Stainless steel isn’t necessarily difficult to machine, but stainless steel welding requires special attention to detail. It does not dissipate heat like mild steel or aluminum and loses some of its corrosion resistance if it gets too hot. Best practices help maintain its corrosion resistance.
The corrosion resistance of stainless steel makes it an attractive choice for many important piping applications, including high purity food and beverage, pharmaceuticals, pressure vessels and petrochemicals. However, this material does not dissipate heat like mild steel or aluminum, and improper welding techniques can reduce its corrosion resistance. Applying too much heat and using the wrong filler metal are two culprits.
Adhering to some of the best stainless steel welding practices can help improve results and ensure that the metal’s corrosion resistance is maintained. In addition, upgrading welding processes can increase productivity without sacrificing quality.
When welding stainless steel, the choice of filler metal is critical to controlling the carbon content. The filler metal used to weld stainless steel pipe must improve the welding performance and meet the performance requirements.
Look for “L” designation filler metals such as ER308L as they provide a lower maximum carbon content which helps maintain the corrosion resistance of low carbon stainless steel alloys. Welding low carbon materials with standard filler metals increases the carbon content of the weld and thus increases the risk of corrosion. Avoid “H” filler metals as they have a higher carbon content and are intended for applications requiring higher strength at elevated temperatures.
When welding stainless steel, it is also important to choose a filler metal that is low in trace elements (also known as junk). These are residual elements from the raw materials used to make filler metals and include antimony, arsenic, phosphorus and sulfur. They can significantly affect the corrosion resistance of the material.
Because stainless steel is very sensitive to heat input, joint preparation and proper assembly play a key role in managing heat to maintain material properties. Gaps between parts or uneven fit require the torch to stay in one place longer, and more filler metal is needed to fill those gaps. This causes heat to build up in the affected area, causing the component to overheat. Incorrect installation can also make it difficult to close the gaps and achieve the required penetration of the weld. We have made sure that the parts come as close to the stainless steel as possible.
The purity of this material is also very important. Even the smallest amount of contaminants or dirt in the weld can lead to defects that reduce the strength and corrosion resistance of the final product. To clean the base metal before welding, use a special brush for stainless steel that has not been used for carbon steel or aluminum.
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, resulting in a change in the microstructure of the material.
This external weld on stainless steel pipe was welded with GMAW and controlled metal spray (RMD) and the root weld was not backflushed and was similar in appearance and quality to GTAW backflush welding.
A key part of the corrosion resistance of stainless steel is chromium oxide. But if the carbon content in the weld is too high, chromium carbides are formed. They bind chromium and prevent the formation of the necessary chromium oxide, which makes stainless steel resistant to corrosion. Without enough chromium oxide, the material will not have the desired properties and corrosion will occur.
Prevention of sensitization comes down to filler metal selection and control of heat input. As mentioned earlier, it is important to select a filler metal with a low carbon content when welding stainless steel. However, carbon is sometimes required to provide strength for certain applications. Heat control is especially important when low carbon filler metals are not suitable.
Minimize the time that the weld and HAZ are at high temperatures, typically 950 to 1500 degrees Fahrenheit (500 to 800 degrees Celsius). The less time you spend soldering in this range, the less heat you will generate. Always check and observe the interpass temperature in the welding procedure being used.
Another option is to use filler metals with alloying components 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.
Root pass welding using gas tungsten arc welding (GTAW) is a traditional method for welding pipes and stainless steel pipes. This usually requires an argon backflush to prevent oxidation on the underside of the weld. However, for stainless steel tubes and pipes, the use of wire welding processes is becoming more common. In these cases, it is important to understand how different shielding gases affect the corrosion resistance of the material.
Gas arc welding (GMAW) of stainless steel traditionally uses argon and carbon dioxide, a mixture of argon and oxygen, or a three-gas mixture (helium, argon and carbon dioxide). As a rule, these mixtures consist mainly of argon or helium with less than 5% carbon dioxide, since carbon dioxide can introduce carbon into the molten bath and increase the risk of sensitization. Pure argon is not recommended for GMAW stainless steel.
Cored wire for stainless steel is designed to work with a common mixture of 75% argon and 25% carbon dioxide. Fluxes contain ingredients designed to prevent contamination of the weld by carbon from the shielding gas.
As the GMAW processes evolved, they made it easier to weld tubes and stainless steel pipes. While some applications may still require the GTAW process, advanced wire processing can provide similar quality and higher productivity in many stainless steel applications.
I.D. stainless steel welds made with GMAW RMD are similar in quality and appearance to the corresponding OD welds.
Root passes using a modified short circuit GMAW process such as Miller’s controlled metal deposition (RMD) eliminate backflushing in some austenitic stainless steel applications. The RMD root pass can be followed by pulsed GMAW or flux-cored arc welding, saving time and money compared to backflush GTAW, especially on large pipes.
RMD uses precisely controlled short circuit metal transfer to create a quiet, stable arc and weld pool. This reduces the chance of cold laps or non-fusion, reduces spatter and improves pipe root quality. Precisely controlled metal transfer also ensures uniform droplet deposition and easier control of the weld pool, thereby controlling heat input and welding speed.
Non-traditional processes can improve welding productivity. Welding speed can be varied from 6 to 12 ipm when using RMD. Because this process improves performance without applying heat to the part, it helps maintain the properties and corrosion resistance of stainless steel. Reducing the heat input of the process also helps control substrate deformation.
This pulsed GMAW process offers shorter arc lengths, narrower arc cones, and less heat input than conventional pulsed jet. Since the process is closed, arc drift and fluctuations in the distance from the tip to the workplace are virtually eliminated. This simplifies the control of the weld pool both when welding on site and when welding outside the workplace. Finally, the combination of pulsed GMAW for filler and cover passes with RMD for the root pass allows welding procedures to be performed with one wire and one gas, reducing process changeover times.
Tube & Pipe Journal was launched in 1990 as the first magazine dedicated to the metal pipe industry. Today, it remains the only industry publication in North America and has become the most trusted source of information for tubing professionals.
Post time: May-22-2023