Pipe Welding Techniques Pdf
Restraint cracking occurs when small welds are made on thick metal sections, such as heavy-wall pipe. Horizontal pipe welding will be treated in detail in a later chapter. In shielded metal-arc weld- ing, an electrical circuit is established between the workpiece and the welding machine. 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.
While welding the root bead, the welder must pay careful attention to the keyhole and watch for changes in its size. Align the two sections to form the desired bevelling. To restart the weld, the arc should be struck on the part of the bead that has been cleaned. The heavier current flow increases the penetration of the weld.
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The length of time for each pulse is also adjustable making possible optimum welding conditions over a wide variety of pipe welding applications. Welding current should be lower than that used in the root and is determined by the pipe size.
This causes the metal in the area of the weld to cool slowly jf the speed of welding and the heat input are the same. Courtesy of the Hobart Brothers Co.
No current can flow in this case because the resistance is too great for the voltage or electrical pressure to overcome. The filler metal should be placed between the root opening where the tip of the rod is in contact with the apex of the groove. This causes the filler metal to rise, which can result in incomplete fusion along the edge of the bead, as shown in Fig.
This serves to preheat this surface as well as to provide time for the arc to stabilize and for the gaseous shield to form. Underpenetration can be corrected by increasing the welding current and by using a somewhat slower welding speed.
However, first there will be a brief discussion on how to practice pipe welding. In doing this the welding torch is pivoted around the end of the nozzle, arcsight user guide pdf which is held in position on the bevels of the weld joint.
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. The current in this case flows from the electrode to the work. When the arc is correctly started, the electrode laid between the edges with insufficient penetration. While this is being done, the welding current should, of course, be shut off.
In passing from the electrode to the puddle, the force of gravity also acts on the globule. To prevent this problem, a possible remedy is to decrease feed speed or reduce the electrode drive angle to increase the temperature of the bath and therefore the penetration. The composition of the weld metal is also affected by the edge preparation.
As the electrode is moved down along the joint the molten puddle and it's slag covering will tend to smother the arc caus- ing porosity and slag inclusions in the weld. The droplet or globule is transferred from the electrode to the base metal by the propellant force of the arc and by an attractive force exerted on the globule by the base metal. The arc should be struck in the joint and never on a tack weld. For wall, but without notching the base metal. In addition to preheating the bevel.
Suitable for semiautomatic and fully automatic orbital welding machines. The chapters that treat the various welding procedures describe in detail the correct welding techniques that should be used as the weld progresses around the pipe joint. The objective of this book is to describe the techniques that will result in a successful pipe weld, which must be sound throughout as well as look good. Those who will take the time and effort to read this book will learn the correct techniques which, if practiced, will result in obtain- ing the skills required to be a successful pipe welder. Steel has a tendency to develop a higher level of hardness with an increasing cooling rate.
The welder must be careful at all times to prevent the electrode from making contact with the sides of the joint and the molten puddle. 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. If it is excessive, the hot liquid metal in the puddle will be exposed to the atmosphere as a result of the removal of the gaseous shield. Single-vee butt joints are usually used to weld sections of pipe together.
The reasoning for such apprehen- sion is clear. The following instructions describe how this may be done. Such electrodes will also require higher operating currents thus increasing the chance of bum though. The combined effect of the slower cooling rate and the lower viscosity of the liquid will cause the molten puddle to drip readily. This requires the welder to be alert in order to maintain the correct electrode angle at all times and to change his welding technique as necessary.
With the electrode held at the correct angle see Fig. The metal at the end of the electrode forms a droplet or globule which is transferred across the arc to the workpiece. Alloying steel along with steel making practices in recent years has led to improvements in terms of toughness.
When the high pulse is initiated, the rod should be inserted, making contact where the rod touches the root bead. If this does not prevent overheating, it may be necessary to discontinue the arc at short intervals to allow the weld to cool before welding is again resumed. To some extent the heat input can also be controlled by the arc length. Finally, we must draw your attention to a series of welding defects caused by incorrect joint preparation. If the puddle is always kept liquid, the slag will seldom be trapped because it has a lower melting point than the weld metal.
Other slightly longer than normal. Welder ready to start a bead. The presence of oil or dirt on the moving towards the other end and should be kept surfaces to be welded causes spherical porosity.
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