WELD OVERLAY CLADDING – ADAPTABLE, FLEXIBLE PROTECTION FOR OFFSHORE EQUIPMENT
As offshore drilling technology advances and wells become ever deeper, the problem of corrosion increases proportionately. The presence of hydrogen sulphide, carbon dioxide and chlorides creates a potentially catastrophic corrosive mixture. Add to this the extremely high product temperatures from deep wells and there are significant problems that need to be overcome.
When assessing the corrosion protection of any production system, piping and process engineers have a number of options to consider. The effectiveness of each will vary dependent on a number of factors including: the aggressive nature the product; pressure and temperature; size and complexity of the system; well-life expectancy; available development period and the budget.
A production pipeline, from wellhead to topside processing, will typically include pipe, various types of connectors, fittings (tees, elbows etc.), complex valve blocks, pig launcher/receivers, etc., all of which will be subject to corrosive and possibly erosive attacks on their internal wetted surfaces. So how do engineers design a system to resist these attacks?
Protective materials
Protection methods where risk of attack is low and life-cycle short, may be as simple as an injected inhibitor used with conventional high-strength carbon or low alloy steel.
Where greater protection is needed corrosion resistant alloys (CRAs) such as austenitic (300 series); ferritic/martensitic (400 series); and duplex stainless steels or the more complex high nickel chromium alloys, must be considered.
With apologies to the manufacturers of austenitic stainless steels, it is unlikely they would have the resistance required for the very worst conditions. They would have to be used in very heavy wall section to match the pressure retention achieved by the carbon steels in common use (API 5L X60 or X65 for instance).
Duplex steels and nickel based alloys, such as alloy 625, are the only materials in general production which, when welded, will achieve the strength to match carbon steels. However, there are constraints to their use in solid form – namely cost, availability and the need for very rigid fabrication procedures.
Cost is particularly relevant where large quantities of pipe and fittings or large forgings or castings are needed. Wellhead valve systems and pipe bundle bulkheads are typical examples.
Protection methods
The use of carbon and low alloy steels clad with a corrosion resistant alloy has been common practice for some years and is a proven, economical and technical alternative to solid alloys.
The term ‘cladding’ covers a wide range of processes including: hot roll bonding, explosive bonding, diffusion bonding, centricast pipe, co-extruded pipe and weld overlay cladding.
Each has particular merits, so the processes are not necessarily competing for the same market. For example, whilst hot roll bonded plate, rolled and welded into pipe, may be economical for a 12m length, co-extruded or centricast would offer savings if 12km are required. Also, in periods of high demand, the lead times for some of these techniques may preclude them from use in a fast track or refurbishment project.
Weld overlay cladding
Weld overlay cladding technology presents the materials engineer with a wide choice of welding processes and immense flexibility. An almost infinite range of component shapes and sizes can be protected, with an equally wide range of base material/cladding alloy alternatives.
The combination of high strength low alloy steels (AISI 4130 or 8630 for example) and alloy 625 (Er-NiCrMo-3) is probably the most common for high pressure retaining wellhead equipment.
Weld procedures are normally qualified to ASME IX, as are the welding operators. Additional testing to prove conformity with API 6A and NACE MR01-75 is also completed, along with any contract specific requirements from the end user.
Welding processes
Selection of the most appropriate welding process is largely dependent on factors such as the size of the clad area; access to the area to be clad; alloy type, specified clad thickness; chemical composition limits; welding position; and NDT acceptance standards.
Welding processes in common use internationally include:
- Electroslag strip cladding
- Submerged arc
- GMAW spray transfer
- GMAW pulse transfer
- FCAW
- GTAW hot wire mechanised
- GTAW cold wire mechanised
- GTAW manual
- PTA
- SMAW
Given that the process used must be practical, viable and provide the mechanical and chemical conditions to achieve service requirements, economics dictate that the higher deposition rate processes should prevail. Details are available to optimise processes and deposition rates while taking into account the limitations that may apply.
Automated or mechanised processes generally offer the best deposition rates and provide the most consistent quality of deposit. This enables the finished cladding to closely match the results provided during procedure qualification testing. Mechanised equipment can also be designed to access areas that simply cannot be reached by manual methods – for example through a small-bore pipe.
GTAW processes can be used in bores as small as 15mm, and are ideal for components of varied geometry where the position of the welding head requires frequent adjustment, from a simple flange that needs to be clad through the bore and across the sealing face, to a complex valve body with several interconnecting bores.
Often equipment also needs cladding to RTJ grooves. The control available with the GTAW process means that cladding can follow the profile of the groove rather than filling it completely. This not only saves time and material but also reduces the cost of finish machining.
This flexibility also lends itself to cladding irregular shaped components such as pipefittings. Elbows and tees as small as 2” NB can be clad, particularly where specifications do not allow for a mixture of base materials – for example a carbon steel pressure vessel, where fittings in solid alloys are not permitted due to risks from the use of materials with different thermal expansion rates.
Using this process the chemical composition of the welding consumable can be achieved at 2.5mm from the base material/cladding interface (this can be reduced to 1.5mm in the case of 300 series stainless steels, where over alloyed wires are available).
Where plain bores (in pipe or flanges for example) are greater than 250 - 300mm, the faster depositing electroslag and submerged arc processes can be considered. Equipment is available to enable pipe lengths of 12m to be successfully clad.
The electroslag process utilises a large weld pool that requires substantial base metal backing (generally a minimum of 20mm) to prevent burn through and support the edge of the weld pool to avoid collapse of the molten weld/flux covering.
It is ideal for areas of plain, open access. It is not ideal for cladding adjacent to convex or concave edges. The deposit thickness is nominally 5mm with the strip widths discussed here. With 60mm strip, deposition rates of up to 22kg per hour can be achieved.
To enable the chemical composition of the deposit to match that of the consumable specification within the first layer (3mm from the interface), over-alloyed strip and ‘loaded’ metal containing fluxes, are available.
Where a strict limitation is imposed on iron dilution into the cladding, a second layer can be added to give entirely undiluted weld metal. However, the use of a 9 -10mm thickness of cladding may negate the commercial advantage of the high deposition rate process.
In these circumstances additional production test plates have been produced, and corrosion tests carried out on the single layer to prove the acceptability of the material for known service conditions.
Another option is the use of a combination of processes. A recent example required a final layer of alloy 400 over a significant surface area. The first layer was pure nickel, deposited by spray GMAW. The second, with 30mm electroslag strip (to ER NiCu-7) ensured that the chemistry (in particular the low iron requirement) was achieved and a total thickness of 7mm was deposited. A material saving of up to 30% was achieved with only a small increase in production time over the two layers of strip.
Submerged arc welding using a solid wire consumable, while not as fast, is a useful ‘halfway house’ between strip cladding and slower GTAW and pulsed GMAW. The welding heads used are not as large as strip heads, and the consumable delivery method is much more flexible. Hence the ability to use this in smaller bore diameters. Traditionally larger diameter consumables (2.4mm +) have been used for this process, again resulting in the need for fairly thick substrates to accept the high heat and large weld deposits.
Recently, procedures have been developed using 1.2mm wires allowing use on thinner section components, and giving more controlled thickness of deposit while maintaining deposition rates of approximately 5kg per hour. As with strip cladding, consumable/flux combinations are available to make single layer deposits viable, especially with duplex and ferritic/martensitic stainless steels.
When weld overlay cladding was first employed, re-machining after cladding was the norm. However, as techniques and equipment have improved, the ‘as welded’ finish has become much smoother and many areas of clad equipment are now left ‘as clad’. This would not apply to sealing/gasket areas, which have to be produced to the very finest of tolerances.
Without doubt the GTAW (and PTA) processes give the least contoured deposits, so procedures have been developed to use the quicker submerged arc or GMAW processes for the first layer and finish with GTAW – combining the benefits of two processes.
When cladding high strength (and therefore more hardenable) low alloy steels such as AISI 4130, 21/4 Cr 1 Mo and, potentially, martensitic stainless steels such as A182 F6NM or AISI 410; PWHT is invariably adopted. This stress relief ensures that the layer of base material immediately below the weld (heat affected zone) is within the recommended hardness levels for the service conditions (as required by NACE).
Test procedures
The level of NDT will be in accordance with the specification to which the equipment is being produced, plus any client or contract requirements detailed in quality plans and purchase orders.
This will almost invariably include liquid penetrant inspection, usually after any machining has taken place. Ultrasonic inspection is less often required, but is used to confirm sound fusion and the absence of volumetric defects.
Chemical analysis of the clad surface is sometimes requested and can be tested in a number of ways, the most common being by analysis of swarf samples from the component, or by use of an X ray spectrograph (PMI) machine, or similar.
Where austenitic or duplex steels have been used, reporting the phase balance may be an additional requirement. This can be calculated from chemical analysis using one of the internationally recognised formulae, or by use of a suitably calibrated magnetic ferrite detector.
Conclusions
The ability to clad ‘off the shelf’ components of any shape and size with a wide variety of corrosion resistant alloys, has made weld overlay cladding the most adaptable and flexible in use - whether you need a one-off special or a large production run.
