Hardfacing is a welding technique where a wear-resistant alloy is deposited onto the surface of a softer base metal to protect it from abrasion, impact, erosion, or corrosion. Unlike standard welding, which joins two pieces together, hardfacing is purely about building up a protective layer on a single component. It can restore a worn part to its original dimensions or make a brand-new part last several times longer than it otherwise would.
How Hardfacing Works
The basic idea is straightforward: you melt a harder material onto the surface of a softer one. The filler alloy bonds metallurgically with the base metal, creating a surface layer that can withstand conditions the original material cannot. This overlay is typically one to several layers thick, depending on the severity of wear the part will face and the properties you need.
One critical factor in any hardfacing job is dilution, which refers to how much the base metal mixes into the deposited layer. When too much of the softer base metal blends into the overlay, it reduces the hardness and wear resistance of the finished surface. Welders control dilution by adjusting heat input, travel speed, and technique. The first layer deposited directly on the base metal always has the highest dilution, which is why multiple layers often perform significantly better. Each subsequent pass has less base-metal contamination and gets closer to the full hardness of the filler alloy.
Welding Processes Used for Hardfacing
Several standard welding methods can apply a hardfacing overlay. The best choice depends on the size of the part, the production volume, and how precise the deposit needs to be.
- Flux-cored arc welding (FCAW) is one of the most popular choices for hardfacing because it delivers high deposition rates and good penetration. It works well for large surface areas where speed matters more than a perfect finish.
- MIG welding (GMAW) also offers high welding speeds and better control on thinner materials, though it struggles with very thick base metals and certain positions like vertical or overhead.
- Stick welding (SMAW) is the most portable and accessible option, requiring no shielding gas or wire feeder. It works well for field repairs on mining or agricultural equipment, though it produces lower deposition rates and generally rougher deposits.
- TIG welding (GTAW) produces the highest quality deposits with precise control over the weld pool, making it ideal for critical components like valve seats. The tradeoff is a much slower welding speed and a steeper skill requirement.
- Plasma transferred arc welding and laser cladding are used in specialized industrial settings, such as nuclear reactor components, where extremely controlled deposits and minimal dilution are essential.
Types of Hardfacing Alloys
Hardfacing alloys fall into several families, each suited to different operating conditions.
Iron-Based Alloys
These are the most common and cost-effective hardfacing materials. Iron-chromium alloys with high carbide content are the workhorses for abrasion resistance in mining, agriculture, and earthmoving equipment. They can reach hardness levels of 61 to 67 on the Rockwell C scale, which is comparable to a hardened file. Iron-based alloys are also being developed as replacements for cobalt alloys in nuclear applications, since cobalt becomes radioactive under neutron bombardment. Newer iron-based formulations have achieved hardness levels exceeding their cobalt-based competitors while maintaining good corrosion resistance.
Cobalt-Based Alloys
Stellite 6 is the most well-known cobalt hardfacing alloy. It excels in environments with high contact pressures, elevated temperatures (up to 300°C), and corrosive conditions. It has been the traditional choice for valve components in nuclear reactors, petrochemical plants, and marine engines. Cobalt alloys maintain their hardness at high temperatures better than most alternatives, which is why they remain the standard for hot-working applications like forging dies and shearing blades.
Nickel-Based Alloys
Nickel alloys are chosen where corrosion resistance is the primary concern, particularly in chemical and petrochemical processing. They perform well in pumps and valves handling aggressive chemicals. Their as-welded hardness is lower than iron-chromium or cobalt options, but they offer superior resistance to chemical attack.
Tungsten Carbide
For the most extreme abrasion, tungsten carbide particles are embedded in a metal matrix. Individual tungsten carbide particles reach hardness values of 2000 to 3000 on the Vickers scale, far exceeding any of the alloy families above. These overlays dramatically outperform chromium carbide deposits in severe abrasion and are used on drill bits, dredge cutters, and other components that grind directly against rock or highly abrasive material. The hardness of the deposit increases with each additional layer, as more carbide dissolves into the surrounding matrix and transforms it into something resembling tool steel.
Types of Wear Hardfacing Protects Against
Not all wear is the same, and matching the right alloy to the right wear mechanism is the core skill of hardfacing selection.
Abrasion is the most common target. It occurs when hard particles like sand, rock, or ore slide or gouge across a surface. High-hardness alloys with hard carbide phases resist this best, because the surface is simply too hard for the abrasive particles to cut into efficiently.
Impact wear happens when repeated blows deform or fracture a surface. Here, hardness alone can backfire. Very hard deposits tend to be brittle, so they crack under heavy impact. Parts subject to impact, like crusher hammers, need alloys that balance hardness with fracture toughness, the ability to absorb energy without cracking. More ductile materials deform under impact rather than shattering, which reduces material loss over time.
Erosion is caused by particles or fluid striking a surface at speed, common in fans, conveyor systems, and piping. Resistance to erosion depends on a combination of hardness, yield strength, and ductility. Harder materials resist the compressive forces of impact, while ductile materials absorb kinetic energy and deform rather than losing chunks of material.
Corrosion and combinations of corrosion with abrasion or erosion demand alloys with specific chemistry, typically nickel or cobalt-based, that can resist chemical attack while still providing a hard surface.
Where Hardfacing Is Used
Mining and earthmoving consume more hardfacing consumables than any other sector. Excavator teeth, crusher jaws, grinding mill liners, conveyor screws, and vertical crushers all operate in constant contact with abrasive rock and soil. Without hardfacing, these parts would need replacement far more frequently.
Agricultural equipment faces similar challenges. Plowshares, tillage discs, and harvester components work directly against soil and crop residue, wearing down quickly. Hardfacing these parts before first use or rebuilding them after wear is standard practice.
Power generation relies on hardfacing for boiler tubes exposed to erosive fly ash, turbine components, and in nuclear plants, valve seats and control rod sheaths that must resist both wear and corrosion at elevated temperatures.
The petrochemical industry uses cobalt and nickel-based hardfacing on valves, pumps, and fittings that handle corrosive fluids under high pressure. In these applications, a failed component can mean a plant shutdown costing far more than the hardfacing itself.
Cost Savings Over Part Replacement
The economics of hardfacing are often dramatic. Research published in Tribology in Industry examined real-world repair cases and found that hardfacing a worn industrial part cost less than 6% of the price of a new replacement in one case, and less than 13% in another. A new part priced at roughly €84,000 was restored for under €5,000. In another example, a €26,500 component was repaired for €3,380.
Beyond the direct cost of the part, hardfacing reduces downtime. Ordering a new component means waiting for manufacturing, shipping, and customs. A hardfacing repair can often be done on-site or at a local shop within days. Energy consumption tells a similar story: repairing a worn forging hammer ram through hardfacing used 2.6 times less electricity than manufacturing a new one from scratch.
The performance case is equally strong. Properly hardfaced parts frequently match or exceed the service life of brand-new parts. New parts that are hardfaced before entering service can last several times longer than identical parts without an overlay. Research estimates that roughly three-quarters of all worn mechanical parts are candidates for hardfacing restoration.
Thermal Management During Hardfacing
Controlling heat before, during, and after hardfacing is essential to preventing cracks. Preheating the base metal before welding reduces the temperature difference between the hot deposit and the cooler surrounding material, which lowers thermal stress. The required preheat temperature depends on the base metal composition and thickness, and is specified in the welding procedure for each job.
Interpass temperature, the temperature of the part between successive weld passes, also needs to stay within a defined range. If the part gets too hot, the metallurgy of the deposit can change in undesirable ways. If it cools too much between passes, cracking risk increases. Both the preheat and interpass temperatures should be maintained for a distance at least equal to the thickness of the part (no less than 75 mm) in all directions from the weld zone.
High-hardness deposits, particularly chromium carbide overlays, are inherently prone to a network of fine surface cracks called check cracks or stress-relief cracks. These are normal and expected in many abrasion-resistant overlays. They relieve internal stress without compromising the overlay’s ability to resist wear. However, cracks that propagate into the base metal are a real problem, which is why proper preheat and controlled cooling matter so much.