TWI Knowledge Summary

Weldability of low carbon steel sheet

by Geert Verhaeghe

The definition of 'thin sheet' is very subjective and varies from industry to industry. What is covered below describes weldability of low carbon steel sheet with a thickness below 5mm.

Low carbon steels have a reasonably low hardenability. Using good welding practice (moderate restraint, removal of excessive mill-oil or lubricants, etc), they can easily be joined by conventional arc welding processes without too much risk of cracking. Further information can be found in Fabrication cracking mechanisms in ferritic steels - a guide to best practice.

A number of welding processes can be used for welding low carbon steel sheet, including MIG/MAG (GMAW), TIG (GTAW) and plasma, laser, electron beam, friction stir and resistance. Some of these processes are more applicable than others. Resistance spot welding for instance, can produce low ductility welds in steels with a carbon equivalent greater than about 0.25, because the very high cooling rates associated with the process produce relatively high nugget hardness. Cracking is not normally a problem, but such welds lack strength in peel or tension.

Weld metal integrity is not normally a problem for low carbon steel. However, de-oxidised steels are preferred, otherwise carbon dioxide gas can form in the welds especially when welding autogenously. For higher speed processes, such as laser welding or mechanised MIG (GMAW), there is insufficient time for these gases to escape from the solidifying weld pool, causing weld metal porosity.

Current filler wires contain sufficient levels of deoxidising elements to compensate for, and eliminate, excessive weld metal porosity. They have been developed for use with arc welding processes or lasers to match the parent material properties. The mechanical properties of the resulting joint - such as hardness, static tensile, shear or peel, fatigue or formability - determine the suitability of the weld in a particular joint configuration for a given application.

Low carbon steel sheets can be welded with a wide variety of shielding gases such as CO ₂ , argon or helium with CO ₂ or O ₂ additions and nitrogen or argon-helium mixtures. Choosing the most appropriate gas composition will depend on the process used, the application and the required joint quality. For instance, laser welding can be carried out without shielding gas, but higher quality top and under-bead profiles are achieved with shielding gas.

Thin steel sheets, for automotive or white-goods applications, are commonly supplied with a protective coating. This coating can be aluminium, zinc-aluminium, zinc-nickel based or even organic, but is usually a zinc-based coating, which provides corrosion protection to the underlying material. Care is required when welding these coated steels as zinc vapour may cause porosity in the welds with high speed welding processes such as MIG (GMAW) or laser. A number of processing techniques have been accepted as good welding practice to avoid excessive coating-related porosity.

It is important to note that without suitable fume extraction, zinc vapour may case cause operators to suffer from metal fume fever.

Further information

You can use the Weldasearch literature database to supplement what you find in JoinIT.