heated in order to reduce the yield strength of the material. Higher bending temperatures result in lowering the yield strength and reduction of the bending energy required.

Cold bending normally infers bending at ambient temperature, while hot bending infers the application of heat. However, definitions given in B31.1 and ASME Section III create an exception to this for ferritic materials. These codes define cold bending of ferritic steels as any operation where the bending is performed at a temperature 100_F (55_C) below the lower critical or lower. Ferritic materials undergo a phase change on heating and cooling. On heating, this change starts at a temperature called the lower critical. (See Heat Treatment—Ferritic Steels).

Ferrous Pipe and Tubes
1. Cold bending: Where sufficient quantities of repetitive bends are required, ferrous pipes and tubes up to NPS 10 or 12 (DN 250 or 300) with wall thickness of 1⁄₂ in (12.7 mm) or less are most often bent at ambient temperature using some type of bending machine.

There are a great variety of cold bending machines available, with degrees of sophistication varying from simple manually operated single-plane bending devices to numerically controlled hydraulically operated machines capable of multiplane bends.
In ram-type bending, two pressure dies which are free to rotate are mounted in a fixed position on the machine frame. The pipe to be bent is positioned against these dies. A ram then presses a forming die against the pipe and the pressure dies wipe the pipe around the forming die. See Fig. A6.7. Ram bending is usually applied to heavier wall thicknesses.

In compression bending the pipe is clamped to a stationary bending die and wiped around it by a follower. As in all bends, the extrados thins and the intrados thickens or compresses. The degree of compression is greater than the thinning in this method. Compression bending is usually limited to heavier walls and larger bending radii. See Fig. A6.8 to compare compression and draw bending.

In rotary draw bending the pipe is clamped to a rotating bending form and drawn past a pressure die which is usually fixed. See Fig. A6.9. The degree of thinning of the extrados is greater than the compression of the intrados. This method permits bending of thinner wall pipe and tubes at smaller bending radii. To accommodate lighter walls and tighter radii it is often advisable to provide internal support to minimize flattening or buckling. Usually this takes the form of an internal mandrel. As the diameter-to-thickness ratio increases and the bending radius decreases, mandrels using follower balls are employed.

Roll bending is often used for coiling. One of its great advantages is that the bending radius is not dependent on a fixed radius die, and consequently there is great flexibility in choosing a bending radius. In roll bending three power-driven rolls, usually in pyramid form, are used. The pipe to be bent is placed between the two lower rolls and the upper roll. Bending is accomplished by adjusting the rolls relative to each other as necessary to attain the required diameter.

Pipe can also be cold bent on a bending table in the manner described for hot bending below, except that for ferritic materials the bending temperature is kept at least 100°F (56°C) below the lower critical. A postbending heat treatment for cold bends may be advisable for some alloys, degree of deformation, certain service conditions, or when mandated by code.

Bending & Limitations on Piping System

Bending
Economics. The use of bends versus welding fittings for changes in direction should be carefully evaluated from an economic viewpoint. Bends whose radii range from 3 to 5 times the nominal pipe diameter will offer the least pressure drop while still affording adequate flexibility to the system. Since each bend eliminates a welding fitting and at least one weld with its attendant examination, bending is very often the economic choice. In the case of special pipe sizes which are frequently used for main steam, reheat, and feed water lines in large central power generating units, bending may be the only option available.

Limitations. The metal being bent should preferably exhibit good ductility and a low rate of strain hardening. Most metals used in piping systems fulfill these requirements. A successful bend is also a function of its diameter, thickness, and bending radius. As the diameter-to-thickness ratio increases and the bending radius decreases, there is greater probability of flattening and buckling. Each bending process has differing capabilities, so the selection of a bending process rests on the availability of equipment and/or practices capable of handling the material, diameter, thickness, and bending radius involved.

FIGURE A6.5 Bend ovality

Accept and Reject Criteria. The codes have certain requirements for the acceptability of finished bends:
1. Thinning: In every bending operation the outer portion of the bend (extrados) stretches and the inner portion (intrados) compresses. This results in a thinning of the extrados and a thickening of the intrados. Because of uncertainties introduced by the pipe-manufacturing method, by the pipe tolerances, and by those introduced by the pipe-bending operation itself, it is not possible to exactly predetermine the degree of thinning. However, it can be approximated by multiplying the thickness before bending by the ratio:
The codes require that the wall thickness at the extrados after bending be at least equal to the minimum wall thickness required for straight pipe. Accordingly, the fabricator must assure that the wall thickness ordered has sufficient margin for this effect.

Although the codes do not comment on the resulting increased thickness of the intrados, this thickness does serve to offset a portion of the increased stresses caused by internal pressure which are found at this location. (See Theory and Design of Modern Pressure Vessels.17)

FIGURE A6.6 Suggested pipe buckling tolerance. (Pipe Fabrication Institute PFI ES-24)

2. Ovality: A second acceptance criteria is ovality. During the bending operation, the cross section of the bend arc frequently assumes an oval shape whose major axis is perpendicular to the plane of the bend. The degree of ovality is determined by the difference between the major and minor axes divided by the nominal diameter of the pipe.
Where the bend is subject to internal pressure, the pressure tries to reround the cross section by creating secondary stresses in the hoop direction. Some codes consider an ovality of 8 percent acceptable in this case. Where the bend is subject to external pressure, the pressure tries to collapse the cross section. The ASME B31.3 Code18 recommends a 3 percent maximum ovality when the bend is subject to external pressure.

3. Buckling: Bending of pipe with large diameter-to-thickness ratios often results in buckling rather than thickening of the intrados, even where internal mandrels or other devices are employed to minimize it. The codes do not address this subject. It is, however, often the subject of ‘‘good workmanship’’ debates. The PFI gives a criterion which has been generally accepted. This appears in PFI ES-24.19 An acceptable
buckle is one where the ratio of the distance between two crests divided by the depth of the average crest to valley is equal to or greater than 12. See Fig. A6.6

Fabrication Practices and Forming

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Fabrication Practices
Cutting and Beveling. The methods of cutting plate or pipe to length can be classed as mechanical or thermal. Mechanical methods involve the use of saws, abrasive discs, lathes, and pipecutting machines or tools.

FIGURE A6.4 Pipe-cutting machine. (Pullman Power Products Corporation)

Thermal methods are oxyfuel gas cutting or electric arc cutting. Oxyfuel gas cutting is a process wherein severing of the metal is effected by the chemical reaction of the base metal with oxygen at an elevated temperature. In the cutting torch, a fuel such as acetylene, propane, or natural gas is used to preheat the base metal to cutting temperature. A high-velocity stream of oxygen is then directed at the heated area resulting in an exothermic reaction and severing of the material. Oxyfuel gas cutting is widely used for cutting carbon steels and low alloys. It does, however, lose its effectiveness with increasing alloy content.

For higher alloy materials, some form of arc cutting is required. Plasma arc cutting is the process most frequently employed. It involves an extremely high temperature (30,000 to 50,000_K), a constricted arc, and a high-velocity gas. The torch generates an arc which is forced to pass through a small-diameter orifice and concentrate its energy on a small area to melt the metal. At the same time a gas such as argon, hydrogen, or a nitrogen-hydrogen mixture is also introduced at the orifice where it expands and is accelerated through the orifice. The melted metal is removed by the jetlike action of the gas stream. Because oxyfuel gas and arc cutting involve the application of heat, preheating may be advisable in some cases.

A very detailed description of oxyfuel gas and arc cutting is presented in The Welding Handbook.16 Weld end bevels can also be prepared by the mechanical or thermal methods just described. Both mechanical and thermal methods are used to apply the V bevel, which is used in the vast majority of piping applications. For compound and U bevels or those which may involve a counterboring requirement, horizontal boring mills are most appropriate. Various factors to be considered in selecting a weld end bevel are discussed in the section, ‘‘Welding Joint Design.’’

Forming. The term forming as it relates to piping fabrication encompasses bending, extruding, swaging, lapping, and expanding. All of these operations entail the use of equipment normally only available in pipe fabrication shops. Although the availability of welding fittings in the form of elbows, tees, reducers, and lappedjoint stub ends may reduce the need for certain of these operations, economics may dictate their use, especially where special pipe sizes are involved.