Stainless steel is a type of metal that is somewhat paradoxical in nature. It is exceptionally corrosion and abrasion-resistant, making it ideal for high-temperature applications. However, it also has a significantly slimmer margin of error and is less forgiving of mistakes made in welding operations. Because of this duality, it’s important to learn about the distinctive characteristics of stainless steel and the techniques required to achieve high-quality welds with this unique material.
Jump to a section: What is Stainless Steel | Welding Applications | Stainless Steel Grades | Challenges | Welding Methods | Preparation | Safety | Step-by-Step Guide | Common Defects | FAQ
What is Stainless Steel?
Stainless steel is essentially a low-carbon steel alloy that acquires its distinctive corrosion-resistant qualities through the incorporation of chromium and nickel.
Stainless Steel Welding Applications
Because of its versatility, stainless steel can be used in a wide array of industrial applications. In food and beverage processing facilities, stainless steel is used for pipes and vessels because of the purity of the metal. Additionally, its resistance to corrosion and abrasion makes it a popular choice in pharmaceutical plants, chemical plants, oil and gas facilities, and marine environments.
Stainless Steel Grades for Welding
Austenitic
Austenitic stainless steel is the most widely used grade of stainless steel, known for its excellent corrosion resistance, high ductility, and good weldability. It’s commonly used in kitchen appliances, food processing equipment, and architectural applications
Ferritic
Ferritic stainless steel is a non-hardenable alloy with elevated chromium content, which allows for excellent corrosion resistance. This alloy is also highly malleable and is frequently used in appliances, automotive trim, and decorative applications.
Martensitic
Martensitic stainless steel offers moderate corrosion resistance, and the ability to be hardened through heat treatment. It’s commonly used in the manufacturing of valves, pumps, and knives.
Precipitation-Hardening
Precipitation-hardening stainless steel is a martensitic stainless steel that can be hardened by heat treating and the precipitation of additional alloying elements. It is frequently found in aerospace applications, nuclear reactors, and the production of turbine blades.
Duplex
Duplex stainless steels were specifically formulated to combine the advantages of austenitic and ferritic microstructures. This unique blend results in high corrosion resistance, exceptional strength, and good weldability.
Other grades of stainless steel
Recent innovations in stainless steel have introduced “Lean Duplex Stainless Steels,” which feature reduced nickel content compared to traditional duplex stainless steel. The result is a more economical material with elevated strength and robust corrosion resistance, making it well-suited for applications such as marine usage and storage tanks.
Nitronic stainless steels are austenitic stainless steel variants infused with nitrogen, which results in exceptional strength and corrosion resistance. These alloys are versatile and are often used in marine environments, aerospace components, as well as wear-resistant applications.
Challenges in Stainless Steel Welding
Distinct Features of Stainless Steel Affecting Weldability
Because of its nature, stainless steel has unique attributes that can pose challenges to welders. Primarily, stainless steel is heat-sensitive and holds heat more effectively than other metals. This can result in localized heating during welding procedures, resulting in distortion, warping and even cracking if heat input is not properly managed and addressed.
Common Challenges
Stainless steel, owing to its chromium content, can form a thin layer of chromium oxide in the weld area during welding. This is due to the elevated temperatures in welding and can impair the mechanical properties and corrosion resistance of the material. To avoid these issues, welders must take great care to maintain proper shielding and workmanship practices, including a meticulous pre-weld cleaning of the base metal and inter-pass cleaning (usually performed using a handheld stainless steel brush) to minimize the formation of oxide layers.
Another obstacle you may encounter when welding stainless steel is carbide precipitation. Carbide precipitation occurs when chromium, the primary defining characteristic of stainless steel, is effectively baked out of the material due to excessive heat and atmospheric contaminants in the weld zone. To avoid this, it is often necessary to employ purging techniques, especially when welding open root joints or in situations where excessive penetration into the weld joint could lead to carbide precipitation on the rear side of the weld.
Stainless steel is sensitive to contamination from a variety of sources, such as dirt, grease, oils, and even carbon steel particles which can lead to weld defects like porosity and incomplete fusion. To maintain the integrity of the weld, keep the base metal clean and clean regularly between weld passes.
Types of Welding Methods for Stainless Steel
TIG
TIG welding, also known as Gas Tungsten Arc welding, is a commonly utilized method in the welding of stainless-steel process pipes. It’s a popular method because of its greater degree of operator control, portability, and the ability of the process to ensure clean welds, largely free from defects when properly performed. A good TIG root pass in stainless-steel process piping should be clean, smooth, and produce no slag. This results in minimal obstruction or places where products and fluids may become trapped and hold potential contaminants.
MIG
MIG welding, or Gas Metal Arc Welding, is often used in stainless-steel welding involving thicker sections of metal. This method is ideal for tasks demanding higher deposition rates, such as the welding of structural stainless components or the fabrication of large tanks and vessels.
SMAW
SMAW, or “stick” welding, is often the preferred welding process for many structural applications performed outdoors where accessibility is limited or the weather may interfere with the shielding gas used in the TIG or MIG welding processes.
Preparing Stainless Steel for Welding
Importance of Pre-Weld Cleaning
To ensure quality welds, cleaning the materials is an important step in any welding process. Contaminants such as paints, dirt, grease, or oils left on the surface at or near the weld area can lead to weld defects like porosity and incomplete fusion.
Techniques for Effective Pre-Welding Cleaning
There are several methods commonly employed to clean stainless steel. A wire brush on a grinder or drill can make fast work out of the cleaning process. Be sure to use a stainless-steel brush to prevent contamination.
Sanding pads or Tiger discs on a grinder are an effective way to clean off surface contaminants. They can also aid in shaping and refining the part, enabling the creation of features like beveled edges for precise fitting and welding.
Edge Prep
To prepare the edges of the stainless-steel parts to be welded, it may be necessary to put a bevel on the plate or pipe pieces being joined. While using a conventional grinding wheel is acceptable, it’s important the wheel is new and has not been previously used on carbon steel to avoid contamination. The same rule applies to sanding discs and other abrasives. It is common practice to keep grinding wheels and abrasives specifically designated for stainless-steel separate from those used for mild steel to eliminate the risk of cross-contamination.
Safety Guidelines For Welding Stainless Steel
Personal Protective Equipment
As with any other form of welding, great care should be taken to ensure safety from the potentially harmful effects of UV rays, sparks, and fumes emitted from the welding process. Always use a welding hood with the appropriate shade lens. While a number 10 shade lens is generally adequate for most stainless-steel applications, you may want to consider shade 11 or even a shade 12, particularly in situations involving high amperage and voltage, such as in certain flux-core arc welding processes.
Stainless-steel welding, like any other welding process, can produce toxic fumes. Don’t assume that because stainless is comparatively clean, it doesn’t give off fumes. It can, and does, so be sure to wear a mask capable of filtering particulates or a respirator and work in a well-ventilated area.
Workspace Safety Considerations
For shops and facilities that weld large volumes of stainless steel regularly, fume extraction fans are a good investment. Affordable and highly portable, these devices can be used to pull toxic fumes and particles away from the welder's work area and greatly reduce the risk of long-term health complications involved with extended exposure to welding fumes.
For work in plants and other facilities, it may be more practical for the welders to wear PAPR (powered air purifying respirators) to help protect the welder from fumes generated by the welding process.
Welding Stainless Steel Step-by-Step Guide
Preparation Process
Before the commencement of welding operations, you should first choose the appropriate filler metal for the type and grade of stainless steel you want to weld. Next, be sure to clean the pieces thoroughly to avoid drawing contaminants into the weld area.
You should also consider which welding process should be selected to perform the necessary welds. If you are welding process pipes requiring high-purity welds, TIG welding is a good choice. For larger vessels, tanks, and thicker structural stainless-steel applications, the higher deposition rates in MIG or FCAW welding could be more advantageous. For outdoor applications, where wind may be present, SMAW welding may be the way to go.
Welding Process
As you begin the actual welding process, position the parts to be tacked up and welded using clamps or fixtures to hold them firmly in place. Apply as many tack welds as necessary to hold the parts in place. Adjust the welding machine to the proper parameters and begin the welding operations. Take care to weld with the appropriate amount of heat and add filler metal, as necessary, to ensure the proper weld.
To prevent distortion and warping due to excess heat, try to shift your welding location when possible to help distribute and dissipate heat. Additionally, if you are welding multiple passes, be sure not to exceed maximum interpass temperatures. Given that stainless can hold heat for longer, it may be advisable to stop and wait for a brief period to allow the material to cool a bit before welding the next pass.
Post-Weld Process
After you have completed welding, you should inspect the completed weld, looking for any cracks, fusion voids, or indications of porosity and repair as necessary. Once satisfied with the completed weld, you can go ahead and clean it. Use a wire brush, either hand-held or on a drill or grinder to remove slag, spatter, or any potential residue to achieve a good, clean, uniform surface on the finished weldment.
Common Weld Defects in Stainless Steel and How to Avoid Them
Carbide Precipitation
Carbide precipitation, also known as “sugaring,” is a common defect found in welding stainless steel. Carbide precipitation usually occurs due to excessive heat and the absence of purging. In essence, the high heat causes the essential chromium component of stainless steel to be depleted, leading to a severe reduction in corrosion resistance properties at the affected site. Typically, this defect occurs on the reverse side of the weld joint or internally in stainless-steel process piping.
To address “sugaring,” it is important to ensure a proper and tight fit in common filet weld joints. In open-root butt-welded joints, particularly in stainless process piping, using an argon purge is a common method to shield the weld area from atmospheric contaminants and prevent carbide precipitation. If you notice the backside of the area turning dark gray or black, it indicates a need to either reduce the heat or employ a purging technique to mitigate the issue.
Incomplete fusion
Incomplete fusion is another common problem that may arise while welding stainless steel. It occurs when the weld does not adequately fuse into the base metal or other welding passes. To prevent incomplete fusion from occurring, first ensure the joint has been prepared properly, which includes cleaning the pieces to be welded and beveling. Next, be sure to employ the proper welding technique and make sure the welding parameters are right for the thicknesses of the material to be welded. Always maintain the proper arc length and travel speed.
Porosity
Porosity can occur in stainless-steel welding on both the surface and between weld passes below the outer surface of the weld. This can weaken the integrity of the completed weld and even compromise the corrosion resistance of the stainless steel.
To prevent porosity from happening, make sure the pieces to be welded are free of contaminants and impurities. When TIG welding, you can clean the exposed filler wires with a clean cloth and an appropriate cleaning solution, like acetone. Check to make sure the shielding gas (if applicable) is flowing at the right rate and that the TIG torch or MIG/ FCAW gun is in proper working order and free of obstructions that will inhibit the flow of the shielding gas. Lastly, make sure there is no wind or excessive draft in the welding area that could blow away the shielding gas, which is essential to protect the molten weld from atmospheric contaminants.
Distortion
Distortion or warping occurs when the heat input from the welding operation is substantial enough to deform the base metal of the welded parts. To minimize distortion, be sure to use a sufficient amount of tack welds to hold the sections to be welded firmly in place.
Well-planned welding sequences may be necessary to minimize heat concentration in a particular area. Employing a strategy like “skip-around” welding, where you periodically shift to different sections, can help maintain uniform heat distribution and reduce distortion. Another useful technique is “backstep” welding, which involves welding for a set period, then skipping a distance of roughly the same length, and resuming the process in a similar pattern until the welding is complete. This method effectively dissipates heat input in the base metal.
FAQ
Can You Weld Stainless Steel?
Welding stainless steel is entirely possible. The key to success is selecting the right welding process, filler metal, and stainless-steel welding electrode. Many welding consumable manufacturers have guides that can help recommend the correct filler metals, electrodes, and even processes for the type of stainless steel you need to weld in your specific application.
Is Welding Stainless Steel Difficult?
Welding stainless steel can be arguably more difficult than welding on carbon steel due to the smaller margin of error. Stainless steel contains chromium that can be compromised if exposed to excessive heat, which will jeopardize the stainless steel’s defining corrosion-resistant properties.
What Kind of Welder is Best for Stainless Steel?
The best welder for stainless steel can depend greatly on many factors. However, you will want to use a machine capable of holding a smooth, steady, and consistent arc capable of maintaining the required amperage and voltage outputs necessary for the chosen welding process for the required amount of time.
Can You Write on Stainless Steel With a Sharpie or Common Paint Pen?
If you need to write on stainless steel, it is highly recommended to use a specifically made low-chloride paint pen, available at any welding supply distributor. Common pens and permanent markers may contain chlorides and sulfur, which will likely result in contamination and a loss in the corrosion-resistant properties of stainless steel.
Can You Weld Stainless Steel to Mild Steel or Carbon Steel?
Welding stainless steel to another metal is not only possible, but often necessary. Stainless steel can be welded to mild steel very effectively, resulting in strong, quality welds. The welding electrode and filler metal most commonly used in welding stainless to mild steel is E309, although E308 can work as well.
Another technique to consider is “buttering” the weld joint before joining the two dissimilar metals. Buttering is performed by welding a pass on both the stainless part and the mild steel part before welding the two dissimilar metals together. This technique may help provide a buffer zone between the two dissimilar metals and minimize potential issues that could arise due to the metallurgical differences that exist between stainless and mild steel.
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