TWI Knowledge Summary

Arc surfacing

by Gene Mathers

Surfacing by the arc welding processes may be carried out for a number of reasons -

Weld build-up:

replacing worn or damaged surfaces by building up the surface with a weld metal which approximately matches the composition and/or mechanical properties of the parent metal.

Hard-facing:

giving a softer material a wear, abrasion or erosion resistant surface

Cladding:

providing a corrosion or oxidation resistant surface on a less corrosion resistant material. An example of this is the deposition of a stainless steel or nickel-based layer on a carbon steel base. One advantage of this technique is cost saving arising when surfacing a relatively inexpensive metal, such as a carbon steel, with a more expensive but corrosion resistant layer of stainless steel. Material and weight savings may be gained when a clad, high strength, quenched and tempered steel is used in a corrosive environment.

Buttering:

depositing a layer of weld metal onto the face of a weld preparation or surface which will then form part of a welded joint. An example of this is the buttering of an alloy steel weld preparation with a nickel-based weld metal and post-weld heat treating this part before making the joining weld between the buttering and a steel, which would be degraded by heat treatment.

Hard-facing and cladding are, as a rule, not included in the calculations for component design but weld build-up frequently is, and buttering must be included since it forms part of the load-carrying portion of a joint.

Surfacing techniques have been used in a variety of applications for thousands of years but it is only since the 1940s that arc welding has been used. Since that time, all of the arc welding processes have been employed. Every sector of industry - oil and gas, automotive, aerospace, power generation, yellow goods etc - uses arc surfacing techniques for repair and recovery and to improve service performance.

Of the arc welding processes, the most commonly used are manual metal arc (MMA), metal arc gas shielded (MIG/MAG) and submerged arc (SAW) welding, the last process using either a wire or a flat strip consumable. Flux cored arc welding (FCAW) is also being used in increasing amounts because of the ease of tailoring the composition of the consumable to the application. Drawing a hard-facing wire down to a small diameter for MIG/MAG or SAW is, in many instances, impossible and cored wires are normally used for this application.

Tungsten arc inert gas shielded (TIG) and plasma-arc (PAW) welding are also used, the latter being capable of high deposition rates and low dilution. Cold wire TIG is used infrequently because of its low deposition rate, but the addition of a hot wire facility can increase deposition rates to match those of the MIG/MAG process but with a TIG quality deposit. Fully automated cladding units are available, off the shelf, that use this technology.

Although electro-slag welding (ESW) is not an arc welding process it is appropriate to mention it here as it is being used very successfully as a replacement for SAW strip cladding. It has the advantages over SAW of a higher deposition rate and reduced dilution.

The main consideration with the surfacing process is achieving correct composition of the surfacing material. Selection of the most appropriate alloy is paramount but the amount of parent metal melted and mixed in with the filler metal (the degree of dilution) is also of crucial importance. This is generally expressed as a 'percentage dilution' of parent metal in the surfacing. Dilution varies from process to process and is influenced by welding parameters, in particular electrode polarity, welding current and travel speed. These need to be closely controlled to achieve consistency.

The composition of the surfacing alloy is selected for the specific application, ranging from tungsten carbide rods or martensitic steel wires for wear resistance, to austenitic stainless steels for corrosion resistance, and from nickel alloys for oxidation resistance to copper-nickel alloys for saltwater service.

Further information

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

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