Hardfacing: Building Erosion-Resistant Overlays - Part 1
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.
Benefits of hardfacing:
- 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 a longer time and 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, onto a common, less expensive base metal that provides 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 a 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 for build-up of worn areas or lining of original surfaces or the finishing, 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.
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 relief or slow cooling. 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 mostly are sold as proprietary alloys. They are only partially covered by specifications. The American Welding Society offers:
- 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 come as a surprise that the iron-base offerings 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 machineable, they offer higher impact resistance, but provide 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 essentially are cast irons.
Austenitic manganese steel, including 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 percent.
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 usually is so high that the deposit is non-machineable.
Cobalt-base alloys with high proportions of chromium and tungsten often are 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.