Knowledge Corner : February 2010
BACKING OR PURGING
The earths atmosphere is composed of about four fifths nitrogen and one-fifth oxygen. When exposed to air, most metals will combine with oxygen, and to a lesser extent with nitrogen especially when these metals are heated, and particularly when they are in the molten state. The rate of oxide formation will vary widely with the different metals, but even a thin film of oxide on the surface of metals can cause difficulty.
In fact, a layer of oxide can actually prevent the joining of two work pieces by welding. Other problems may occur as well. Quantities of oxide, if entrapped in a weld, may lower its strength significantly. In some instances, a portion of the oxides formed may dissolve in the molten metal and cause embrittlement of the solid weld.
Yet, if proper control is not exercised, oxygen and nitrogen can interfere with the making of a quality weld.
BACKING OR PURGING :
In order to obtain the smooth interior surface, it is necessary to protect the back as well as the face of the weld bead from oxidation. This may be done by introducing Argon or Helium into the pipe. This operation is known as purging. The gas shields the molten pool on the inside of the pipe.
Welds backed up by flux are high quality, and they are almost equal in physical properties to those made with gas purging. Flux permits more penetration through the back side, whereas gas purging reduces the inside buildup and causes a very smooth inside bead.
INERT GAS PURGING :
Elimination of an oxidizing interior atmosphere is a requirement when using the Gas Tungsten Arc Welding process for root bead welding of austenitic stainless steel. The purge gas protects the root surface of the weld and adjacent base metal surfaces from oxidation during welding. Because of oxidation protection and the related effect on surface tension and weld pool characteristics, purge gas aids in obtaining complete fusion in the root bead and also good contour and surface uniformity. It also lessens the tendency for root bead cracking.
PRELIMINARY STEPS :
- All weld joints of the assembly should be tape sealed.
- The end of all branch connections should be vented to eliminate air entrapment.
- The venting arrangement should be determined to be adequate to accomodate the flow rate of the purging gas. The approximate time for adequate purging of a pipe run can be determined by pipe size.
DURING WELDING :
- Seal tape should be left on all joints except the one to be welded.
- In joints, the tape should be removed just in advance of welding progression around the joint. This is to minimize purge gas loss and atmospheric contamination through the root opening.
Another procedure is use a tape which will burn off as welding proceeds.
- As a rule of thumb, a relatively inert atmosphere will be obtained by flushing with four times the volume to be purged. After purging is completed, the flow of backing gas during welding should be reduced until only a slight positive pressure exists in the purged area.
After the root and first filler passes are completed, the back purge may be discontinued. When using argon or nitrogen, the backing gas should preferably enter the system at a low point, to displace the atmosphere upwards and be vented at a point beyond the joint to be welded.
PURGING SET UP :
Purging is achieved on short pipe lengths or tubing by sealing off or capping the open ends and introducing a liberal amount of argon through the cap at a slightly higher pressure than atmospheric.
On tubing of large lengths, special sealing arrangements can be employed, which produce an enclosed area around the joint. Such arrangements minimise the consumption of argon.
Purge blocks or soluble paper dams are frequently used on each side of the joint to localize the area under purge for large dia pipe.
The amount of argon required for purging can be reduced by constructing internal baffles from paper or similar material that disintegrates during hydrostatic testing or subsequent operations. At other times, it may be advisable to employ a special baffle assembly such as the one shown in figure.
PURGING GAS :
The gases used for weld root purging are generally argon. It has been established that nitrogen may be used satisfactorily for purging purposes when welding stainless steel pipe. Where weld discoloration due to slight surface oxidation is not objectionable, use of commercial or standard dry nitrogen is acceptable. It should be recognized that nitrogen absorption can reduce the ferrite content of the root pass.
Purge gas flow should be adjusted to maintain zero interior gage pressure, normally between 6 to 10ft3/h (3 to 5 litres per min).
FILLER METAL :
Flux cored GTAW filler metals for root pass welding where internal gas shielding (purge gas) is not practicable. Recently developed flux cored GTAW filler metal permits open root pass welding without gas backing. However, use of this filler metal is limited to the root pass and consideration must be given to residual slag on the root side.
The area of vents through which the backup gas is exhausted to the atmosphere should be atleast squal to the area of the opening through which the gas is admitted to the system. Extra care should be taken to ensure that the backup puge pressure is not excessive when welding the last inch or two on the root pass, to prevent weld pool blow, out or root concavity.
- Welding Metallurgy, Linnert Volume.1 4th edition 1994 page 707.
- Welding Principles and Practices 3rd edition 2005-pages 722, 723.
- AWS welding hand book, Welding Processes Volume 2, 8th edition 1991 page 90.
- AWS Welding hand book, materials and applications volume 4, part-2 8th edition 1998-page 282.
- Modern Arc welding technology, ADOR welding edition 2008- page 272, 286.
- Welding Technology for Engineers Edition 2008- Page 146.
- ANSI/AWS D10.4-86, Recommended practices for welding Austenitic CR-Ni stainless steel piping and tubing- pages 16, 17, 18 and 29.