5 Tips for Welding Stainless Steel Pipe

The corrosion resistance of stainless steel makes it a competitive choice for many important pipe applications. However, poor welding operations can reduce the corrosion resistance of stainless steel pipes. Following a few tips for stainless steel pipe welding can help improve the results and ensure the metal retains its corrosion resistance. Next, we will explore in detail 5 tips for welding stainless steel pipes.

Tip 1: Choose a low carbon filler metal

In stainless steel pipe welding, the choice of filler metal is critical to control the carbon content. The filler metal used for stainless steel pipe welding should enhance the weld performance and meet the service requirements.

Look for filler metals with an "L" designation, such as ER308L, as their lower maximum carbon content helps maintain the corrosion resistance of low-carbon stainless steel alloys. Welding low-carbon base materials with standard filler metals increases the carbon content of the weld and thus increases the risk of corrosion. Avoid filler metals with an "H" designation as they offer higher carbon content and are designed for applications requiring greater strength at high temperatures.

When welding stainless steel, it is also important to choose a filler metal that is low in trace (also known as trash) elements. These are residual elements from the raw materials used to make filler metals and include antimony, arsenic, phosphorus and sulfur. They can greatly affect the corrosion resistance of the material.

Tip 2: Pay attention to solder preparation and proper assembly

Since stainless steel is very sensitive to heat input, joint preparation and proper assembly play a key role in controlling heat to maintain material properties. Because of gaps between parts or uneven fit, the torch has to stay in one position longer and more filler metal is needed to fill those gaps. This causes heat to build up in the affected area, causing the part to overheat. Poor fit can also make it more difficult to close gaps and achieve the necessary weld penetration. Take care to ensure that the fit of the stainless steel parts is as close to perfect as possible.

Cleanliness is also very important with this material. Even the smallest amount of contamination or dirt in a weld can cause defects that reduce the strength and corrosion resistance of the final product. To clean the substrate before welding, use a stainless steel-specific brush not used on carbon steel or aluminum.

Tip 3: Control Sensitization Through Temperature and Filler Metal

In stainless steels, sensitization is the main cause of loss of corrosion resistance. This occurs when the welding temperature and cooling rate fluctuate too much, altering the microstructure of the material.

This OD weld on stainless steel pipe was welded with GMAW and Regulated Metal Deposition (RMD), the root pass was not back-purged, and was of the same appearance and quality as a back-purged weld using gas arc welding (GTAW) resemblance.

A key component of stainless steel's corrosion resistance is chromium oxide. But if the carbon content in the weld is too high, chromium carbide will form. These substances bind chromium and prevent the formation of the chromium oxide needed to make stainless steel corrosion-resistant. Without enough chromium oxide, the material will not have the required properties and corrosion will occur.

Preventing sensitization depends on filler metal selection and controlled heat input. As mentioned previously, it is important to select a low carbon filler metal for stainless steel welding. However, carbon is sometimes needed to provide strength for certain applications. Controlling heat is especially important when low-carbon filler metals are not an option.

Minimize the time the weld and heat-affected zone is exposed to high temperatures—generally considered 950 to 1,500 degrees Fahrenheit (500 to 800 degrees Celsius). The less time the welding takes in this range, the less heat is generated. Always check and adhere to the interpass temperatures in the applied welding procedure.

Another option is to use filler metals containing alloying ingredients such as titanium and niobium to prevent the formation of chromium carbides. Because these components also affect strength and toughness, these filler metals cannot be used in all applications.

Tip 4: Understand how shielding gas affects corrosion resistance

Root pass using gas tungsten arc welding (GTAW) is the traditional method for welding stainless steel pipe. This usually requires a back purge with argon to help prevent oxidation on the back side of the weld. However, for stainless steel pipes, the use of wire welding processes is becoming more and more common. In these applications, it is important to understand how various shielding gases affect the corrosion resistance of the material.

When welding stainless steel using the gas metal arc welding (GMAW) process, traditionally a mixture of argon and carbon dioxide, argon and oxygen, or a three-gas mixture (helium, argon and carbon dioxide) is used. Typically, these mixtures contain primarily argon or helium and less than 5% carbon dioxide, since carbon dioxide can contribute carbon to the weld pool and increase the risk of sensitization. Pure argon is not recommended for GMAW on stainless steel.

Flux-cored wire for stainless steel is designed to operate with a conventional mixture of 75% argon and 25% carbon dioxide. Flux contains ingredients designed to prevent carbon from the shielding gas from contaminating the weld.

Tip 5: Consider different processes and waveforms

As GMAW processes evolved, they simplified the welding of stainless steel tubing and pipe. While some applications may still require the GTAW process, advanced wire processes can provide similar quality and higher productivity in many stainless steel applications.

Stainless steel ID welds made with GMAW RMD are similar in quality and appearance to the corresponding OD welds.

Using a modified short-circuit GMAW process such as Miller's Regulated Metal Deposition (RMD) for the root pass can eliminate back purge in some austenitic stainless steel applications. The RMD root pass can be followed by pulsed GMAW or flux-cored arc welding filler and cap passes, a change that can save time and money compared to using GTAW with backpurge, especially on larger pipes

The RMD uses precisely controlled short-circuit metal transfer to produce a calm, stable arc and weld pool. This reduces the chance of cold laps or lack of fusion, reduces spatter, and improves the quality of the pipe root pass. Precisely controlled metal transfer also provides uniform droplet deposition and easier control of the weld pool, making it easier to control heat input and welding speed.

Unconventional processes can increase welding productivity. Welding speeds can be 6 to 12 in/min when using the RMD. Because the process increases productivity without applying additional heat to the part, it helps maintain the performance and corrosion resistance of stainless steel. The reduced heat input of the process also helps control deformation of the substrate.

This pulsed GMAW process offers shorter arc lengths, narrower arc cones and less heat input than conventional jet pulse delivery. Because the process is closed loop, arc drift and tip-to-workpiece distance variations are virtually eliminated. This provides easier weld pool control for in-situ and out-of-situ welding. Finally, coupling pulsed GMAW for filler and cap passes with RMD for the root pass allows the welding process to be performed with one wire and one gas, thereby eliminating process changeover time.