Welding of Ferrous Piping Materials

Welding of Ferrous Piping Materials is the topics that Piping & Fabrication will explore and please enjoy it.

Carbon Steels. Carbon steels are classed as P-No.1 by ASME Section IX. The vast majority of carbon steel pipe is used for services below 775°F (413°C). Joints are most often V bevels with commercial backing rings or open butt roots and are welded out with SMAW, SAW, GMAW, and FCAW. For services which require high quality, GTAW root welds with SMAW, SAW, and FCAW weld-outs are most prevalent. Most carbon steel filler metal is produced to weld 60,000- and 70,000-psi material. More often than not fabricators use the 70,000-psi filler for all carbon steel welding. For SMAW the most popular electrode is E-7018, although for open-butt root pass welding using SMAW, E-6010 is still the choice.

FCAW welding is rapidly replacing SMAW because it can deposit at a much higher rate. Preheating and postweld heat treating are required depending on the carbon content and wall thickness. For typical preheat and postweld heat treatment requirements. When working to a specific code, be sure to use the requirements found in that code.

Carbon Molybdenum Steels. Carbon molybdenum steels are classed as P-No 3. Currently this material has very little use because of unfavorable experience with graphitization at temperatures over 800°F (427°C).

Chromium Molybdenum Steels. The chromium molybdenum steels are primarily used for service temperatures from 800 to 1050°F (427 to 565°C). They range from 1⁄₂ Cr-1⁄₂ Mo to 9 CR-1 Mo-V and are classed by ASME Section IX as P-No. 3, P-No. 4, and P-No. 5 A and 5 B. The preponderance of usage is in the 11⁄4 Cr-1⁄2 Mo-Si and 21⁄ Cr-1 Mo grades. Welding usually consists of GTAW root welds with filler metal added or preplaced inserts. The balance of the weld is made by SAW for welds which can be performed in the 1G position and SMAW for fixed position welds. FCAW is rapidly overtaking SMAW for these materials also.

The 9Cr-1Mo-V material is a relatively recent addition to the list of chromium molybdenum steels for use in high-temperature service. Its great advantage over other chrome moly steels is its high-temperature strength. It has allowable stresses comparable with those of austenitic stainless steels. This results in a lesser wall thickness and consequently less weight to support and considerably less volume of filler material. A tighter line configuration can be anticipated because the lesser section modulous will result in smaller reactions at the terminals due to expansion loadings. This material also has an advantage over austenitic stainless steels in that its coefficient of thermal expansion is less than that of the stainlesses, again resulting in lower end reactions for the same configuration. On the down side, 9Cr-1Mo-V is typically amartensitic structure at room temperature and requires great care in bending, welding, and postbending and welding heat treatment.

For hot bending, a temperature of 1740 to 1920°F (950 to 1050°C) is preferred. Bending in the temperature range of 1560 to 1740°F (850 to 950°C) should be avoided. After hot bending, a normalize at 1900 to 1990°F (1040 to 1090°C) is required to put carbides back into solution. The normalize is followed by a tempering heat treatment between 1350 and 1440_F (730 and 780_C). Both are followed by cooling in still air.

Welding is extremely critical. The latest ASME Section II Part C, lists 9Cr-1Mo- V filler materials. SFA 5.5 lists E9018-B9 for SMAW electrodes, and SFA 5.28 lists ER90S-B9 for rods and electrodes for gas-shielded welding. Storage and handling of electrodes is very critical (see Filler Metals). Preheat and interpass temperatures and postwelding cooling should be scrupulously observed (see Preheat and Interpass Temperature). Postwelding stress relief is a necessity. The current ASME B31.1 Code requires a range of 1300 to 1400°F (700 to 760°C), but some literature indicates that a range of 1360 to 1440°F (740 to 780°C) may be more desirable for reasonable hardness and good ductility. The time at temperature should be 1 h per in of thickness, and heating and cooling rates above 800°F (427°C) should be limited to 100°F (55°C) per h.

Martensitic and Ferritic Stainless Steels. The martensitic and ferritic grades of stainless steels are not often encountered in piping systems. They are a group of steels with chromium contents ranging from 11.5 to 30 percent. Martensitic stainless steels are those which are capable of transformation to martensite under most cooling conditions and therefore can be hardened. Ferritic stainless steels on the
other hand contain sufficient chromium and other ferrite formers such as aluminium, niobium, molybdenum, and titanium so that they cannot be hardened by heat treatment. ASME Section IX classes martensitic stainless steels as P-No.6 and ferritic stainless steels as P-No.7. The user should consult the Welding Handbook16 for suggested welding processes and the applicable code for specific preheating and postweld heat-treatment requirements.

Austenitic Stainless Steels. Austenitic stainless steels are classed as P No. 8. Piping systems of austenitic stainless steels represent a fairly significant proportion of a fabricator’s and/or installer’s work, since they appear in nuclear power plants, chemical plants, paper mills, food processing facilities, and other applications where cleanliness and corrosion resistance are mandatory and even in fossil power plants where their high-temperature properties are needed. Most root welding is done by the GTAW process, and the inside of the root is protected by purging with argon, helium, or nitrogen to prevent formation of hard chromic oxides. GTAW is used for weld-out in lighter walls, and combinations of GTAW, SMAW, and SAW are used for heavier sections. Filler metal must contain some ferrite to preclude microfissuring as described in the section ‘‘Filler Metals.’’ To minimize the precipitation of carbides (sensitization) during welding, interpass temperatures are usually limited to 300 to 350°F (150 to 175°C). Heat treatment after welding is not mandatory. For corrosion services, heating during fabrication could be detrimental since it would serve to enhance sensitization. The effects of sensitization can be mitigated by a carbide solution heat treatment as described in the section ‘‘Heat Treatment.’’ Low-carbon grades of stainless steels welded with L grade electrodes are also used in services where sensitization can be a problem.

Low-Temperature Steels. The term low-temperature steel is applied to a variety of steels which exhibit good notch toughness properties at temperatures down to cryogenic levels. The B31.1 and B31.3 Codes permit the use of most steel down to -20°F (-29°C). Below this, certain grades of carbon and nickel steel with good toughness and austenitic stainless steels are needed. Welding procedures and welding filler metals must be tested to assure suitability for the intended service. B31.3 gives details of such requirements. Root pass welding using GTAW, with SMAW and SAW weldout, is commonly used. Some FCAW is used in the carbon steels and low-nickel steels. A preheat of 200°F (95°C) is suggested by B31.3 for low-nickel steels followed by a postweld heat treatment consisting of a stress relieve at 1100 to 1175°F (600 to 630°C) when the wall exceeds 3⁄₄ in (19 mm). For 9 percent nickel steel a preheat of 50°F (10°C) and a stress relieve at 1025 to 1085°F (552 to 585°C) followed by cooling at a rate greater than 300°F/h (167°C/h) down to 600°F (316°C) is required. Certain nonferrous materials are also suitable for low-temperature service.

Welding Type, Root Pass Welding, Backing Rings

Still with Piping & Fabrication and once again we still discuss about welding type and system, and all those welding type will always very important for us, and we hope all of this will be useful to all us.

Root Pass Weldings. The integrity of any weld rests primarily with the quality of the root pass. In double-welded joints the root pass serves as a backing for passes welded from the first side. Before welding begins from the opposite side, the root area is usually removed to sound metal. In most cases, however, pipe welds must be made from one side only, and the inside surface of the root weld is not accessible for conditioning.

Backing Rings. The earliest solution to root pass welding was the use of a backing ring using the SMAW process. This usually assured good penetration and is still used for many applications. However, commercial rings used with nominal pipe dimensions may result in unwanted flow restriction, crevices for entrapment of corrosion products, and notch conditions which could result in cracking during service. Prior to the introduction of GTAW root welding, piping systems which required the highest possible quality were welded using counterboring of the pipe to close tolerances and machined backing rings. This reduced problems significantly, but the crack potential still remained. See PFI ES-132

Open Butt Root Welds. In petrochemical services backing rings often could not be used, and the practice of open butt welding with shielded metal arc electrodes was and still is used. Welders require considerably more skill. Welding is most often performed with E-XX10 electrodes, which are more controllable than the lowhydrogen types but are also more prone to porosity.

GTAW Root Welds. The introduction of GTAW represented a breakthrough in root pass welding. Because of the greater expense involved, its application is usually limited to applications requiring high-quality root welds. The weld end bevels are carefully prepared by machining and counterboring where necessary to meet the close tolerances required. The joint involves butted or open lands, and the weld is made with filler metal added or with a preplaced consumable insert. The latter have a decided advantage in that they eliminate a good deal of the variability introduced by hand feeding of filler wire. Consumable inserts come in a variety of shapes, each requiring somewhat differing fit-up tolerances. See PFI ES-21. Some types can be used for root pass welding in lighter wall materials (1⁄2 in and less) without the need for counterboring. Depending on the service, the inside surface of the molten weld puddle is often shielded from oxidation by an inert gas inside the pipe contained between dams. A small, controlled, positive pressure on the backing gas can aid in better controlling the shape of the root inside diameter.

Typical shop purging arrangement

When the root pass is made by the GTAW process, the resulting finished weld is relatively thin. In depositing the second and third passes, the first pass may be remelted. As it resolidifies, it shrinks radially, resulting in a small concave depression on the inside of the weld. This condition is usually considered acceptable provided the resulting thickness through the finished weld is equal to or greater than the required minimum wall, and the concavity blends smoothly into the adjacent base metal.

GMAW Root Welds. Many fabricators and/or installers take advantage of the low penetrating power of GMAW in the short-circuiting mode to use it for openbutt root pass welding where the quality level of GTAW root pass welding is not required. The balance of the weld is made by other processes. Care must be taken to assure that unmelted wire does not penetrate the joint and remain.

Unequal Wall Thickness

Let's see, Unequal wall thickness, I think this is one of the important thing that we have to understand in the Piping System, and Piping & Fabrication will explain it.

In most piping systems there are components such as valves, castings, heavier header sections, and equipment nozzles which are welded to the pipe. In such instances the heavier sections are machined to match the lighter pipe wall and the excess thickness tapered both internally and externally to form a transition zone. Limits imposed by the various codes for this transition zone are fairly uniform. The external surface of the heavier component is tapered at an angle of 30° maximum for a minimum length equal to 11⁄2 times the pipe minimum wall thickness and then at 45° for a minimum of 1⁄2 times the pipe minimum wall.

Slip-on and socket welding flange welds

Internally, either a straight bore followed by a 30° slope or a taper bore at a maximum slope of 1 to 3 for a minimum distance of 2 times the pipe minimum wall are required. The surface of the weld can also be tapered to accommodate differing thickness. This taper should not exceed 30°, although some codes limit the taper to 1 to 4. It may be necessary to deposit weld metal to assure that these limits are not violated.

Fillet Welds. Circumferential fillet welds are used in piping systems to join slipon flanges and socket welding fittings and flanges to pipe. In welding slip-on flanges to pipe, the pipe is inserted into the flange and welded with two fillet welds, one between the outside surface of the pipe and the hub of the flange and the other between the inside surface of the flange and the thickness of the pipe.  Alignment is relatively simple since the pipe fits inside the flange. The B31.1 Code requires that the fillet between the hub and the pipe have a minimum weld leg of 1.09 times the pipe nominal wall or the thickness of the hub, whichever is smaller. The weld leg of the front weld must be equal to the pipe nominal wall or 1⁄4 in, whichever is smaller. The gap between the outside diameter of the pipe and flange inside diameter may increase with size, so the size of the fillet leg should be adjusted to compensate for this situation. Fillet welds are also used for circumferential welding of pipe to socket fittings.

Socket weld fittings and flanges are available in sizes up to NPS 4 (DN 100) but are most frequently used in sizes NPS 2 (DN 50) and smaller. Alignment is not a problem since the pipe fits into the fitting socket. Some codes require that the fillet have uniform leg sizes equal to 1.09 times the pipe nominal wall or be equal to the socket wall, whichever is smaller. In making up socket joints it is recommended that the pipe not be bottomed in the socket before welding. B31.1 and ASME Section III suggest a 1⁄16-in (2.0 mm) gap. In high-temperature service especially, the pipe inside the socket will expand to a greater degree than the socket itself, and the differential expansion may result in unwanted shear stress in the fillet and possible cracking during operation.

Intersection-Type Weld Joints. Intersection-type weld joints occur when the longitudinal axes of the two components meet at some angle. Such is the case where nozzle, lateral, and wye intersections are fabricated by welding. Weld joints in these cases may be butt, fillet, or a combination thereof. Nozzles are made either by seton or set-through construction. In set-on construction, the opening in the header pipe is made equal to the inside diameter of the branch pipe. The branch pipe is contoured to the outside diameter of the header and beveled so that the weld is made between the outside surface of the header and through the thickness of the branch. The through thickness weld is covered by a fillet weld to blend it into the header pipe surface. In set-through construction an opening is cut in the header pipe equal to the outside diameter of the branch pipe and beveled. The branch pipe is contoured to match the inside diameter of the header. See Fig. A6.19. The weld is between the outside surface of the branch and through the thickness of the header and is covered with a fillet weld to blend it into the outside surface of the branch. Either type of construction is acceptable; the usual practice is to use seton since the volume of required weld metal is less. However, when the header is is preferred.

Small nozzles are frequently made with socket welding or threaded couplings set on the header. In these cases it is difficult to assure complete root penetration, and specially designed couplings which permit drilling through the bore to remove the root of the weld are often used.  Welded-nozzle construction cannot be used at the full rating of the pipe involved, and suitability for particular pressure temperatures must be verified by component design methods found in Part B of this book. In all cases there must be a through thickness weld of the branch to the header. Where reinforcing pads are used, they should also be joined to the header by a weld through their thickness. See Fig. A6.19 for typical details. In designing headers with multiple outlet nozzles, sufficient clearance is needed between adjacent nozzles to provide accessibility for welding. Nozzles with reinforcing pads or flanges need greater clearance. PFI ES-731 gives suggested minimum spacings.

Weld Joint Design

It's too many thing that we have too learn about Weld Joint Design, but Piping & Fabrication will step by step explain about it.

Butt Welds. A butt joint is defined as one in which the members being joined are in the same plane. The circumferential butt joint is the most universally used method of joining pipe to itself, fittings, flanges, valves, and other equipment. The type of end preparation may vary depending on the particular preferences of the individual, but in general the bevel shape is governed by a compromise between a root sufficiently wide to assure a full-penetration weld but not so wide as to require a great deal of filler metal.

Typical weld and bevel

In the shop, the inside surface of large-diameter pipe joints is often accessible. In this case the joint is most often double-welded (welded from both sides), and a double V bevel is used. For heavier walls, machined double U bevels can be used. However, the vast majority of piping butt welds must be made from one side only. For this situation the most frequently specified shapes are the V bevel, compound bevel, and U bevel, all of which can have varying angles, lands, and tolerances. Recent advances in SAW narrow-gap welding as applied to piping butt welds have cut the volume of filler metal significantly in pipe walls that are 2 in (51 mm) and thicker. The 30 or 371⁄2 (60 or 75° included angle) V bevel is most often performed integrally with the cutting operation by machine, oxyfuel gas, or arc cutting. Other bevel shapes such as the compound V, U, J bevels, or combinations thereof require machining in lathes or boring mills.

1. Alignment: Alignment for butt welding can often be a frustrating task since it is influenced by the material; pipe diameter, wall thickness, out-of-roundness tolerances; welding process needs; and design requirements.

When a joint can be double welded, the effects of misalignment are minimized since both inner and outer weld surfaces can be blended into the base metal, and any remaining offsets can be faired out. ASME Section III gives a table of allowable offsets due to misalignment in double-welded joints. All resulting offsets must be faired to a 3:1 taper over the finished weld. For single-welded joints alignment can be more difficult, since the inside surface is not accessible. The degree of misalignment is influenced by many factors and depending on the type of service application may or may not be significant. The various codes impose limits on inside-diameter misalignment. This is to assure that the stress intensification resulting from the misalignment is kept within a reasonable value. The B31.1 Code requires that the misalignment between ends to be joined not exceed 1⁄16 in (2.0 mm), unless the design specifically permits greater amounts. The B31.4 and B31.8 Codes do not require special treatment unless the difference in the nominal walls of the adjoining ends exceeds 3⁄32 in (2.5 mm).

ASME Section III on the other hand requires that the inside diameters of the adjoining sections match within 1⁄₁₆ in (2.0 mm) to assure good alignment. Counterboring is usually required to attain this degree of alignment. The welding process and NDEs to be employed also bear on misalignment limits. Some welding processes can tolerate fairly large misalignments while others, notably gas tungsten arc root pass welding with and without consumable inserts require closer tolerances. Radiographic or ultrasonic examinations of misaligned areas may show unacceptable indications if the degree of misalignment is too great.

A review of the tolerances permitted in the manufacture of various types of pipe, fittings, and forgings immediately reveals that in many situations the probable inside diameter and wall thickness variations will produce unacceptable misalignment situations. Out-of-roundness in lighter wall materials can add to the problem. When most of the pipe comes from the same rolling and the fittings from the same manufacturing lot, variations in tolerances are minimal and the pipe and fittings can be assembled for most common applications without a great deal of adjustment. Out-of-round problems in lighter walls are handled with internal or external round-up devices.

To assure that all components will be capable of alignment in the field, it is common practice for the designer to specify that the inside diameters of all matching components be machine counterbored to some specified dimension. This practice is also desirable for shop welding of heavier wall piping subassemblies. PFI ES-21 contains a set of uniform dimensions for counterboring of seamless hot-rolled pipe ordered to A106 or A335 by NPS and schedule number.

Preheat and Interpass Temperature

Preheat and Interpass Temperature, that's the topics that Piping & Fabrication will discuss today. Ferritic materials undergo metallurgical phase changes when cooling from welding to ambient temperature. Mild steels which contain no more than 0.20 percent carbon and 1 percent manganese can be welded without preheat when the thickness is 1 in (25 mm) or less. However, as the chemical composition changes by increases of carbon, manganese, and silicon or the addition of chromium and certain other alloying elements, preheating becomes increasingly important since the higher carbon and chrome molybdenum steels can develop more crack-sensitive martensitic, matensitic-bainitic, and other mixed phase structures when cooled rapidly from welding temperatures.

There is also a potential for hydrogen from SMAW electrode coatings or from moisture on the base metal surface to be dissolved in the weld. Also as the weld cools, stresses caused by shrinkage are imposed on the parts and distortion can result; and as thickness increases, thermal shock from the heat of welding can induce cracking more readily.

Preheating prior to welding is a solution to most of these problems. Preheating slows the cooling rate of the weld joint and results in a more ductile metallurigical structure in the weld metal and HAZ. It permits dissolved hydrogen to diffuse more readily and helps to reduce shrinkage, distortion, and possible cracking caused by the resultant residual stresses. It raises the temperature of the material sufficiently high to be above the brittle fracture transition zone for most materials.

The codes vary regarding preheat requirements. Some have mandatory require ments while others give suggested levels. For example, for carbon steel welding, the B31.1 Code requires preheating to a temperature of 175_F (80°C) when the carbon content exceeds 0.30 percent and the thickness of the joint exceeds 1 in. B31.3 recommends preheating to 175°F (80°C) when the base metal specified strength exceeds 71 ksi or the wall thickness is equal to or greater than 1 in (25 mm). ASME III Section suggests a preheat of 200_F (95_C) when the maximum carbon content is 0.30 percent or less and the wall thickness exceeds 11/2  in for P No. 1 Gr. No. 1, or 1 in (25 mm) for P No. 1 Gr. No. 2. It also suggests a 250°F (120°C) preheat for materials with carbon in excess of 0.30 percent and wall thicknesses exceeding 1 in (25 mm). The ASME B31.4 and B31.8 Codes require preheat based on carbon equivalents. When the carbon content (by ladle analysis) exceeds 0.32 percent, or the carbon equivalent (C + 1/4 Mn) exceeds 0.65 percent, preheating is required. The reader is advised to consult the specific codes for preheating requirements. See Table A6.4 for some typical preheat requirements. It should be noted that for the 9Cr-1Mo-V (P No. 5B Gr. 2) material some manufacturers suggest a preheat of 350_F (177_C) for GTAW and 400 to 450°F (204 to 232°C) for other types of welding regardless of thickness.

While it is preferred that preheat be maintained during welding and into the postweld heat treatment cycle without cooling, this may not always be practical. The B31.1 Code permits slow cooling of the weld to room temperature provided the completed weld deposit is a minimum of 3/s in (9.5 mm) or 25 percent of the final thickness, whichever is less. For P No. 5B and P No. 6 materials some type of intermediate stress relief is required. For the 9Cr-1Mo-V material it is recommended that the finished weld be heated to 500°F (260°C), held at that temperature for 2 hours, and allowed to cool slowly in still air by wrapping it with insulating material.

Too much heat during welding can also be a problem. Where notch toughness is a requirement, prolonged exposure to temperatures exceeding 600°F (316°C) can temper the base metal. Controlling the interpass temperature is required to minimize this problem. Interpass temperature control means allowing the temperature of the joint to cool below some specified level before the next pass is deposited.

Because of its martensitic structure, a maximum interpass temperature of 600°F (316°C) should be observed when welding 9Cr-1Mo-V material. In welding of austenitic stainless steels, sensitization of the base metal HAZ will result from the heat and welding. Here the solution is to weld with as low a heat input as possible at the highest possible speed to minimize the precipitation of carbides (sensitization). A maximum interpass temperature of 300 to 350°F (149 to 177°C) is usually employed.

Base Metals and Filler Metals

There is a lot of thing that we have to know on the piping system, and Piping & Fabrication try to share the knowledge of the piping. Hope that will be useful to all of us, and let's continue with this post.

Base Metal. Base metal is one of the essential variables for welding qualification. Because there are so many base metals to be welded, ASME Section IX has established a system of P Numbers and Group Numbers. Each base metal is assigned to a specific P Number depending on characteristics such as composition, weldability, and mechanical properties. Each P Number is further subdivided into Group Numbers depending on fracture toughness properties. See Table A6.3. When a procedure is qualified with a base metal within a particular P Number, it is also qualified for all other base metals within that P Number. When fracture toughness is a requirement, qualification is limited to base metals within the same P Number and Group Number. For example: A 106 Gr. B pipe is P No. 1 Gr. No. 1, while an A 105 flange is P No. 1 Gr. No. 2. Since both are P No. 1, qualification on either qualifies both when fracture toughness is not a factor. However, should fracture toughness become a requirement, a separate qualification would be required for each to itself and to each other.

Filler Metals. Electrodes, bare wire, wire-flux combinations, and consumable inserts which form a part of the finished weld are classed as filler materials. Most are covered by AWS and ASME specifications. See ASME Section II Part C.29 When the filler material is part of the electric circuit, it is designated as an electrode. If it is fed externally and melted by the heat of the arc, it is designated as a rod. Coated electrodes for SMAW come in straight lengths. Bare rods for GTAW come in straight lengths or spools. Electrode wire for GMAW and SAW are in spools or coils, while composite electrodes for FCAW are in spools. Each specification incorporates a system of identification so that the filler materials manufactured by different suppliers which have equivalent characteristics are identified by the same number.

For qualification purposes, they are classified in ASME Section IX with F Numbers and A Numbers. Changes in filler metal from one F Number or A Number to another require requalification. One of the problems associated with coated electrodes for SMAW is the introduction of hydrogen into the arc atmosphere and finished weld, resulting in hydrogen induced cracking. To minimize this problem, low-hydrogen-type coatings are used, but these can absorb moisture from the atmosphere. Once a sealed can of electrodes is opened, the electrodes should be stored in an oven at about 250 to 350°F (120 to 175°C) or other temperature recommended by the manufacturer. Once removed from the oven, low-hydrogen electrodes should be maintained at 175°F (80°C) minimum until consumed. Baking to remove moisture is recommended for electrodes which have been out of the oven for several hours. Refer to the manufacturers’ recommendations.

A problem associated with welding of fully austenitic stainless steel is microfissuring. To combat this problem the chemical composition of the filler material is adjusted to produce a weld deposit with small amounts of ferrite. ASME III requires that filler materials used in welding austenitic stainless steels contain a minimum of 5 percent ferrite. Ferrite, however, can be a problem at cryogenic and high temperatures. For cryogenic services the weld metal may not possess the fracture toughness capabilities of the base metal, and the ferrite content should be kept as low as possible. Alternatively, fully austenitic fillers may be required, but these are more crack-sensitive. For very high temperatures ferrite in the weld may convert to a brittle phase called sigma. For this reason applications over about 800°F (427°C) usually require a minimum of 3 percent ferrite for weldability but not exceeding 7 percent to minimize sigma formation.

Welding Processes: SAW, GTAW, GMAW and FCAW

Now we can continue with Welding Processes, and in this post Piping & Fabrication will explain the Welding Method starting from SAW, GTAW, GMAW and FCAW.

Currently the most commonly used welding processes for fabrication of piping are SMAW, submerged arc welding (SAW), GTAW, GMAW, and flux core arc welding (FCAW). Some special applications may involve plasma arc welding (PAW) or electron beam welding (EBW), but their application to piping is still rare. However, any welding process which can be qualified under the requirements of ASME Section IX is acceptable. Detailed descriptions of these various processes and their variations may be found in the Welding Handbook.

This section will limit discussion to their application to piping. For shop work, the best efficiency in all welding processes is attained when the pipe axis is horizontal and the piece is rotated so that welding is always done in the flat position. This is referred to as the 1G position. Other positions are 2G (pipe vertical and fixed, weld horizontal); 5G (pipe horizontal and fixed, weld a combination of flat, vertical, and overhead); and 6G (pipe inclined at 45° and fixed). See ASME Section IX.

Shielded Metal Arc Welding. SMAW has been the mainstay for pipe welding for many years, but it is rapidly being displaced by newer, more efficient processes. It is a process where an arc is manually struck between the work and a flux-coated electrode which is consumed in the weld. The core wire serves as the filler material, and the flux coating disintegrates to provide shielding gases for the molten metal, scavengers, and deoxidizers for the weld puddle and a slag blanket to protect the molten metal until it is sufficiently cool to prevent oxidation. It can be used in all positions, for upward or downward progression, and for root pass welding depending on the flux composition. Each weld pass is about 1⁄8 in thick, and before subsequent passes are made the slag must be removed and the surface prepared by removing irregularities which could entrap slag during subsequent passes.

Submerged Arc Welding. Unlike SMAW, SAW is an automatic or semiautomatic process. For circumferential welds in pipe the welding head is fixed for flat welding and the work is rotated under the head (1G position). It is used most efficiently in groove butt welds in heavy wall materials with pipe sizes NPS 6 (DN 150) and larger. The arc is created between the work and a bare solid wire or composite electrode which is consumed during the operation. The electrode comes in coils. Shielding is accomplished by a blanket of granular, fusible material called a flux which covers the arc and molten metal by forming a slag blanket to prevent oxidation of the molten metal until it has sufficiently cooled. Particular wire-flux combinations are required to assure that the deposited weld has the needed chemical and physical properties. This process has the greatest deposition rate and accordingly is the preferred process wherever possible. Because of the high heat input, care must be taken to assure that the interpass temperature is controlled to minimize sensitization in austenitic stainless steels or loss of notch toughness in ferritic steels. High heat input can also result in excessive penetration, so this process cannot be used effectively for root pass welding unless the root is deposited against a backing
ring or sufficient backing is provided by two or more weld passes made by the shielded metal arc or a gas-shielded arc process.

Gas Shielded Arc Welding. The term gas-shielded arc welding applies to those welding processes where the arc and molten metal are shielded from oxidation by some type of inert gas rather than by a flux.

1. Gas tungsten arc welding: GTAW is a form of gas-shielded arc welding where the arc is generated between the work and a tungsten electrode which is not consumed. The filler metal must be added from an external source, usually as bare filler rod or preplaced consumable insert. The filler metal is melted by the heat of the arc, and shielding gases are usually argon or helium. Alloying elements are always in the filler material. GTAW is considered to be the most desirable process for making root welds of highest quality. Techniques using added filler metal or preplaced filler metal as inserts are equally effective in manual and automatic applications.

Automatic versions can be used in all positions provided sufficient clearance is available for the equipment. Automatic versions also require tighter fit-up requirements since the equipment is set to specific parameters and will not recognize variations outside of these limits, such as a welder would do in manual applications. In automatic GTAW, the welding head orbits the weld joint on a guide track placed on the pipe adjacent to the joint to be welded. The welding head contains motors and drive wheels needed to move the head around the track, a torch to create the arc, and a spool of filler wire. Welding current, voltage, travel speed, wire feed rate, and oscillation are controlled from an external source. These parameters may be varied by the operator as the welding head traverses the weld. Oscillation and arc energy can be adjusted to permit greater dwell time and heat input into the side walls. Automatic GTAW welds are usually deposited as a series of stringer beads to minimize the effects of high interpass temperature.

2. Gas metal arc welding: GMAW is a type of gas-shielded welding generally used in the manual mode but adaptable to automation. The filler wire is the electrode and is furnished in coils or spools of solid wire. Is is fed automatically into the joint, melted in the arc, and deposited in the weld groove. Alloying elements arc in the wire, and shielding gas may be argon, helium, nitrogen, carbon dioxide, or combinations thereof, depending on the application. Depending on the equipment and the heat input settings, filler metal can be transferred across the arc is several modes. In short-circuiting transfer, the electrode actually touches the work where it short-circuits, melts, and restarts the arc. This process has low heat input and accordingly low penetrating power. It can often result in lack of fusion. Because of the low heat input, however, it can be effectively used for open-butt root pass welding. In spray transfer, the heat input parameters are sufficiently high to transfer the molten electrode across the arc as small droplets. Argon or argon-rich gases are used for shielding, resulting in a very stable spatterfree arc. Because of the high arc energy, it is normally used in the flat (1G) position. For all-position welding, a procedure which superimposes high amplitude pulses of current on a low-level steady-state current at regular intervals is often used. This results in a discrete transfer of metal with lower heat input needed for all-position welding.

3. Flux core arc welding: FCAW is a variation of GMAW where a composite electrode is substituted for the solid wire. The electrode is a tubular wire containing a flux material. Depending on the application, the arc may be self-shielding, or shielding gases may be used. Because of its high deposition rate this process is rapidly being developed for shop and field welding of piping.