Assessing the weldability of primers

TWI Bulletin, May - June 1996

John Seldon joined TWI in 1990, and is currently a member of its Fabrication and Welding Engineers Group. John holds European Engineer and European Welding Engineer status, and is registered with the International Register of Certified Auditors as a Lead Auditor.

In recent years considerable effort has been made by paint manufacturers to produce a 'weldable' primer, that is, coated steel which can be welded without the need to remove the primer prior to welding. Bill Lucas and John Seldon report.

Commercially available primers can be conveniently grouped into one of the following categories:

• iron oxide poly-vinyl-butyral (PV13)
• iron oxide epoxy
• zinc silicate - high zinc
• zinc silicate - medium zinc

As zinc is the primary cause of porosity, most developments have been directed at reducing the zinc content. The original zinc silicate primers contained up to 90% by weight of zinc. Then the first generation of reduced zinc silicate primers contained 60 to 70% zinc, the second generation contained 40 to 50% zinc and in the so-called weldable, third generation primers, the zinc content can be as low as 20 to 30%. [1] Although the third generation primers are considered to be weldable, their performance will depend, to a large extent, on the coating thickness and the type of welding application.

It should be emphasised when arc welding over a primer coating, gases will be generated which can be trapped in the weld metal, forming porosity. The factors which will influence the likelihood of porosity are:

• primer composition
• thickness of coating
• welding process
• welding procedure
• joint type
• welding position

A factor not generally considered in production situations is joint fit-up and in particular, the closeness of the joint gap. In tightly closed joints, as there is no escape path, the gases generated are more likely to become trapped in the weld metal.

There is no generally agreed best method for testing the weldability of primers. The various national standards apply quite different methods for testing primers [2-5] and the relative performance of the primers will differ according to the standard being used. The limitations of the national standards were highlighted in a recent group sponsored project at TWI: in particular they lacked the capacity to compare and quantify the tendency of primers to create porosity. A further limitation was that the porosity levels recorded were significantly different to those being observed in production.

A new weldable test has been derived. It has proved to be capable of comparing the relative weldability of primers and also of providing a measure of the type and amount of porosity likely to be generated under production welding conditions. The suitability of the new weldability test, which is described in this paper, has been proposed by the British Standards Institute (WEE/42 Committee) in a revision of BS 6084. [2[

Weldability Test

In designing the new weldability test, the following factors were considered to be crucial in producing a more realistic and repeatable test:

• Joint types more susceptible to porosity are the fillet and T-butt joints.
• A closely fitting joint should be welded which will provide a positive barrier to the escaping gas.
• The joint should be welded in the horizontal-vertical position ie with one of the plates standing vertical to reproduce one of the most severe welding positions.
• Welding should be carried out using the approved or recommended procedure so as to reproduce the amount of porosity likely to be generated under production conditions.

In producing a viable and reproducible testing procedure, consideration was given to the measurement of coating thickness, testpiece design, arrangement for clamping and tack welding the testpiece, welding procedure and the assessment of weld quality.

Coating thickness measurement

The coating thickness is measured according to the paint manufacturer's recommended technique, or using the technique agreed in the contract. When a technique has not been specified, several techniques are available, detailed in ISO 2808, [6] which are suitable for dry films, including mechanical (micrometer, dial gauge, profilometric microscope), electromagnetic and microscope methods.

Electromagnetic techniques (magnetic flux, magnetic pull-off, eddy current and dielectric principles) are non-destructive and provide a simple, quick means of measuring the mean point thickness on magnetic substrates. Sectioning the test-plate and measuring the paint thickness using a microscope has the advantage that the minimum and maximum thickness can be measured which can provide useful information for coatings deposited onto a rough (eg shot blasted) surface. Mechanical methods are generally less accurate.

Testpiece design

The testpiece is a T-butt joint made up from two flat bars, 1000mm long x 50-100mm wide x 10mm thick, of grade S275 to BS EN 10025, or equivalent, material, Fig.1. The plate is shot blasted and coated with primer in accordance withthe manufacturer's recommendations. The standing plate should also be coated along the edge which will be in contact with the lower plate surface.

Clamping and tack welding

The arrangement for clamping and tack welding the flat bars for the testpiece is shown in Fig.2. The clamping force should be sufficient to ensure that the joint gap is less than 0.1mm.

Tack welding is carried out at each end, on alternate sides, with additional tacks at equal spacing along the length of the testpiece, Fig.3.

Welding procedure

On tack welding in the jig, the testpiece is then welded using the welding procedure specification to be used in production. Welding should be carried out in the horizontal-vertical (PB) position, one pass each side to give a 6mm leg length fillet weld (Fig.1).

The length of weld is at least 500mm but for the submerged-arc process, a 1000mm long weld is recommended.

Assessment of Weld Quality

Examination of weld

Weld quality is assessed by recording the type of gas pores and quantifying the level of porosity produced in the weld metal. The welds are examined visually for surface breaking pores. Sub-surface pores are examined by first removing three sections, each 100mm long, from between the tacks. On removing the first side fillet weld by air arc gouging, the testpieces are broken open as shown in Fig.4.

Evaluation of welds

Guidance on levels of imperfections in arc welded joints in steel are defined in BS EN 25817. [7] However, because of the high level of porosity which can be generated by some primers, it has been deemed necessary to propose a new lower quality level, Level E. The four quality levels and the maximum units for imperfections are defined in the Table.

It is proposed that the new Level E will permit the presence of a single pore of up to 6mm, or localised clustered porosity up to 5mm, in fillet welds. The typical appearance of the weld surface and fracture surface of proposed quality levels are shown in Fig. 5.

Table: Quality level limits for imperfections when welding over primers

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Discussion

It is important to note that even the so-called weldable, or 3rd generation, primers have a tendency to form porosity in the weld bead. The level of porosity will be determined principally by the composition and thickness of the primer coating, the welding process and the welding procedure. It is, therefore, essential that in comparing the weldability of specific primers, there should be agreed methods of measuring the coating thickness, welding and standardised testpiece welding procedure and determining the level of porosity generated.

A number of techniques have been used at TWI for measuring the coating thickness. Depositing the primer on a glass slide is, no doubt, the most accurate method, but depositing on a shot blasted plate surface will be more representative of the situation in production. In the latter, the thickness of the coating will vary according to whether the measurement is made at the peaks or within the troughs of the surface roughness. The fabricator often requires that the minimum coating thickness is measured at the peaks in order to ensure adequate corrosion protection. For convenience, electromagnetic techniques which provide a mean value, are generally preferred but care must be taken to calibrate the instrument.

With regard to assessment of the weldability of primers, it is considered that for the test to be meaningful to fabricators, the test procedure must either reproduce the production situation or be representative of the worst case conditions. Therefore, it is recommended that primers are assessed using the double-sided fillet joint and the welding processes to be used in production. The standardised test employed in a group sponsored project, and subsequently adopted by the British Standards Institute, was a 6mm leg length fillet weld. For reproducibility, it was necessary to clamp firmly and tack the plate together to prevent escape of gases during welding.

The assessment of weld quality should include a measurement of the porosity level. Here, it was deemed necessary to introduce a new quality Level, E, (in BS EN 25817:1992), to accommodate the higher level of porosity which can be generated when arc welding primed steelwork. The level of porosity is quantified by simple visual examination of the welds for surface breaking pores, or breaking open the welds to reveal sub-surface pores. It should be noted that the proposed levels of imperfections refer only to weld quality and not to fitness-for-purpose of the product being manufactured.

Experience at TWI has shown that the testing technique described in this paper is a simple but reliable means of classifying the tendency of the primer to generate porosity. Fabricators can use testing techniques and assessment criteria to ensure that weldable primers can be used with confidence in production.

References