The Worn-Out Valve That Changed Everything
The exhaust valve had seen better days.
the practitioner, a maintenance engineer at a large fleet depot, held the valve up to the light and traced his finger along the pitted, eroded contact surface. Six months in service and it looked like it had survived a decade. The replacement cost was manageable for one valve — but across a fleet of 200 heavy-duty engines, the math was brutal.
"We're replacing these every six months," the practitioner told his supervisor. "That's 400 valves a year, plus downtime."
His supervisor slid a catalog across the desk. "What if you didn't have to replace them at all?"
That catalog contained something the practitioner had only vaguely heard of in engineering school: metal surfacing processes. Hard facing. Flame spraying. Plasma arc coating. Techniques that could deposit wear-resistant, corrosion-resistant, heat-resistant alloys onto metal components — extending their service life by factors of 5x, 10x, or more.
What the practitioner discovered over the next several months would transform the way he thought about metal components entirely. You don't always need a new part. Sometimes you need a better surface.
This guide contains everything the practitioner learned — and everything you need to know to master metal surfacing processes from first principles.
What Is Metal Surfacing — and Why Does It Matter?
Metal surfacing is the practice of depositing a layer of specialized metal or alloy onto the surface of a component to enhance its resistance to abrasion, corrosion, heat, or impact. The deposited layer may be harder, more chemically resistant, or better suited to elevated temperatures than the base metal underneath.
You can apply surfacing to new parts (to prevent wear before it starts) or worn parts (to restore and improve them beyond their original condition).
The core methods include:
- Hard Facing — Weld-depositing specialized alloys onto surfaces using oxyacetylene, shielded-metal arc, submerged arc, plasma arc, or inert-gas-shielded arc processes
- Flame Spraying — Melting wire or powder feedstock and propelling it onto a prepared substrate
- Plasma Arc Surface Coating — Using extreme-temperature plasma to deposit alloys with metallurgical or mechanical bonds
- Chromium Plating — Electrolytically depositing chromium for wear resistance and low friction
- Laser Cladding — Using focused laser energy to fuse coating materials to surfaces
Each process has distinct characteristics, advantages, and ideal applications. Choosing the wrong one costs you time, money, and performance. Choosing the right one can make a component outlast the machine it sits in.
Flame Spraying — Building Surfaces Atom by Atom
The Process That Started It All
Flame spraying (originally called "metal spraying") is the forerunner of all modern thermal spray technologies. In this process, metals, alloys, ceramics, and cermets are deposited on metallic or other surfaces to build up worn or undersized parts, provide wear-resistant or corrosion-resistant surfaces, and correct defective castings.
How Flame Spraying Works
There are three primary equipment configurations, each using different feedstock forms and heat sources.
Wire-Fed Flame Spraying
In this configuration, wire is fed automatically through the nozzle of a spray gun. A combustible gas (typically acetylene, though other gases may be used), oxygen, and compressed air serve to melt and blow the atomized metal against the surface to be coated.
Key characteristics:
- Any desired thickness of metal may be deposited
- Depositable metals include steels (low to high carbon), brass and bronze compositions, babbitt metal, tin, zinc, lead, nickel, copper, and aluminum
- Movement control can be mechanical or manual
- Common practice for shafts, spindles, and cylindrical components involves clamping the gun in a lathe toolholder and using the feed mechanism to traverse the gun at a uniform rate while the workpiece rotates
Typical production applications using wire-fed flame spraying:
| Application | Industry | Purpose |
|---|---|---|
| Exhaust valve coating | Automotive | Wear resistance at high temperatures |
| Transfer ink roller refinishing | Printing | Restore dimensional accuracy |
| Clutch plate rebuilding | Trucking/Heavy Equipment | Restore worn friction surfaces |
| Glass meter box metallizing | Utilities | Apply conductive coatings |
| Aluminum on cloth gauze | Electronics | Produce electrolytic condenser plates |
| Zinc/copper on ceramics | Electrical | Coat ceramic insulators |
Powder-Fed Flame Spraying
Instead of wire, this equipment uses metal, refractory, and ceramic powder as the feedstock. The system ordinarily employs two gases — oxygen and a fuel gas (usually acetylene, sometimes hydrogen).
Powder reservoirs vary by application:
- Hand-held equipment — Small reservoir
- Lathe-mounted or production equipment — Larger reservoir
The four basic types of coating powders are:
- Ceramics — For extreme heat and wear resistance
- Oxidation-resistant metals and alloys — For high-temperature service
- Self-bonding alloys — For simplified surface preparation
- Alloys for fused coatings — For metallurgically bonded deposits
These powders produce wear-resistant, corrosion-resistant, heat-resistant, and electrically conductive coatings depending on the composition selected.
Plasma Flame Spraying
This is the most advanced flame spraying technology. Plasma flame equipment raises materials to a higher energy level than the ordinary gaseous state by employing an inert or chemically inactive gas. This provides two critical advantages:
- A dramatically higher temperature ceiling — commercially available plasma equipment can exceed 30,000°F, though the optimum range for most applications is 12,000 to 20,000°F
- A controlled atmosphere — chemical reactions like oxidation during heating and application can be controlled
Plasma flame spray materials include:
| Material Category | Examples |
|---|---|
| Oxides | Alumina, Zirconia |
| Refractory metals | Tungsten, Molybdenum, Tantalum |
| Conductive metals | Copper, Aluminum |
| Compounds | Carbides |
| Alloys | Nickel-base alloys |
Surface Preparation: The Critical Step You Cannot Skip
Regardless of the equipment used, the most critical factor is proper preparation of the surface that will receive the sprayed coating. Skip this step or do it poorly, and even the most expensive coating material will fail.
Required preparation activities include:
- Degreasing or solvent cleaning — Remove all oils, greases, and contaminants
- Undercutting the surface — Provide room for proper coating thickness
- Abrasive or grit blasting — Create a roughened surface profile for mechanical adhesion
- Grooving (flat surfaces) or rough threading (cylindrical work) — Provide additional mechanical keying
- Preheating the base metal — Improve bonding conditions
Methods of obtaining a bond between sprayed material and substrate:
| Bonding Method | Mechanism | Notes |
|---|---|---|
| Heating the base | Thermal activation | Seldom used alone in machine element work — causes warpage and surface corrosion |
| Roughening the base | Mechanical interlocking | Most common approach |
| Self-bonding material | Chemical/metallurgical bonding | Sprayed onto a smooth surface |
Engineering Insight: Heating alone is seldom used in machine element work because the elevated temperatures required to obtain a proper bond cause problems of warpage and surface corrosion. Combine it with roughening or self-bonding materials instead.
Plasma Arc Surface Coating — The High-Temperature Workhorse
Two Distinct Processes Under One Name
Plasma arc surface coating operates in two fundamentally different modes, and understanding which one you need is critical to specifying the right process.
Transferred Arc Process (Plasma Arc Surfacing)
In this mode, the arc is struck between the electrode and the workpiece — meaning the arc transfers directly to the work surface. This creates a true metallurgical bond rather than just a mechanical one.
Process characteristics:
- Arc temperatures: 25,000 to 50,000°F (14,000 to 28,000°C)
- Deposition rate from powder: Up to 15 lb/h (6.8 kg/h)
- Deposition rate from wire: Up to 28 lb/h (12.7 kg/h) — much higher than oxygen/fuel or gas metal/arc methods
- Base metal dilution: Can be held below 5% if required
- Weld quality: Homogeneous welds with low porosity
The ionized plasma gas stream melts both the work surface and a stream of powdered alloy or filler wire fed into the arc. This produces a coating that is metallurgically integrated with the base metal.
Nontransferred Arc Process (Plasma Spraying)
In this mode, the arc is struck between the electrode and the torch nozzle, so it does not attach to the work surface. This is sometimes called "metal spraying."
Process characteristics:
- Coating material (powder or wire) enters the plasma column, is melted thoroughly, and propelled toward the work at high velocity
- Bond type: Mechanical bond with the work surface
- Available materials: Over 500 different powder combinations
- Deposition rates: Up to 100 lb/h (45 kg/h)
- Plasma gas: Argon is frequently used
Applications include:
- Building up surfaces for hard facing
- Application of anticorrosion and barrier layers
- Recovery of worn parts
- Providing surfaces with unique properties on new or existing components
High-Velocity Plasma Systems
Some advanced systems can use either metal powder or wire as the spray material and operate at higher voltage settings that produce:
| Parameter | Specification |
|---|---|
| Arc temperatures | Over 10,000°F (5,537°C) |
| Plasma velocity | Approximately 12,000 ft/s (3,658 m/s) |
| Coating porosity | Less than 1% |
| Current range | 30 to 500 amps |
| Plasma gas | Nitrogen (frequently) |
| Shielding gas | CO₂, nitrogen, or compressed air |
| Gas flow rates | 50 to 350 cu ft/h (24 to 165 l/min) |
These systems produce extremely dense coatings suitable for the most demanding applications, and they can coat large or small surface areas at low cost with minimum heat input.
Deciding Between Transferred and Nontransferred Arc
| Factor | Transferred Arc | Nontransferred Arc |
|---|---|---|
| Bond type | Metallurgical (fusion) | Mechanical |
| Dilution control | Below 5% possible | No base metal dilution |
| Deposition rate | Up to 28 lb/h (wire) | Up to 100 lb/h |
| Heat input to workpiece | Higher | Lower |
| Porosity | Low | Low (< 1% with high-velocity systems) |
| Best for | Critical wear surfaces requiring fusion bonding | Large areas, anticorrosion layers, barrier coatings |
| Manual or automated | Both | Both |
Decision Framework: If a mechanical bond is acceptable for your application, the nontransferred arc process offers higher deposition rates and lower heat input. If you need metallurgical integrity — especially for critical wear surfaces — the transferred arc process provides superior bonding at the cost of higher heat input and lower deposition rates.
Hard Facing — The Art of Making Surfaces Indestructible
Key engineering insight
When the practitioner first opened the hard-facing materials catalog, he was overwhelmed. Dozens of alloy designations, hardness ranges, temperature limits, and resistance ratings stared back at him. It took him weeks to build a mental framework that made the selection process intuitive.
Here is that framework, organized so you can build the same understanding in minutes instead of weeks.
The Selection Principle
The first thing to consider when selecting a hard-facing material is the type of service the part will undergo. Beyond that, you must evaluate:
- Machinability of the deposited material
- Cost of the hard-facing material
- Porosity of the deposit
- Appearance in service
- Ease of application
The general rule: The greater the hardness of the facing material, the greater its resistance to abrasion — but the lower its impact resistance and the more difficult it becomes to machine.
Many hardenable materials can be used for hard facing — carbon steels, low-alloy steels, medium-alloy steels, and medium-high alloys — but none of these is outstanding. The materials that deliver genuinely superior performance fall into six categories:
- High-Speed Steels
- Austenitic Manganese Steels
- Austenitic High-Chromium Irons
- Cobalt-Base Alloys (Cobalt-Chromium)
- Copper-Base Alloys
- Nickel-Chromium-Boron Alloys
. High-Speed Steels (RFe5 / EFe5)
Available forms: Welding rods (RFe5) and electrodes (EFe5)
Primary applications: Hard facing where hardness is required at service temperatures up to 1100°F and where both wear resistance and toughness are needed.
Typical surfacing operations: Cutting tools, shear blades, reamers, forming dies, shearing dies, guides, ingot tongs, and broaches.
Hardness Properties
| Condition | Rockwell C Hardness |
|---|---|
| As-welded | 55 to 60 HRC |
| Annealed | 30 HRC |
| At 1100°F (as-deposited) | Drops slowly to 47 HRC |
| At 1200°F | Maximum 30 HRC |
Resistance Properties
- Impact resistance (as deposited): Medium only — but increases appreciably when tempered
- Oxidation resistance: Will oxidize readily due to high molybdenum content
- Atmospheric corrosion: Can withstand
- Liquid corrosives: Cannot withstand
Other Key Characteristics
- Excellent metal-to-metal wear — especially at elevated temperatures
- Retains hardness at elevated temperatures
- Takes a high polish
- Machining: Must be annealed first; full hardness can be regained by subsequent heat treatment
Engineering Insight: High-speed steel deposits give you the best combination of hot hardness and metal-to-metal wear resistance up to 1100°F. Above that temperature, their advantage disappears rapidly.
. Austenitic Manganese Steels (EFeMn)
Available forms: Electrodes (EFeMn)
Primary applications: Metal-to-metal wear combined with heavy impact. Rock-crushing equipment, railway frogs and crossings.
Hardness Properties
| Condition | Hardness |
|---|---|
| As-deposited | 170 to 230 BHN |
| Work-hardened | 450 to 550 BHN |
| Above 500–600°F | Becomes brittle — no practical hot hardness |
This is the critical property of austenitic manganese steels: They are relatively soft when deposited, but work-harden dramatically under impact and compressive loading. The surface gets harder the more it is pounded — which is exactly what happens in rock-crushing and railway service.
Resistance Properties
- Impact resistance: High — the highest of any hard-facing material
- Corrosion and oxidation resistance: Similar to ordinary carbon steels (not special)
- Abrasion resistance vs. hard abrasives (like quartz): Only mediocre
Other Key Characteristics
- Yield strength in compression: Low initially, but any compressive deformation rapidly raises it until plastic flow ceases — a major asset in impact wear situations
- Machining: Difficult with ordinary tools; finished surfaces are usually ground
- Temperature limit: Must never be reheated above 500–600°F or the deposit becomes brittle and loses its work-hardening capability
Critical Warning: The temperature sensitivity of austenitic manganese steels cannot be overstated. If you reheat a deposit above 500–600°F — whether through welding procedures, nearby operations, or service conditions — you will permanently destroy the very properties that make this material valuable.
. Austenitic High-Chromium Irons (RFeCr-A / EFeCr-A)
Available forms: Rods (RFeCr-A) and electrodes (EFeCr-A)
Primary applications: Agricultural machinery parts, coke chutes, steel mill guides, sand-blasting equipment, and brick-making machinery.
Hardness Properties
| Condition | Rockwell C Hardness |
|---|---|
| As-welded | 51 to 62 HRC |
| Under impact | Work hardens somewhat, but also leads to cracking |
| At 800–900°F | Decreases slowly |
| At 900°F (instantaneous) | 43 HRC |
| At 900°F (3 min under load) | 37 HRC |
| At 1200°F (instantaneous) | 5 HRC |
| After cooling to ambient | Hardness practically recovered |
Resistance Properties
- Impact resistance: Light impact only — deposits crack under dynamic compression stresses above 60,000 psi
- Oxidation resistance: Good, up to 1800°F
- Liquid corrosion: Not very resistant; will rust in moist air (but more stable than ordinary iron and steel)
- Low-stress scratching abrasion: Outstanding — related to the amount of hard carbides present
- High-stress grinding abrasion: Only mediocre — not suitable for grinding abrasion service
Mechanical Properties in Compression
| Property | Value |
|---|---|
| Yield strength (0.1% offset) | 80,000 to 140,000 psi |
| Ultimate strength | 150,000 to 280,000 psi |
| Tensile strength | Low — avoid tension applications in design |
Machining and Finishing
These deposits are considered commercially unmachinable and are also very difficult to grind.
Recommended grinding specification:
| Parameter | Specification |
|---|---|
| Abrasive | Aluminum oxide |
| Grit size | 24 |
| Bond (off-hand high-speed) | Hard (Q), medium-spaced, resinoid |
| Bond (off-hand low-speed) | Slightly softer (P), vitrified |
Engineering Insight: The distinction between low-stress scratching abrasion (outstanding) and high-stress grinding abrasion (mediocre) is critical. Just because a material resists scratches doesn't mean it will survive grinding forces. Match the alloy to the actual wear mechanism in your application.
. Cobalt-Base Alloys — CoCr (Cobalt-Chromium)
Available forms: Rods (RCoCr) and electrodes (ECoCr)
Primary applications: Contact surfaces of exhaust valves in aircraft, truck, and bus engines. Valve trim in steam engines. Pump shafts where corrosion and erosion occur simultaneously.
This is the alloy family the practitioner eventually selected for his fleet's exhaust valves — and the results were transformative.
Alloy Designations and Applications
| Designation | Carbon Content | Application Focus |
|---|---|---|
| CoCr-A | Lower | General-purpose; balance of hardness and toughness |
| CoCr-B | Medium | Higher hardness and abrasion resistance |
| CoCr-C | Higher | Maximum hardness where impact is not expected |
Hardness Properties
| Alloy | Gas-Welded (HRC) | Arc-Welded (HRC) |
|---|---|---|
| CoCr-A | 38 to 47 | 23 to 47 |
| CoCr-B | 45 to 49 | 34 to 47 |
| CoCr-C | 48 to 58 | 43 to 58 |
Critical note on arc-welded deposits: The wide hardness range for arc-welded deposits is caused by base metal dilution. The greater the dilution, the lower the hardness. This means your welding technique directly determines the performance of the finished deposit.
The Exceptional Temperature Property
Most surfacing alloys are softened permanently by heating to elevated temperatures. Cobalt-base alloys are the exception.
They do exhibit lower hardness values when hot, but they return to their approximate original hardness values upon cooling. This makes them exceptional for cyclic thermal service like exhaust valves.
Temperature performance guidelines:
| Temperature Range | Performance |
|---|---|
| Above 1200°F | Considered advantageous — this is their sweet spot |
| 1000 to 1200°F | Advantages not definitively established over other alloys |
| Below 1000°F | Other surfacing metals may prove better |
| 1000 to 1200°F (creep resistance) | Great resistance to creep in weld deposits |
Resistance Properties
- Scaling resistance: The chromium promotes a thin, tightly adherent scale resistant to combustion products of internal combustion engines, including leaded fuels
- Corrosion resistance: Resistant in air, food, and certain acids — but field testing is advised for specific applications
- Metal-to-metal wear: Excellent — deposits take a high polish and have a low coefficient of friction
- Creep resistance: In the 1000–1200°F range, tough martensitic steel deposits are considered superior to cobalt-base deposits in both flow resistance and toughness
Machining Characteristics
| Alloy | Machining Method |
|---|---|
| CoCr-A | Sintered carbide tools preferred |
| CoCr-C | Grinding only (too hard to machine) |
| General rule | Difficulty increases proportionally with carbon content |
. Copper-Base Alloys
Available 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
Primary applications: Overlays and inlays for bearing, corrosion-resistant, and wear-resistant surfaces.
Alloy Selection by Application
| Alloy Group | Hardness Range (BHN) | Application |
|---|---|---|
| CuAl-A2 | 130 to 190 | Bearing surfaces, corrosion-resistant surfaces |
| CuAl-B, CuAl-C | 140 to 290 | Bearing surfaces (medium hardness) |
| CuAl-D, CuAl-E | 230 to 390 | Gears, cams, wear plates, dies — high-hardness applications |
| CuSn (copper-tin) | Lower range | Lower-hardness surfacing, corrosion resistance, occasional wear applications |
Hardness: The Process Matters
Hardness of a copper-base alloy deposit depends directly on the welding process used. Deposits made by the inert-gas metal-arc process (both consumable and nonconsumable electrode) will be higher in hardness than deposits made with gas, metal-arc, and carbon-arc processes. This occurs because lower losses of aluminum, tin, silicon, and zinc are achieved through better shielding from oxidation.
Temperature limitation: Copper-base alloys are not recommended for elevated temperatures because their hardness and mechanical properties decrease consistently above 400°F.
Impact Resistance by Alloy
| Alloy | Impact Resistance |
|---|---|
| CuAl-A2 | Highest of all copper-base alloys |
| CuAl (higher aluminum content) | Decreases markedly with increasing aluminum |
| CuSi | Good impact properties |
| CuSn | Low impact resistance |
| CuZn-E | Very low impact resistance |
Corrosion Resistance
| Alloy | Corrosion Performance |
|---|---|
| CuAl | Forms protective oxide coating on atmospheric exposure; widely used for acids, mild alkalies, salt water |
| CuSi | Fair oxidation resistance |
| CuSn | Comparable to pure copper |
| CuSn-E, CuZn-E | Exception — not recommended for acid/alkali/saltwater service |
Important: Copper-base alloy deposits are not recommended where severe abrasion is encountered. Their strength is in metal-to-metal contact wear — gears, cams, sheaves, wear plates, and dies.
Bearing Surface Selection Rule
When selecting copper-base alloys for bearing surfaces, the deposit should have a Brinell hardness of 50 to 75 units below that of the mating metal surface. Slight porosity is actually acceptable — even desirable — in bearing service, as a porous deposit retains oil for lubrication.
Mechanical Properties in Compression
| Alloy | Elastic Limit (psi) | Ultimate Strength (psi) |
|---|---|---|
| CuAl | 25,000 to 65,000 | 120,000 to 171,000 |
| CuSi | 22,000 | 60,000 |
| CuZn-E | ~5,000 | 20,000 |
All copper-base alloy deposits can be machined — a significant advantage over many other hard-facing materials.
. Nickel-Chromium-Boron Alloys (NiCr)
Available forms: Rods (RNiCr) and electrodes (ENiCr)
Primary applications: Seal rings, cement pump screws, valves, screw conveyors, and cams — applications requiring good metal-to-metal wear resistance, low-stress scratch-abrasion resistance, corrosion resistance, and elevated-temperature hardness retention.
Three Formulations
| Designation | Characteristics |
|---|---|
| NiCr-A | Softest; best impact tolerance |
| NiCr-B | Medium hardness |
| NiCr-C | Hardest; best wear and abrasion resistance; most prone to cracking under deformation |
Hardness Properties — Rods vs. Electrodes
Rod deposits:
| Alloy | Hardness (HRC) |
|---|---|
| NiCr-A | 35 to 40 |
| NiCr-B | 45 to 50 |
| NiCr-C | 56 to 62 |
Electrode deposits:
| Alloy | Hardness (HRC) |
|---|---|
| NiCr-A | 24 to 35 |
| NiCr-B | 30 to 45 |
| NiCr-C | 35 to 56 |
The lower hardness values and greater ranges in electrode deposits are attributed to dilution of deposit and base metals — the same phenomenon seen with cobalt-base alloys.
Hot Hardness Performance
NiCr-A electrode deposits:
| Temperature | Instantaneous (HRC) | 3-Minute Load (HRC) |
|---|---|---|
| 600°F | 30 | — |
| 1000°F | — | 19 |
NiCr-A rod deposits:
| Temperature | Instantaneous (HRC) | 3-Minute Load (HRC) |
|---|---|---|
| 600°F | 34 | — |
| 1000°F | — | 24 |
NiCr-B electrode deposits:
| Temperature | Instantaneous (HRC) | 3-Minute Load (HRC) |
|---|---|---|
| 600°F | 41 | — |
| 1000°F | — | 26 |
NiCr-B rod deposits:
| Temperature | Instantaneous (HRC) | 3-Minute Load (HRC) |
|---|---|---|
| 600°F | 46 | — |
| 1000°F | — | 37 |
NiCr-C electrode deposits:
| Temperature | Instantaneous (HRC) | 3-Minute Load (HRC) |
|---|---|---|
| 600°F | 49 | — |
| 1000°F | — | 31 |
NiCr-C rod deposits:
| Temperature | Instantaneous (HRC) | 3-Minute Load (HRC) |
|---|---|---|
| 600°F | 55 | — |
| 1000°F | — | 40 |
Resistance Properties
- Impact: Withstands light impact fairly well; NiCr-C is more prone to cracking than NiCr-A and NiCr-B
- Oxidation resistance: Up to 1800°F — but do not use above 1750°F because fusion may begin
- Corrosion resistance: Completely resistant to atmospheric, steam, salt water, salt spray corrosion, and milder acids and common corrosive chemicals
- Grinding abrasion: Not recommended for high-stress grinding
- Metal-to-metal wear: Good — deposits take a high polish and are particularly resistant to galling (especially NiCr-C)
Mechanical Properties in Compression
| Property | Value |
|---|---|
| Elastic limit | 42,000 psi |
| Yield strength (0.01% offset) | 92,000 psi |
| Yield strength (0.10% offset) | 150,000 psi |
| Yield strength (0.20% offset) | 210,000 psi |
Machining
NiCr deposits may be machined with tungsten carbide tools using slow speeds, light feeds, and heavy tool shanks. They can also be finished by grinding using a soft-to-medium vitrified silicon carbide wheel.
The Complete Hard-Facing Material Comparison
This is the reference table the practitioner ultimately built to guide every hard-facing decision at his depot. Use it as your decision-making starting point.
| Property | High-Speed Steel | Austenitic Mn Steel | High-Cr Iron | Cobalt-Cr | Copper-Base | Nickel-Cr-B |
|---|---|---|---|---|---|---|
| Max Service Temp | 1100°F | 500–600°F | 1800°F (oxidation) | 1200°F+ | 400°F | 1750°F |
| As-Welded Hardness | 55–60 HRC | 170–230 BHN | 51–62 HRC | 38–58 HRC | 130–390 BHN | 35–62 HRC |
| Impact Resistance | Medium (better tempered) | Highest | Light only | Good (lower grades) | Varies by alloy | Light |
| Abrasion Resistance | Good | Mediocre vs. quartz | Outstanding (low-stress) | Good | Not for severe abrasion | Good (low-stress) |
| Corrosion Resistance | Atmospheric only | Like carbon steel | Moist air — moderate | Good (air, food, acids) | Good (acids, alkalis, salt water) | Excellent (atmospheric, steam, salt) |
| Metal-to-Metal Wear | Excellent | Via work hardening | Good | Excellent | Good (CuAl for gears/cams) | Good (anti-galling) |
| Machinability | Anneal first | Difficult — grind | Unmachinable — grind | CoCr-A: carbide tools; CoCr-C: grind | All can be machined | Carbide tools, slow speeds |
| Hot Hardness Retention | Good to 1100°F | None above 500°F | Recovers on cooling | Recovers on cooling | Degrades above 400°F | Degrades but usable |
| Welding Process | Rod/Electrode | Electrode only | Rod/Electrode | Rod/Electrode | Rod/Electrode | Rod/Electrode |
Chromium Plating — The Precision Surface Treatment
How It Works
Chromium plating is an electrolytic process of depositing chromium on metals either as protection against corrosion or to increase surface-wearing qualities. Unlike the weld-based hard-facing processes, chromium plating provides a thin, precisely controlled layer with exceptional properties.
Proven Applications
Gages: The value of chromium-plating plug and ring gages has been more thoroughly demonstrated than perhaps 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 — a remarkable reclaimability advantage.
Cutting tools that benefit from chromium plating:
- Drills, taps, reamers
- Files, broaches, tool tips
- Saws, thread chasers
Dies that benefit from chromium plating:
- Stamping dies
- Drawing dies
- Hot forging dies
- Die casting dies
- Plastic molding dies
Plating Thickness
The thickness of chromium plating varies based on application:
| Application | Thickness Range |
|---|---|
| Thin functional coatings | 0.0001 inch |
| Standard wear coatings | 0.001 inch |
| Build-up of undersize tools (taps, reamers) | Up to 0.002 inch |
The Chromium Plating Procedure
A common procedure for hard chromium plating of tools and salvage parts follows these steps:
- Degrease with solvent
- Mount tools on racks
- Clean in anodic alkali bath at 82°C for 3 to 5 minutes
- Rinse in boiling water
- Immerse in 20% hydrochloric acid solution for 2 to 3 seconds
- Rinse in cold water
- Rinse in hot water
- Etch in reverse-current chromic acid bath for 2 to 5 minutes
- Plate — place work immediately in chromium plating bath
- Remove hydrogen embrittlement (if necessary) by immersing plated tools for 2 hours in an oil bath at 177°C
Friction Properties
Chromium has an exceptionally low coefficient of friction. Here is how it compares:
| Surface Combination | Static CoF | Sliding CoF |
|---|---|---|
| Steel on steel | 0.30 | 0.20 |
| Steel on babbitt | 0.25 | 0.20 |
| Steel on chromium-plated steel | 0.17 | 0.16 |
| Chromium-plated steel on babbitt | 0.15 | 0.13 |
Practical Requirement: Articles to be chromium plated for frictional wear resistance should be highly polished before plating so that full advantage can be taken of chromium's low friction coefficient.
Chemical and Thermal Resistance
- Chemical resistance: Resists attack by almost all organic and inorganic compounds, except muriatic (hydrochloric) and sulfuric acids
- Melting point: 2930°F
- Remains bright: Up to 1200°F
- Above 1200°F: A light, adherent oxide forms that does not readily detach — suitable for protecting articles that must resist high temperatures, even above 2000°F
Surface Preparation Warning
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 preparation leads to poor plating, regardless of plating bath quality.
Cutting Metals — The Destructive Side of Surfacing
While most of this guide covers building surfaces up, the ability to precisely remove metal through thermal cutting is an equally essential manufacturing capability. The processes share much of the same thermal physics, and understanding cutting processes deepens your grasp of surfacing processes.
Cutting Metals with an Oxidizing Flame
The Principle
When iron or steel is heated to a high temperature, it has a powerful affinity for oxygen and readily combines with it to form oxides. This reaction is rapid and violent enough to disintegrate the metal. The cutting torch exploits this principle.
A torch tip preheats the metal to a sufficiently high temperature, then a jet of pure oxygen is directed onto the heated metal. The oxygen combines with the iron, burning and oxidizing it with great speed. The kerf (cut path) resembles a saw cut when the torch is properly adjusted.
The Cutting Torch
The ordinary cutting torch consists of:
- A heating jet using oxygen and acetylene (or hydrogen, or another fuel gas)
- An auxiliary jet of pure oxygen that performs the actual cutting
- Some designs include multiple preheating flame ports surrounding the central oxygen port, ensuring the preheating flame precedes the oxygen regardless of torch direction
Torch Adjustment and Use
For cutting steel plate:
- The preheating flame contacts the edge of the plate
- The metal is raised to white-hot temperature
- The oxygen valve is opened
- As pure oxygen contacts the heated metal, oxidation (burning) occurs rapidly
- The torch traverses the cut line either by hand or mechanically
What Can Be Cut — and What Cannot
| Material | Cuttability | Notes |
|---|---|---|
| Wrought iron | Readily cut | — |
| Low-carbon steel | Readily cut | — |
| High-carbon steel | Can be cut with preheating | Higher carbon = more preheating needed |
| Ordinary tool steel | Preheated to black heat | — |
| Alloy tool steels | May require low red heat preheat | — |
| Brass and bronze | Can be cut by interposing between steel plates | Not directly cuttable |
| Stainless steel | Requires flux-injection method | See below |
| Cast iron | Practicable but difficult | See below |