Welding stainless steel can be a challenge at first as it’s a bit trickier of a material than its close cousin, carbon steel. Learning a whole new process for welding the metal might seem ominous, but it is a skill well-worth learning. As the metal continues to increase in popularity due to its strength and corrosion resistance, knowing how to weld stainless is quickly becoming a cornerstone in the fabrication industry.
Within the stainless-steel category, you have different families that all exhibit unique properties. Materials from each family must be treated differently according to chemical composition, so it’s essential to have a good grasp on what you’re working with to ensure an effective weld. Stainless steel also has insulating properties that make it difficult for heat to diffuse during welding. This uneven distribution causes pockets of concentrated heat, an issue that can lead to warping and burn-through. Hydrogen cracking is also a big concern when welding stainless steel.
Stainless steel isn’t as cost effective a material as carbon steel. What might be a simple mistake on carbon steel could make for a costly lesson on stainless steel, so it’s important to know what you’re doing before you get started.
Advantages of Stainless Steel Over Carbon / Mild Steel
The main difference between these two metals has to do with corrosion resistance. When exposed to air and moisture, carbon steel reacts to create iron oxide. The formation of this active chemical layer serves as a catalyst for further corrosion. Stainless steel, on the other hand, doesn’t pose this risk. The high level of chromium (+11.5%) in stainless steel makes it far more resistant to corrosion and rust than carbon steel. Chromium also reacts to exposure to air and moisture; however, the resulting chromium oxide layer isn’t active like iron oxide. This passive chemical layer protects the stainless steel from degrading further.
As its name suggests, carbon steel has a significantly higher level of carbon than stainless steel. The resulting material exhibits many different qualities, one of which is the ductility of the material. Carbon steel is much more malleable that stainless steel, making it the better choice for projects requiring bending or warping. On the other hand, stainless steel exhibits higher electrical resistance properties than carbon steel. This lower thermal conductivity makes stainless steel a much better insulator.
While there are similarities between the two materials, each metal does pose unique benefits well-suited to different fabrication applications. Choosing which material to work with will heavily rely on the properties you are looking to bring to a specific project.
When to Choose Carbon Steel vs. Stainless Steel
Choose carbon / mild steel over stainless steel if:
- The volume of material for the project needed is high (carbon steel is much more cost effective than stainless)
- You are planning on covering the surface of the material with paint, powder coating, sealant, etc.
- The project requires the material to be bent
Choose stainless steel over carbon / mild steel if:
- Corrosion protection is necessary
- The finished product will be exposed to extreme temperatures
- Resistance to bacterial growth or contamination is a factor (e.g. food service materials)
Types of Stainless Steel
There are five unique alloy families of stainless steel, each with its own individual set of properties.
Austenitic, martensitic, and ferritic are the most commonly used alloys. Duplex and precipitation hardening are more typically used for aerospace, process, and other high-performance industry applications.
This 200 & 300 series alloy family contains high amounts of chromium and nickel and is the most popular choice for welding stainless steel. More specifically, 304 is the most used alloy in modern fabrication. Although there is no matching filler metal for 304, most fabricators choose 308L because it works well with the base.
While it isn’t necessary to preheat the base material prior to welding, austenitic stainless does have a strict 300-degree F (150 C) maximum interpass temperature. Staying underneath this temperature minimizes the metal’s risk of weakening and cracking.
This 400 series alloy family is the most difficult of the stainless steel categories to weld as its high carbon content raises the risk of cracking. Proper preheating, as well as ensuring the metal stays above the minimum interpass temperature, will minimize the risk of cracking. Heat treatment hardens martensitic stainless, so it’s necessary to use proper heating techniques to keep the steel from cooling too quickly and becoming brittle.
These properties make martensitic stainless the perfect metal for overlays and resurfacing. However, the same characteristics make it uncommon to see the material used for joints. 401 is the standard filler metal of choice for martensitic stainless steel.
400 series ferritic alloys are largely used in the automotive industry because of their low carbon content and non-hardenable properties. This is the cheapest of all the stainless-steel options and generally comes in thin sheets (¼ in or less). Because of the material’s typical thinness, single pass welding is common to ensure low heat application. Staying under the 300-degree F maximum interpass temperature will prevent the steel from becoming grainy and brittle. Ferritic stainless steel is also the only of the alloy families to be naturally magnetic.
Duplex stainless steel gets its name from its possession of both austenitic and ferritic qualities. Although duplex stainless contains both chromium and nickel, just like austenitic, the proportions are much different. Duplex stainless is a much stronger material and far more resistant to corrosion when exposed to chlorides than both austenitic and ferritic stainless steels. This complex composition can cause trouble during welding as the material is easily ruined by too much heat. As with ferritic stainless, low heat application creates the best result. Too much heat too quickly can compromise the structure and strength of the metal.
The last of the stainless-steel alloy families is the 600 series precipitation-hardening stainless. As with duplex stainless, precipitation-hardening stainless steel isn’t commonly used. Precipitation-hardening stainless grows stronger with heat treatment, like martensitic, and has the corrosion resistance of austenitic stainless. This combination of properties makes the metal easily machinable and well-suited for high-performance applications.
What to Watch Out For When Welding Stainless
Hydrogen embrittlement of stainless steel—the absorption and bonding of hydrogen to stainless steel’s carbon content—can take place in two ways. The first is through hydrogen absorption at room temperature, resulting in transgranular cracks in the stainless steel. Absorption can also occur in a high temperature environment, creating intergranular cracks. The higher the chromium content in the stainless steel, the better contained the hydrogen will be. This absorption prevention safeguards the metal against cracking. Heat treating the stainless steel properly will also help prevent hydrogen cracking when welding.
The two main differences between welding stainless steel and carbon / mild steel are technique for heat application and filler metals.
As mentioned above, stainless steel’s physical and mechanical properties make it very temperature sensitive. Consequently, if you don’t stay above a minimum interpass temperature the material won’t properly fuse. On the other hand, if you overshoot the maximum interpass temperature you run the risk of cracking, warping, and burn-through.
Because stainless steel is a better insulator than carbon steel, the direct application of heat is a bit trickier to get right. Instead of distributing evenly, heat can become trapped in the weld pool. This risk is compounded further with thinner sheets of stainless steel. Uneven heat distribution increases the material’s risk of oxidation, warping, or burning.
Paying close attention to minimum and maximum interpass temperatures, as well as starting with low heat, can help prevent these defects. Increase the temperature slowly as needed to ensure proper fusion. Jumping into the weld with high heat has the potential to ruin the material.
Two simple ways to prevent warping when welding stainless steel are:
- Tack Welding: Start with tack welds to outline your pass before filling it in. This low application of heat ensures the material stays underneath the maximum interpass temperature. Additionally, tack welding will help you align your material more precisely to further combat the risk of warping.
- Intermittent Welding: Maintaining direct application of heat increases your risk of overheating the material and ending up with defects. Practicing intermittent welds followed by cooling intervals will help keep the metal below max interpass temperature.
The use of temperature gauges is highly recommended when working with stainless steel and can make a huge difference in end weld result. A surface temperature probe is the best tool for the job as it allows for the continuous assessment of the material’s temperature during welding. Some electronic probes also offer data readouts that can help you visualize the process and better learn how to work with the material. Temperature-indicating sticks and infrared thermometers are other great options to consider, although both have their limitations and learning curves.
Utilizing specific machines and techniques can help minimize the defect risks associated with welding stainless steel. 304 is the most commonly used stainless steel, so most welding processes for stainless are based around this alloy. Different alloy families may require different machines or processes in order to achieve the most successful weld possible.
Shielding Metal Arc Welding (SMAW):
SMAW, or stick welding, is likely the best place to start for welders working with stainless steel for the first time. The machine’s portability makes it a popular tool for stainless due to the nature of the projects it is generally chosen for. SMAW also offers good weld strength and high cracking resistance. This method is the easiest way to deal with the repair work and maintenance heavily associated with stainless steel projects.
Gas Metal AC Welding (GMAW):
GMAW, or MIG welding, provides a level of versatility that can make working with stainless steel projects much more accessible. Additionally, the use of a shielding gas stabilizes the arc, allowing for a more consistent application of heat. The resulting weld is stronger and more aesthetically pleasing. Stainless steel requires different shielding gasses than carbon steel, the most common of which is a gas tri-mix of helium, argon, and CO2. A dual mix gas of argon and either oxygen or CO2 also works well. Be sure to pick the mixture that is right for the material and project you are working on.
Submerged Arc Welding (SAW):
SAW is most effective when welding thick materials, making it a good choice for bulkier stainless steel projects. However, the limitations this technique presents make it more of an alternative method than an effective rule of thumb.
Gas Tungsten Arc Welding (GTAW):
The low heat application characteristic of GTAW (also known as TIG welding) makes it well suited for working with thin sheets of stainless steel. GTAW typically produces the cleanest and most precise welds of all the processes listed. While SMAW may be a great place to start learning to weld stainless steel, GTAW is the best option for creating strong, neat welds on a stainless project. When welding GTAW, you will want to use 100% argon—the tri-mix gases that are a great fit for GMAW will not work with GTAW.
It is imperative that tools used for stainless steel not be used for carbon steel. Even minor impurities from the carbon steel will contaminate the stainless, causing it to rust. Sharing tools between the two metals will ruin perhaps the most significant reason for choosing stainless steel in the first place. This applies to all tools: hammers, brushes, grinding discs, etc. Stainless steel is so sensitive to this kind of contamination that it’s recommended you don’t even grind carbon steel in the presence of stainless. Planning your work station and materials accordingly will safeguard your project and prevent some of the costly errors associated with welding stainless steel.
Welding Stainless Steel Like a Pro
Strength, corrosion resistance, and insulating properties make stainless steel an effective material for fabrication spanning various industries. While it may not be as straightforward to work with as carbon / mild steel, stainless steel has a lot of unique physical and mechanical properties to offer. Learning to effectively weld stainless is a skill that will definitely come in handy, especially as the material grows in popularity. With a little bit of research and careful practice, you’ll be welding stainless steel like a pro in no time.