How to weld mild steel to stainless steel?

Welding mild steel to stainless steel can be a challenge for new welders who are unsure of their ability to create a high-quality joint between these two dissimilar metals. The properties and compositions of these metals are different, making the welding process more difficult than welding pure stainless steel. In this article, we will discuss the challenges of welding mild steel to stainless steel and provide some tips to ensure a successful weld.

Note: Mild steel (iron containing a small percentage of carbon, strong and tough but not readily tempered), also known as plain-carbon steel and low-carbon steel, is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications.

One of the most critical aspects of welding mild steel to stainless steel is to avoid over-welding and deep penetration. This is because these metals have different melting points and thermal expansion rates, which can cause cracking, distortion, and brittleness in the joint. To prevent these issues, it's important to focus the arc more on the stainless steel portion and maintain low heat.

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Reasons You Want to Avoid Welding Stainless Steel to Carbon Steel

Maintaining low heat is essential to prevent deep penetration, preserve corrosion resistance, and prevent carbon contamination. To achieve this, you can set the lowest amperage that will melt the filler metal, travel at a fast speed, use stringer beads instead of weaving, use chill bars under or on the metals, and create a symmetrical joint that requires the least amount of weld metal. Despite the uneven angle, pointing the arc towards the stainless steel portion will result in a symmetrical bead with good toe fusion.

Differences between mild and austenitic steel for welding

Property	Mild steel	Austenitic (304) steel
Thermal expansion	65	100
Thermal conductivity	100	33
Electrical resistance	12.5	72
High-temperature strength	900 °F (480 °C)	1300 °F (700 °C )
Tensile strength	60-70ksi	85ksi
Ductility	25	55
Melting point	2800 °F (1540 °C)	2600 °F (1425 °C)
Galvanic corrosion	High	Low

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 Another challenge in welding mild steel to stainless steel is avoiding slag inclusions, which can occur when the welding process produces slag on the bead. To prevent this, the customized angle becomes even more crucial. It's also a good idea to test on similar scrap metals before welding your main project.

Contamination of the stainless steel portion with iron particles is another critical consideration in welding mild steel to stainless steel. Iron particles can cause rusting of the stainless steel portion, leading to corrosion over time. To avoid this, it's essential to use a separate set of tools to clean and bevel the stainless steel portion and avoid scratching it on any carbon steel surfaces, such as the steel portion or the welding table.

In conclusion, welding mild steel to stainless steel can be a challenging task for new welders. It requires a proper understanding of the properties and compositions of these metals and specialized techniques to ensure a successful weld. By following the tips outlined in this article, you can minimize the risks and create a strong, durable joint between these two dissimilar metals.