Sometimes a combi- nation of narrow and wide root openings is encountered in a pipe joint. This can be the result of the ends of the pipe being cut incorrectly or of the pipes being misaligned - that is, oriented at a slight angle to each other. Welding a combination narrow and wide root opening requires the application of both of the methods previously described.
The method used obviously depends upon the root opening encountered. First of all, of course, the four tack welds should be welded in place. To prevent restraint cracking, the first tack welds should be made in the region of the narrow root openings. This is not always possible, however, because the basic method of welding the pipe from the bottom toward the top should not be abandoned.
For this reason it may be necessary to start the root bead in a region having a wide root opening. When the root face is too wide, it may be possible to correct this condition by recutting the end of the pipe.
Normally, however, the pipes are welded together in the usual manner, using a higher current setting to obtain complete penetra- tion.
In this case the edge, at the root, will melt very rapidly while the rest of the weld is relatively cold. The method of overcoming this difficulty is to reduce the current flow and to use a U-weave for depositing the bead. By reducing the current setting less heat is generated by the arc and the possibility of burning through the thin edges is diminished.
The U-weave preheats the bevel ahead of the edge, thereby insuring that the weld deposit will fuse properly with the parent metaL As before, when using the U-weave, it may be necessary to allow the puddle to become mushy before the arc is returned each time. Root beads are seldom welded with low-hydrogen electrodes because very highly skilled welders are required to make welds that are free from defects. High-pressure pipe joints are usually welded by welding the root bead with a deeply penetrating type electrode, such as E60I0, or by the Gas Tungsten Arc-Weld process GTAW, described in Chapter 6 , and the remainder of the joint with a low-hydrogen electrode.
However, with skill and care it is possible to weld root beads with low-hydrogen electrodes. The following instructions describe how this may be done. As shown in Fig. The heavier coating of the E elec- trode does not allow the arc to be taken close enough to the root face, thereby making it difficult to establish the correct arc length.
Welding with an arc that is too long can cause suck-in see Fig. Diameters across the coatings of. Diameter across the coating of a. E low-hydrogen electrode. Furthermore, the heavier coating interferes with the manipu- lation of the electrode when making a weave.
For this reason a smaller size. When welding with low-hydrogen electrodes, pinholes can be caused by incorrect arc striking, chipped flux coating, moisture in the weld joints, or wet electrodes. To avoid pinholes while striking the arc, strike just ahead of the starting point and shorten the arc as quickly as possible to the proper length. Then back-up the arc to the starting point and proceed to weld as soon as the molten pool of metal has formed.
Chipped spots on the electrode coating will cause the arc to be erratic at that point, resulting in pinholes and a hard zone in the weld. For this reason electrode containers should be handled with care and damaged electrodes discarded. Wet joints should be heated with an oxyacetylene torch to drive off all mois- ture.
The electrode flux coating is sensitive to moisture and must be kept dry. Open containers should be stored in a "dry box" or controlled-humidity storage oven, where the electrodes are kept at a temperature of to F. Electrodes that have been exposed to moisture can be reconditioned by drying for one hour at a tempera- ture of to F, the exact temperature depending on the make of the electrode. A higher current setting is almost always used for welding with low-hydrogen electrodes and, therefore.
The arc characteristic is also different. The low-hydrogen electrode pro- duces an arc that is relatively smooth but lacks the penetrating power of the more lightly coated electrodes.
The heavy electrode coating will form a heavy blanket of slag over the liquid puddle of molten metal, which causes the cooling rate to be slower and the metal to remain liquid for a longer period. The viscosity of the molten slag and weld metal is lower; i. The combined effect of the slower cooling rate and the lower viscosity of the liquid will cause the molten puddle to drip readily.
For this reason, difficulty is experienced when welding in the overhead and vertical positions with electrodes. The general procedure for welding the root bead with low- hydrogen electrodes is the same as before.
Tack welds are made around the pipe, after which both sides of the joint are welded from the bottom to the top of the pipe. The welding technique. A very short arc should be used at aU times when welding with low-hydrogen electrodes. The electrode must be kept very close to the root face in order to obtain adequate penetration.
When the arc is struck. The whipping procedure should never be used when welding with low-hydrogen electrodes. A V-shaped weave, Fig. The objective of this weave is to allow the molten slag and metal in the puddle to cool and to lose some of its fluidity or to increase its viscosity in order to prevent dripping.
Using this weave also preheats the metal ahead of the weld. The weave should be made by smooth and precise movements of the wrist. The arc should be brought out of the puddle and up along the bevel with a quick movement.
Allowing just enough time for the puddle to lose some of its fluidity by slowing down the return movement, the arc is returned to the puddle and held there for a short pause. This movement is then repeated up and along the other bevel. The liquid metal must never be allowed to solidify during the weave. Some of the slag may also solidify and, if this occurs along Fig. If the puddle is always kept liquid, the slag will seldom be trapped because it has a lower melting point than the weld metal.
Since the arc is smooth and lacks deep penetrating power, the movements must be smooth and precise to avoid solidification of the puddle. Summary of Root Bead Welding A perfect root bead should be free from undercuts, porosity, incomplete fusion, insufficient penetration, and excessive penetra- tion see Fig. All of these defects can be avoided by learning and practicing the correct welding procedures.
It should be kept in mind that these defects are the responsibility of the welder. Porosity cannot be blamed on the equipment, but rather on not cleaning the weld sufficiently when grease, oil, and rust are present prior to welding. Other causes of porosity are defective electrode coatings such as chipped coatings, flaking of the coating, and coatings containing an ex.
All of these electrode defects can be detected by the welder before he strikes the arc. Proper root opening and edge penetration help to attain complete penetration. The composition of the weld metal is also affected by the edge preparation.
When the spacing and the edge preparation are correct, the welder will be able to manipulate the electrode comfortably so that there will be a better intermixing of the base metal and the filler metal. Restraint cracking occurs when small welds are made on thick metal sections, such as heavy-wall pipe. This subject is discussed at length in Chapter Suffice to say here that the size of the root bead must be large enough to withstand the shrinkage stresses without cracking.
This calls for careful attention to the condition of the joint before welding. The presence of hydrogen in the weld or in the base metal heat- affected zone becomes dangerous ifthe microstructure in both ofthese areas becomes martensite, with hardness exceeding 30 Rockwell. The danger increases as the carbon content increases. Hydrogen alone cannot be blamed for under bead cracking; other contributing variables in the immediate vicinity of the weld area must be considered as well.
This transformation is influenced by the alloying elements, especially the carbon content in respect to higher stress levels; it is likely to increase further from the melting of the base metal and inducement of filler metal into the weld. The use of premium electrodes carrying the prefix LC Low Carbon is strongly recommended. Multiaxial stress develops in those alloys with limited toughness and ductility. There are limited options for accommodating this high stress.
If hydrogen is present in such a weld, then very little energy will be needed to promote cracking. Hydrogen in a weld of hardenable steel is a powerful promoter of cracking. Carbon steel generally contains less than 0. When joined alloy steel containing 0. Even the addition of manganese to a level of about 1.
It is not difficult to decide when countermeasures are needed against hydrogen if consideration is given to the principle of anticipated microstructure, as discussed earlier. The reasoning for such apprehen- sion is clear. Such steel in heavy sections with tend to form martensite in the heat-effective zone, unless welding was conducted with con- trolled heat input to secure a relatively slow cooling rate.
Welding procedure are not just for information, but must be imple- mented. The specifics cannot be ignored during welding of an alloy material, nor can the welder fail to follow these written instruction. For example, if preheating and the interpass temperature are ignored, the metal experiences a cooling rate that would likely lead to a martensite structure with poor ductility and toughness. Steel has a tendency to develop a higher level of hardness with an increasing cooling rate.
Therefore, it can be difficult to detennine if a service failure is due to defective welding or simply weld induced brittleness. Sometimes adding a single alloying element will work; in other cases multiple elements are more effective. For instance, it is uncommon to 54 Uphill Welding the Root Bead on Heavy-Wall Pipe find iron without carbon as a strengthening additive.
Carbon is more powerful and effective than any other element when added in small quantities. Its effectiveness increases as it is increased for tensile strength and hardness. Carbon, as an alloying element, is soluble in high-temperature iron to an appreciable extent, but is soluble in the low-temperature form only to a limited extent.
With the addition of alloy to a point where its high temperature crystalline form exists and the alloying element enters the lattices to form a solid solution. Upon cooling the alloy to the point where the crystalline transformation occurs, the alloying ele- ment suddenly experiences the full effects of limited solution solid , and precipitation takes place. This results in embrittlement and reduced ductility in the HAZ. The cooling rate must be regulated to produce a fine dispersion of precipitation throughout the metal so that it has high ductility and a fine grain structure.
Otherwise, the structural condition will result in a marked decrease in ductility. Transfonnation hardening is the princi- pal mechanism used to increase the hardness and tensile strength of carbon and alloy steel. Tensile strength moves to higher levels when the carbon and manganese content increase. In the alloy steels, mod- erate addition of carbon also increase these mechanical properties, but reduces ductility and toughness. Depending upon the welding process employed, the conditions under which steel hardens to its martensite structure varies.
Also, residual stress, reaction stress, and hydrogen can cause delayed crack- ing during extended service, particularly in large heavy weldments. Alloying steel along with steel making practices in recent years has led to improvements in terms of toughness. This is especially true when welding high strength, low alloy material and medium-high alloys.
A minute fissure or void in such a weld can or will be accepted, depend- ing on the, the joint design, and the severity and variation of load car- rying capacity. In recent years, toughness has received much more attention within the microstructure and the heat affected zone, along with the fusion line. Molten alloy exhibits much more of the stronger carbide forming elements; it has a slower dissolving rate from its higher temperatures, and higher preheating, requirements, approximately to 0 F.
Caution may be necessary to control grain growth by carefully preheating and the use of temp sticks to indicate when the appropriate preheat has been reached. The success in welding carbon and alloy steel is deter- mined by the familiarity with the microstructural characteristics of each particular type of steel, and avoiding the development of an 55 Chapter 5 unsuitable structure low toughness in or adjacent to the weld joint.
In merging the chemical composition of carbon steel with low and high alloy, the addition of an alloying element from a filler metal can improve a particular quality or may unexpectedly appear as a hin- drance to improving toughness. Other important factors include the microstructural changes involving the transformation of austenite, fer- rite, and martensite. Grain size, cooling rate, residual elements, and multiple alloy may also bring about synergistic effects, though little is yet known about such synergy.
Preheating and postheating can also help attain the required level of toughness while avoiding cracking and other metallurgical difficulties. Emphasis must be given to the type ofthe base metal chemical com- position to be welded, steel making practices, wall thickness, joint design, and, in most cases, the work function of such a welded com- ponent.
In addition, the welding engineer and metallurgist need to determine the effects of microstructures of a weld based on a hydrogen free weld, or how to maintain a hydrogen-free condition. Preheating, interpass temperature, and post weld heat treatment are very important.
Even considering which electrode to use for a weld is important to the welding engineer in tenus of dilution, pick-Up, and recovery. The welding engineer must consider all these facts and write an operation sheet which adequately reflects the above concerns.
A qualified welder can prevent defects such as porosity, slag inclusion, incomplete fusion, excessive penetration, incomplete penetration, undercutting, root bead cracking during welding, and distortion caused by shrinkage. Of all of these, cracking seem to be the most unwanted defect. Defects in the heat affected zone can be caused by the lack of adherence to proper procedure in terms of preheating and interpass temperatures. When root beads of exceptional quality must be made, the GTAW process is very frequently used.
Entire welds are seldom made by this process except in situations where unusually stringent requirements must be met, such as in the case of space vehicles. Usually, only the root bead is welded by the GTAW process.
However, sometimes the second pass is also made by this process because GTAW welded root beads tcnd to be somewhat thin. Stainless steel and high-alloy steel pipes, as well as mild steel pipes, are welded by the GTAW process, especially for high-pressure pipe joints that require high-quality welds.
The outstanding features of the GTAW process arc: 1. Welds of exceptional quality can be made in almost aU metals used by industry 2. Practically no post-weld cleaning is required 3. The arc and the pool of molten metal are readily visible by the welder 4. No filler metal is transported across the arc stream; thus there is no spatter 5. Welding is possible in all positions 6. There is no slag which might be trapped in the weld.
Moreover, the outside surface of the weld Fig. This eliminates the necessity for deslagging, grindingo, f chipping after the root bead has been completed. The 57 Chapter 6 Fig. In many cases thi'i is a very important factor, such as in nuclear power plant piping sys- tems. A root bead made by the GTAW process is metallurgically sound throughout By choosing the correct rod to deposit the filler metal, it is possible to obtain a weld having the same chemical, metallurgical, and physical properties as the base metal in the pipe.
Defects such as oxidation are eliminated because the blanket of inert gas covers the weld and whipping is not used to deposit the bead. The GTAW Process Gas tungsten arc welding is a process whereby the base metal and the filler metal are melted by the intense heat of an arc that is maintained between the work and a non-melting tungsten electrode.
Filler metal, supplied by a rod, is used to supplement the base metal except when welding very thin sheets of metal, for which no filler metal is used. This process is shown in Fig. The tungsten electrode is held within a welding torch, which also supplies the inert gas that is expelled from the end of the gas nozzle at a rate of 15 to 30 cubic feet per hour to form a shield over the hot metal. Right-handed welders hold the GTAW welding torch in the right hand while the rod that supplies the filler metal is held in the left hand.
For left-handed welders these positions are usually reversed. In either case, both hands are used for GTAW welding. The welding machine 2. The shielding ga'i and the gas controls 3. The GTAW welding torch 4. The tungsten electrode. The welding machines used for the GTAW process are specially designed for this purpose. Those welding machines designed for consumable arc welding, either AC or DC, can also be used if they are equipped with a special high-frequency atlachment; however, the best welds are obtained by using machines designed specifically for the GTAW process.
Either straight or reversed polarity can be used with direct current. High- frequency current is used only for starting the welding arc when using DC current; it is always used with AC current. The welding current is turned on by a foot or a hand control. The current characteristic used depends upon the type of metal to be welded. Specific recommendations arc given in Table To prevent oxygen and nitrogen in the air from contaminating the weld, either argon Or helium, or a mixture of both, is used as a shielding gas.
Argon is more widely used since it is easier to obtain and because it is a heavier gas, thus providing better protection, or shielding, at a lower flow rate. A gas flow of 15 to 30 cubic feet per hour CFH is normally used. The gas is stored in a cylinder, Fig. IT"yP:U;:cl' llC. Alternating current -- high frequency; [l '. Direct current- straight polarity.
Welding Currenttt Shieldin! Argon or argon and helium preferred tor hc"VXJ'. COil rt en "l'iI,,, ffohtlrl 8ml hers Co. Major equipment components for Gf AW welding. The OTAW welding torch.
Components of the GTAW welding torch. Beyond this gage, the gas to the torch and the weld is controlled either by a switch mounted on the torch or by a foot pedal. When the gas leaves the tank it is fed through an electrical control valve that is actuated by the switch which allows the gas to flow only when the welding current is turned on. The gas can be made to flow contin- uously by means of manual control but this can be very costly. Torches can be water-cooled or air-cooled, depending on the welding current amperage.
For root bead welding, which is normally deposited using a current range of 75 to amps, water-cooling is not required. The parts of a torch are shown in Fig. A nozzle surrounds the electrode, which is held in place by a coUet chuck. Different collets, ranging in size from. To insert an electrode, the cap is removed and the correct-size collet is inserted in the torch. Insert the electrode in the torch and push it about Vz inch beyond the end of the nozzle, using the wrench furnished for this purpose to adjust the collet.
Attach the cap to the torch and tighten it lightly. Then adjust the electrode so that it extends beyond the nozzle the correct distance, which is usually about I liz to 2 times the electrode diameter, and finger tighten the cap. The torch is then ready to be used. Two different types of nozzles are available. One type is made of ceramic, which is not transparent. Another is of glass, which offers better visibility of the pool of molten metal when welding.
The electrodes used with the GTAW process are made from tungsten alloys. They have a very high melting point F and are practically non-consumable. When properly positioned, the electrode is located over the puddle and the intense heat of the arc keeps the puddle liquid. The electrode, which must be kept clean at all times, must never touch the molten metal to avoid the possibility of contaminating its tip with metal from the puddle.
Should it become contaminated, the electrode must be removed from the torch and the contaminant removed either by grinding or by break ing off the end of the electrode to remove the contaminated portion.
There are three types of electrodes: I. Pure tungsten 2. I or 2 percent thoriated tungsten 3. Zirconiated tungsten. Recommendations for the applications of the types of electrodes are provided in Table Thoriated tungsten electrodes are used for most pipe welding applications, including mild steel pipe.
The shape of the electrode tip has a marked influence on the contour, penetration, and width of the face of the weld deposit. It is 62 Welding the Root Bead by GTAW especially important to shape the electrode tip correctly, similar to a sharpened pencil, when it is to be used to weld a root bead on a pipe joint. The working end, or tip, of the electrode is shaped by grinding it with a very fine grinding wheel that should be used only for this purpose.
A fine-grained wheel should be used in order to obtam a very smooth surface finish on the tip of the electrode, which is helpful in maintaining a more stabilized arc. For this purpose, a 60 grain, 0 to M grade silicon carbide grinding wheel should be used, such as a CM-V wheel. If the electrode becomes contaminated with metal, it should be reground immediately. Exact specifications for the shape of the electrode tip are given in Fig.
The included angle of the point should be about 22 to 23 degrees, or the length should be ground back to a distance equal to about 21jz electrode diameters. It is important to blunt the tip slightly by grinding it flat at the end for a distance slightly less than V64 inch from the point.
Shielding the Weld Metal. In all welding processes, the very hot metal in the region of the puddle is protected by some kind of shielding. A gas formed by heating the coating of the electrode protects the top of the weld in the shielded metal-arc process. In addition, the molten metal is protected by a coating of liquid slag that rises to the surface.
On open butt joints, this coating protects the top of the weld and some of the slag flows through the liquid metal to the bottom of the joint where it also protects the exposed surface. Grinding the lhoriated tungslen electrode ltp for rOOI bead welding Point angle is shown as 22 to 23 0.
Mild steel pipe can also be welded by the GTAW process without special precautions. A sufficient quantity of the inert gas reaches the bottom of the joint to provide adequate protection and the top of the weld is covered with a heavy blanket of inert gas.
Highly alloyed steel pipe, however, when welded with the GTAW process, will require extra protection at the bottom of the joint. This is done by filling the inside of the pipes, in the region of the pipe joint, with an inert gas.
Several methods are used for containing the inert gas in the pipes; two of these are shown in Fig. Two "pistons" having rubber seals are inserted on each side of the pipe joint see Fig. Tlle pipe joint itself is taped shut to prevent the inert gas, which is blown into the pipe at a very low pressure, from escaping. A small portion of the joint is left open to allow the air to escape.
When welding, the sealing tape around the pipe is removed in sections just ahead of the weld, and additional inert gas is blown inside of the pipe to make up for the gas lost through this opening. This method can be used when welding short lengths of pipe; however, it is somewhat awkward to use on longer pipe lengths and on larger-diameter pipes. Methods of containing the inert gas shield inside a pipe. Two pistons used for short, smalJer-diameter pipes.
Plastic gas bags used in conjunction with longer pipe lengths. In field conditions, where the pipe diameters and lengths are frequently large, the joint can be sealed by inflating two plastic balloons in the pipe, as shown in Fig. They must be positioned far enough from the weld joint so that the heat from the weld will not burst them.
The joint itself is sealed with tape, as before, and the inert gas is blown into the pipe through a rubber hose or a length of small-diameter pipe or tubing. Another method of sealing the inside of the pipe is to tape a wall of polystyrene to the inside of the pipe. The advantage of these two sealing methods is that the l1aterials used to make the seal can be blown out by compressed air or a stream of water after the pipe joint is finished.
Preparation ofthe Weld Joint. The procedure for preparing the weld joint is the same as for preparing the joint when welding' by the Shielded Metal-Arc process. The principal difference is in the dimensions of the joint. The pipe joint should be carefully fitted together to obtain an accurate alignment and the correct width of root opening.
If alloy steel pipes are being welded, an inert gas should be blown into the inside of the pipe as described in the previous section. Weld joint specification for GTAW welding of the rool bead. When all of the preparations have been made, four evenly spaced tack welds are made around the pipe joints.
The content is intended to serve as the useful reference source for refreshing the steps and procedures. The newcomers to the field will also find this volume useful. The book has been serving as a pretty standard reference volume for several decades.
This latest released is continuing to enforce the proper technical understanding of the welding procedures The "Read Later" function allows you to add material to this block with just one click. Carousel Next. What is Scribd? Uploaded by Sachein Anand. Did you find this document useful? Is this content inappropriate? Report this Document. Flag for inappropriate content. Download now. Pipe Welding Procedures by by Hoobasar Rampaul.
Related titles. Carousel Previous Carousel Next. When and why do translators add connectives? A corpus-based study. Jump to Page. Search inside document. R27 62 I. The Intermediate and Cover Passes 81 8. Acknowledgment by the Publisher Industrial Press wishes to express its sincere appreciation to Robert O'Con for his invaluable assistance in the preparation of the second edition of this book. It cannot be Fig. Example of a high quality pipe weld. In preparing to make the weld, the welder is concerned with the following matters: I.
In recent years as the demand for larger diameter pipe with thicker Fast moving welding crew. Cross country pipeline laying. Welded joints in pipes also play a vital role in the transportation of liquids and gases, as exemplified Fig. Typical plant requiring many pipe-weld joints. Anonymous 6tuR1hz. Sergio Luis Blas Mercado. Febrian D' Shinnosuke. Benjamin Fallado. Yash Gavate. Mohammed Gorji. Joel Wilson. Arjun Reghu. Celeste Fremon.
Appointment of Machakos Executive Committee Members - Anonymous jK4p5X. Zeeshan Mazhar. More From Sachein Anand. Sachein Anand. Sampath Kumar. Pradeep Aneja. Technical Specifications and Drawings - Mdpe Laying. Indranil Chakraborty. Danem Halas. Sankar Cdm. Valli Raju. Nasa Gauthami. Fabricio Pedriel. Popular in Welding. Fikri Makhluf. Download Download PDF. Translate PDF. Any WPS must be qualified by the manufacturer. WPS specifies the condition ranges under which welding must be performed called variables.
WPS establishes the properties of the weldment and not the skill of the welder. Procedure Qualification Record It documents what occurred during welding the test coupon and the results of the test coupon. PQR documents the essential variables and other specific information and the results of the required testing.
In addition, when notch toughness is required for procedure qualification, the applicable supplementary essential variables shall be recorded. Procedure Qualification PQR is a record of welding data to weld a test coupon.
It also contains test results.
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