Can You MIG Weld Mild Steel to Stainless Steel

Can you MIG weld mild steel to stainless steel? The short answer is, in most cases, yes and an ER309L filler metal is typically used. However, understanding the nature of stainless steel and MIG is helpful to best tackle this dissimilar-metal joining application. In this article we will discuss how this process is possible and what its applications are. 

Why Join Mild Steel to Stainless Steel

Mild steel, as used in this article, refers to a wide range of steel grades/compositions having a relatively low overall alloy composition. On the other hand, stainless steel has a chromium content above 11% so that the surface of the steel forms a protective layer of chromium oxide. The chromium oxide layer provides enhanced corrosion resistance compared to mild steel in many applications. Despite this benefit, there are many applications where you might mix mild steel and stainless steel. 

Cost is often the driving force behind the dissimilar joining: stainless steel is significantly more expensive than mild steel. Combining mild steel and carbon steel is one way to control the cost of a component while still ensuring that corrosion resistance is available in key areas. However, dissimilar-metal welding can sometimes be used to better allow components to carry required stresses, and in some cases, do so at a minimal component weight.

An Introduction to MIG

MIG is an acronym that represents Metal Inert Gas welding. MIG is one of many arc welding processes—processes that utilize an electric arc to melt the base metal and filler metal. A defining feature of the process is that it is a “wire-fed process” meaning that a continuously fed wire is used to maintain the electrical arc and provide filler metal into the weld joint. As the acronym suggests, MIG is also defined by the use of a shielding gas having generally inert characteristics that displaces the atmosphere from the weld zone to protect against detrimental reactions.

MIG is a very common process for welding mild steel, welding stainless steel, and welding these two metals to each other. MIG can be used with a range of shielding gasses and wire diameters to fine-tune performance for a wide range of material thicknesses. Because there is no slag, deposition efficiency (the ratio of filler metal consumed versus completed weld weight) is quite high. Because there is limited need for stop/starts and post-weld cleanup, operator factor (time spent welding versus total project time) can be much higher than other processes. These factors combine with the ability to achieve high travel speeds to produce a productive process. If you don’t currently own the equipment needed for MIG welding, welder rentals can be a way for you to become familiar with the process or tackle short-run production with limited investment.

How to Weld Mild Steel to Stainless Steel

Typically, ER309L filler metals are used to join mild steel to stainless steel. ER309L is a filler metal classification that designates:

• That the filler metal can be used as an electrode for MIG or as a rod for TIG
• That the filler metal has a 309 nominal alloy composition
• That the filler metal is a low-carbon variant of the 309 nominal composition

ER309L is an austenitic stainless steel that is high in both chromium and nickel. The presence and quantity of nickel in this alloy helps to form a ductile weld microstructure. ER308/308L is a popular choice for joining 304/304L stainless steel to itself, but attempting to use this alloy for steel to stainless steel instead of ER309L may result in a crack-susceptible microstructure.

Shielding gas selection can influence the ease of welding and the quality of the results. Typically high-argon shielding gasses are used. 98% Argon/2% Oxygen or 98% Argon/2% Carbon Dioxide (CO2) are used for welding thicker materials, since these gasses help to achieve a smooth, stable spray transfer with minimal chemical interaction. Three-component gas mixtures, known as “tri-mixes”, typically consist primarily of helium with varying additions of argon and carbon dioxide. While not required for thin materials, they can offer improved performance when welding using the “short-circuit” mode of transfer commonly employed to help prevent burn-through or weld out of position.

Selecting parameters for steel to stainless steel weld joints is very similar to selecting parameters for welding stainless. The stainless filler metal requires a lower current to melt-off than mild steel filler metal, so expect to utilize lower wire feed speeds than you may be used to. Likewise, the weld pool will be more “sluggish” than when welding using mild steel filler metal, and penetration will be reduced. This means that you may need to use a wider included/bevel angle depending on the application to ensure good root and sidewall fusion. Try to avoid excessive heat input to minimize the risk of sensitization of the stainless steel base metal that can negatively affect corrosion resistance of that base metal.

Be aware that the finished weld is a mixture of alloys; the corrosion resistance of the weld metal will not be equivalent to the stainless steel base metal, and it may be important to locate these dissimilar weld joints away from sources of corrosion in some situations. Also be aware that not all stainless alloys are created equal. Stainless steel can be austenitic, martensitic, or ferritic which provides insight into their microstructure and typical compositions. Austenitic stainless steels—one of the most common types by tonnage—are generally easy to weld, while martensitic and ferritic stainless steels can be more of a challenge.

Next Steps

Contact us today to find the MIG welder that is best suited for the stainless to mild steel welding that you are looking to perform. “Sizing” a machine to your application can help you to get the feature set you need without unneeded expense. Our knowledgeable team can also provide guidance into the world of stainless steel and dissimilar-metal welding to help you select the best consumables, accessories, and knowledge.