It would be possible to weld the root bead with the pipe nipples placed on end on a welding table. However, it may prove awkward to weld in this position, especially if the welding table is large. A better method would be to clamp the pipe nipples onto a welding stand, as shown in Fig. 9-2.

Fig. 9-2. Method of clamping pipe nipples in pipe stand for welding in the horizontal position.

The current setting should be adjusted so that it is just sufficient to provide good fusion of the base metal and the filler metal at the root opening. Usually, slightly less current is used when the weld is a horizontal weld (2G position) than when the pipe is in the 5G position. Too much current can cause serious trouble in welding by making the molten metal in the puddle and on the upper edge of the weld difficult to control. A safe procedure before starting to weld the pipe joint is to make a test weld on two pieces of scrap metal that have been beveled, to determine the best current setting.

The electrode angle for welding the root bead in the hori­zontal (2G) position is shown in Fig. 9-3. This angle should be

Fig. 9-3. Correct electrode angle for welding the root bead in the horizontal position.

Fig. 9-4. Correct arc length for welding in the horizontal position.

maintained when welding around the entire pipe. The side angle of the electrode should not deviate more than 5 degrees from the horizontal plane. Any further deviation can result in undercutting, which, in turn, can cause cracking, especially on heavy-wall pipe.

For welding the root bead, the correct arc length is about y32 inch above the edge of the root face (Fig. 9-4), which is shorter than that used to weld the root bead in the 5G position. The weld should not
be started against a tack weld; usually it is started about 2 inches away from a tack weld.

The arc should be struck in the joint ahead of the weld. As usual, a long arc is maintained until it has stabilized and the gaseous shield has formed. It is then shortened to the normal arc length (y32 inch above the edge of the root face) and held in place until the keyhole is formed. When this occurs the weld is started, using the whipping procedure.

Figure 9-5 illustrates the whipping procedure used to weld the

Fig. 9-5. Whipping procedure used to weid the root bead in the horizontal

TOP VIEW

position.

WORK

root bead in the horizontal position. It is the same whipping proce­dure used when welding pipe in the 5G position. The arc is moved out of the puddle momentarily to allow the molten metal in the puddle to lose some of its fluidity. It is then returned to the edge of the keyhole, or that part of the keyhole that is adjacent to the weld bead. The arc is held in this position for a short period of time to allow filler metal to be transferred from the electrode to the puddle and to maintain the puddle in a liquid state. This action can be described as repeated “whip and pause.” The arc should not be held directly over the keyhole, however, as this will cause excessive penetration, or possible burn-through.

The object of whipping is, of course, to control the pool of molten metal and prevent it from sagging. The length of stroke should be about one or two electrode diameters. An excessive stroke length should not be used as this will cause the gaseous shield to be removed from the liquid metal with a resultant harmful effect on the quality of the weld.

While welding the root bead attention must be paid to the keyhole and the molten metal. If the keyhole shows signs of increasing, the speed of welding should be increased slightly and the electrode angle decreased slightly. By increasing the speed of welding the heat is not

retained in one position as long as when welding at a slower rate and the heat build-up in the weld is decreased somewhat. Slightly decreasing the electrode angle (see Fig. 9-3) allows some of the heat from the arc to escape through the root opening during whipping. To control the keyhole, the welder should maintain a short (Узг-inch) arc length at all times. By not using the prescribed shorter arc length the keyhole may become enlarged.

These measures, together with the whipping procedure, also help to control the size of the molten pool of metal. If they fail to control the keyhole and if sagging of the molten metal occurs, the weld should be stopped and the current setting lowered before continuing the weld.

The welder must avoid fatigue. If his arms become tired he is apt to slow the speed of welding and he will have greater difficulty in controlling the arc length. Fatigue can cause the welder to become erratic in his manipulation of the electrode. If fatigue should occur, it is best to stop work and rest for a few moments, rather than to continue and make a poor weld.

With practice, and by following the instructions given here, a good root bead can be made. Figure 9-6 shows the outer and inner surfaces of a good root bead.

Stop and Restart. It is necessary to stop and restart the weld several times when welding around the pipe joint. When welding a root bead the arc is quenched by a quick stab through the keyhole. For other beads the arc is reversed a short distance and then quenched by a quick withdrawal from the weld, leaving a crater behind.

Before restarting the weld, ail of the slag coating should be removed from the end for a distance of about y2 inch. When welding the root bead, the arc is struck in the weld joint y2 inch in back of the keyhole. A long arc is maintained until it has stabilized and the gaseous shield has formed. The arc is then brought to the end of the bead and shortened. The welder then watches the development of the puddle of liquid metal. As soon as it is large enough, and certainly when it shows signs of sagging, he will start the whipping procedure and continue to weld as before.

When welding the intermediate and cover layers, the arc should be struck ahead of the crater of the bead being welded. After the arc is stabilized and the gaseous shield has formed, the arc is brought into the crater and shortened. The arc is slowly moved from side to side within the crater several times until a pool of liquid metal has formed. When the molten metal shows signs of sagging, the welder must start to manipulate the electrode, using the weave pattern described further on in this chapter.

Fig. 9-6. A perfect root bead deposited in the horizontal welding position. A. Outside surface. B. Inside surface.

Making a Tie-in. Tie-ins must be made while welding toward the keyhole and toward the heavy end of a root bead. The procedures that are used in these cases are essentially the same as those for welding in the 5G position, described in Chapter 5.

When welding toward the keyhole, the weld can be continued at the normal speed of welding, using the whipping procedure. When the keyhole begins to close, the whipping motion is carried over the end of the bead to which the tie-in is being made. The welder must watch the molten metal as it fills the space between the two beads and when this space is neatly filled with metal the arc must be quenched by a sudden movement of the electrode away from the weld.

The procedure for welding toward the heavy end of a bead is very similar, except that the speed of welding must be reduced somewhat. This allows time for the heavy end of the weld to get hot enough to obtain good fusion with the oncoming molten metal. As soon as the bead that is being deposited gets close enough, the whipping motion should be directed toward the heavy end of the bead to which the tie-in is being made. It may even be advisable to pause momentarily a few times when the arc is over the heavy end. Again, as the weld metal begins to fill up the space between the beads, the welder will continue the whipping procedure. When the weld metal forms a good blend between the beads the arc is quenched.

Poor Fit-up. Poor fit-ups are also encountered when welding in the horizontal (2G) position. The welding procedures used when a poor fit-up is encountered are nearly the same as those used in the 5G position. These procedures are explained in detail in Chapter 5.

When a wide root opening occurs, a bridge across this opening must first be built by depositing several nuggets of weld metal on the root face. These nuggets need not be welded perfectly; their sole purpose is to form a bridge across which the arc can be carried to
start the weld. Before the second half of the pipe joint is welded, the bridge of weld nuggets and a short length (about */2 inch) of the adjacent root bead must be removed by grinding or with a hammer and chisel.

The bridge must be built in order to weld the tack welds in place. When welding a bead across a wide root opening (which may be a part of the tack weld or the remainder of the root bead), the current setting should be reduced somewhat and a U-weave must be used. The tack weld is started at the bridge. Contrary to the procedure used for a normal root opening, when the root opening is wide, the regular root bead is started at the end of a tack weld.

The U-weave used to weld the bead should be long enough to carry the arc completely out of the puddle of molten metal. The puddle must be allowed sufficient time to solidify completely before the electrode is reversed and returned to the weld to deposit addi­tional filler metal. When the arc is brought out of the puddle it should be made to travel up the face of the bevel, away from the edge of the bevel at the root face. If the arc is concentrated on the edge of the bevel at the root face, the edge can easily be melted away, thereby widening the root opening even more. As shown in Fig. 9-7, the electrode should be held so that it points directly at the pipe and the arc length must be kept short.

Difficulties can be encountered when welding the root bead across the wide root opening. Often they are the result of unsteadiness in manipulating the electrode while making the long U-weave, The arc length may have been irregular or the pipe bevel around the puddle

Fig. 9-7. A. U-weave used to weld a horizontal root bead when the root opening is too wide. B. Top view of electrode angle used when welding with a U-weave.

may have overheated - It is then necessary to discontinue welding for a short time to permit the weld to cool.

When the root opening is too narrow or is closed entirely, the root bead can be welded by using a higher current setting and a normal length arc. This will increase the heat input into the weld and the penetration. The puddle must not be allowed to become too large or it will sag. To prevent sagging, it may be necessary to resort to the whipping procedure.

A narrow root face can be welded by reducing the current setting and by using the U-weave to weld the bead. When the root face is too wide, the current setting should be increased slightly. If the puddle tends to sag, it may be necessary to resort to the whipping procedure or to the use of the U-weaye.

Root Bead with Low-Hydrogen Electrodes. Although low-hydrogen electrodes are seldom used to weld root beads, they can be employed if the correct technique is used expertly. An entirely different proce­dure is used as compared to that described for welding the root bead with E6010 and similar electrodes.

First of all, poorly fitted joints must be avoided. The heavy flux coating and the slow cooling rate resulting from the heavy blanket will cause the weld metal to sag if the root opening is too wide.

The electrode coating must be dry and it must not be chipped. Figure 9-8 shows the position at which the electrode must be held. It is held at an angle of approximately 10 degrees with respect to the horizontal plane, so that the end of the electrode points toward the upper pipe. When welding, the end of the electrode may be lightly dragged along the edges of the pipe* By positioning the electrode in this manner, the deposit of weld metal may droop slightly; however, it will very nearly have the proportion of a perfect root bead.

Low-hydrogen electrodes are not deeply penetrating and a short arc must be maintained at all times. For this reason a slightly higher current setting should be used. Moreover, the higher current setting prevents the electrode from sticking as it is being dragged along the edges of the pipe.

The arc should be struck ahead of the starting point and short­ened as quickly as possible. It is then brought back to the starting point, where the root bead is started as soon as enough molten metal appears to form a puddle. A short arc must be used at all times but the whipping procedure must not be used. The bead is welded by lightly dragging the electrode along the edges of the joint while holding the electrode as shown in Fig. 9-8. By following these recommendations, an exceptionally good root bead can be welded.

Fig. 9-8 Position of electrode when welding a root bead in the horizontal position with a low-hydrogen electrode.

The Second Pass. Before each layer of weld metal is deposited, the surface of the weld must be deslagged and thoroughly cleaned. All defective portions of the weld and extremely high humps should also be removed.

Again, the tendency of the molten metal to sag under the influence of gravity must be considered. The puddle must not be allowed to get too large. To achieve a good result, the welder must use the correct electrode angle, arc length, weave pattern, and speed of travel.

Figure 9-9 illustrates the correct welding technique that is used to weld the second pass. As before, a short arc length should be used to

Fig, 9-9 Correct welding procedurefor welding the second pass of a horizontal weld. Note the slanted “loop” weaving technique.

A В

weld in this position. Instead of pointing toward the upper pipe, the electrode should be held in a horizontal position, with an electrode angle of 5 to 10 degrees. The current setting should be higher than that used in the 5G position.

A slanted looped weave, shown in Fig. 9-9, is used to weld the second layer. This weave resembles a handwritten letter “1” that leans slightly toward the left. The electrode should be advanced about one-half of its diameter for each weave.

When this weave is used, the metal has a tendency to flow in two directions; the molten metai just deposited tends to trail the arc while that portion of the puddle not in the arc vicinity tends to flow in a straight downward direction. This combined action causes the puddle to flow sluggishly in any direction and sagging can be prevented if the dectrodc is advanced one-half of its diameter at every weave. The weave, then, is an important factor in controlling the puddle and in preventing it from sagging.

The width of the bead for the second pass is controlled by observing the edges of the root bead. The molten metal should be allowed to penetrate into the beveled surfaces of both pipes to obtain good fusion and to prevent undercut. For heavy-wall pipe (% inch, and larger) the width of each layer of weld metal should not exceed three times the diameter of the electrode, when welding in the horizontal (2G) position.

On low carbon steel thin-wall pipe it is possible to use a somewhat larger weave* as shown in Fig. 9-10. In order to keep the puddle from sagging, the arc is moved downward, at a slant, and back to the upper edge of the pipe by weaving the electrode completely out of the puddle. When the arc is returned to the upper edge it is held in this position momentarily to allow the puddle to reform and the molten metal to flow into the corner of the joint. Because the heat is retained in the thin-wall pipe, the puddle usually does not freeze completely and reforms very rapidly.

This technique cannot be used on heavy-wall pipe because the

Fig. 9-Ю. Longer weave used to weld second pass on low-carbon-steel thin-wall pipe.

heat is withdrawn from the weld more rapidly by the thicker metal and the puddle cannot be maintained. Using this weave on heavy - wall pipe will result in incomplete fusion of the weld metal with the base metal and the metal in the adjacent weld beads. One other difficulty occurs when using this technique; when the electrode is completely out of the puddle, the weld metal is left exposed to the atmosphere, causing oxidation and porosity. This occurs very read­ily in high-alloy steel pipe and, for this reason, a large weave should never be used to weld such pipes.

Third and Fourth Passes. The third pass is deposited on the bevel of the lower pipe, as shown in Fig. 9-1 1 A. A deep crevice is formed above the third bead in which the fourth bead is deposited, as shown in Fig. 9-11B.

Fig. 9-11 Procedure for weidingthird and fourth passes in the horizontal position.

The end of the electrode should point slightly (about 5 to 10 degrees) toward the lower edge when welding the third layer as shown at the top of Fig. 9-11A. The electrode angle can be 5 to 10 degrees, as before.

The same slanted loop weave used to weld the second bead is used to weld the third bead. In fact, this weave is also used to weld all of the remaining layers because it enables the welder to control the pool of molten metal while at the same time depositing a sound bead.

When welding the third pass, care must be taken to prevent the third pass from being deposited close to the upper pipe. An open crevice must be left in which the fourth pass can be deposited. If this crevice is too narrow, it may not be possible to weld a sound fourth pass, as it may lack good fusion and have slag inclusions trapped in the weld.

Precautions - maintaining uniform preheating practices and maintaining for interpass tem­peratures during welding. ( See page 140 for bead sequences. )

When the third and fourth passes are being deposited, the arc should be kept away from the extreme edges of the pipe bevel as they are easily burnt away by the intense heat of the arc, resulting in an undercut. The molten pool of metal should be allowed to wash up to these edges when the electrode is brought close to the edges, but not quite onto the edges. In this manner good fusion can be obtained at the edges without undercut.

The fourth pass is deposited within the deep crevice that is located over the third pass. As shown in Fig. 9-11 A, the side angle (5° to 7°) of the electrode is very important when welding the fourth pass because the force of the arc, or arc-force, must be used to help maintain the puddle of molten metal in the desired place within the crevice. If left to itself, the molten metal would spill downward, and out of this crevice. The arc-force pushes the molten metal upward, holding it in place.

The size of the puddle should be kept relatively small, about two or three times the diameter of the electrode. Then, by using the slanted “loop” weave with a steady speed of travel, a good-looking and metallurgically sound bead can be deposited.

Fifth, Sixth, and Seventh Passes (Cover Passes). These passes act as a cover pass when welding in the horizontal (2G) position. On very heavy-wall pipe the fifth, sixth, and seventh passes may be intermedi­ate or filler passes, in which case they are welded in a manner similar to that used to weld the third and fourth pass. However, in the discussion to follow, it will be assumed that the beads to be deposited are cover passes.

The cover passes must be neat in appearance and they should form a crown, or reinforcement, of about y8 inch. As before, they are deposited by building from the bottom up; i. e., the fifth pass is deposited over the third pass as shown in Fig. 9-12. This pass should extend beyond the third pass about one electrode diameter at the lower edge of the pipe. Although not desirable, a certain amount of drooping of this weld will probably be unavoidable. It is important, however, that there is no undercut into the pipe when welding this bead.

The sixth and seventh passes are built up on top of the fifth pass, as shown. When welding all of these passes, the side angle of the electrode should be approximately as shown in Fig. 9-13. Again the slanted “loop” weave is used to control the puddle, which should be two or three electrode diameters in size, and a short arc length should be used. Since these beads must be sound and have a good appearance, the welder should get into the most comfortable posi-

Fig, 9-і2. (Left) Disposition of intermediate and cover passes when welding in the horizontal position. (Right) View of weld from inside. Courtesy of Hobart Brothers Co, Fig. 9-13. Correct side angle for welding the intermediate and cover passes shown in Fig. 9-12.

tion possible under the circumstances. Manipulating the arc smoothly and with consistency, the cover passes can be welded to fulfill all of these requirements.

The technique just described for depositing the filler passes and the cap, or cover pass, is the same for many types of welding, such as:

(1) Carbon steel of all composition and heavy-wall pipe

(2) Low-alloy, high-strength heavy-wall and thin-wall pipe

(3) Medium alloy, both heavy-wall and thin-wall pipe

(4) Heat-resistance steel (low - and medium-alloy steel pipe)

(5) Steel for very low temperature services (cryogenic services-

pipe)

(6) High alloy steel (stainless steel pipe)

Caution is needed at all times. It starts first with the preparation of the base metal, in terms of oxyacetylene cutting and beveling, and second, with the tacking (short weld) of the pipe to be welded.

Many times, when preparing edges of heavy-wall pipe by flame cut­ting, the fabricator either forgets or ignores preheating the heavy-walls of low - and medium-alloy pipe. Without preheating, the effects of the alloy on high carbon pipe leads to a hardening of the bevel surface. This surface may then develop microcracks due to the very fast cool­ing rate. This effect is similar to other materials that are welded with­out preheating, though they should be preheated. With some pipe alloys the HAZ(Heat Affected Zone) may not be affected if no preheat is used. Yet, it is important to use a grinder, and remove the oxide and surface metal approximately.014 inches deep in order to avoid a hard­ened bevel surface.

The inside surface of the pipe edges at the root should be addressed first, before tacking the pipe together A file or stiff wire may be used to remove scale and rust. This will ensure proper fusion in the root. Tacking the pipe together for welding must not be undertaken as a tem­porary weld. Instead, it will be part of the entire weld or root bead. It should be given ali the attention that allows it to be part of a perfect weld.

High carbon pipe materials, as well as those of the alloy type with the propensity to harden, should be preheated to avoid cracking of those short tack welds and to avoid shrinkage stress cracking. Before starting, the welder should examine both edges of the tacks to be cer­tain there is no initial porosity or tear at the edges where the tack was discontinued.

These elongated tack welds, besides being part of the initial root pass, also serve to maintain the root opening as the root pass is subsequent­ly completed. It is important for the welder not start or discontinue the rest of this root pass on one of these tacks. Not only is there a good chance for a cold lap to take place but the key hole penetration or rein­forcement inside the pipe may not be continuously fused into that of the tack.

When welding in the 2G position, weld the pipe joint by using the stringer bead method, because stringer bead is smaller and is deposit­ed at a faster rate. If the pipe material being welded has high harden - ability, faster travel speed around the pipe ispossible so it will be important to determine if any stipulated preheat is adequate. When alloy elements capable of inducing hardenability are used in two dif­ferent welded joints—with one weld being made in the 5G position at a speed of 6 inches per minute, and the other being welded in the 2G

position at 11 inches per minute using the stringer bead method—there is a difference in the heat buildup in the joint,

When preheating is applied initially on hardenable steel, the inter­pass temperature is much higher. It should be maintained during weld­ing to prevent a hardened microstructure and a weld with a high resid­ual stress, which can lead to cracking. Preheating and the interpass temperature during welding slow down heat dissipation from the weld and the surrounding area, thus providing both an allowance for hydro­gen diffusion and a weld with the required toughness.

It is well known that hydrogen gas, above a certain level in the weld­ing electrode coating, and through other ways of inducement, can be observed in its atomic form in the molten metal during welding. In molten metal, its solubility is high. But as solidification or freezing begins, its solubility decreases as the temperature begins to drop. This process is slow; most likely hydrogen in its atomic form will become trapped. The second possibility would be for hydrogen to escape into the heat affected zone during welding. The heat affected zone is with­in the high and low critical range, and is itself a spongy reserve wait­ing the inducement of (atomic) hydrogen. The problem, however, is that hydrogen gas can find its way into micro fissures, slag pockets, inclusions, and other type of voids. The accumulation of this gas in its atomic form recombines to form molecules, and both the incubation and the diffusion phenomenon begins to exert tremendous pressures to produce cracking. Hydrogen and its activities alone may not cause failures in welds, but can contribute to cracking when the structure of a weld and its heat affected zone consist of hardened structure, high residual stress, and low toughness.

Low hydrogen electrodes are baked at high temperatures to reduce their moisture content. The material and clay used to manufacture electrodes are analyzed for their moisture levels before they are processed. Limestone is used in forming low hydrogen electrode coat­ing; it gives out 40 percent of its weight in gaseous form, which pro­tects the molten pool and the metal transfer with the arc stream. The rest of its content forms a white powdery dust, which helps form slag as well as being a fluxing agent.

The E-6010 electrode used for pipe welding is restricted to root bead on certain low-aiioy steels. However, its use on carbon steel is quite popular; it can be used on any diameter size in the carbon steel cate­gory for completing the weld. Still, it is recommended that when the thickness of the pipe wall exceeds half an inch, its root bead should be deposited by the E-6010 electrode, then filled and capped by the low hydrogen E-7018 electrode. The reason is that a low hydrogen weld

deposit is ductile. The E-7018 electrode has a low hydrogen content and its heavy coating is an excellent insulation to oxidation. In terms of low-alioy steel, with respect to the E-6010 electrode, the root bead dillution will depend on the percentage of alloying element in the par­ticular steel. The dilution of the weld deposit is given consideration accordingly,

Parts of this text have described the effects of carbon and other alloy­ing elements on steel. The discussion of carbon steel has considered steel containing as much as.10 to .25 percent carbon. Above this range, there is little doubt that precaution must be taken to plan a weld­ing procedure that avoids cracking, brittleness, and hardening in the heat affected zone.

The properties that obviously exert the greatest influence on the weld are the propensity to harden when heated to a high temperature and quickly cooled, the manner in which the hardness in the heat-affected zone is controlled by the carbon content, and the ability to harden when cooling was controlled by the carbon, manganese, and silicon content. The formation of the martensite structure is also a factor.

The carbon range over which steel appears to face the greatest changes appears to be.25 to.50 percent. Below this range, there appears to be little cause for concern about the hardenability of the steel, under bead cracking, or a brittle heat affected zone, unless small quantities of other carbide forming elements, such as columbium, vanadium, chromium, and molybdenum, are present. When alloying elements are added, they influence hardenability.

Higher strength, toughness, and other properties are achieved by adding alloying elements and heat treatment. The welding engineer must continually seek ways of welding steel without risk of cracking and without impairing ductility and toughness. There are no simple procedures or systems to predict the behavior of a steel or alloy pipe during welding, although some progress has been made. Whatever the reason for their presence, the addition of alloying elements should be considered in any appraisal of the steel’s composition from the stand­point of hardenability.

There is a comprehensive formula that takes into consideration the carbon content and its influence on hardenability as well as the marten­site structure that contributes to short range stress and becomes inten­sified as the carbon content increases. The formula also factors in other alloying elements that add to hardenability, such as manganese, nick­el, and silicon. This carbon equivalent (CE) formula is:

CE - %C+(MI) + (Ш) + (Cr) + (CLJL) + (MO) + (V) 6 20 10 40 50 10

When CE exceeds.40, cracking is possible.

Welding most high carbon pipe material, and those that incorporate alloying elements in the low-alloy and medium steel, requires control­ling the cooling rate. As welding progresses, welds cool at a rate known as the critical cooling rate, increasing the hardness of the heat affected zone. The structure known as martensite is known for its hardness, poor ductility, toughness, and short range stress, eventually leading to cold cracking.

The application of supplementary heat to the joint before welding is an important part of the welding procedure. It elevates the temperature of the pipe immediately before welding, and is called preheating. Preheating lowers the cooling rate as the welding arc moves along. For a given set of welding conditions such as current setting, speed of trav­el, and materia! thickness, the cooling rate will be faster for a weld made without preheating than with preheating. If the preheating is high, the cooling rate will be slower* When a piece of metal heated to 1100°F, its terminal conductivity is half its capacity at room temperature. When preheating is applied to a joint to be welded, the heat dissipation by dilution from the heat affected гопе and the surrounding area is lower. Therefore, the slow drop in temperature allows for transformation, by diffusion, to a final structure called perlite instead of the hardened structure called martensite, which is more likely to form if the joint was not preheated.

This treatment in the welding operation is due to a number of reasons, the principal ones being as follows (a) to avoid cracking in the heat affected zone, (b) to increase the toughness and improve the ability to withstand adverse conditions involving impact loading at low tempera­tures, (c) to reduce residual stress and internal stress from shrinkage, phase transformation and reaction to restraint, (d) to minimize shrink­age and distortion, (e) to prevent cooling at elevated temperatures that would allow a ductile structure that is softer with higher toughness, and (f) to slow down the cooling rate in order to give time for hydrogen to diffuse from the hot weld, and thus avoid cracking.

When a welding procedure is stipulated as requiring preheating, it is complemented by a stated interpass temperature as well. When work­ing with an alloy steel, the interpass temperature has an effect on grain refinement of the weld metal. The time between beads affects the extent to which the grain size is refined. Depositing weld metal immediately after another may result in no grain refinement of the weld metal. Allowing the previous bead to cool to room temperature before depositing the next bead will yield less grain refinement than deposit­ing the next while the previous one is hot, but below the critical range.

The significance of refining successive weld beads is reflected in the notch toughness impact value.

An important question to consider asks what interpass temperature should be maintained when welding an alloy pipe. Establishing appro­priate preheating and interpass temperatures depends on certain fac­tors: (a) the carbon content and the alloying element in the particular steel, (b) the temperature at which martensite structure begins, and (c) the temperature at which 90 percent of the martensite structure has formed. An interpass temperature should be maintained at a higher temperature than that at which the martensite begins.

With regard to hardenability, a steel that is considered to be shallow hardened will have a faster cooling rate, and a steel that is considered to be deep hardened will coo! at a slower rate. The alloying elements in steel, starting with those most effective in increasing the harden­ability, are carbon, manganese, molybdenum, chromium, nickel, vana­dium, and silicon. These elements in various quantities will determine the cooling rate, applicable to the particular alloy. In addition, heat treatment plays an important role in complementing the needed microstructure. Many formulas and booklets published by steel man­ufacturers and agencies establish other important criteria.

While the maniplitive techniques used when welding mild steel are similar to those required when welding the alloy steels, the welder must be aware of the unique problems that can appear as the alloy steels are subjected to the heat of welding and then the subse­quent cooling down. The job operation sheet as developed by the welding engineer must be carefully followed. Not only in regard to the proper joint preparation, but also in respect to preheating and the maintaining of inter-pass temperatures.

In most cases, the root bead on medium alloy piping and other alloy materials are welded by the gas tungsten arc welding process. This process is considered hydrogen free. It protects the molten metal from oxidation by the inert gas used for shielding. In most instances when thin-wall pipe (alloy) is being welded—which requires just one or two passes—the entire joint is welded by this process. Heavy-wall pipe is most likely to have the root bead deposited by the gas tungsten arc welding process, while the rest of the weld is completed by the shield­ed metal-arc welding process.