Material Conditions and Filler Metal Selection
When proper cleaning and part preparation of dirty or coated materials prior to welding isn’t possible, choosing the right filler metal for the base material conditions can help address quality issues and improve results.
Some of the most common material conditions encountered in welding applications are mill scale, galvanized coatings, rust, and oil. Removing these coatings from the material takes time and adds cost, so many operations don’t undertake the process of cleaning. Instead, operators weld over the material coatings or rust, which can add difficulty to the welding process and affect weld quality.
Welding Mill Scale
Mill scale is a protective oxide layer that forms on metal when it is hot rolled during the manufacturing process. Cold rolling the steel eliminates mill scale formation but adds costs to the production process, so many operations don’t choose cold-rolled over hot-rolled material. The mill scale layer can also be blasted off prior to welding.
A much slower travel speed is one of the pitfalls of welding on mill scale. In some cases, it requires half the speed compared to welding base material that has been cleaned and prepped. A slow travel speed is necessary to burn off the mill scale layer with the arc. Welding too fast on mill scale will cause an erratic weld pool, resulting in an inconsistent or wavy weld bead. Welding over mill scale also causes more spatter compared to welding clean metal.
When choosing a filler metal to tackle material with mill scale, a flux cored wire can help. Flux cored wires typically have the most deoxidizers, which help bring impurities to the surface. Good flux cored wires for welding mill scale are AWS classes E71T-1C/M and E71T-9C/M.
Metal cored wires, such as AWS E70C-6M, are also good choices because they are high in deoxidizers, like silicon, which feature great impurity-gathering capabilities thathelp filler metals perform better on dirtier materials.
A shielding gas combination, such as 90% argon and 10% carbon dioxide, produces a hotter arc and offers benefits for welding steel with mill scale, rust, or primer coating. The higher temperature can help burn away the coating or rust faster. Increasing the welding voltage slightly can provide the same benefits.
Welding Galvanized Steel
Galvanized base metals are very common in many welding applications. The material is galvanized through either a dipping or an electroplating process that forms a coating designed to prevent rust and oxide from forming on the metal. The thicker the coating, the better the protection but the harder it is to weld over.
Welding galvanized steel requires slower travel speeds and can cause porosity and weld cracking. If an operation can avoid galvanizing before welding and instead complete the dipping process after the material has been welded, that is preferred and helps avoid these issues. If there are high residual stresses present after welding, operators may experience liquid metal embrittlement, which is cracking in the weld or heat-affected zone after hot dipping.
In applications where it’s necessary to weld galvanized steel, there are filler metals that perform better on galvanized and coated materials. A flux cored wire with an AWS E71T-11 classification works well on coated steels because of the added aluminum and other deoxidizers that help reduce impurities and cracking.
Welding over Rust
Rust differs from mill scale in that mill scale is formed during the manufacturing process while rust forms when the metal is exposed to the atmosphere. Moisture and humidity can speed up the rusting process, so proper storage of the material is important to consider.
Rust has less electrical conductivity than steel, so it will be harder to establish a stable arc on very rusty material. The thicker the rust, the more difficult welding will be. Welding over rust can cause porosity and high levels of spatter. Look to flux cored and metal cored wires with high levels of deoxidizers to weld through rust. For welding A36 steel, AWS E71T-1C/M and E71T-9C/M flux cored wires are good choices. For metal cored wires, choose the AWS E70C-6M classification.
Many plates and other metal pieces come into the welding process with oil on them. The oil is often left on the material as lubrication used in the cutting process.
There is little reason to ever weld over oil since removing it from metal parts is much faster and easier than for any of the other material conditions listed above. Simply use some acetone and a rag to wipe the oil from the weld joint and surrounding area.
This article was written by Jake Balogh (welding engineer and CWI at Hobart, Troy, Ohio) for the American Welding Society.