Stainless steel is not necessarily difficult to work with, but welding it requires careful attention to detail.It doesn’t dissipate heat like mild steel or aluminum, and it may lose some corrosion resistance if you put too much heat into it.Best practices help maintain its corrosion resistance.Image: Miller Electric
The corrosion resistance of stainless steel makes it an attractive choice for many critical tubing 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 input and using the wrong filler metal are two culprits.
Following some best practices for stainless steel welding can help improve results and ensure the metal retains its corrosion resistance.Additionally, upgrading the welding process can bring productivity benefits without compromising quality.
In stainless steel welding, filler metal selection is critical to controlling carbon content.Filler metals used for stainless steel pipe welding should enhance weld performance and meet application requirements.
Look for filler metals with an “L” designation, such as ER308L, as they provide a lower maximum carbon content that helps maintain the corrosion resistance of low-carbon stainless steel alloys.Welding a low carbon base metal with standard filler metals increases the carbon content of the welded joint, increasing the risk of corrosion.Avoid filler metals marked with an “H” as these provide higher carbon content and are designed for applications requiring higher strength at elevated temperatures.
When welding stainless steel, it is also important to choose a filler metal with low trace levels (also known as impurities) of 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.
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.Due to gaps between parts or uneven fit, the torch must stay in one location longer and more filler metal is required to fill those gaps.This can cause heat to build up in the affected area, which can overheat the part.Poor fit can also make it more difficult to bridge the gap and obtain the necessary weld penetration.Take care to ensure that the parts fit into the stainless steel as close to perfect as possible.
The cleanliness of this material is also very important.Very small amounts of contamination 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 stainless steel special brush that has not been used on carbon steel or aluminum.
In stainless steel, sensitization is the main cause of loss of corrosion resistance.This can happen when the welding temperature and cooling rate fluctuate too much, changing the microstructure of the material.
This OD weld on stainless steel pipe, welded using GMAW and regulated metal deposition (RMD) without backflushing of the root pass, is similar in appearance and quality to welds made with backflushed GTAW.
A key part 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 bind the chromium and prevent the formation of the desired chromium oxide, which gives stainless steel 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 control of heat input.As mentioned earlier, it is important to choose a low carbon filler metal for stainless steel welding.However, carbon is sometimes required to provide strength for certain applications.Heat control is especially important when low carbon filler metals are not an option.
Minimize the amount of time the weld and heat-affected zone remain at elevated temperatures—typically considered 950 to 1,500 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 in the application soldering procedure.
Another option is to use filler metals designed with alloying components such as titanium and niobium to prevent chromium carbide formation.Because these components also affect strength and toughness, these filler metals cannot be used in all applications.
Gas tungsten arc welding (GTAW) for the root pass is the traditional method of welding stainless steel pipe.This usually requires backflushing of argon to help prevent oxidation on the backside of the weld.However, the use of wire welding processes in stainless steel tubing is becoming more and more common.In these applications, it is important to understand how the various shielding gases affect the corrosion resistance of the material.
When welding stainless steel using the gas metal arc welding (GMAW) process, argon and carbon dioxide, a mixture of argon and oxygen, or a three-gas mixture (helium, argon, and carbon dioxide) are traditionally used.Typically, these mixtures contain mostly argon or helium and less than 5% carbon dioxide, as carbon dioxide provides carbon to the weld pool and increases the risk of sensitization.Pure argon is not recommended for GMAW on stainless steel.
Flux-cored wire for stainless steel is designed to run with a traditional mixture of 75% argon and 25% carbon dioxide.Flux contains ingredients designed to prevent carbon from the shielding gas from contaminating the weld.
As GMAW processes have evolved, they have simplified the welding of stainless steel tubes and pipes.While some applications may still require GTAW processes, 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 corresponding OD welds.
The root pass using a modified short-circuit GMAW process such as Miller’s Regulated Metal Deposition (RMD) eliminates backflushing in some austenitic stainless steel applications.The RMD root pass can be followed by pulsed GMAW or flux-cored arc welding fill and cap passes—a change that saves time and money compared to using GTAW with back-purging, especially on larger pipes​​​ .
RMD uses precisely controlled short-circuit metal transfer to produce a calm, stable arc and weld puddle.This provides less chance of cold laps or lack of fusion, less spatter and a higher quality pipe root pass.Precisely controlled metal transfer also provides uniform droplet deposition and easier control of the weld pool and therefore heat input and welding speed.
Unconventional processes can increase welding productivity.When using an RMD, the welding speed can be 6 to 12 in./min.Because the process increases productivity without additional heating of parts, it helps maintain the properties and corrosion resistance of stainless steel.The reduced heat input of the process also helps control deformation of the substrate.
This pulsed GMAW process provides shorter arc lengths, narrower arc cones and less heat input than conventional spray pulse transfer.Since the process is closed-loop, arc drift and tip-to-workpiece distance variations are virtually eliminated.This provides easier puddle control for in-place and out-of-place welding.Finally, coupling pulsed GMAW for fill and cap bead with RMD for root bead allows the welding procedure to be performed using one wire and one gas, eliminating process changeover times.
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