Back purging is a process in welding where the back side or under side of a weld is shielded from the atmosphere during welding..
Correspondingly, why is purging necessary in welding?
With purging, a gas is used to protect the weld seam until it has cooled to the point that oxidation no longer can occur. Usually, an inert gas, such as argon which is heavier than air, is used (see Figure 1). Other purging gases such as nitrogen and nitrogen/hydrogen blends can also be used.
Secondly, why do you purge stainless steel? When you are welding stainless steel pipe for the food service,industrial or automotive industries back purging is done to prevent the formation of oxides inside of the pipe, which are caused by the presence of oxygen. Weld defects include lack of fusion to the base metal, root-pass cracking and incomplete penetration.
Keeping this in consideration, do you have to back purge stainless?
Welding stainless steel tubes and pipes typically requires back purging with argon gas when using traditional processes, such as gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW).
How is purging done?
Purging with inert gas is normally done after column steaming during startup because atmospheric condensation of the residual steam can pull a vacuum which may suck in air. The converse applied at shutdown - i.e. purge the column with inert prior to steaming to eliminate combustibles.
Related Question Answers
What is the purpose of purging?
Purging is the act of removing the contents of a pipe or container and replacing it with another gas or liquid. Purging is crucial in pipeline, piping, welding and industrial process. It removes contaminants from the piping and vessels, which reduces chances of corrosion.What happens if you don't purge stainless?
Non-purged stainless steel could crack, especially in high-vibration conditions. Caustic materials could become wedged in the line and eat away at the weld. Purging removes oxygen and nitrogen and replaces those gases with a 100% argon gas atmosphere.What is the process of purging?
Purging is the act of removing the contents of a pipe or container and replacing it with another gas or liquid. Purging is crucial in pipeline, piping, welding and industrial process. It removes contaminants from the piping and vessels, which reduces chances of corrosion.What is oxidation in welding?
Metal oxidation takes place when an ionic chemical reaction occurs on a metal's surface while oxygen is present. Electrons move from the metal to the oxygen molecules during this process. Oxidation is a form of metal corrosion.What is backing gas in welding?
Backing" gas is the term to describe the application of a shielding gas to protect the other side of a welded joint while welding is taking place on the front side of the same joint, generally used when welding open-root pipe joints, but not always, and applied to a variety of materials, titanium, monel, inconel,Why does nitrogen purge pipeline?
Put simply, nitrogen purging is an air-cleansing method used to ensure that no trace of harmful contaminants or gases is left in the atmosphere. The ultimate goal here is to re-stabilize the environment to prevent an explosion. That is why the industrial use of nitrogen in the oil and gas industry is so common.What is oxidation in SS welding?
It is simply oxidation on the root side of the welded joint because the root side wasn't protected from oxygen. The blackened surface is simply oxides of chrome, iron, and nickel (assuming it is austenitic stainless).Why do we purge gas?
A purge is carried out in order to remove the hazardous gas from the line. This is done by injecting an inert gas into the pipe. The inert gas mixes with the gas in the pipework. This stops the hazard of a combustible mixture which could cause harm to workers on the site.What is a purge block?
During MultiColor 3D printing the default way of purging the old color during transition between colors is to use a purge block or wipe tower. The main problems with purge blocks are that they must be printed on every layer even if there are no color changes, and they take up space on the print bed.What is a purge plug?
Purge plugs are manufactured from Duravar-Ultra High Molecular Polyethylene. A high Temp, high performance plastic to meet the needs of today's industrial applications. Available in a variety of high visibility colors. The purge process is instantaneous. Just push it in, hit the gas, and begin to weld!What is root pass in welding?
Root pass simply refers to the 1st pass in any weld process, which makes the two members being welded, one structure.Can you purge stainless with nitrogen?
Argon is generally used for this purpose but helium is an alternative depending on gas cost and availability. Nitrogen has been used as a purge gas but is unsuitable for some stainless steels. Purge gases must be of a certain quality, in order that the welds are made correctly.Why is carbon steel not necessary for purging?
Generally, any steel with a Chromium content % of more than 5% will require an inert or other purging gas to reduce/eliminate Oxygen contact with the molten internal root weld pool to prevent this and also for any filler passes when the 'skin' is likely to be melted from heat i.e. hot pass and maybe first fill.Do you purge aluminum?
purging will help aluminum weld better in open root situations but is not always needed or called for. but stainless steel open butt welds need a back purge because of the granulation that occurs on the back side that is a surface flaw and also causes loss of corrosion resistance.What is back purging of a weld?
Back purging refers to the practice of ensuring that the root run of a weld is protected by the same inert gas that is being used to protect the weld pool. The reason is that this avoids oxidation of the root run from the non-weld side.Why is purging necessary?
Purging is an important part of a good calibration procedure and should always be practised when handling dangerous gases. Purging ensures that you have control over which gases are in your gas delivery system, and therefore which gases are being exposed to internal components, sensors, or other equipment.What is purging in GTAW Welding?
Weld purging. Weld purging is the act of removing, from the vicinity of the joint, oxygen, water vapour and any other gases or vapours that might be harmful to a welding joint as it is being welded and immediately after welding.What is the difference between argon and nitrogen?
Argon is an inert gas and reacts with absolutely nothing. Less likely to leak than much smaller/lighter Helium. Nitrogen is >75% of air and hence cheaper (just have to get rid of the <25% to make it pure). Nitrogen is not inert and hence will combine with other elements that want to give up/share electons.How do welders prevent oxidation?
Pipe manufacturers typically use inert gases, such as a mixture of argon with carbon dioxide, to prevent external oxidation during welding. But internal microbial corrosion can occur even at very low oxygen levels, and preventing this is a more complex process. 
