TWI ran a Group Sponsored Project to evaluate the welding performance of zinc silicate primers. This project prompted requests for TWI advice on current practice when welding primed plate, as summarised below.
Steel fabrications are protected from corrosion during service by an appropriate paint system depending on the service conditions - industrial, marine, underwater. Before painting, steel is grit-blasted to remove millscale. This operation is usually carried out before fabrication so that plates or rolled sections can be treated in mechanised grit-blasting installations. This treatment leaves the steel in a clean but active condition, and rusting would usually be apparent within a few hours unless it is immediately coated with a primer.
Known by various names - shop primer, weld primer, pre-fabrication primer - a primer will protect the steel during fabrication, which may take several months. It then serves as a first coat for a subsequent paint system. All primers are designed to be quick drying so that they can be applied by mechanised spray guns as the plates are moved through a paint spraying cabinet.
However, using primers may have adverse effects on welding behaviour of structural steel. Primer may be removed locally before welding, but this operation is expensive. Thus, there are many advantages in directly welding primed plate. The principal problem when welding primed plate is porosity. The organic binder used in primers can break down to form hydrogen, water and carbon dioxide, and hydrogen or water vapour can be trapped in solidifying weld metal, depending on the solidification rate of the weld.
The general experience of fabricators is as follows:
Butt joints
There are generally no problems even when the prepared edges are coated with primer. A possible exception is close square edge butt joints, if the edges have an excess coating thickness and a high speed welding process is used giving a high rate of weld solidification.
T joints
Fillet welds may contain porosity generally in the form of wormholes situated at the root of the weld. Whether porosity occurs, and its extent, depend on primer type and coating thickness, welding process and conditions, joint fit-up etc. In double fillet welds porosity is generally more extensive in the second side weld because the first weld seals one avenue of escape for any gases formed. Recent developments in zinc contents have been reported to reduce the porosity formed to very low levels.
Welding position
Welding in the vertical down or overhead positions produces higher levels of porosity than other welding positions because the buoyancy of gas pores prevents them from moving towards the weld surface where they would escape.
Welding Process
Manual metal arc welding usually produces sound welds on primed plate when either basic or rutile electrodes are used. Exceptions are when welding in the vertical-down or overhead positions as described above but the extent of porosity is fairly low.
MIG welding gives more extensive porosity because it is generally used at higher welding speeds.
Flux-cored arc welding with basic wires tends to produce lower levels of porosity than rutile cored wires because the higher fluidity of the slag from the basic flux allows gases to escape more readily.
Mechanised welding processes used at higher travel speeds than manual or semi-automatic welding tend to produce welds containing more extensive porosity because of the higher freezing rates of the weld metal. The problem can be overcome by restricting the travel speed, by careful control of the primer thickness, and also by leaving a 1.5mm gap between the plates in a T joint.
Acceptance standards
There is a multitude of acceptance standards for welds on primed plate with purely arbitrary levels of allowable defects. This is in spite of extensive work over many years which has shown that porosity generally has no adverse effect on mechanical properties.
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