In pipe welding, back purging is a primary concern during root pass, which is used to produce good quality welds having sound mechanical and corrosion properties. This process totally depends on material type and its thickness, process heat input and cleanliness of joint surface. Primarily in case of austenitic stainless steel pipes, purge of inert gas with specific flow rate is required along with the TIG (Tungsten Inert Gas Welding) process, in order to avoid chromium depletion. Soundness of root pass can be identified by the level of discoloration in weld metal. (also see the reference AWS D 18.2 that guides
discoloration level in austenitic stainless steel). Due to high cost of inert gases, we always want to prepare alternatives for expensive purging. Here we
are sharing five tactics that can eliminate or minimize the cost of back purging during root pass and not
to forget, a skillful welder is the foremost requirement that can give you quality results.
1. Welding Flux
Fine powder of chemical solar flux in conjunction with alcohol, is applied on back side of the joint. It assists the ease of filler metal flow to the base
metal and forms a shield to prevent oxidation.
Types of Solar Flux available
- Type I used for high nickel super alloys
- Type B for Alloy steels and stainless steels
Welding Flux is preferred usually for large
diameter pipes where high cost purging is a serious issue.
2. Flux cored wires
Now flux cored TIG filler rods are being used for
root pass to save the cost of back purging. Nevertheless the use of these alloy rods results in slag formation which has to be removed before the next weld pass in case of multipass welding. Available grades with the brand name are; TGX 308L, TGX 309L, TGX 316L,
TGX347.
3. Water Soluble Purge dam
Water soluble papers that are simply
installed inside the pipe. It saves time and one of the best mean, to minimize the purging cost. In this process, purging is localized to particular section in order to prevent any loss of expensive inert gas inside the large pipes. The welder simply cut the paper according to
the dimensions and wrap it around the inner diameter of pipe, at few distance from the heat affected zone. Weld tape is applied to fill the root gap and the localized section is purged. After installation, welding process is commenced with consistent purging. Actually this one is a timely method that also yields quality results for large diameter pipes.
4. Weld Backing Tape
Fiberglass weld backing tapes are now in
use either to eliminate or to minimize the purging cost in carbon steel and stainless
steel pipe welding. The tape has two adhesive strips connected with a central fiberglass strip. The adhesive strips stick to the edges while that of fiberglass covers
the root gap. This tape is applied on the backside of the joint (gap) before making the weld pass.
5. Copper paste lubricant
This paste gives satisfactory and
economical results without purging but the negative aspect of that method is the formation of hazardous smoke which can be injurious to operator's health. For this welder needs a hyperbaric helmet to minimize the risk. According
to some evidences, welds produced by application of copper paste along with some sort of lubricants, had amazing level of
purification and quality.