Stainless steel is not necessarily difficult to machine, but welding it requires careful attention to details. It does not dissipate heat like mild steel or aluminum, and if it is heated too much, it will lose some of its corrosion resistance. Best practices help maintain its corrosion resistance. 
The corrosion resistance of stainless steel makes it an attractive choice for many key pipe applications, including high-purity food and beverages, pharmaceuticals, pressure vessels, and petrochemical applications. However, the material's heat dissipation is not as good as mild steel or aluminum, and improper welding operations will reduce its corrosion resistance. Excessive heat input and the wrong filler metal are the two culprits.
Following some best practices for stainless steel welding can help improve results and ensure that the metal maintains its corrosion resistance. In addition, upgrading the welding process can increase productivity without compromising quality.
In stainless steel welding, the choice of filler metal is very important to control the carbon content. The filler metal used for the welding of stainless steel pipes should enhance the welding performance to meet the requirements of use.
Look for filler metals with an "L" mark, such as ER308L, because they have a lower maximum carbon content and help maintain the corrosion resistance of low-carbon stainless steel alloys. Welding low-carbon base metals with standard filler metals will increase the carbon content of the welded joint, thereby increasing the risk of corrosion. Avoid using filler metals with the "H" mark because they provide a higher carbon content and are designed for applications that require higher strength at high temperatures.
When welding stainless steel, it is also important to choose filler metals with low trace (also called impurities) elements. These are residual elements in the raw materials used to make filler metals, including antimony, arsenic, phosphorus, and sulfur. They can significantly affect the corrosion resistance of the material.
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. If there are gaps or uneven assembly between parts, the torch must stay at one point for longer and more filler metal is required to fill these gaps. This can cause heat to build up in the affected area, which can overheat the part. Improper assembly will also make it more difficult to bridge the gap and obtain the necessary weld penetration. Take care to ensure that the parts are assembled as close to perfect stainless steel as possible.
The cleanliness of this material is also very important. Very small amounts of contaminants or dirt in welded joints can cause defects, thereby reducing the strength and corrosion resistance of the final product. To clean the substrate before welding, use special stainless steel brushes that are not used for carbon steel or aluminum.
In stainless steel, sensitization is the main cause of loss of corrosion resistance. This happens when the welding temperature and cooling rate fluctuate too much, which changes the microstructure of the material.
The OD weld on the stainless steel pipe is welded using GMAW and Regulated Metal Deposition (RMD), and the root weld is not back blown, and is similar in appearance and quality to the weld made by GTAW using back blow gas.
A key part of the corrosion resistance of stainless steel is chromium oxide. However, if the carbon content in the weld is too high, chromium carbide will be formed. They bind the chromium and prevent the formation of the required chromium oxide, thus making the stainless steel corrosion resistant. If there is not enough chromium oxide, the material will not have the required properties and corrosion will occur.
Preventing sensitization comes down to the choice of filler metal and control of heat input. As mentioned earlier, it is important to choose low-carbon filler metals for stainless steel welding. However, carbon is sometimes needed to provide strength for certain applications. When low-carbon filler metals cannot be selected, controlling the heat is particularly important.
Minimize the amount of time the weld and heat-affected zone remain at high temperatures-usually considered to be 950 to 1,500 degrees Fahrenheit (500 to 800 degrees Celsius). The less time it takes to weld in this range, the less heat will accumulate. Always check and observe the interlayer temperature during the applied welding procedure.
Another option is to use filler metals designed with alloying components such as titanium and niobium to prevent the formation of chromium carbide. Because these ingredients also affect strength and toughness, these filler metals cannot be used in all applications.
Gas tungsten arc welding (GTAW) is the traditional method of welding stainless steel pipes for the root weld. This usually requires back-blowing of argon gas 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 materials.
When welding stainless steel using the gas shielded metal arc welding (GMAW) process, argon and carbon dioxide, a mixture of argon and oxygen, or a mixture of three gases (helium, argon, and carbon dioxide) are traditionally used. Usually, these mixtures mainly contain argon or helium and less than 5% carbon dioxide, because carbon dioxide will provide carbon to the weld pool and increase the risk of sensitization. It is not recommended to use pure argon for GMAW on stainless steel.
The stainless steel flux-cored wire is designed to use a traditional mixture of 75% argon and 25% carbon dioxide. The flux contains ingredients designed to prevent carbon in the shielding gas from contaminating the weld.
With the development of the GMAW process, they simplify the welding of stainless steel pipes. Although some applications may still require the GTAW process, advanced wire processes can provide similar quality and higher productivity in many stainless steel applications.
The stainless steel inner diameter weld made with GMAW RMD is similar in quality and appearance to the corresponding outer diameter weld.
The use of a modified short-circuit GMAW process (such as Miller's Regulated Metal Deposition (RMD)) for the root bead can eliminate back-purging in some austenitic stainless steel applications. After the RMD root pass, pulsed GMAW or flux-cored arc welding can be used to fill the bead and cover the bead. Compared with the use of GTAW with back-flushing, this change can save time and money, especially in larger ones. On the pipeline.
RMD uses precisely controlled short-circuit metal transfer to produce a calm and stable arc and molten pool. This provides fewer chances for cold laps or lack of fusion, less splashing and higher quality pipe roots to pass through. Precisely controlled metal transfer also provides uniform droplet deposition, making it easier to control the molten pool, thereby controlling heat input and welding speed.
Unconventional processes can improve welding productivity. When using RMD, the welding speed can be from 6 to 12 inches per minute. Since this process can increase productivity without increasing the heat of the parts, it helps to maintain the characteristics and corrosion resistance of stainless steel. The reduced heat input during this process also helps control the deformation of the substrate.
Compared with the traditional spray pulse transmission, this pulse GMAW process provides a shorter arc length, narrower arc cone and less heat input. Since the process is closed loop, it almost eliminates arc drift and changes in the distance from the tip to the workpiece. This provides easier weld pool control for in-place and out-of-place welding. Finally, coupling the pulsed GMAW for the fill and cap weld bead with the RMD for the root bead allows the use of one wire and one gas for the welding procedure, thereby eliminating process changeover time.
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Post time: Jun-25-2021