How to weld stainless steel to mild steel

Welding of stainless steel to mild steel/ Carbon steel/ Low alloy steel

Dissimilar metal welding (DMW)

Dissimilar metal welding (DMW) is applied for welding of different metallurgical alloys for example welding of stainless steel to carbon steel. The requirements of DMW are quite common in oil & gas, power plant, and chemical plants to weld transition welds. E.g., austenitic stainless-steel tubing material is generally used for high-temperature applications in refineries.

But below some temperatures, carbon steel is also suitable to withstand this temperature. And it’s not practical to use stainless steel for these areas. The best option as economical is to join the stainless steel pipeline to a carbon steel pipeline instead of using the full stainless steel option. Such type of combinations occurs frequently in power plant and oil & gas plants.

Consideration for Dissimilar Welding

The essential features to be considered when welding dissimilar metals are:

The melting temperatures of the two metals are concerned: Widely differing temperatures can mean that the higher melting point may not melt or could provide a disproportionately small amount of the weld pool.

Coefficients of Expansion: Widely differing coefficients of thermal expansion can provide undue stressing on the low expansion alloy during any thermal cycling subsequent to welding. There may even be a problem during initial cooling that could leave undesirable internal stress in the join.

Sometimes it is necessary to provide a buttering layer with an intermediate coefficient of expansion.

Coefficients of thermal conductivity: If one metal in a bond has a high thermal conductivity this side of the weld will be at a lower temperature and this could lead to non-uniform join properties.

Welding of stainless steel to mild steel/ Carbon steel/ Low alloy steel (Temperature upto 600 °F (315 °C)

The choice of consumable must cover not only the strength requirement in the weld. Other factors such as melting point relative to the parents, color, harmful phases that might occur, and corrosion possibility all have to be included in the considerations.

The most common Electrode/ filler wire used to join stainless steel or duplex stainless steel to mild steel is E309L/ ER309L (Stick welding/ TIG-MIG rod) for normal temperature service. For high-temperature Nickel, base consumables are preferred.

Here, E309L, ENiCrFe-2, ENiCeFe-3 & ENICrMo-3 are SMAW Consumables. Their equivalent for TIG/ MIG shall be: ER309L, ERNiCr-3 & ERNiCrMo-3.

The table below shows the combination of various dissimilar welding combinations between stainless, duplex stainless steel & carbon steel, and Cr-Mo steels.

Click here to Get Guide on How to Weld various materials

Welding of ferritic steel ( P No. 1, 2, 3, 4 & 5) to other ferritic steel, the welding wire should have matching chemistry of base metal or match to diluted weld chemistry. Welding of pressure parts, the filler wire must match the chemistry of the pressure part material.

Welding of ferritic steels (P-No. 1 to P-No. 5 and P-No. 15E) to:

  1. Martensitic stainless steels (P-No. 6); or
  2. Ferritic stainless steels (P-No. 7); or
  3. Austenitic stainless steels (P-No. 8), the welding wire shall be selected based on:

1) Welding rod/ filler wire 309 and Type 309L can be used for design temperatures up to 600 °F (315 °C). For temperatures above 600 °F (315 °C), Nickel base welding consumables (E.g. ENiCrFe-3, ERNiCr-3, etc.) shall be used. 

Welding of Mild steel or low alloy steel (P1, P3, P4 & P5) to Stainless steel (P6, P7 & P8) for high-temperature applications

2. Nickel-base alloy filler materials may be selected using design conditions shown in Table below:

Use of the Schaeffler diagram – Joining stainless steel and mild steel

The Schaeffler diagram is helpful to estimate/predicting the microstructure for dissimilar weld joints. Based on the filler wire, and base metal chemistry microstructure prediction can be carried out.

It is an indication of the constituents that will be present in the microstructure for various values of the nickel and chromium equivalent percentages.

Watch this interactive video on Schaeffler Diagram and its practical uses:

Dissimilar metal welding with Nickel Alloys

These alloys are most sensitive to sulfur as well as phosphorus, lead, and zinc. In determining whether a weld is likely to give a problem, the composition of the weld pool should be estimated using the proportional method.

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