Stainless steel is not necessarily difficult to work with, but its welding requires special attention to detail. It does not dissipate heat like mild steel or aluminum and may lose some corrosion resistance if you heat it up too much. Best practices help maintain its corrosion resistance. Image: Miller Electric
The corrosion resistance of stainless steel makes it an attractive choice for many critical pipe applications, including high purity food and beverage, pharmaceutical, pressure vessel and petrochemical applications. However, this material does not dissipate heat like mild steel or aluminum, and improper welding can reduce its corrosion resistance. Applying too much heat and using the wrong filler metal are two culprits.
Following some of the best stainless steel welding practices can help improve results and ensure that the metal remains corrosion resistant. In addition, upgrading the welding process can increase productivity without sacrificing quality.
When welding stainless steel, the choice of filler metal is critical to controlling the carbon content. Filler metals used to weld stainless steel pipe must improve the welding performance and be suitable for the application.
Look for “L” designation filler metals such as ER308L as they provide a lower maximum carbon content which helps maintain corrosion resistance in low carbon stainless steel alloys. Welding a low carbon base metal with standard filler metals increases the carbon content of the weld joint, increasing the risk of corrosion. Avoid filler metals marked “H” as they provide higher carbon content and are intended for applications requiring higher strength at elevated temperatures.
When welding stainless steel, it is also important to select a filler metal with low trace levels (also known as impurities) of the elements. These are residual elements in the raw materials used to make filler metals, including antimony, arsenic, phosphorus and sulfur. They can greatly 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 controlling 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 can cause heat to build up in the affected area, which can cause the part to overheat. A poor fit can also make it difficult to bridge the gap and obtain the required penetration of the weld. Take care to match the parts to the stainless steel as closely as possible.
The purity of this material is also very important. Very small amounts of contaminants or dirt in welded joints can cause defects that reduce the strength and corrosion resistance of the final product. To clean the substrate before welding, use a special stainless steel brush that has not been used on carbon steel or aluminum.
In stainless steel, sensitization is the main reason for the loss of corrosion resistance. This can happen 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, welded using GMAW and controlled deposition metal (RMD) without root backwash, is similar in appearance and quality to welds made with GTAW backwash.
A key part of the corrosion resistance of stainless steel is chromium oxide. But if the carbon content of the weld is too high, chromium carbide is formed. They bind chromium and prevent the formation of the desired chromium oxide, which gives stainless steel its corrosion resistance. If there is not 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 heat input control. 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. Temperature control is especially important when low carbon filler metals are not suitable.
Minimize the time the weld and HAZ are at elevated temperatures, typically 950 to 1500 degrees Fahrenheit (500 to 800 degrees Celsius). The less time soldering spends in this range, the less heat it generates. Always check and observe the interpass temperature during the soldering process.
Another option is to use filler metals with alloying components such as titanium and niobium to prevent the formation of chromium carbide. Because these components also affect strength and toughness, these filler metals cannot be used in all applications.
Root weld tungsten arc welding (GTAW) is a traditional welding method for stainless steel pipe. This usually requires an argon backflush to prevent oxidation on the underside of the weld. However, the use of wire welding processes in stainless steel pipes is becoming more common. In these cases, it is important to understand how different shielding gases affect the corrosion resistance of the material.
When welding stainless steel using gas arc welding (GMAW) traditionally used argon and carbon dioxide, a mixture of argon and oxygen or a three-gas mixture (helium, argon and carbon dioxide). Typically, these mixtures contain mostly argon or helium and less than 5% carbon dioxide because carbon dioxide supplies carbon to the weld pool and increases the risk of sensitization. Pure argon is not recommended for GMAW on stainless steel.
Cored wire for stainless steel is designed to work with a traditional mixture of 75% argon and 25% carbon dioxide. The flux contains ingredients designed to prevent contamination of the weld by carbon from the shielding gas.
As GMAW processes evolved, they made it easier to weld stainless steel pipes. While some applications may still require the GTAW process, advanced wire processing processes 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.
A root pass using a modified short circuit GMAW process such as Miller’s controlled metal deposition (RMD) eliminates backwash in some austenitic stainless steel applications. The RMD root pass can be followed by pulsed GMAW or flux-cored arc welding to fill and close the passes, a change that saves time and money compared to using backflushed GTAW, especially on larger diameter pipes.
RMD uses precisely controlled short circuit metal transfer to produce a quiet, stable arc and weld pool. This results in less chance of cold run-in or non-melting, less spatter, and better pipe root pass quality. Precisely controlled metal transfer also ensures uniform droplet deposition and easier control of the weld pool and thus heat input and welding speed.
Non-traditional processes can improve welding productivity. When using RMD, the welding speed can be from 6 to 12 in/min. Because the process improves productivity without additional heating of the parts, 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 provides a shorter arc length, narrower arc cone, and less heat input than conventional pulsed spraying. Since the process is closed, arc drift and fluctuations in the distance between the tip and the workpiece are virtually eliminated. This simplifies the management of the weld pool with and without welding on site. Finally, the combination of pulsed GMAW for fill and top roll with RMD for root roll allows a welding procedure to be performed using a single wire and a single gas, reducing process changeover time.
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