The process of hardfacing has evolved from using the oxyacetylene rods invented by the Stoody brothers – Shelley and Winston – back in 1921 to a complex material science that provides application-driven solutions. Hardfacing improves profitability by reducing unplanned downtime and allowing for scheduled maintenance procedures. It does so by using lower cost base metals that are overlaid with a specialized alloy of superior durability to rebuild worn parts rather than replacing them.
With a focus on cost reductions and productivity enhancements, the selection of the correct alloy and proper application is critical. What follows are the five basic steps to maximize hardfacing effectiveness: base metal identification, wear factor identification, alloy selection, weld process selection and bead patterns.
Step 1. Identify the base metal
Base metal type and its carbon and total alloy content are important factors in the hardfacing process. Some metals tolerate extremes in heat or cold well, while others do not. Common base metals include low, medium and high carbon steels; wear- and abrasion-resistant steels; manganese steel (Mn-steel); stainless steels (300 and 400 series); tool, die and mold steels; and cast irons.
Materials with higher amounts of carbon and alloy content, wear- or abrasion-resistant steels, tool steels, cast irons and 400 series stainless steels may require pre- or post-heat, slow-cool or stress-relief processes to minimize distortion, shrinkage, cracking and spalling (lifting of the weld deposit) and to reduce thermal shock to the part. Rapid air quenching, the abrupt air cooling from welding temperature, embrittles the part in the heat-affected zone. For higher carbon and alloy steels, preheating greatly reduces this tendency.
The carbon and alloy content of the base metal determines the recommended preheat temperature; the higher the carbon and alloy content of the base metal, the higher the preheat temperatures required. To determine the best preheat procedure for an application, a trusted hardfacing alloy product representative or distributor can help.
Austenitic Mn-steel requires special provisions to prevent brittleness. Mn-steel is non-magnetic and tough, and it workhardens under high-impact loads. For this reason, Mn-steel parts are often found in machinery that uses high-impact loads to render and crush. Examples include rock crushers, roll and impact crushers, swing hammers and car shredders. Mn-steel parts are often used without hardfacing. Prior to workhardening, Mn-steel has low abrasion resistance. Applying a suitable hardfacing on top of a manganese buildup deposit significantly increases the part life.
The toughness of Mn-steel can be lost if the base metal temperature exceeds 500 F (260 C). Therefore, avoid applying prolonged and concentrated heat to any single area on the part because overheating causes embrittlement. Using higher travel speeds can also help reduce the size of the heat-affected zone.
Weld Mn-steel base metals only with manganese alloy consumables. Avoid using carbon steels as they cause brittle weld deposits that will spall. Stoody Nicromang and Dynamang products in wire or stick electrode forms are commonly applied for buildup and hardfacing in earth moving and crusher applications for impact resistance. Stoody 110 and 2110 can also be used for this as well as for joining Mn-steel to carbon steel.
Step 2. Identify the wear factor
Once the base metal has been identified, the next step is to identify the type of wear to which the part is subjected. This is an important factor when determining the best hardfacing alloy to use. There are several wear categories:
- Abrasion-gouging (high impact), accounting for 55 to 60 percent of wear, is caused by high-stress grinding, repeated grinding or crushing against equipment. It can also be caused by low-stress scratching and slow wear from scouring action across equipment.
- Adhesive wear (usually metal-to-metal) occurs from scuffing, scoring, galling and seizing (frictional wear).
- Cavitation erosion is metal loss caused by the implosion of vapor cavities at the metal part surface. This occurs in liquid handling systems, pumps, impellers, propellers and hydro-turbine runners.
- Fretting corrosion is encountered when closely fitted metal parts are subjected to vibration loading and stress.
- Corrosion, both galvanic and chemical.
- High-temperature wear results from rapid cooling following high heat.
- High-compression wear results from mechanical stress caused by large compressive loads.
Individually, or in combination, these wear categories determine the best choice of hardfacing alloy to minimize wear and extend part life.
Step 3. Select an alloy
If restoration of a dimension is needed prior to hardfacing, select a buildup material compatible with the base metal and the final overlay. The alloy content of the hardfacing material determines hardness and abrasion resistance. The hardness of the hardfacing bead is not the major factor providing wear resistance; the percentage of carbide and the type of carbide structure in the matrix determine how well a hardfacing overlay will resist wear.
Hardfacing alloys may contain one or more of the following carbide types in the matrix: iron, chromium, molybdenum, tungsten, columbium (niobium), vanadium and titanium. When a hardfacing product forms more than one type of carbide, it is referred to as a “complex carbide.”
High-carbon, high-chromium alloys have good abrasion resistance with moderate resistance to impact and corrosion. Where abrasion resistance is the primary performance requirement and deposit cost is a concern, using iron-based, high-carbon and high-chromium hardfacing alloys is a good option.
A reduction in deposit hardness and wear resistance may occur if a hardfacing is subjected to a maximum service temperature that exceeds its recommended usage. For applications involving abrasion with high temperature and corrosion, use cobalt- or nickel-based alloys. Tungsten carbide is the best choice to resist metal-to-earth and fine-particle abrasion. Nickel-based tungsten carbide alloys, such as Stoody 160FC or the latest Stoody 160FS, with spherical tungsten carbides, offer superior performance when abrasion resistance in ground engaging applications is warranted. However, it should not be used above 1,000 F (538 C). At higher temperatures, the matrix softens, thus reducing wear resistance.
Deposits with higher alloy contents (and wear resistance) tend to cross check (form hairline cracks across the weld beads). A regular check pattern is beneficial in many applications. It reduces or eliminates distortion. Irregular crack patterns, such as cracks running in the direction of the weld beads, can result in spalling, especially under high impact.
In addition, some hardfacing wires are designed to reduce the amount of inter-pass cleaning. These alloys produce little or no slag and allow a second pass to be applied without cleaning.
Step 4. Select the weld process
Process selection likely depends on the welding equipment available and operator skill. While hardfacing may be performed using most welding processes, larger scale operations most commonly use the stick, flux-cored, open arc or submerged arc welding process. There are advantages and disadvantages to using each welding process.
Stick welding is the most common process. Hardfacing alloys are easily accessible, can be used for welding on a range of thicknesses and for welding in or out of position. In addition, stick welding equipment is simple and mobile, making it especially suited for field work.
As with stick welding, there are many alloys available for hardsurfacing using the flux-cored or open arc welding process. These wire welding processes increase deposition rates and produce welds with good integrity.
While most welding is performed in the flat or horizontal position, wires are available for all-position welding. Stoody’s iron-based non-martensitic all-position wires 964 AP-G, 965 AP-G and buildup AP-G, for instance, can be applied without repositioning of a part, freeing welders to choose whether to apply hardfacing alloys in situ or in a shop environment. Because repositioning is no longer necessary, mobile welding systems can be employed in situ – on location where the part or equipment is in service. This can further reduce costs attributable to longer periods of downtime and resources traditionally expended to disassemble, transport and reassemble heavy equipment requiring maintenance.
Submerged arc welding may make sense when buildup on larger parts is required and they can be welded in the flat position (or, in the case of cylindrical parts, rotated). The process is typically automated. Using the correct flux and wire combination provides the required performance. Using incorrect flux and wire combinations can, however, result in deposit chemistry issues and even part failure.
Step 5. Apply bead patterns
A cost-saving alternative to hardfacing an entire surface is to apply weld beads spaced from 1/4 in. (6.3 mm) to 5/8 in. (15.9 mm) apart and not in the same direction as the flow of abrasive material across the part. Weld beads applied perpendicular to abrasive flow work particularly well for fine abrasive materials like sand or soils.
For shovel teeth subjected to wear from rock, ore, slag or similar material, apply hardfacing beads parallel to the abrasive material flow. These weld beads will act as directional runners, allowing the abrasive material to ride along the weld beads, protecting the base metal from erosion.
A waffle (crisscross) or herringbone hardfacing bead pattern works well with sand or soil containing clay. Form the herringbone pattern by laying weld beads at varying angles. The overburden packs into the spaces between the weld beads, affording further base metal surface protection.
Apply a dot pattern to part areas subjected to wear, but not heavy abrasion. Use dot patterns on parts where distortion may be a problem, such as with base metals that are sensitive to overheating. Applications may include Mn-steel or thinner material where embrittlement may occur.
In some applications, hardfacing deposits applied to part surfaces and edges may spall from impact and high-compression side loadings. By applying the hardfacing into grooves cut into the base metal or surrounding impact-resistant weld metals, such as Mn-steel, spalling potential is greatly reduced. This method also extends service life of more brittle hardfacings used under high-impact conditions.