TIG Welding Stainless Steel
Gas Tungsten Arc Welding (GTAW), also known as Tungsten Inert Gas, or TIG Welding, is a joining process used on stainless steel. An arc welding process, TIG welding stainless steel involves the use of an non-consumable tungsten electrode in order to deliver a weld.
Most commonly used for welding thin stainless steel sections, TIG welding enables manufacturers a greater control over the weld when compared to other welding processes such as Gas Metal Arc Welding (GMAW) and Shielded Metal Arc Welding (SMAW).
TIG welding is also the most commonly used process because of its versatility, the high quality weld it delivers as well as the aesthetically pleasing appearance of the finished weld. The ability of the process to weld at low currents (hence a lower temperature input) and to add filler wire when required, makes it an ideal process for both thin materials as well as for root runs in the one sided welding of thicker plate and pipe sections.
TIG welding of stainless steel can be done with or without the use of filler wire (known as autogenous welding) and is thus the chosen process for the orbital welding of fixed pipe.
The most popular shielding gas used in TIG welding stainless steel pipe and other sections, is pure Argon, but Argon rich mixtures with additional helium, hydrogen or nitrogen is also used to achieve specific results.
Inert backing gas protection of the weld under bead is used when single sided welding is performed. This protection is employed in order to prevent oxidation as well as the loss of the stainless steel’s resistance to corrosion.
Below you will find more information concerning:
- The Process of TIG Welding Stainless Steel
- TIG Welding Stainless Steel Pipe
- The Advantages of TIG Welding Stainless Steel
The Process of TIG Welding Stainless Steel
The energy required for melting the stainless steel is delivered in the form of an electric arc. This electric arc is struck and maintained between the tungsten or tungsten alloy electrode and the stainless steel work piece. This process is completed under an inert or slightly reducing atmosphere.
Welds produced on stainless steels are always done in the Direct Current Electrode Negative (DCEN) or Direct Current Straight Polarity (DCSP) mode.
The process involves the stainless steel work piece being struck by the electrons, thus enhancing penetration. The electrode itself undergoes very little wear.
In cases where filler metals are used, it is done so in the form of bare rods or, in automatic welding processes, coiled wire.
The flow of the inert gas used protects the arc zone from ambient air, which in turn enables the maintenance of a stable arc. Based on the materials being welded, an appropriate shielding gas will be selected.
The selection of the shielding gas influences the following factors:
- Shielding efficiency.
- The stainless steel’s resistance to corrosion.
- The emission of gasses and fumes during the welding process.
- The arc’s ignition and stability.
- The weld’s geometry.
- The resulting surface appearance (in terms of oxidation, spatters etc.).
- The loss of alloying elements and the pickup of atmospheric gasses.
- And in some cases, metal transfer.
TIG Welding Stainless Steel Pipe
When it comes to TIG welding stainless steel pipe, the selection of the filler metal to be use is important. Selecting the filler metal is all about enhancing the properties of the weld produced as well as meeting the requirements set out by the final application of the fabricated pipe.
Choosing filler metals with a low carbon content can help retain the resistance to corrosion in low carbon stainless steel alloys. Filler metals with higher carbon content is used in applications that require a higher strength, whereas filler metals with a higher silicon level is able to increase the puddle fluidity of the weld, increase the travel speeds as well as improve the tie-ins.
When selecting filler metals, a filler metal with low trace (also known as tramp) elements are important. These trace elements include phosphorus, tin, sulfur, arsenic an antimony - all which have an effect on resistance to corrosion.
In TIG welding stainless steel pipe, sensitization is the most common cause of the loss of resistance to corrosion.
Sensitization is influenced by:
- The base metals (being joined)
- The filler being used, and
- The temperature at which the weld cools down.
When carbon levels in the weld, as well as the heat affected zones, are raised, chromium carbides form. This in turns results in the prevention of the formation of the chromium layer, leading to corrosion or a loss in the required resistance to corrosion.
The three main ways in which sensitization is fought:
The Use of Low Carbon Base and Filler Metal:
The use of low carbon base and filler metal will help to either reduce or completely eliminate carbon in the welding process. Note that this method is not always sufficient as carbon is an alloying factor in a number of applications.
Minimizing the time the weld and heat affected zone spends at sensitization temperature ranges:
Generally, this temperature range will be between 500°C and 800°C. By minimizing the time spent at sensitization temperature zones or ranges, the less damage will occur as a result of the heat of the weld. Welding at the lowest possible temperature will aid in a faster cooling rate.
he Use of Filler Metals with Special Alloying Elements:
By using these types of filler metals, the formation of chromium carbides can be prevented. An example of this would be niobium and titanium alloyed into the filler metal. Thus helps to prevent a reaction between carbon and chromium. Note that this elements do have a strong impact on toughness and strength and is subsequently limited in application. It is also important to note that this method delivers no benefits to the areas the farthest away from the zones affected by heat.
TIG welding stainless steel pipe and tube requires a back purge of argon. In processes where cost does play an important role, nitrogen is also used, but note that nitrogen can lead to the formation of nitride compounds in the root of the weld. This formation of nitride compounds does influence resistance to corrosion.
This loss of corrosion resistance may be acceptable in certain applications (such as hydraulic fluid systems) where a risk of interior corrosion is low.
Normally, straight argon is recommended for stainless steel tube and pipe TIG welding processes.
Joint preparation is also a vital factor when it comes to welding stainless steel tubes and pipes. Trace elements of other (or foreign) materials incorporated into the weld joint will results in joint flaws, a reduced resistance to corrosion and a reduced strength.
The way in which the pipe is cut and beveled will also play a role on the weld. A lack of fit-up and the occurrence of gaps will lead to the welder having to add more filler metal. This adding of filler metal does not only reduce productivity, but also leads to a heat buildup in the work area.
Today, TIG welding is the best solution for joining stainless steel pipes and tubes, especially for high purity application tube or pipe with a 6 inch and under diameter and a schedule 10 wall thickness.
Food grade stainless steel pipes and tubes are joined by using an Autogenous TIG Square Butt Weld, because of its ability to join pieces without the use of a filler metal. This elimination of filler metal aids in eliminating any changes in chemistry.
An Autogenous TIG Square Butt Weld will typically work on any pipe that is thinner than 1/8 inches. When the pipe gets thicker (in the range of schedule 10 to schedule 40), the pipe will need to be beveled and the use of filler metal will be required.
The Advantages of TIG Welding Stainless Steel
Using TIG Welding on stainless steel offers the following advantages to manufacturers:
- A very low electrode wear.
- The creation of pore-free, sound welds.
- TIG welding offers a concentrated heat source, which in turn leads to a narrow fusion zone.
- It offers a precise control over penetration as weld as the weld’s shape in all positions.
- TIG welding has a very stable arc.
- There is no spatter as no flux is needed in the welding process.
- The residue of oxidation is eliminated, resulting in a simplified final cleaning process.
You might also like TIG Welding Aluminum.
Back to Main Page: Custom Stainless Steel Fabrication.
Further Suggested Reading:
- Stainless Steel Grades
- Stainless Steel Fabrication Process
- Stainless Steel Tube Fabrication
- Tolerances in Stainless Steel Fabrication
- Stainless Steel Finishes
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