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.
Related post:
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 |
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.
1. Use 309L Filler Metal
Whether you are using MIG or TIG, 309L (like ER309L) is specifically alloyed to join mild (carbon) steel to austenitic stainless steels like 304 or 316.
Why not 308L? 308L is great for welding stainless-to-stainless, but if you use it to tie into mild steel, the carbon from the mild steel dilutes the weld puddle. This results in a brittle, crack-susceptible microstructure.
Why the "L"? The "L" stands for Extra Low Carbon. Keeping the carbon content under 0.03% helps prevent carbide precipitation—a process that robs the stainless steel of its corrosion resistance when heated.
2. Manage the Heat Input
Stainless steel and mild steel handle heat very differently. Stainless steel holds onto heat much longer (lower thermal conductivity) and expands significantly more when heated (higher thermal expansion).
This mismatch means the stainless side of your joint is going to warp and distort much faster than the mild steel side.
The fix: Keep your overall heat input as low as possible. Use techniques like back-stepping or stitch welding, allow the piece to cool between passes, and use thick aluminum or copper chill bars clamped to the stainless side to draw the heat out.
3. Account for Corrosion
While the weld bead itself (being 309L stainless) will resist rust, the mild steel side will not. Furthermore, joining these two metals can sometimes accelerate galvanic corrosion on the mild steel side if the joint is exposed to an electrolyte like rainwater.
The fix: Once the weld is cool and cleaned, you must protect the mild steel and the heat-affected zone. Apply a high-quality paint or specialized coating to seal the mild steel away from oxygen and moisture.