Welding Stainless Steel to Mild Steel [Solved] With 9 Tips

Welding stainless steel to mild steel is a common requirement in many DIY projects, as it combines the corrosion resistance of stainless steel with the strength and cost-effectiveness of mild steel. However, welding these dissimilar metals presents some challenges due to differences in their thermal expansion, melting points, and chemical composition.

In this comprehensive guide, we will delve into the techniques and considerations necessary to achieve a strong, corrosion-resistant joint when welding stainless steel to mild steel.

Challenges for Welding Stainless Steel to Mild Steel

Before we discuss the welding process, it’s crucial to understand the key challenges associated with welding stainless steel to mild steel:

Thermal Expansion Differences:

• Stainless steel has a thermal expansion coefficient of 17.3 μm/m-K, while mild steel has a thermal expansion coefficient of 12.0 μm/m-K.
• This means that for every 1°C rise in temperature, stainless steel will expand 17.3 micrometers per meter, while mild steel will expand only 12.0 micrometers per meter.
• To combat this, use clamping fixtures and proper joint design to minimize distortion and residual stresses.

Melting Point Variations:

• The melting point of stainless steel (1400-1455°C) is slightly lower than that of mild steel (1425-1540°C).
• To account for this difference, use lower heat input settings and maintain a consistent travel speed to avoid overheating the stainless steel.
• A good starting point is to set the welding current 10-15% lower than what you would use for welding mild steel of the same thickness.

Chemical Composition Disparities:

• Stainless steel contains 10-30% chromium and 8-25% nickel, while mild steel has less than 0.15% carbon and only trace amounts of other alloying elements.
• Welding these dissimilar metals without proper filler metal can result in the formation of brittle intermetallic phases and chromium carbide precipitation, which reduce the corrosion resistance and mechanical properties of the weld.
• Always use a filler metal with a composition that is compatible with both base metals, such as 309L or 312 (discussed in the next section).

Filler Metal Selection

Choosing the right filler metal is crucial for successful welding of stainless steel to mild steel. The recommended filler metals are:

• 309L:
  • Composition: 23-25% Cr, 12-14% Ni, 0.03% max C
  • Tensile Strength: 550-600 MPa
  • Yield Strength: 415-450 MPa
  • Elongation: 35-40%
  • Recommended for welding austenitic stainless steels (e.g., 304, 316) to mild steel
• 312:
  • Composition: 28-32% Cr, 8-10.5% Ni, 0.12% max C
  • Tensile Strength: 620-680 MPa
  • Yield Strength: 480-530 MPa
  • Elongation: 30-35%
  • Recommended for welding martensitic (e.g., 410, 420) or ferritic (e.g., 430, 439) stainless steels to mild steel

Welding Procedure Guidelines

To ensure a successful weld between stainless steel and mild steel, follow these guidelines:

Heat Input and Interpass Temperature Control:

• Calculate heat input using the formula: Heat Input (kJ/mm) = (Voltage (V) × Current (A) × 60) / (Travel Speed (mm/min) × 1000)
• Aim for a heat input range of 0.5-1.5 kJ/mm for thickness up to 6 mm, and 1.0-2.0 kJ/mm for thickness above 6 mm.
• Measure interpass temperature using a thermocouple or infrared thermometer, and ensure it stays below 150°C for austenitic stainless steels and 200°C for martensitic/ferritic stainless steels.
• If necessary, allow the weld to cool naturally or use forced air cooling to maintain the desired interpass temperature.

Joint Preparation:

• For butt joints, use a 60-75° included angle with a 1-2 mm root face and a 2-3 mm root gap.
• For fillet joints, maintain a minimum root opening of 1 mm and a maximum of 3 mm.
• Clean the joint area with a stainless steel wire brush, followed by wiping with acetone or isopropyl alcohol to remove any oil, grease, or dirt.
• Use a grinding disc with a maximum speed of 25 m/s and a feed rate of 50-100 mm/min to avoid overheating and surface contamination.

Welding Technique:

• Use the GTAW (TIG) process with direct current electrode negative (DCEN) polarity for precise heat control and minimal spatter.
• Set the welding current to 50-80 A for 1.6 mm diameter filler wire, and 80-120 A for 2.4 mm diameter filler wire.
• Maintain an arc length of 2-3 mm and a travel speed of 100-150 mm/min.
• Use argon shielding gas with a flow rate of 10-15 L/min for the torch and 5-7 L/min for back purging.
• For root passes, use a gas lens cup with a diameter of 8-12 mm, and for fill passes, use a standard cup with a diameter of 10-16 mm.

Post-Weld Inspection:

• Perform visual inspection under adequate lighting (500-1000 lux) and magnification (3-5X) to detect any surface defects.
• Use liquid penetrant testing (PT) or magnetic particle testing (MT) to detect any subsurface defects.
• For critical applications, perform radiographic testing (RT) or ultrasonic testing (UT) to verify the internal soundness of the weld.
• If required, conduct guided bend tests as per ASME Section IX or ISO 5173 standards to assess the ductility and soundness of the weld.

Weld Properties and Performance

When the appropriate filler metal and welding procedures are used, the resulting weld between stainless steel and mild steel will exhibit the following properties:

Property	Value	Testing Standard
Tensile Strength	550-620 MPa	ASTM E8
Yield Strength	415-480 MPa	ASTM E8
Elongation	30-35%	ASTM E8
Charpy Impact Toughness @ -40°C	27-32 J	ASTM E23
Ferrite Number (FN)	5-15 FN	AWS A4.2

These properties ensure that the welded joint will have adequate strength, ductility, and toughness for most structural applications in corrosive environments.

Conclusion

Welding stainless steel to mild steel requires careful selection of filler metal, precise control of welding parameters, and adherence to proper joint preparation and welding techniques. By following the guidelines outlined in this guide, DIY welders can successfully join these dissimilar metals and achieve welds with excellent mechanical properties and corrosion resistance.

Remember to:

• Use 309L filler metal for austenitic stainless steels and 312 filler metal for martensitic/ferritic stainless steels.
• Control heat input and interpass temperature to avoid distortion and detrimental microstructural changes.
• Prepare the joint properly and maintain cleanliness to ensure good fusion and prevent contamination.
• Use the GTAW process with the recommended welding parameters and shielding gas.
• Perform appropriate non-destructive testing and guided bend tests to verify the quality of the welded joint.

References:

Tips for Welding Dissimilar Metals for a Quality Result

Can You MIG Weld Mild Steel to Stainless Steel