The corrosion resistance of stainless steel makes it an attractive choice for many critical tube and pipe applications, including high purity food and beverage, pharmaceuticals, pressure vessels, and petrochemical uses. However, the material does not dissipate heat as well as mild steel or aluminum, and poor welding practices can decrease its ability to resist corrosion. Applying too much heat and using the wrong filler metal are two culprits.
Following some of the best practices for welding stainless steel can help improve results and ensure the metal retains its corrosion resistance. In addition, upgrading the welding process can offer productivity gains without affecting quality.
Tip 1: Select a low carbon filler metal
In welding stainless steel, the selection of the filler metal is crucial in controlling carbon levels. The filler metal used for welding stainless steel pipe and tube must improve weld properties and meet the requirements of the application.
Look for filler metals with an “L” designation, such as ER308L, as they provide a lower maximum carbon content, which helps maintain corrosion resistance in low carbon stainless alloys. Welding a low carbon base material with a standard filler metal can increase the carbon content of the weld seam and thus increase the risk of corrosion. Avoid filler metals with an “H” designation, as they provide a higher carbon content designed for applications that require greater resistance to high temperatures.
When welding stainless steels, it is also important to choose a low trace filler metal (also called Hobo) items. These are residual elements, including antimony, arsenic, phosphorus and sulfur, present in the raw materials used to make filler metals. They can drastically affect the corrosion resistance of the material.
Tip 2: Pay attention to weld preparation and a good fit
Because stainless steel is so sensitive to heat input, joint preparation and proper fit play a key role in heat control to maintain material properties. With gaps or an uneven fit between parts, the torch must 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, which can overheat the room. A poor fit can also make it more difficult to fill in gaps and achieve the necessary weld penetration. Make sure that the fit of the parts is as perfect as possible with the stainless steel.
Cleanliness is also very important with this material. Very small amounts of contaminants or dirt in the weld joint can cause defects which reduce the strength and corrosion resistance of the final product. To clean the base material before welding, use a dedicated brush specifically for stainless steel that has not been used on carbon steel or aluminum.
Tip 3: Control sensitization with temperatures and filler metal
In stainless steel, sensitization is the main cause of the loss of corrosion resistance. This can happen when welding temperatures and cooling rates fluctuate too much, changing the microstructure of the material.
Chromium oxide is a key component in the corrosion resistance of stainless steel. But if the carbon levels in the weld are too high, chromium carbides are formed. These bind the chromium and prevent the formation of the necessary chromium oxide which gives stainless steel its corrosion resistance. Without enough chromium oxide, the material does not have the desired properties and corrosion can set in.
The prevention of sensitization comes down to the selection of the filler metal and the control of the heat input. As stated earlier, it is important to choose a low carbon filler metal for welding stainless steel. However, sometimes carbon is needed to provide strength in some applications. When it is not possible to choose a low carbon filler metal, heat control is particularly important.
Minimize the time that the weld and the heat affected area are held at elevated temperatures, generally considered to be 500 to 800 degrees C (950 to 1,500 degrees F). The less time a weld spends in this range, the less heat can build up. Always check and observe the temperatures between passes in the welding procedure for the application.
Another option is to use filler metals designed with alloying ingredients such as titanium and niobium which prevent the formation of chromium carbides. Since these ingredients also affect strength and toughness, these filler metals cannot be used in all applications.
Tip 4: Understand how shielding gas affects corrosion resistance
The use of Tungsten Gas Arc Welding (GTAW) for the root pass is the traditional method of welding stainless steel pipe and tube. This usually requires a back purge of argon gas to help prevent oxidation on the back side of the weld. However, the use of wire welding processes is increasingly common with stainless steel tubing and pipes. In these applications, it is important to understand how different shielding gases affect the corrosion resistance of the material.
Mixtures of argon and carbon dioxide, argon and oxygen, or mixtures of three gases (helium, argon and carbon dioxide) have traditionally been used when welding stainless steel with the welding process. gas arc welding (GMAW). Often, these mixtures contain mostly argon or helium and less than 5% carbon dioxide, as carbon dioxide can add carbon to the weld puddle and increase the risk of sensitization. Pure argon is not recommended for GMAW on stainless steel.
Flux-cored wires for stainless steel are designed to work with traditional mixtures of 75% argon and 25% carbon dioxide. The flux contains ingredients designed to prevent carbon in the shielding gas from contaminating the weld.
Tip 5: Consider different processes and waveforms
As GMAW processes have evolved, they have simplified the welding of stainless steel pipe and tube. While some applications may still require the GTAW process, advanced wire processes can provide similar quality and much better productivity in many stainless applications.
Using a modified short circuit GMAW process such as Miller Regulated Metal Deposition (RMD) for the root pass eliminates back purging in some austenitic stainless steel applications. The RMD root pass can be followed by pulse or flux-cored GMAW fill and cap welding passes, a change that saves time and money compared to using GTAW with purge rear, especially on larger pipes.
RMD uses precisely controlled short-circuited metal transfer that creates a calm and stable arc and weld puddle. This offers less chance of cold overlap or lack of fusion, less spattering and a better root passage on the pipe. Precisely controlled metal transfer also allows for uniform droplet deposition and aids in control of the puddle and therefore heat input and welding speeds.
An unconventional process can increase the productivity of welding. When using the RMD, the weld speed can be 6 to 12 in / min. Because the process allows for increased productivity without putting additional heat into the room, it helps maintain the properties and corrosion resistance of stainless steel. The reduced heat input of the process also helps control the deformation of the base material.
This pulsed GMAW process provides a shorter arc length, a narrower arc cone and less heat input compared to traditional spray pulse transfer. Since the process is in a closed loop, arc slippage and variations in tip-to-work distances are virtually eliminated. This allows easier control of puddles for in-position and out-position welding. Finally, the pulsed GMAW coupling for the fill and close passes with RMD for the root pass enables welding procedures with wire and gas, eliminating process change time.