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What are the considerations for choosing welding materials for strength and efficiency?

From the CWB Archives: We’re revisiting past articles from the “How It Works” series.

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Question: What are the considerations for choosing welding materials for strength and efficiency

Answer: Welding materials are defined as any materials used for making or protecting a weld such as filler metals (coated electrodes, rods, solid wires, tubular wires), fluxes, shielding gases and consumable inserts. Welding materials selection is one of the most important steps of the weld design. It will directly influence not only the performance and efficiency of the welding operation during fabrication but most importantly the integrity and performance of the weld during service. The selection is made considering a multitude of factors including strength, metallurgical, corrosion, temperature, welding position, diffusible hydrogen, efficiency, etc. 

Strength criteria is key for selection of welding materials and their combinations, and relies on matching the minimum specified tensile strength of the filler metal with the minimum specified tensile strength of the base metal. The selection can be made so that the two strengths can be matching, under matching or overmatching. 

Matching. Most welding applications require matching filler metal strength to that of the base metal’s and it implies selection of the filler metal in such manner that the filler metal strength will be equal to or greater than of the base metal. 

It should be mentioned that chemical compatibility between the filler metal – as deposited - and base metal is a prerequisite for filler metal selection based on matching strength.

Under matching. Under matching filler metal strength is generally considered when joining high strength steels and when making certain fillet welds and partial joint penetration welds. In these cases under matching is more economical, it helps to improve ductility, and may help in minimizing the residual stresses in the weldment.

Overmatching. Overmatching filler metal strength to a base metal’s strength is generally not encouraged and it often happens when matching yield strengths of base and filler metals having different yield-to-tensile strength ratios. 

When dissimilar metals are being joined by welding, as a general rule, the strength of the filler metal should be matched to the lower strength of the two base metals. 

In terms of efficiency, proper filler metal selection depends on the welding process employed. For GMAW the electrode selection must be combined with the shielding gas selection to achieve efficient electrode-gas combinations. Cored wires are also an efficient alternative for an increased efficiency due to higher melt off and deposition rates. For SMAW higher deposition rates can be achieved with iron powder electrodes that use iron powder as an additive in the coating. 

Author: Zoltan Tanacs P.Eng, CWI – Director Robotics Manufacturing – National Steel Car

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