Welding stainless steel to carbon steel: dissimilar joints and the right filler metal

By Robert Lahnsteiner, International Welding Engineer (IWE) · Updated: November 2026

When stainless steel is welded to carbon steel, both materials mix with the filler metal in the weld. The filler metal must be chosen so that this mixture becomes neither hard and crack-prone nor susceptible to hot cracking. Such joints are known as dissimilar joints (black-and-white joints).

The weld has to meet the requirements of both base materials. Depending on the composition of the weld metal, the risks are:

• hardening cracks, if there is too little chromium and nickel in the weld metal and martensite forms,
• hot cracks, if the weld metal solidifies without ferrite,
• embrittlement by sigma phase, if there is too much ferrite and chromium.

The basics of the microstructure are covered in Microstructure, delta ferrite and hot cracking.

Without filler metal, the weld would consist of roughly half carbon steel and half stainless steel. In the Schaeffler diagram, this mixture lies in the martensite region and would harden. An over-alloyed filler metal pulls the weld metal back into the safe region.

This is how the check with the Schaeffler diagram works:

1. Calculate the chromium and nickel equivalents of both base materials from their analysis, ideally from the inspection certificate 3.1, otherwise from the standard analysis.
2. The midpoint between the two points is the base material mixture.
3. Plot the point of the filler metal and connect it to the mixture.
4. The weld metal lies on this line, closer to the filler metal or to the mixture depending on the dilution. The ISER information sheet uses 25 % base material in its example.

The Schaeffler calculator for dissimilar base materials does this, including dilution.

With typical analyses, the mixture of S355 and 1.4301 has a chromium equivalent of about 9.6 and a nickel equivalent of about 8.1, which is in the martensite region. With filler metal, the result depends on the dilution:

Own calculation with the Schaeffler diagram and typical analyses; real values depend on the certificates.

• 309L retains a few per cent of ferrite at normal dilution and is therefore safe against hot cracking. It tolerates the most dilution and is therefore the standard for dissimilar joints.
• 308L works at low dilution, but loses its ferrite at high dilution and approaches the martensite region.
• 307 solidifies fully austenitic. The manganese stabilises the austenite and binds sulphur as manganese sulphide, which counteracts the risk of hot cracking.

At low dilution, filler metals of type 18 8 Mn or 20 10 3 are sufficient; at higher dilution, the higher-alloyed types 23 12 L or 23 12 2 L are needed.

Based on ISER information sheet 823 (2019) and DVS 0931. For ferritic chromium steels in thick components, the last passes on the medium side should be welded with ferritic filler. For construction products subject to building regulations, only approved filler metals may be used.

Type 29 9 (312) with a very high ferrite content is used for joints with steels that are difficult to weld or of unknown composition. More on filler metal selection in Choosing the filler metal.

The aim is dilution that is as uniform as possible and not too high.

• Process: Dissimilar joints are mainly welded with manual metal arc and gas-shielded processes, see Processes at a glance.
• Limit dilution: The penetration into the base materials determines the dilution. With more passes and less penetration, it stays lower.
• Heat control according to the more sensitive material: if the carbon steel needs preheating, this also applies here. Preheat temperatures are provided by the Preheat temperature calculator.
• Keep tools separate: Grinding dust from carbon steel must not get onto the stainless steel, otherwise extraneous rust forms, see Joint preparation and tack welding.

Stainless steel is the more noble material. If both come into contact with an electrolyte, the carbon steel can corrode more heavily at the joint.

This is critical above all when the area of the stainless steel is large compared with the area of the carbon steel. It can be avoided by insulating the materials from each other or by protecting the carbon steel with a coating that extends across the weld some way onto the stainless steel.

Can I weld stainless steel to carbon steel with 308L?

Often yes at low dilution, but 309L is safer. At high dilution, for example in the root, 308L loses its ferrite and can approach the martensite region.

Can I weld stainless steel to carbon steel with a carbon steel wire?

No. The weld metal would not be corrosion-resistant, and dilution with chromium and nickel from the stainless steel can make it harden and crack.

Do I need to purge dissimilar joints?

Yes, if the root side of the stainless steel has to remain corrosion-resistant. Otherwise temper colours form there, see Root protection and purging.

The series from the material via process, gas and root protection to the tested weld. For an overview, see Welding of stainless steel.

1. What makes stainless steel stainless
2. Microstructure, delta ferrite and hot cracking
3. Choosing the filler metal
4. Processes at a glance
5. Shielding gases
6. Heat input and interpass temperature
7. Joint preparation and tack welding
8. Root protection and purging
9. Temper colours, pickling, passivation
10. Duplex and super duplex
11. Weld defects and corrosion damage
12. Testing, health and safety