Hard Facing

A worn-out part does not always mean a scrapped part. Hard facing transforms components headed for the scrap bin into assets that outperform the originals — and the difference between choosing the right alloy and the wrong one is the difference between a machine that runs for years and one that fails in weeks.

The Day the Crusher Stopped

Marcus Obi ran a mid-size aggregate processing operation. His primary jaw crusher — the workhorse responsible for breaking down 400 tons of raw limestone per shift — had been grinding slower for weeks. The manganese jaw plates were worn past their service limit. Replacement plates would cost a significant portion of his annual maintenance budget and take three weeks to ship.

His welder, Priya Nazari, walked the floor with him and pointed at the worn contact surfaces. "We don't replace these," she said. "We rebuild them."

What Priya proposed was hard facing — a process that would deposit a wear-resistant alloy directly onto the degraded surfaces, restoring the jaw plates to full service condition in days rather than weeks, at a fraction of the replacement cost.

This is not a niche trick. Hard facing is one of the most powerful — and most underutilized — surface engineering methods in manufacturing. Whether you are maintaining mining equipment, rebuilding agricultural implements, extending the life of cutting tools, or engineering new components with built-in wear resistance, hard facing gives you a decisive advantage.

This guide covers everything: the process fundamentals, every major alloy family, hardness data, resistance properties, application methods, selection criteria, and the critical role of chromium plating as a complementary finishing process. No theory is left untested. No alloy is left unexplained.

What Is Hard Facing?

Hard facing is a method of adding a coating, edge, or point of a metal or alloy capable of resisting abrasion, corrosion, heat, or impact to a metal component.

The process applies equally well to:

• New parts — engineering superior wear surfaces from the start
• Old worn parts — restoring and even upgrading degraded components

Think of it as giving your metal parts an armor layer — one engineered specifically for the punishment they will endure in service.

The Core Application Methods

Hard facing materials are deposited using welding and spraying processes. Each method has distinct advantages depending on the geometry of the part, the alloy being deposited, and the production environment.

Welding vs. Spraying: The Critical Distinction

Welding-based hard facing creates a metallurgical bond between the deposited alloy and the substrate. The two metals fuse together at the atomic level. This bond is extremely strong but introduces dilution — the mixing of base metal into the deposited layer that can reduce hardness and alter alloy properties.

Spraying creates a mechanical bond. The coating adheres to the surface through interlocking of sprayed particles. Porosity is generally higher, but the process allows for rapid coverage of large areas with minimal heat input to the workpiece.

Laser cladding occupies a unique position. It produces a dense, homogeneous, nonporous clad layer that is metallurgically bonded to the substrate — with dilution rates below 2%, compared to 5–15% for plasma arc and 20–25% for stick electrode processes. The result is superior coating integrity with minimal alteration of the deposited alloy's designed properties.

The Dilution Problem

Dilution is the total volume of the surface layer contributed by melting of the substrate. It is the single most important variable affecting the final hardness and performance of a hard-facing deposit.

\text{Dilution (%)} = \frac{\text{Volume of Base Metal Melted into Deposit}}{\text{Total Volume of Deposit}} \times 100

The greater the dilution, the lower the hardness. This is why arc-welded deposits consistently show wider hardness ranges than gas-welded deposits — more base metal mixing means less predictable alloy properties in the deposited layer.

How to Select a Hard-Facing Material

The first thing to be considered in the selection of a hard-facing material is the type of service the part in question is to undergo.

Beyond service conditions, you must evaluate:

• Machinability — Can you finish-machine the deposit after application?
• Cost of the hard-facing material — Does the alloy cost justify the performance gain?
• Porosity of the deposit — Is a dense coating critical, or is slight porosity acceptable (as in bearing surfaces where oil retention is desirable)?
• Appearance in use — Does the surface need to maintain a polished or finished look?
• Ease of application — Can your shop apply the material with existing equipment?

The Fundamental Rule of Hard Facing

Generally, the greater the hardness of the facing material, the greater is its resistance to abrasion and shock or impact wear.

But hardness alone does not tell the full story. Some alloys sacrifice machinability for hardness. Others trade impact resistance for abrasion resistance. The selection always involves trade-offs — and the following alloy guide gives you the data to make those trade-offs intelligently.

The Hardenable Base Metals

Many hardenable materials can be used for hard facing, including:

• Carbon steels
• Low-alloy steels
• Medium-alloy steels
• Medium-high alloys

However, none of these is outstanding for hard-facing applications. They serve as functional overlays where extreme wear resistance is not required, but for demanding service, you need one of the six specialized alloy families detailed below.

The Six Major Hard-Facing Alloy Families

This is the core of hard-facing material science. Each alloy family occupies a distinct performance envelope defined by hardness, impact resistance, temperature capability, corrosion resistance, and machinability.

1. High-Speed Steels (RFe5 / EFe5)

The Struggle: Tools That Lose Their Edge at High Temperatures

Imagine a production line where forming dies shape hot metal at temperatures approaching 1100°F. Standard tool steel overlays soften and wear rapidly. The dies need regrinding every few shifts, and production stops each time.

High-speed steels solve this problem by maintaining exceptional hardness at elevated temperatures — the same property that makes them dominant in cutting tool metallurgy.

Designations and Forms

Typical Applications

• Cutting tools
• Shear blades
• Reamers
• Forming dies
• Shearing dies
• Guides
• Ingot tongs
• Broaches

Hardness Data

Key insight: The as-deposited hardness of 60 HRC falls off very slowly up to 1100°F, dropping only to 47 HRC. This retained hot hardness is what makes high-speed steels invaluable for elevated-temperature tooling overlays. Above 1200°F, the advantage disappears — hardness drops to 30 HRC.

Resistance Properties

Other Critical Characteristics

• Metal-to-metal wear: Excellent, especially at elevated temperatures
• Hot hardness retention: Outstanding — the defining property of this alloy family
• Surface polish: Can take a high polish
• Machinability: Must be annealed first before machining; full hardness can be regained through subsequent heat treatment

The Transformation for Marcus's Operation

High-speed steels were not the right choice for Marcus's jaw crusher — the service involved heavy impact, not elevated temperature. But Priya filed this alloy away for another client who needed forming die overlays running at 900°F. The retained hardness of 55+ HRC at that temperature meant die life tripled.

2. Austenitic Manganese Steels (EFeMn)

The Inciting Incident: When Impact Is the Primary Destroyer

Marcus's jaw crusher plates took a pounding — literal impact from boulders dropping into the crushing chamber. This is the domain of austenitic manganese steel, the only hard-facing alloy specifically engineered for high-impact, metal-to-metal wear.

Designations and Forms

Note: Austenitic manganese steels are available primarily in electrode form for hard facing.

Typical Applications

• Rock-crushing equipment
• Railway frogs and crossings
• Impact-wear surfaces
• Heavy equipment buckets and teeth

Hardness Data

This is the critical insight: The as-deposited hardness is relatively low — only 170 to 230 BHN. But austenitic manganese steels work-harden rapidly under impact. Every blow from a rock drives the surface hardness upward, eventually reaching 450 to 550 BHN in service.

The Work-Hardening Mechanism

\text{Surface Hardness}_{\text{in service}} = f(\text{Impact Energy} \times \text{Cycles})

The yield strength of the deposited metal in compression starts low, but any compressive deformation rapidly raises it until plastic flow ceases. This self-strengthening behavior is an extraordinary asset in impact wear situations — the harder the service punishes the surface, the harder the surface becomes.

Resistance Properties

Critical limitation: These metals have no practical hot hardness. They become brittle when reheated above 500 to 600°F. Hard facing with austenitic manganese steels must avoid overheating the deposit, and service temperatures must remain well below this threshold.

Machinability

Machining is difficult with ordinary tools and equipment. Finished surfaces are usually ground. The work-hardened surface resists conventional cutting tools — the same property that makes the alloy excellent in service makes it challenging in the machine shop.

Priya's Decision

This was the alloy Priya chose for Marcus's jaw crusher plates. The impact loading was severe, temperatures were ambient, and the self-hardening property meant the rebuilt plates would actually get tougher in service. She deposited multiple passes of EFeMn electrode, building up the worn surfaces to original dimensions.

Within a week, the crusher was back in service. Within a month, the work-hardened surfaces had reached nearly 500 BHN — harder than the original plates had been when new.

3. Austenitic High-Chromium Irons (RFeCr-A / EFeCr-A)

The Scene: Relentless Abrasion Without Heavy Impact

Not every wear problem involves impact. In agricultural equipment, coke chutes, steel mill guides, sand-blasting cabinets, and brick-making machinery, the dominant failure mode is low-stress scratching abrasion — a constant grinding away of surface material by hard particles moving across the surface.

Austenitic high-chromium irons are engineered for exactly this service.

Designations and Forms

Typical Applications

• Agricultural machinery parts
• Coke chutes
• Steel mill guides
• Sand-blasting equipment
• Brick-making machinery

Hardness Data

The hot hardness profile reveals a critical design boundary. These alloys maintain useful hardness up to about 800–900°F. Above that, hardness degrades rapidly — at 1200°F, instantaneous hardness drops to just 5 HRC. However, the decrease in hardness during hot testing is practically recovered on cooling — this is not a permanent softening like most alloys experience.

Resistance Properties

The Abrasion Paradox

Here is where many engineers make costly mistakes: Low-stress scratching resistance is outstanding, but high-stress grinding abrasion performance is only mediocre.

The distinction matters enormously:

• Low-stress scratching: Particles slide across the surface at low contact pressures (sand flowing through a chute, grain moving over a plow blade)
• High-stress grinding: Particles are crushed between two surfaces at high contact pressures (crushing chamber walls, ball mill liners)

If you specify austenitic high-chromium iron for a grinding application, the deposit will wear far faster than expected. The hard carbides that provide outstanding scratch resistance cannot withstand the fracture mechanics of high-stress grinding.

Mechanical Properties

Machinability

These deposits are considered commercially unmachinable and are also very difficult to grind.

When grinding is required:

4. Cobalt-Base Alloys (RCoCr / ECoCr)

The Aha Moment: When Nothing Else Survives the Heat

At 1200°F and above, most hard-facing alloys have surrendered their hardness. Cobalt-base alloys are the exception — they maintain outstanding elevated-temperature strength and hardness where other materials fail.

Designations and Forms

Typical Applications

• Exhaust valve contact surfaces (aircraft, truck, bus engines)
• Valve trim in steam engines
• Pump shafts (corrosion + erosion service)
• High-temperature sliding wear surfaces

The Three Cobalt-Chromium Grades

Three formulations cover the range from impact-tolerant to maximum-hardness:

Hardness Data

Gas-Welded Deposits:

Arc-Welded Deposits:

Why the wider range for arc-welded deposits? The values depend primarily on base metal dilution. The greater the dilution, the lower the hardness. Gas welding produces tighter hardness ranges because it generally results in less dilution than arc welding.

The Exceptional Hot Hardness Property

Many surfacing alloys are softened permanently by heating to elevated temperatures. Cobalt-base alloys are exceptional. They do exhibit lower hardness values when hot, but they return to their approximate original hardness values upon cooling.

This reversible hardness behavior makes cobalt-base alloys uniquely suited for thermal cycling environments like internal combustion engine exhaust valves.

Temperature Service Guidelines

Resistance Properties

Machinability

• CoCr-A: Preferably machined with sintered carbide tools
• CoCr-B and CoCr-C: Increasingly difficult as carbon content increases
• CoCr-C deposits: Finished by grinding (machining is impractical)

5. Copper-Base Alloys

The Scene: Bearing Surfaces, Corrosion, and the Art of Controlled Wear

Not all hard-facing applications involve extreme hardness. Bearing surfaces require a specific hardness relationship with their mating surface. Corrosion-resistant overlays need chemical stability. Copper-base alloys fill these roles with a versatility that spans from soft bearing surfaces to hard wear plates.

Designations and Forms

Copper-base alloys are available in an extensive range of rod and electrode forms:

Rods: RCuAl-A2, RCuAl-B, RCuAl-C, RCuAl-D, RCuAl-E, RCuSi-A, RCuSn, RCuSn-D, RCuSn-E, RCuZn-E

Electrodes: ECuAl-A2, ECuAl-B, ECuAl-C, ECuAl-D, ECuAl-E, ECuSi, ECuSn-A, ECuSn-C, ECuSn-E, ECuZn-E

Application Guide by Alloy Group

Hardness: The Welding Process Factor

Hardness of a deposit depends upon the welding process employed and the manner of depositing the metal.

Critical temperature limitation: Copper-base alloys are not recommended for use at elevated temperatures because their hardness and mechanical properties decrease consistently as the temperature goes above 400°F.

Resistance Properties

Corrosion resistance: With the exception of CuSn-E and CuZn-E, copper-base alloys are widely used to resist many acids, mild alkalies, and salt water. The CuAl filler metals form a protective oxide coating upon exposure to the atmosphere.

Abrasion limitation: Copper-base alloy deposits are not recommended for use where severe abrasion is encountered in service. Their strength lies in metal-to-metal wear resistance, bearing service, and corrosion protection — not in resisting hard-particle abrasion.

Bearing Surface Selection Rule

Metals selected for bearing surfaces should have a Brinell hardness of 50 to 75 units below that of the mating metal surface.

This hardness differential ensures that any wear occurs preferentially on the bearing surface (which can be rebuilt via hard facing) rather than on the more expensive shaft or housing.

Slight porosity is generally acceptable in bearing service because a porous deposit is able to retain oil for lubricating purposes — a deliberate design advantage.

Mechanical Properties in Compression

Machinability

All copper-base alloy deposits can be machined. This is a significant advantage over many other hard-facing alloy families, particularly the austenitic high-chromium irons and the harder grades of nickel-chromium-boron alloys.

6. Nickel-Chromium-Boron Alloys (RNiCr / ENiCr)

The Transformation: Corrosion Resistance Meets Wear Resistance

When a cement plant needed to rebuild the screws on its slurry pumps, conventional hard-facing alloys presented a dilemma. The screws faced simultaneous metal-to-metal wear, abrasive scratching from suspended solite particles, and constant corrosion from the alkaline cement slurry. No single alloy family seemed to cover all three failure modes.

Nickel-chromium-boron alloys answered the call. Their combination of metal-to-metal wear resistance, scratch-abrasion resistance, corrosion resistance, and retained hardness at elevated temperatures makes them the multi-threat alloy of the hard-facing world.

Designations and Forms

The Three NiCr Formulations

The lower hardness values and greater hardness ranges of electrode deposits are attributed to the dilution of deposit and base metals — the same phenomenon observed across all hard-facing alloy families.

Typical Applications

• Seal rings
• Cement pump screws
• Valves
• Screw conveyors
• Cams

Hot Hardness Data

The following data shows Rockwell C hardness values across temperature ranges from 600 to 1000°F, under conditions from instantaneous loading to 3-minute loading intervals:

NiCr-A:

NiCr-B:

NiCr-C:

Rod deposits consistently outperform electrode deposits in hot hardness because of the lower dilution inherent in rod (gas welding) applications.

Resistance Properties

The galling resistance of NiCr alloys — especially NiCr-C — is a standout property. In applications where two metal surfaces slide against each other under high load (seal rings, valve seats), galling can destroy components in hours. NiCr deposits resist this failure mode exceptionally well.

Mechanical Properties

Machinability

Deposits of NiCr filler metals may be machined with tungsten carbide tools using:

• Slow speeds
• Light feeds
• Heavy tool shanks (rigidity is critical)

They are also finished by grinding using a soft-to-medium vitrified silicon carbide wheel.

Master Comparison: All Six Alloy Families at a Glance

This is the table you print and hang in the shop. When a worn part comes across your workbench, this comparison tells you where to start.

Plasma Arc Surface Coating: The High-Volume Hard-Facing Process

When production volumes demand rapid deposition or when the substrate cannot tolerate high heat input, plasma arc surface coating becomes the process of choice.

Transferred Arc Process

The transferred arc strikes between the electrode and the workpiece, producing a true metallurgical weld. Arc temperatures range from 25,000 to 50,000°F, and deposition occurs rapidly:

Dilution of the base metal can be held below 5% if required — a significant advantage over conventional arc welding methods.

Non-Transferred Arc Process (Metal Spraying)

In the non-transferred arc process, the arc strikes between the electrode and the torch nozzle — it does not attach to the workpiece. This process is used for:

• Building up surfaces for hard facing
• Application of anticorrosion and barrier layers

Over 500 different powder combinations are available, and deposition rates can reach up to 100 lb/h (45 kg/h).

High-Velocity Plasma Systems

Advanced systems operating at higher voltage settings produce:

• Plasma arc lengths at temperatures over 10,000°F
• Plasma velocity of approximately 12,000 ft/s
• Extremely dense coatings with less than 1% porosity
• Current ranges of 30 to 500 amps

Laser Cladding: The Precision Alternative

Laser cladding represents the highest-precision method for applying hard-facing materials. A shaped or defocused laser beam heats either preplaced or gravity-fed powdered alloys, which melt and flow across the substrate surface, rapidly solidifying when laser power is removed.

Process Variables

• Laser power
• Beam or part travel speed
• Clad thickness
• Substrate thickness
• Powder feed rate
• Shielding gas

Compatible Alloys

Many alloys currently used in plasma arc or metal inert gas cladding can be used with laser cladding:

• Stellites (cobalt-base)
• Colmonoys (nickel-base)
• Carbide-containing alloys
• Inconel
• Triballoy
• Fe-Cr-C-X alloys
• Tungsten and titanium carbides

The Key Technical Advantage: Minimal Dilution

The result is a dense, homogeneous, nonporous clad layer that is metallurgically bonded to the substrate — in contrast to the mechanically bonded, more porous layer produced by spraying methods.

Chromium Plating: The Finishing Complement to Hard Facing

Hard facing and chromium plating are not competing processes — they are complementary. Hard facing rebuilds geometry and provides bulk wear resistance. Chromium plating adds a final surface layer of extreme hardness and corrosion protection.

What It Is

Chromium plating is an electrolytic process of depositing chromium on metals either as a protection against corrosion or to increase the surface-wearing qualities.

The Gage Application: Proven Beyond Doubt

The value of chromium-plating plug and ring gages has been more thoroughly demonstrated than any other single application. Chromium-plated gages not only wear longer, but when worn, the chromium may be removed and the gage replated and reground to size. This makes the gage effectively immortal — it can be recycled through plating and grinding cycles indefinitely.

Performance Across Materials

Chromium-plated tools have given greatly improved performance on nearly all classes of materials:

• Brass, bronze, copper
• Nickel, aluminum
• Cast iron, steel
• Plastics, asbestos compositions

Tool Types That Benefit

Critical Preparation Requirements

Special care is essential in grinding and lapping tools preparatory to plating the cutting edges. The chromium deposit is influenced materially by the grain structure and hardness of the base metal. Poor surface preparation leads to poor adhesion and premature flaking of the chromium layer.

Plating Thickness Guidelines

Decision Framework: Choosing the Right Hard-Facing Strategy

When a worn or new part arrives at your workbench, run through this decision tree:

Step 1: Identify the Primary Wear Mechanism

Step 2: Evaluate Secondary Requirements

• Must you machine after deposition? Eliminate high-chromium irons (unmachinable). Prefer copper-base (all machinable) or high-speed steels (machinable after annealing).
• Is the part exposed to corrosive media? Eliminate high-speed steels and manganese steels. Prefer NiCr-boron (completely resistant to atmospheric, steam, and salt corrosion) or copper-base alloys (resist acids, alkalies, salt water).
• Does the part experience thermal cycling? Cobalt-base alloys recover hardness on cooling. Most other alloys soften permanently.
• Is dilution control critical? Use laser cladding (<2%) or plasma arc surfacing (<5%) rather than SMAW (20–25%).

Step 3: Select the Deposition Method

Step 4: Specify Post-Processing

The Universal Takeaway

Marcus's rebuilt jaw crusher plates ran for fourteen months before needing attention again — longer than the original plates had lasted. The cost of hard facing was roughly one-fifth of full replacement. More importantly, the downtime was measured in days rather than weeks.

Hard facing is not a repair technique. It is a design strategy.

When you specify hard facing on new components, you are engineering the wear surface independently of the base metal. You can use inexpensive, tough base materials for structural strength and deposit exactly the alloy needed for surface performance. When you use hard facing for rebuilds, you are not merely restoring a part — you are upgrading it.

The six alloy families in this guide cover the vast majority of industrial wear scenarios. The application methods span from manual field repair to precision laser cladding. The decision framework gives you a systematic path from worn part to rebuilt asset.

Every worn surface is a decision point. Replace and spend, or rebuild and invest. The metallurgy is proven. The processes are mature. The only variable left is whether you apply them.

Your Next Step

Pull one component from your current maintenance backlog — the part that wears out most frequently, costs the most to replace, or causes the most downtime when it fails. Run it through the decision framework above. Identify the primary wear mechanism, match it to the right alloy family, and get a quote from a qualified hard-facing shop.

That single rebuilt component will teach you more about hard facing than any reference manual ever could. And once you see the cost savings and performance gains firsthand, you will never look at a worn part the same way again.

What is the most expensive wear problem in your operation right now — and which of these six alloy families would you use to solve it?