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Hardfacing, info about
Hardfacing is a group of welding related techniques, called also cladding or overlaying processes, having for purpose to restore dimensions of worn surfaces of used implements by depositing new and improved material to extend their useful working life. Alternatively, when applied to a new part, to protect a common metal with a layer of a complex alloy presenting different characteristic properties that better resist wear, abrasion, impact or corrosion, to improve the usefulness of the original item.
The minimum thickness of Hardfacing layers is about 1 mm (0.040"): if thin overlays are required one should investigate the applicability of Thermal-spray as a method of depositing materials with designated properties, without welding the base material or even heating it much.
What is in here for me?
Hardfacing benefits:
* Building new parts with assured longer life of elements subjected to wear and abrasion.
* Rebuilding worn parts at a fraction of replacement cost.
* Savings in maintenance costs when rebuilding at the equipment operation place.
* Producing more economic parts by placing hardfacing alloy only where needed.
* Reducing breakdown time and increasing work efficiency.
Essentially the process of Hardfacing or Surfacing consists in depositing a welded overlay, providing hardness, abrasion-, erosion-, galling-, impact-, corrosion- or heat resistance as required, to cover the original or the worn out surface so that it might better perform its function in a harsh environment, for longer time with less maintenance.
The economic importance of Hardfacing derives from the feasibility of selectively applying expensive material, chosen for its properties, exactly where it is required for best performing its specialized function, unto a common inexpensive base metal providing the bulk of the structure. Also the local applicability of Hardfacing using portable equipment means that repairs can be done at the point of utilization, avoiding excessive costs of transportation to the repair facility.
Selection depends on many factors, like the type of friction, heat, corrosion and impact, that generate wear in the particular application. Other constraining conditions are the base metal involved, preparation if build up of worn areas or lining of original surfaces, the finishing or machining requirements, if any, and the welding process available.
More than one selection will be likely to provide adequate service, although the most economical solution may not be evident if experience is lacking.
Tip!: Material suppliers may be willing and able to suggest their choice, although it is normal to expect some bias in favor of their products. One should investigate more than one proposal.
Adequate thought has to be dedicated to the type of base metal, to preparation and preheating, if needed, and to final stress relieve or slow cooling. As welded hardness is a useful datum to know and check, although it may not be the most important element determining the success of alloy and process selection for the application.
Despite the documented capability of Hardfacing as an important source of savings, the cost of its application can and should be estimated, with some assumptions, so that the comparison of alternatives becomes possible. Selection of process and of welding position has a major influence on the total cost. The following cost elements should also be taken into account:
* Volume of material to be deposited
* Process to be used
* Deposit efficiency (Ratio of deposited material to consumable material used)
* Operating efficiency (Ratio of deposit time to total time including setup, preparation, preheating, transport, finishing etc.)
* Consumable costs (Flux, gas, power, welding material, labor and overhead)
Materials for Hardfacing are mostly sold as proprietary alloys. They are only partially covered by specifications. See:
AWS A5.13 - SPECIFICATION FOR SURFACING ELECTRODES FOR SHIELDED METAL ARC WELDING.
AWS A5.21 - SPECIFICATION FOR BARE ELECTRODES AND RODS FOR SURFACING.
Of the Hardfacing filler materials available, it will not surprise that the Iron base are the least expensive, while their different compositions present characteristics useful in a large range of situations. They should always be considered as the first choice.
Low alloy steel for Hardfacing containing Chromium, Molybdenum and Manganese (total alloy content of 6 to 12%) can be used as a support for more abrasion resistant layers: moderate in price and machinable, they offer higher impact resistance but only moderate improvement over base metal abrasion resistance.
Next come higher iron base alloys (alloy content of 12 to 25%) of Chromium and Molybdenum, with Manganese and Silicon. Alloys with High Carbon content are essentially cast irons.
Austenitic Manganese Steel, including also Nickel and Molybdenum, are impact resistant: they develop higher hardness and abrasion resistance through mechanical deformation or work hardening, usually in operation: however the application is more difficult because one must avoid overheating which tends to embrittle the overlay. Alloy content can reach almost 40%.
More expensive high Carbon and higher alloy content (25 to 50%) alloys have Chromium and Molybdenum which form massive carbides. Hardness depends on the substrate but it is usually so high that the deposit is non machinable.
Cobalt base alloys with high proportions of Chromium and Tungsten are often described as the most versatile alloys, capable of resisting abrasion, corrosion, heat, oxidation, impact and wear.
Nickel base Hardfacing alloys are selected for heat and corrosion resistance when metal to metal contact wear is present.
The last group of Hardfacing alloys presents Tungsten Carbide (WC) particles embedded in one of any kind of matrix metal like Iron, Steel, Bronze, Nickel or Cobalt: these alloys have the highest abrasion resistance when impact is low or moderate.
Special Hardfacing processes have been developed where hard tungsten carbide particles are deposited from a funnel, right on the molten pool, to be embedded there, avoiding their passage into the high temperature of the process (flame or arc) which might affect them negatively.
The best Hardfacing selection would be the one that provides the minimum requirements (expressed in acceptable working time or tons of processed materials, before repair is needed) at the least total cost, including consumable materials, loss of production time and repair.
The Hardfacing selection is more based on the application than strictly on composition. The use of more expensive materials where less expensive ones would have provided acceptable results, should be avoided unless it is shown that a longer operating time and economic benefit is achieved.
The shapes in which Hardfacing materials are presented to welding depend on their properties established by composition. For the more ductile alloys coiled wire is usually the preferred supply form. Less ductile materials, subject to cracking on bending, are supplied in straight rod lengths, cast or rolled.
Totally brittle Hardfacing materials are provided as powders, or are packed in the hollow of roll formed sheet foil tubes, possibly intermixed with special fluxes. Depending on application, manual or semiautomatic, rods or wires may be provided as covered electrodes. Some alloys are available in more than one form for different processes.
An Article on Hardfacing Filler Materials, with more details and practical advice, was published in the January 2004 issue #05 of Practical Welding Letter.
Click here for reading.
Surface preparation for Hardfacing consists in cleaning the affected area and usually providing a machined or ground groove or channel into which the Hardfacing alloy will be deposited and fused. Smooth and gradual transition is imperative to protect the edges, because Hardfacing materials may tend to be chipped away under impact.
All welding processes have been used for Hardfacing with overlay application. The selection of the proper welding process depends on many factors and has a major influence on the total operation cost. The shape, size and weight of the workpiece dictates if it can be moved to the production facility or if welding equipment must be made available at the place of the structure.
In case of bulky implements, difficult to move, the process will preferably be manual, to be performed in place by a skilled welder using portable equipment. Mechanized setups can sometimes be implemented, when applicable, if long stretches of weld deposit are needed, using either Gas Metal Arc Welding (GMAW also known as MIG) or Submerged Arc Welding (SAW), because of their higher deposition rate when compared to manual Shielded Metal Arc Welding (SMAW or Stick).
Conversely small parts to be processed in large quantities will usually be more economically manufactured using semi automatic or machine Hardfacing in a properly set up industrial environment.
The base metal will dictate the procedure to be used, including preheating for materials prone to cracking. Overlay materials may have thermal expansion characteristics very different from the base metal in which case special procedures must be put in place, like possibly interposing a buffer layer of a third material with intermediate properties.
The process selected should provide the best deposition rate of the thinnest layer required for the Hardfacing application while limiting the dilution of the deposited Hardfacing alloy layer into the base metal by controlling the heat input to a minimum. Slow cooling or stress relieving after surfacing may be required for mitigating the effects of residual stresses.
Oxyacetylene Flame.
The equipment which uses the oxyacetylene flame is relatively simple and versatile and more economic than arc systems. The limitations are that a certain skill is required from the welder, who should have good coordination of both hands movements, and that the rate of deposition of Hardfacing is rather slow when compared to other ones. Typical applications are for items where the surface to be overlaid is relatively small. The process can easily be applied in the field, even for replacement of broken sections of tools or implements.
When the filler metal is in the form of wire or rod, the gas welding torch is that used for regular oxyacetylene welding. See Gas Welding Process and Gas Welding Equipment. If the Hardfacing material is supplied in powder form, then the torch includes a special dispenser to spread the powder ahead of the flame. The type of flame is usually neutral or slightly reducing (with excess acetylene), as recommended by the manufacturer of the particular filler employed.
If preheating is needed, it is done before Hardfacing either in a furnace or by manipulating the welding torch. The filler material, already preheated in the edge of the flame, is melted by introducing it in the hottest part of the flame when the base metal is already superficially in molten state.
Arc welding.
Hardfacing by arc welding is performed using all of the common processes and equipment, see Arc Welding Process and Arc Welding Equipment.
Of the arc welding group, Shielded Metal Arc Welding (SMAW) or stick welding is the most common and versatile process although it does not provide the highest deposition rate. Rate of dilution depends on materials and on welder's skill.
Submerged arc welding can provide much higher deposition rate if the conditions are correct for uninterrupted alloy deposition of Hardfacing filler wire. The limitations are that dilution tends to be higher unless speed is kept as high as possible, and that the process is not readily adapted to field conditions.
Gas Metal Arc Welding (GMAW) or Mig, where shielding is provided only by inert gas, is readily applicable but only for those fillers supplied in wire form, and usefully complements the range of applications of the preceding process.
Gas Tungsten Arc Welding (GTAW) or Tig is used either manually or in some mechanized form, but is used only for small parts, because dilution cannot be reduced under 10%, which is sometimes unacceptable.
Plasma Arc Welding (PAW) is used for Hardfacing filler material in powder form, even of ceramic materials, which is fed through the arc and reaches very high temperatures. The filler though must be in the appropriate mesh size, must not conduct electricity (not to short the torch) and must not undergo vaporization or sublimation. Besides the elevated cost of equipment the limitation is that it cannot be brought to the field.
This process is similar to Plasma Spray (see Thermal-spray except that the bond with the base metal may be of a different nature.
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