Oxy-Acetylene Gas Welding Technique


There are different welding techniques for welding mild steel. They are as follows:
  • Leftward or forward welding.
  • Rightward or backward welding.  
  • Vertical welding, single or double.
  • Overhead welding. 
  •  Linde welding.
Leftward or Forward Welding
The leftward method of welding is also known as forward welding. It is the oldest and most widely established method for the butt-welding of steel plates. Recent developments have shown that above certain thickness, the leftward method cannot be used successfully, but within its sphere of application, which is limited of thickness, it gives excellent results and is the most satisfactory method, both from the point of view of the economy of the joint and the resultant mechanical properties of the weld.
Welding is commenced at the right hand edge of the plate and proceeds across the plane in a leftward direction, the blowpipe following the welding rod. (Ref Fig 1)
                                                         
Fig 1 Leftward or Forward Welding
It is necessary to bevel the plates upto 1/8” thickness, but there should be an included angle of bevel of at least 80º for thickness between 1/8”and 3/16”, above 3/16” thickness under normal condition, it is not economical to use the leftward method and it should be replaced by rightward method.
Table 1 Data for Leftward or Forward Welding
Sheet thickness (inch)
1/32”
1/16”
3/32”
1/8”
5/32”
3/16”
Welding speed ft./Hr.
20-25
25-30
20-25
18-20
15-18
12-15
Power of blowpipe Cu.ft. of C­2H2/Hr.
1-2
2-3
3-5
5-7
7-10
10-13
Dia of welding Rod (Inch)
1/32
1/16
3/32
3/32
1/8
1/8
Consumption of welding wire per foot of weld (ft.)
1.0
1.75
2.75
1.65
2.1
4.8
Rightward or Backward Welding
During the past few years much publicity has been attached to the rightward method of welding and the results obtained by this method. Investigations show that the all these claims have been adequately sustained.
The rightward method of welding consists of commencing at the left-hand side of the plate and proceeding towards the right, the filler rod following the blowpipe (Refer Fig 2). It will be observed that the blowpipe point in the direction of the completed weld and that it moves regularly along the seam. There is no lateral movement of the blowpipe, rather the end of the filler rod describes a series of loops and doesn’t progress steadily as in case of the leftward welding. It  is not necessary to bevel the edges of the plate between 3/16” to 5/16” and even when a bevel is necessary the included angle should be only 60º and 80º as in the previous case. Further differences, all of which must be learnt and remembered if the method is to be applied successfully, are that the welding rod is held at an angle of 40º-50º to the job and not 30º-40º, which is the case in leftward welding. On other hand, the blowpipe is held much flatter i.e. the angle of the blowpipe to the plate is only 40º-50º as compared with the 60º-70º required in the case of leftward method. These differences in the angle of the bevel, movement of the rod, angle of the filler rod and angle of blowpipe are of real importance and significance.
                                                                                                                                                                                 Fig 2 Rightward Welding
With regard the mechanical properties of the weld, the enhance ductility, elongation etc., are due to the annealing effects. The flame playing on the finished weld prevents the weld zone from cooling down suddenly and so prevents brittle deposit. There is less risk of excessive oxidation. A further advantage is that the heat is more localized and consequently, the effects of distortion are greatly reduced.
Table 2 Data for Rightward or Backward Welding
Sheet thickness (inch)
3/16”
1/4”
5/16”
1/2”
5/8”
¾”
Welding speed ft./Hr.
12-15
10-12
7-8
4.5-5
3.75-4.25
3-3.25
Power of blowpipe Cu.ft. of C­2H2/Hr.
13-18
18-25
25-30
45-50
55-60
60-70
Dia of welding Rod (Inch)
3/32”
1/8”
5/32”
¼”
¼”
¼”
Consumption of welding wire per foot of weld (ft.)
3.4
3.4
3.4
5.75
6.75
9.75
Advantage of Rightward Welding Over Leftward Welding
The following are some of the advantages of rightward welding over leftward welding process:
  • Consumption of filler rod is less.
  • Welds are made faster.
  • Less chance for expansion or contraction.
  • Flame anneals the weld portion; therefore the mechanical properties are superior.
  • Better control of weld as the molten pool can be viewed clearly.
  • Less risk of excessive oxidation as the reducing flame covers the weld.
Vertical Welding
When the welding plane is in a vertical position, the welding method  applied is vertical welding. This method of welding has many advantages. Bevelling is not required upto 5/8” thickness, welds can be made rapidly and distortion is reduced to minimum.
Fig 3 Single and Double Vertical Welding
Annealing effect is also obtained as far rightward welding. Vertical welding is divided into two different methods (Refer Fig 3).
  • Welding from one side only (Single Operator Method).
  • Welding from both sides (Double Operator Method)    
Overhead Welding
In this position the welding is done from the underside of the joint and where the axis of the joint and base metal are both horizontal (Refer Fig 4). This is the most difficult position for welding. In this welding the welder should wear all protective clothing and helmet. The position of the welding rod and welding torch is almost same as the welding in horizontal position. Here, the force from the gas velocity and the surface tension of the metal prevent the molten metal falling from the weld zone. Care should be taken not to over heat the metal.
Fig 4 Overhead Welding
One method of keeping the molten metal from becoming too hot is to use a flip motion of the torch tip (Refer Fig 5). This is a quick movement of the tip from the puddle (molten pool) and back again. The momentary removal of the flame allows the puddle and bead to cool slightly.
Fig 5 Flip Motion in Overhead Welding
Pipe Welding
The method of making butt welds in steel pipes is similar to that of making welds in equivalent thickness of M.S. plates. In straight line or angle joint, tacking in jigs is advisable. For smaller diameter pipes, tacking in two places is enough. For 2” and above, tacking at three or more equidistant places is better. Edge preparation is as in the case of plate welding. Pipes are welded mainly by two methods.
  • Roll welding
  • Fixed position welding.
Roll Welding. In roll welding the weld is started at the three o’clock position and welded to the top of the pipe. The pipe is then rotated 90 degrees. Welding continues at the three o’clock position and ends at the top.This continue until the joint is finished (Refer Fig 6). When using this technique, no overhead and only small amount of vertical welding is required. Most of the welding is carried in flat position. The size of the torch tip is same as for the same thickness of flat sheet or plate. The welding can be done by either fore hand or back hand position. The weld is usually reinforced 1/16”above the pipe wall upto a thickness of 3/8” and 1/8” for 3/8” an above. The width of the bead should be approximately 1 ½ to 2 times the wall thickness.
Fig 6 Rolling Method of Welding Pipe
Fixed position welding. When it is difficult or impossible to roll the pipe, then welding is done, keeping it in a fixed position. Sometimes this must be done in cramped quarters and other adverse condition, but the weld must be as secure as any other weld. In this type the weld must be started at the bottom and welded up both sides of the pipe up to the top (Refer Fig 7). The blowpipe must be held vertically while welding the bottom as this will assist holding the metal ion place.
Fig 7 Fixed Position Pipe Welding
Elbow Joint
To weld at angles, pipes are cut with a hacksaw or ground to the required angles and set so as to give wall contact. Edges are prepared as required for the thickness, and tacking is done after setting in jig available and welded either by the leftward or rightward method according to thickness. The rest of the technique employed is as used for butt joints. (Refer Fig 8)
Fig 8 Elbow Joint
Pipe to Plate Welding
A blowpipe tip, one size larger will normally be chosen for a particular thickness in butt-welding, is selected for this type of welding. The blowpipe is held at a 50° angle and the welding rod at 70-75°. The tip of the blowpipe should be directed slightly more towards the plate than to the pipe. Holding the pipe too high will result in undercut the vertical leg. The main features of a sound weld are:
  • Good fusion of there whole surface of the pieces and then slightly reinforcement at the joint as in the case of fillet weld.
  • If any bevel is made, the chamfering is filled to a level slightly higher than its surface.
  • Absence of any porous metal in the joint.
  • Strong section and no undercut of the parent metal.
Linde Welding
This is a special technique used for the welding of pipes with a carburising flame. The excess of carbon in the carburising flame cause an addition to the carbon content of the outer layer of the steel and thereby, reduces its melting point and also causes it to sweat at a lower temperature lower than its melting point. With specially graded linde rod, it is then possible to carry out welds on mild steel at temperature below its melting point and at greater speeds.
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