Hardfacing, Thermal Spraying and Surface Rebuilding: Cutting Stainless Steel

Cutting Stainless Steel: The Flux-Injection Solution

The elements that give stainless steels their desirable properties produce oxides that reduce the flame cutting operation to a slow melting-away process when conventional oxyacetylene equipment is used. The solution is flux injection.

How it works: A suitable flux powder is injected directly into the stream of cutting oxygen before it enters the torch. This removes the obstructing oxides, allowing normal cutting to proceed.

Equipment: Portable flux feeding units with a vibrator-type dispenser and rheostat control for accurate flux flow regulation.

Result: The operating procedure and cutting speed become practically the same as in cutting mild steel.

Cutting Cast Iron: The Melting Challenge

Cast iron cutting with the oxyacetylene torch is fundamentally different from steel cutting. In steel, the reaction is primarily chemical — oxygen combines with iron. In cast iron, the carbon in graphite form hinders this chemical action, making it partly a melting operation rather than purely an oxidation process.

Key observations:

• Very soft cast iron is more difficult to cut than harder varieties
• The slag contains considerable melted cast iron (unlike steel, where slag is nearly free of metal particles)
• Cost is much higher than steel cutting due to larger preheating flames and higher oxygen consumption

Speed improvement technique: Feed a steel rod (approximately 1/4 inch diameter) into the top of the cut beneath the torch tip. This rod furnishes additional slag that flows over the cut and increases the temperature, improving cutting speed and reducing cost.

Thickness Limitations and Kerf Width

Maximum cutting thickness depends on:

• Type of gases used
• Oxygen pressure (may be as high as 150 psi)

Why oxyhydrogen cuts thicker: The oxyhydrogen flame is longer than the oxyacetylene flame and can penetrate to the full depth of the cut, keeping all oxide molten so it can be easily blown out by the cutting jet.

Kerf width varies with thickness:

Mechanically guided torches cut thick material more satisfactorily than hand-guided torches because the flame is directed straight into the cut without wobble.

Cutting Steel Castings: The Blowhole Hazard

When cutting steel castings, blowholes present a specific danger. If the flame strikes a blowhole:

1. Molten oxide will splash into the cavity
2. The flame will be diverted
3. Excessive sparks will appear (the warning sign)

Recovery procedure:

1. Immediately move the torch back along the cut
2. Direct the torch at an angle to strike the metal beneath the blowhole
3. Burn away the metal beyond the cavity if possible
4. Resume cutting in the normal position

Mechanically Guided Torches

For production cutting of specific outlines, shapes, or patterns, mechanically guided torches provide dramatically better results:

• Pantograph-guided torches trace the outline from a pattern or drawing
• Straight-line designs are used for linear cuts
• Circular cutting fixtures handle round shapes
• Numerically controlled torches provide programmable cutting paths

Arc Cutting of Metals

While the oxyacetylene torch cuts steel easily and accurately, many metals resist oxidation and therefore cannot be efficiently cut with flame processes. For these materials, arc cutting offers a practical solution.

Materials that resist flame cutting but respond to arc cutting:

• Cast iron (carbon in graphite form hinders the oxidation reaction)
• Stainless steels
• Manganese steels
• Nonferrous materials

The fundamental difference: Flame cutting is a chemical action — oxygen combines with iron. Arc cutting uses thermal energy to melt through the material regardless of its chemical reactivity with oxygen.

Plasma Cutting of Metals

Plasma arc cutting (PAC) represents the most advanced thermal cutting technology, operating on DC straight polarity with a transferred arc that melts through the material.

How it works: The nozzle is positioned close to the work surface. The velocity of the plasma jet is greatly increased by a restricting nozzle orifice, blowing away metal as it melts.

Advantages over flame cutting:

• Much faster than oxygen/fuel torch cutting for steel less than 1/2 inch thick
• Works on materials that resist oxidation cutting
• Can be automated with numerical control

Limitations:

• Produces kerfs with some variation in width and bevel angle
• Some molten metal may recast on cut edges and be difficult to remove

Factors affecting plasma cutting quality:

• Type and pressure of the gas
• Gas flow pattern
• Current level
• Size and shape of the nozzle orifice
• Distance from nozzle to work surface

Noise and fume reduction: Mechanized plasma cutting is often performed with the workpiece submerged in water. This also virtually eliminates oxidation of cut surfaces.

Precision Plasma Arc Cutting

A further refinement uses a magnetic field in the cutter head to stabilize the plasma arc. Lorentz forces cause the arc to spin faster and tighter on the electrode tip, confining the spinning plasma to produce a narrower kerf without reducing cutting speed.

Results from precision plasma cutting are comparable to laser cutting and, with numerical control, the process is used for production of small batches of blanks for stamping and similar applications.

Surface quality by material:

The Selection Framework — Putting It All Together

How the practitioner Made the Right Choice

After months of research and testing, the practitioner developed a decision tree that guided every surfacing project at his depot. Here is the logic:

Step 1: Define the wear mechanism

• Abrasion (low-stress scratching)? → High-Chromium Iron or NiCr-C
• Abrasion (high-stress grinding)? → Not high-chromium iron or NiCr; consider cobalt-base or high-speed steel
• Metal-to-metal wear? → Cobalt-base, High-speed steel, or CuAl alloys
• Impact? → Austenitic manganese steel (heavy impact) or CoCr-A (moderate)
• Galling resistance? → NiCr alloys, especially NiCr-C

Step 2: Define the temperature range

Step 3: Determine post-deposition finishing requirements

Step 4: Select the deposition process

Your Next Step

You've just absorbed one of the most comprehensive overviews of metal surfacing processes available anywhere. The question is — what will you do with it?

Here are three concrete actions based on where you are:

If you're a beginner: Pick one process from this guide — flame spraying is the most accessible — and research the equipment requirements for your shop or facility. Start with the simplest application: restoring a worn cylindrical part to dimension using wire-fed flame spraying.

If you're an experienced engineer: Build your own version of the practitioner's selection matrix. Map every component in your operation that fails from wear, corrosion, or heat damage. Identify which surfacing process and alloy could extend its life — and calculate the cost-benefit for the top three candidates.

If you're evaluating surfacing for a client project: Use the comparison tables in this guide to shortlist candidate materials. Then specify field testing for the top two options — because the final selection is always dependent on experience with the particular type of service.

What wear problem are you trying to solve? And which surfacing process looks most promising for your application?

The answer to that question is the beginning of a transformation just like the practitioner's.

Context and scope

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.

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.

. High-Speed Steels (RFe5 / EFe5)

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

. Austenitic Manganese Steels (EFeMn)

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.

the practitioner's Decision

This was the alloy the practitioner chose for the practitioner'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.

. 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:

. Cobalt-Base Alloys (RCoCr / ECoCr)

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)

. 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 in the supplied reference

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

the practitioner 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.

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

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.