Flame Spray Welding: Process, Materials, and Real-World Uses

A hydraulic cylinder ram on a paper mill rolls machine is scored to within a few thousandths of an inch of being scrap — and a new one costs more than the whole press section. The maintenance tech does not weld it. He grit-blasts the surface, lights a combustion gun, feeds a nickel-alloy wire into a hot flame, and in forty minutes the shaft is built back up, hardfaced, and ready for the grinder and lathe. That is flame spray welding, and it is far older and far more useful than most fabricators ever realize.

Flame Spray Welding: What It Actually Is (and What It Is Not)

Start with the hydraulic cylinder ram on the paper mill rolls machine. The maintenance tech never lit an arc, never ran a filler rod, never opened a welding procedure. He rebuilt the surface with a coating, and that coating is what this article is about: “flame spray welding” is the most misleading name in metalworking.

The short answer: flame spray welding is a thermal spray (surfacing) process, not a conventional fusion weld. It does not join two pieces of metal, it does not melt the parent part, and it does not mix with it. It lays a layer of material on a surface, held on by mechanical interlocking rather than a metallurgical bond. Get that distinction right and you get a coating; get it wrong and the layer peels off the first time the part is loaded.

How the process actually works

Strip away the confusing name and the mechanics are easy to picture. A fuel gas, usually acetylene but also propane, propylene or hydrogen, is mixed with oxygen and ignited inside a spray gun. The flame melts the consumable, fed concentrically into the flame as wire or injected into the flame core as powder. Compressed air atomizes the molten material into fine droplets and propels them at the workpiece. On impact, each droplet flattens into a platelet, gives up heat to the cooler substrate, shrinks and solidifies. Layer after layer of these flattened particles, called splats, builds the coating.

That is the concept in four moves: melt, atomize, deposit, solidify. It is also where the process earns its keep: a flame spray pass adds material without driving heat into the part the way an arc does.

The vocabulary you will meet

• Thermal spray — the umbrella term. TWI describes it as a family of processes that deposits metals, alloys, plastics and ceramics, with melting temperatures roughly 2,600 °C to 16,000 °C depending on the process.
• Metalizing — the old name for spraying solid metal in wire form, and why most shops learned the process before the modern terminology.
• Spray welding — a legacy trade term for flame spraying itself, still used interchangeably in older manuals and supplier literature.
• Spray-fuse and spray-weld — the variant where the sprayed coating is reheated afterwards so it fuses and densifies.

Where it came from

The lineage is old. A NIOSH-authored review in Toxicology and Industrial Health records that in 1912 Dr. M.U. Schoop and colleagues patented the first instrument for spraying solid metal in wire form onto surfaces. The process was originally called metallizing. A century later, the same idea repairs hydraulic rams and hardfacing dies on shop floors every day.

The bond is mechanical, not metallurgical

This is the most important fact in the article. TWI states plainly that in thermal spray processes the deposit does not fuse with the substrate and does not form a solid solution with it. The bond is primarily mechanical, created by the melting and acceleration of the coating material. Molten droplets slam into a grit-blasted surface, flow into the peaks and valleys of the anchor profile, and lock together as they cool. No transition zone, no grain boundary, no diffusion.

TWI also reports that thermal spray coatings can be deposited in layers typically from 60 µm to 10 mm thick, with bond strengths from about 10 MPa to greater than 80 MPa depending on process and parameters. Flame spray sits at the accessible end of that range, and the next sections explain why.

Two families, one variant

The Flamespray process family splits cleanly in two. Wire flame spray feeds a continuous wire into the flame through the gun barrel, and it is the workhorse for zinc, aluminum, nickel and self-fluxing alloys. Powder flame spray injects pre-alloyed powder into the flame core, allowing tighter chemistry control, and is how ceramics and carbides are usually applied.

Add a controlled post-spray fusion pass to either family and you get the spray-fuse or spray-weld variant. The reheated coating melts and wets the substrate, closing the porosity a purely as-sprayed flame deposit always contains and producing a denser, more weld-like layer.

Why it solves problems a weld cannot

So why bother with a process that is not a weld? Because a weld solves the wrong problem. Welding a pad onto a worn journal means heat input into a large, expensive steel part, distortion, machining time to bring it back to tolerance, and a repair that may be weaker than the original section. Flame spray puts a few millimetres of material on the surface with almost no heat into the part, builds dimension instead of removing it, and works on site with a hand-held gun and a cylinder of gas.

It is also a different tool from the oxy-fuel torch most shops already own for brazing and cutting. Same combustion chemistry, different result: that flame joins metal with filler metal at the joint, while a spray gun atomizes molten consumable into a coating. Once that difference is clear, everything else here follows from it.

How the Process Works: Melting, Atomizing, and the Splat

Flame spray melts metal, blows it at a part as a cloud of droplets, and freezes those droplets on arrival as a coating. Everything else refines that sequence, and once you have watched it happen you never mistake it for a weld.

Step 1: Mix the gases and light them

Fuel gas (acetylene, propane, propylene or hydrogen) and oxygen meet in the gun’s mixing chamber and are ignited at the nozzle, giving a hard, bright combustion flame. The Fabricator’s thermal spray safety guidance puts that flame’s peak temperature in excess of 3,100 °C. That flame, not an electric arc, does the melting. Gas handling follows the discipline of an air acetylene torch: two regulators, two hoses, a mixer, and never light on an unpurged system.

Step 2: Feed the consumable into the flame

In wire flame spray, a spool of spray-grade wire is pushed through the gun into the flame concentrically, straight into its hottest zone. In powder flame spray, an injector feeds fine powder into the flame core. Either way the flame melts the consumable into a stream of liquid metal and molten droplets.

Step 3: Atomize and propel

A separate compressed-air stream, the atomizing air, is introduced around the flame at the nozzle. It shreds the molten stream into fine droplets and drives them at the substrate. Moisture or oil in that air shows up later as spatter and porosity, and a fouled nozzle breaks the spray cone apart, so the gun and its air supply are part of the process, not plumbing afterthoughts. Correct gas and equipment setup is half the battle.

Step 4: Flatten into splats

The NIOSH review of thermal spray health hazards describes molten particles impacting the roughened surface at high speed, deforming, flattening and spreading. They dump their heat into the cooler substrate, shrink and solidify. TWI calls the result a splat or platelet, and successive splats interlock into the coating. A half-millimetre coating might be a few dozen splats thick: a laminate of frozen droplets, not a cast or welded solid.

Why flame spray bonds less tightly than its siblings

Particle velocity at impact dominates, and flame spray sits at the bottom of the thermal spray family. TWI notes that flame spray coatings can show low bond strength, high oxide content and high porosity, at a moderate spray rate and low cost, while arc spray delivers higher bond strength and spray rate. The accelerated processes sit in another league: the NIOSH review, drawing on ASM Thermal Spray Society and Oerlikon Metco data, reports HVOF driving powder to as much as 700 m/s, detonation gun spraying at 750 to 1,000 m/s, and plasma pushing powder at 300 to 550 m/s through a plume that can reach 16,000 °C.

Process	Impact velocity	Effect on the deposit
Flame spray	Lowest of the family	Rounder splats, high oxide and porosity, low bond strength
Arc spray	Higher than flame spray	Higher bond strength and spray rate
HVOF	Up to about 700 m/s	Dense, tightly bonded, low porosity
Detonation gun	750 to 1,000 m/s	Highest bond strength, lowest porosity
Plasma	300 to 550 m/s (powder)	Dense, oxide-free microstructure

Fast, thin splats flow into every valley of a rough surface and pack tightly together. Slow, fat flame spray splats bridge gaps and leave voids underneath, and that explains most of the coating’s reputation.

The two weaknesses built into the process

Oxide content. Every droplet flies through air, and a hot droplet’s surface oxidizes as it goes. The faster and hotter the process, the shorter the flight and the less oxide forms. Flame spray’s slow droplets cook in the atmosphere, so oxide ends up dispersed through the coating.

Porosity. Gas gets trapped inside a splat as it flattens, and low-density materials such as lithium do not melt completely, leaving unmelted particles and unfilled gaps. Hence the sealing of flame sprayed corrosion coatings before immersion service, and the post-fusing of self-fluxing alloys to close the voids.

How the bond actually forms

Adhesion depends on particle velocity at impact and on surface condition, which must be clean and roughened by grit blasting or machining before spraying, according to TWI. The blasted surface provides sharp peaks and valleys, and each molten splat keys into that roughness. The bond is mechanical interlocking, with a little chemical bonding at the hottest contact points. It is not metallurgical.

Equipment, Gases, and Consumables You Will Need

A flame spray rig is deceptively simple: a torch, a bundle of hoses, a box of gas and a spool of wire. No power source, no high-speed wire feeder, no robot arm. That simplicity is why it is still on today’s shop floor. The kit breaks into four blocks: the gun, the gas train, the atomizing air supply and the consumables.

The Gun

For wire flame spray, the gun is a combustion torch head with a wire path running concentrically through the flame. Wire is pushed in by a separate floor feeder or pulled from a spool on the gun, what shops call a pistol-wire gun. The pistol is the most portable option, since the whole system hangs from one hose bundle and one operator. The powder gun is simpler still: a hollow flame nozzle, a venturi-style powder injector, a feed canister and a flow meter for dialling in feed rate. Either version bolts to a control manifold carrying the fuel and oxygen regulators, the igniter, gauges for each gas and the flame failure safety device that cuts fuel if the flame goes out. A gas cup and heat-shielded handle make long shifts bearable.

The Gas Train

This is what sets flame spray apart from other thermal spray methods, and where most training time belongs. TWI lists acetylene, propylene, propane and hydrogen as the fuel gases burned with oxygen to heat the consumable. Everything about the deposit follows from how hot and clean that flame burns.

• Acetylene gives the hottest, cleanest flame and is the default choice for hardfacing and wear alloys.
• Propane and propylene cost less, store more safely and are far easier to find for field work, but the flame runs cooler.
• Hydrogen burns completely, leaving no carbon soot, at a premium price.

Every cylinder gets a proper regulator, and every fuel line a flashback arrestor that stops a burning gas stream travelling back into the regulator or cylinder. Oxygen and fuel hoses are colour coded and inspected. As The Fabricator points out, thermal spray systems draw on industrial gases including acetylene, argon, propylene, helium, hydrogen, kerosene and oxygen, and those systems sit under OSHA Subpart H for hazardous materials. They are not welding gases you can improvise with.

Atomizing Air

Compressed air does the atomizing and propelling, and deserves its own supply rather than a tap off the shop header. Use a dedicated compressor, oil-free or properly maintained, plus a moisture separator or dryer and a filter-regulator to knock out water and particulate. Moisture in the air stream becomes porosity in the coating, and a wet line or oversize nozzle spits droplets that never melt properly. Set atomizing pressure to the gun manufacturer’s specification: too much pressure overheats the gun body, too little leaves a coarse, weak deposit. A well-built equipment package with matched regulators and hoses saves an afternoon.

Consumables: Wire and Powder

Both forms are available for most alloys. The NIOSH review of thermal spray hazards notes that flame spray is frequently used to reclaim worn parts, with nickel-based, bronze, zinc and tungsten carbide alloys among the most common choices.

Consumable	Form	Typical use
Zinc, aluminium, zinc-aluminium (TSA)	Wire	Corrosion protection, sacrificial anodic coating, galvanic shielding
Nickel, copper, aluminium bronze	Wire or powder	Bearing and journal build-up, electrical conductivity
Tin, babbitt, molybdenum	Wire or powder	Bearing surfaces and babbitt-lined journals
Stainless and nickel alloys	Wire or powder	Corrosion and wear overlays, machineable build-up
Self-fluxing NiCrBSi	Powder	Hardfacing, post-spray fusion for a near-metallic coat
Carbides (WC-Co, WC-Ni)	Powder	Severe wear, erosion, dies and forming tools
Oxides (Al2O3, Cr2O3)	Powder	Oxidation and wear resistance, thermal barrier coats

Two limitations are worth flagging before you order. Low-density metals such as lithium are hard to spray, because a light droplet cannot carry enough momentum to flatten on impact, and oxidation-sensitive materials need inert shielding or a sealed post-treatment to avoid a discoloured, brittle deposit.

What Self-Fluxing Actually Means

A self-fluxing alloy carries boron and silicon in its chemistry. Heated to fusion after spraying, those elements melt first and form a glassy flux that wets the substrate, pulls the particles together and seals the pores as the layer solidifies. The result approaches a weld-like, near-metallic coating that machines well and resists wear far better than as-sprayed flame spray. This spray-fuse step is the single biggest upgrade available to a flame sprayed hardfacing job. If you are buying for hardfacing, plan on a torch or induction heater that can take the part to fusion temperature: self-fluxing powder without fusion is just an expensive porous coating.

Where Flame Spray Welding Is Actually Used

The useful question is what this process is doing on a Tuesday afternoon in a maintenance bay somewhere. The answer is more varied than most people expect.

Start with restoration, the case that made flame spray what it is. A shaft, a journal, a bearing housing, a spindle, a hydraulic cylinder ram: the surface is worn past tolerance, the part is fine, and the options are scrap it, weld a pad on and machine it back, or spray it. The AWS SprayTime feature on thermal spray repair lays out five triggers that push a shop toward spraying:

• The part is no longer manufactured, so there is nothing to order.
• Replacement cost is prohibitive.
• Fabrication lead time is too long for the production line to wait.
• An existing wear-protection coating has deteriorated and needs replacing.
• A machining error removed too much stock and made the part nonconforming.

The last trigger is a shop-floor classic: a shaft gets cut short, the part is scrap on paper, and the only path back into service is build-up and remachine. The AWS article notes that thermal spraying can typically restore an item to like-new condition, and that early practitioners carried wire spray guns and oxygen-acetylene cylinders into factories, mills and utilities to get failed machinery back online quickly. That heritage is why flame spray still feels like a field repair tool rather than a production line process.

Hardfacing and Wear Coatings

Second is wear resistance. Tungsten-carbide-bearing nickel alloys go onto dies, moulds, valve seats, pump housings, mixer blades and wear plates where a surface has to survive abrasion, erosion or sliding contact, and here the choice of consumable matters most.

For thin wear layers on machine ways, the AWS article describes flame wire application using a Metco 12E system to apply molybdenum to lathe ways, in shop and on site. Molybdenum on a lathe way is a classic: it restores the wear surface without the heat input of a hardfacing weld, which matters on a long precision bed that would distort.

Corrosion Protection

Third is corrosion, and this is where flame spray has the longest institutional history. TWI lists protective coatings to prevent wear and corrosion among common thermal spray applications, and the consumables are well established: zinc, aluminum, and zinc-aluminum (TSA) for steel.

These go on bridges, storage tanks, ship decks, pipeline, and offshore structures. Offshore is the interesting case, because the alternative is a huge coating job with containment and scaffolding at enormous cost. Spray in the field, from a platform, with the crew already there, and you avoid most of that.

Corrosion coatings almost always need a sealer and a paint system on top: the sprayed layer provides the sacrificial metal and the barrier, while the sealer and topcoat keep water out and the zinc working.

Dimensional Build-Up and Spray-Fuse

Fourth, large-area dimensional build-up. Spray-fuse with self-fluxing nickel-chromium-boron-silicon alloys puts down a hard, near-metallic layer over a broad area with very little heat going into the base metal. On a large machine frame, weld overlay means grinding a full pad and machining it back. Spraying avoids that.

Fifth, and different in intent: sacrificial zinc on steel in immersion service. Not a hardface, not a dimension fix, just a controlled corrosion allowance.

Application Map

Application	Typical consumable	Typical service life
Worn shaft, journal, ram, bearing housing	Steel or nickel wire; self-fluxing NiCrBSi	Restored to original dimension, back in service
Lathe ways and machine beds	Molybdenum wire	Re-worn sliding surface, no heat distortion
Dies, moulds, valve seats, pump parts	WC-Co bearing nickel alloy (wire or powder)	Abrasion and erosion resistance
Bridge, tank, ship deck, pipework	Zinc, aluminum, or zinc-aluminum (TSA)	Long-term atmospheric protection, sealed and painted
Offshore and marine structures	Zinc-aluminum plus sealer and topcoat	Field protection where access cost dominates
Steel in immersion service	Zinc (sacrificial)	Sacrificial layer around submerged steel
Large-area hardfacing	Self-fluxing NiCrBSi, spray-fused	Weld-like density, minimal distortion

The Economics

Here is the part that gets a maintenance manager to reach for the gun. Spraying restores a worn part for a fraction of the cost and hours of replacing it, and avoids welding a full pad and machining it back off. A shaft needing a new casting might be out of service for weeks and cost thousands. The same shaft, grit-blasted and sprayed, is often back on the machine the same shift.

One note on scope: if a job is really an aluminum repair rather than a coating job, a guide to alloy wheel welding or aluminum MIG welding technique is the right reference, not a spray gun. Flame spray is a surfacing and restoration tool, and knowing which one the job calls for is most of the skill.

The Benefits, the Limits, and Where Flame Spray Fits Best

Every process collects a reputation, and flame spray’s is mixed. It gets called the cheap cousin of a better coating process, and for heavy impact wear that is not far from the truth. Step into a paper mill, a foundry or an offshore platform, though, and you will find flame spray used on purpose: in a specific set of jobs it is the cheapest, fastest and least fussy way to put metal back. Knowing which set of jobs those are is the whole skill.

Here is the honest version, both sides of the ledger.

What flame spray does genuinely well

• Low capital cost and simple, portable gear. A combustion gun, a gas train, a modest compressor and a blasting cabinet. No plasma console, no powder hopper, no robotic spray cell. The ASM-derived summary in the NIOSH review of thermal spray hazards describes flame spray coating processes as characterised by efficiency, high deposition rates, ease of operation and low equipment costs.
• Minimal heat input to the part. Because nothing fuses, the substrate never goes through a metallurgical transformation. There is no heat-affected zone, and on a large roll or a thin housing distortion is effectively nil. You can spray a two-metre shaft that a TIG pass would bow.
• Restoration instead of scrapping. TWI notes that thermal spraying can prevent the need to replace a part and reduces costs tied to maintenance and unexpected downtime. A scored journal or a pitted valve seat becomes a build-up job rather than a purchase order.
• Material flexibility. Zinc, aluminium, zinc-aluminium, nickel, copper, aluminium bronze, babbitt, stainless and nickel alloys, self-fluxing NiCrBSi, plus oxide and carbide powders.
• Coatings that can be finished. A flame-sprayed layer can be ground and machined back to dimension and blended into the surrounding surface, so the repair does not look like one.
• Field deployable. Cylinders, a compressor and a gun travel in a van. The part comes to the crew, not the other way round.

Where the limits bite

The biggest one is the bond. Flame spray adheres by mechanical interlocking rather than metallurgical fusion, so a sprayed layer is not a structural load path. You cannot design a load-bearing joint around it, and you cannot treat “sprayed” as a synonym for “welded.”

TWI’s process comparison is blunt about the rest: flame spray coatings can exhibit low bond strength, high oxide content and high porosity, with a moderate spray rate and low cost. Across thermal spray as a whole, TWI puts bond strengths at about 10 MPa to greater than 80 MPa depending on the process and spray parameters. Flame spray sits at the bottom of that band: fine for corrosion protection and dimensional build-up, a problem for anything abrasion will attack.

High oxide content means the coating is slightly less corrosion-resistant than the parent metal. High porosity means it needs sealing, so flame-sprayed zinc or zinc-aluminium for immersion service is normally finished with a sealer or paint system rather than left bare. Deposition rate is moderate, so very large areas take time. And the self-fluxing nickel alloys that give the best hardfacing results need a post-spray fusion step to close the porosity and reach a weld-like density.

Compare that with arc spray, which achieves higher bond strengths and spray rates but produces arc light, ozone and fume that can cause difficulties in some situations. That fume is why flame spray remains the sane choice inside a gearbox housing or on a ship deck, where a welder would need respiratory protection anyway.

Where flame spray wins on the shop floor

• Field repair of shafts, journals, rams, spindles and bearing housings.
• Low-volume work where setting up a plasma or HVOF rig does not pay.
• Budget and access constraints, including small shops with no capital budget for new equipment.
• Thin or heat-sensitive substrates that a weld would distort.
• Parts too large or costly to move, such as offshore pipework, in-situ coating and machine bases.

The same cylinders and regulator will also run a torch for brazing and light fabrication, so our air-acetylene torch guide is worth a read if the site has no separate brazing setup.

Where flame spray is the wrong answer

Do not reach for it where the job needs structural integrity, severe wear with impact, or thick, hard hardfacing. Impact-loaded wear, valve seats and pump parts want HVOF, detonation gun or plasma coatings, and heavy hardfacing is a weld overlay job built with a hardfacing consumable, covered in our all-position electrode guide. If the deposit must carry load, specify a welded build-up, and remember that conventional welding carries its own fume burden, which is why SMAW still has its fan club for many shops.

I’ll rewrite this section to fit the word count while keeping the heading, facts, and all links.

Flame Spray Welding Compared with Arc Spray, Plasma, HVOF, Detonation and Cold Spray

Every shop that runs a flame spray gun eventually asks the same question: is this good enough, or should we move to something more advanced? It is a fair question, because thermal spray is a family of processes that share a basic idea (melt or accelerate a consumable, build a coating, bond it to a prepared surface) but deliver very different results.

The honest headline: flame spray is not the best coating you can buy. It is the most accessible one. Whether that trade makes sense depends entirely on what the part in front of you has to survive.

Side by side

Process	Heat source	Particle velocity	Bond strength	Porosity / oxide	Feedstock	Capital cost	Portability	Best fit
Flame spray	Combustion of fuel gas and oxygen, material propelled by compressed air or the venturi effect	Low for the thermal spray family	Around the 10 MPa class	High oxide, high porosity	Wire or powder	Lowest	Highly portable	Field repair, dimensional build-up, low-volume restoration, corrosion protection that will be sealed
Arc spray	DC arc between two consumable wires	Moderate, above flame spray	Higher than flame spray	Lower than flame spray, still present	Wire only	Low	Portable, needs a power supply	Large-area corrosion protection and high throughput
HVOF	Combustion in a confined chamber	Supersonic, up to about 700 m/s	High	Low porosity	Powder	Medium to high	Shop-based equipment	Dense wear-resistant coatings, hardfacing, machined journals
Detonation gun	Acetylene and oxygen detonations	750 to 1,000 m/s	Highest of the group	Lowest porosity	Powder	Highest	Fixed installation	Benchmark-quality coatings, aerospace, critical wear surfaces
Plasma	Arc-heated plasma plume, up to about 16,000 °C	300 to 550 m/s powder	High	Dense, mostly oxide-free	Powder	Medium	Shop-based equipment	Aerospace turbine and combustion chamber coatings
Cold spray	Solid-state, no melting; carrier gas roughly 0 to 700 °C	200 to 1,200 m/s	High	Dense, minimal or no oxidation, no heat-affected zone	Powder	High	Shop-based, needs high-pressure gas	Thin dense deposits, repair of heat-sensitive parts

Why flame spray sits at the bottom of the table

TWI’s comparison of the thermal spray processes puts it plainly: flame spray melts the consumable with combustion of fuel gas and oxygen, using compressed air or the venturi effect to propel it, while arc spray uses a DC arc between two consumable wires and is the process with the highest productivity in the family.

The gap between them is mostly physics. All of these processes build adhesion from the same thing: droplets arriving fast enough and hot enough to flatten, interlock and fuse with each other on impact. As TWI’s thermal spraying overview explains, bond quality tracks impact velocity closely. Flame spray droplets are heavy, slow, and cooked in a flame that also fills them with oxide. Arc spray throws them harder and melts them with an arc instead of a flame. HVOF and detonation gun push the numbers to 700 m/s and beyond, and the coatings that come off those guns are dense and tightly bonded.

Cold spray is the outlier. Per TWI’s cold spraying page, particles typically 10 to 40 µm in size are accelerated to 200 to 1,200 m/s through a supersonic compressed gas jet at temperatures below the material’s melting point. The result is a deposit with high density, minimal or no oxidation, and no phase transformation or heat-affected zone. That is attractive for heat-sensitive work, but the trade is real: a smaller process window, tighter control, and a higher equipment bill.

The two heavyweight options

The NIOSH review of thermal spray health hazards states that detonation coating has the highest bond strengths and lowest porosities of the different thermal spray methods, and that nearly all metallic and ceramic materials can be deposited using detonation coating processes. The same review notes that plasma spray produces high bond strengths and mostly dense, oxide-free microstructures, and is widely used in the aircraft engine industry, where coating consistency matters more than cost.

That is why plasma dominates aerospace coating and repair work, and why detonation remains the benchmark other thermal spray processes are measured against. Both are capital-intensive, chamber-bound, and far less suited to a technician standing over a paper mill roll in a dead-end bay than a flame gun is.

So which one should you use?

Ask what the part actually needs. A sacrificial zinc layer on a tank wall that will be painted afterwards does not need the coating quality of a turbine blade. A hydraulic ram journal that will be ground to size and put back into service needs bond strength and density that a flame-sprayed layer cannot honestly promise. That is where combustion gas handling and flame spray still wins on cost and speed, and where HVOF or detonation wins on performance.

Flame spray is the right answer for field repair, low-volume restoration, dimensional build-up, and corrosion protection where a sealed, non-structural coating is acceptable. It is the wrong answer the moment the coating becomes a wear surface, a structural load path, or an aerospace-grade thermal barrier. The choice is not about which process is best in the abstract. It is about matching the coating to the duty, the budget, and the part you cannot afford to replace.

Surface Preparation and Spray Technique: Where Good Coatings Are Won

Here is the uncomfortable truth about flame spray welding: the coating does not care how good your gun work is if the steel underneath was dirty. A thermal spray deposit holds on by interlocking with a deliberately roughened surface, so every hour spent on preparation returns more than any hour spent fine-tuning the flame. TWI’s thermal spraying guidance makes the point plainly: adhesion depends on the molten particle velocities at impact as well as the surface condition, which must be clean and roughened by grit blasting or machining prior to spraying.

One more framing before the practical detail. The NIOSH review of thermal spray hazards describes the coating material simply as any material that will not degrade when melted, and it notes that suitable substrates are materials that can withstand the intense propulsion process that roughens the surface. Not every alloy survives being melted, and not every base metal survives being blasted.

The Preparation Sequence, Step by Step

1. Degrease and desalt. Oil, grease, cutting fluid, hydraulic oil and soluble salt residues have to go first. Every one of them burns into the coating and blocks adhesion. If the part is greasy, the spray job is already in trouble.
2. Strip scale, rust, mill scale and old coating. Mill scale is the hardest enemy: smooth, loosely bonded, and it flicks off with the deposit if you leave it. Remove it by grinding, chipping, wire brushing or shot blasting, and strip back to sound metal to expose the corrosion pits picked up in service.
3. Abrasive blast to a near-white metal finish. The usual reference is ISO 8501-1 Sa 2.5, where 95 percent of the surface is free of visible residue and shows an even matte metal profile. Use angular grit, not round shot; round shot peens the surface smooth, the wrong texture for mechanical interlocking. For most steel work, 20 to 36 mesh angular alumina or garnet is a sensible start.
4. Clean and keep it clean. Blow off dust with clean, dry compressed air immediately before spraying, and do not touch the surface with bare hands. Fingerprints are oil.
5. Mask adjacent surfaces. Mask shafts you need to keep at diameter, bearing bores, threads, machined faces, seals and nearby components. Masking saves you grinding off overspray later.

The Technique Variables That Decide the Result

Standoff distance and angle. Hold the gun at the distance and angle correct for the consumable and nozzle type. Too close or tilted too steeply and you waste material, distort the profile and overheat the local surface. Too far and the droplets arrive with reduced velocity and temperature, so splats do not flow and the coating comes out weak and loosely bonded. A nominal figure of around 100 to 150 mm for wire spray appears in many gun manuals, but the right setting is the one that produces a dense, well-bonded deposit. Mark the handle so every operator holds the same distance.

Traverse speed and overlap. Move the gun steadily. Rushing produces a thin, porous coating. Overlap each pass on the previous one, typically a third to a half of the spray pattern width, so there are no cold joints and no thin spots between passes.

Interpass temperature control. This is what keeps a large shaft straight. Keep the substrate well below the temperature at which it distorts, and a rule of thumb many shops use is to hold the part under about 200 °C for steel, monitored throughout. If a cylinder ram starts to grow and the operator keeps spraying, the final part will be out of tolerance no matter how good the coating is.

Nozzle maintenance. Clean the nozzle between passes. Spatter and partially melted wire build up on the tip, distort the spray pattern, and eventually clog it mid-run.

Coating thickness discipline. TWI notes that thermal spray coating layers typically run from about 60 µm up to 10 mm, so a single spray pass is nowhere near a substitute for heavy weld build-up in structural work. If a part needs a lot of metal, think about whether the base material can take the heat.

The Finishing Sequence

Most jobs resolve in one of a few paths: as-sprayed then machined back to final dimension, or as-sprayed then spray-fused, the standard route for self-fluxing nickel alloys. The NIOSH materials table lists families such as NiCrBSiC and NiCoCrBSiC for high hardness, high fusion and wear resistance. Their boron and silicon form a glassy flux during the fusion pass, wetting the substrate and closing porosity for a dense, weld-like coating. For corrosion work, the coating is sealed and painted before service.

One scenario deserves calling out because it comes up constantly. The AWS SprayTime feature on thermal spray repair describes a classic failure mode: a machining error removes too much stock and the piece is left nonconformant. Spray build-up is the way back to specification, but only if the surface the deposit will sit on was prepared properly in the first place.

Troubleshooting Table

Symptom	Likely cause	Fix
Coating peels or flakes off	Contaminated or insufficiently roughened surface; oil or mill scale left behind	Re-clean with solvent, re-blast to a fresh near-white profile
Coating is powdery and rubs off	Standoff too far, or deposit too thin; substrate overheated	Shorten standoff, slow the traverse, build up in more passes, reduce interpass heat
Coating is too rough	Gun too close or too steep, spatter build-up on the nozzle, low traverse speed	Correct distance and angle, clean the nozzle, steady the traverse
Uneven thickness	Poor overlap, inconsistent speed or distance, material not fully melting	Fix overlap, hold a constant speed, check consumable feed and flame settings
Nozzle clogs mid-run	Spatter and oxidized metal accumulating on the tip	Stop, cool, clean the nozzle, resume, and clean more often between passes

Safety: Fumes, Ultraviolet, Fire, Gas and Noise

Here is the uncomfortable truth about thermal spray: the process that saves a shaft can quietly wreck a set of lungs. The hazards are not the ones welders are trained to expect. There is no blinding arc for most flame work, no obvious smoke plume that everyone instinctively respects, and the whole operation can look like a torch with a bit of dust floating off it. That appearance is exactly why this section is the longest in the guide.

Flame spray is a serious multi-hazard process, and OSHA treats it that way. The rules, the research and one very sad case all point to the same conclusion — the controls are basic, they are known, and people still skip them.

1. Airborne Metal Fume and Dust: The Primary Hazard

The molten droplets that miss or bounce off the substrate do not disappear. They cool into ultrafine metal particulate and, depending on the consumable, toxic metal fume. This is a spray, not a closed weld pool, so the generation rate is high and the exposure is near the operator’s breathing zone.

OSHA’s welding and cutting standard is the baseline. Paragraph 29 CFR 1910.252(c)(1)(iii) requires local exhaust or general ventilating systems arranged to keep toxic fumes, gases and dusts below the maximum allowable concentrations specified in 1910.1000. Paragraph 1910.252(c)(1)(iv) goes further: employers must include the potentially hazardous materials in fluxes, coatings, coverings and filler metals in the Hazard Communication program under 1910.1200. In plain terms, if the wire contains chromium or nickel, the operator has a right to know what they are breathing.

The NIOSH evidence is what should stop an operator cold. Emission measurements summarised in the NIOSH review of thermal spray health effects — including work by Bemer et al. (2010) — recorded concentrations greater than 10⁸ particles/cm³ inside a well-ventilated cabin during thermal spray gun operation, with 85 to 90 percent of those particles below 100 nanometres. These emission rates far exceed those typically encountered during arc welding. Nanometre-scale particles are the ones that travel deep into the alveolar region where gas exchange happens.

It gets more sobering. Industrial hygiene reports cited in the same NIOSH review measured hexavalent chromium above 500 µg/m³ in an open-air work area. That is approaching 100 times OSHA’s permissible exposure limit of 5 µg/m³. Outdoors, with wind, with what looked like adequate dilution. Hexavalent chromium is a known human lung carcinogen.

The most serious documented outcome is a 2003 Washington State fatality. The NIOSH review records a worker who spent two days applying nickel- and chromium-based thermal spray coating at a temporary worksite without proper ventilation and without an appropriate respirator. He developed progressive symptoms and died. Post-mortem findings identified severe lung damage associated with toxic metal inhalation. Two days of work. No exotic process, no unusual material — just a spray gun, a metal powder and the assumption that a temporary job did not need permanent controls. For a deeper look at how fume and gas control is managed across common processes, see the safety guidance in this air arc welding guide, which faces many of the same ventilation problems.

Different metals carry different specific risks. The table below summarises the reported health effects for the main elements found in flame spray consumables.

Element	Reported health hazard
Chromium (esp. hexavalent)	Lung carcinogen; respiratory irritation, ulceration, dermatitis
Nickel	Lung and nasal carcinogen; sensitisation and dermatitis
Cobalt	Respiratory irritant; asthma and hard-metal lung disease
Copper	Metal fume fever; irritation of the upper respiratory tract
Zinc	Metal fume fever (flu-like, usually self-limiting)
Iron	Siderosis — benign iron-oxide pneumoconiosis
Manganese	Neurotoxicity; effects similar to manganism/Parkinsonism

The engineering-control hierarchy does not change with the metal. Move the air at the source, then verify it is working, then protect the operator for whatever remains. Fume extraction that is present but starved of airflow is a decoration, not a control.

2. UV Radiation and Eye Protection

Combustion flame spray flames exceed 3,100 °C. That heat radiates visible light and significant ultraviolet. The cornea absorbs UV readily, so overexposure causes flash burn, commonly called welder’s eye or arc eye. The cruel detail is that the damage can occur without discomfort during exposure — an operator may feel nothing while being injured and discover the consequences hours later when the pain and gritty sensation begin.

The control is straightforward. Use No. 6 green welding goggles for the operator’s direct line of sight, and install UV-dark glass over booth windows so that anyone else in the shop who glances toward the spray station is not exposed to scattered radiation. Anyone working nearby — a machinist, an inspector, another tech — should be treated as a potential exposed person, not just the person holding the gun. If you want to compare eye and skin protection approaches across processes, the Alpha laser welding safety guide covers how a similar invisible-radiation hazard is managed.

3. Respiratory Protection: Engineering Controls First

OSHA’s respiratory protection standard, 29 CFR 1910.134, is explicit that respirators are the last line of defence. Engineering controls — local exhaust, enclosures, general ventilation — come first. Where respirators are still needed, they are not something you hand out at the tool crib. The standard requires a written respiratory protection program that includes medical evaluation, fit testing, training and routine use assessment. A factory would not run a pressure vessel without a procedure, and it should not put a worker in a sprayed-metal aerosol without one either.

Selection comes down to the assigned protection factor. OSHA’s 1910.134 Table 1 lists an APF of 10 for a half-mask air-purifying respirator and 50 for a full facepiece. Powered air-purifying respirators can reach up to about 1,000 under the programme’s requirements for that configuration. HEPA filters are defined as at least 99.97 percent efficient against monodisperse particles of 0.3 µm — the particle size that historically represents the most penetrating aerosol in filter testing. Given that 85 to 90 percent of spray particles are below 100 nm, HEPA filtration is not optional for particulate control.

Practical guidance from The Fabricator’s thermal spray safety guidance frames the tiers: where engineering controls and ventilation are unavailable, a supplied-air positive-pressure respirator or SCBA may be necessary. Where engineering controls are in place, a negative-pressure half mask with OV/P-100 filters is the minimum. The organic vapour and particulate combination matters because coatings and sealers can off-gas as well as emit metal particulate.

One rule that gets broken constantly: never connect plant compressed air to a breathing apparatus. Shop air contains oil, water and contaminants from the compressor and distribution system. Breathing air requires a dedicated supply and air quality verification.

4. Fire and Explosion

Fine metal dust is potentially ignitable. Aluminium and zinc fines are classic reactive dusts, and both metals can react with water to produce explosive hydrogen gas. That means spray residue and dust accumulation in enclosures, filters, ducts and on ledges is a fuel load, not just housekeeping.

OSHA 1910.252(a) sets general fire precautions for hot work. The practical rules include clearing combustibles from the work area — the familiar 35-foot combustible clearance rule — and maintaining a fire watch for at least half an hour after work is completed, because hot metal and coatings can smoulder well after the torch is off. NFPA 51B provides the same framework for fire prevention during hot work and is the reference most shops’ insurance and safety programmes hinge on. For hot work in tanks, pipework and confined spaces, the rules are stricter again.

• Clear or protect all combustible material within the hot work zone.
• Keep a fire watch during and for 30 minutes after the work.
• Wet or otherwise manage reactive dust on a schedule, not when it becomes visible.
• Do not spray near open containers of solvent, fuel or powder.

5. Gas Safety

Flame spray runs on flammable fuel gas and oxygen under pressure. OSHA’s Subpart H covers oxygen-fuel gas welding and cutting. The controls are well established but easy to get lazy about.

• Install flashback arrestors on both the fuel and oxygen lines at the torch or manifold.
• Isolate fuel gas and oxygen at the source when equipment is not in use. Leaking fittings in a busy shop are how a small problem becomes an event.
• Ground equipment properly to control static, especially when handling powders.
• Use sparkproof tools in areas where metal dust can accumulate.
• Never work on a pressurised gas system. Isolate, depressurise and prove it before touching a fitting.

Fuel selection itself is a safety variable. Acetylene gives the hottest, cleanest flame, while propane and propylene offer portability and lower cost, and hydrogen burns cleanly but demands its own handling care. Whichever gas you use, treat the oxygen side with special respect: oxygen speeds combustion in materials you would not normally think of as flammable, including clothing and greasy rags.

6. Compressed Air, Dust Handling and Noise

Flame spray uses compressed air to atomise and propel the molten material. That same air is a cleaning tool, and OSHA references the conditions for safe use in 1910.242. Limit cleaning air pressure to below 30 psi, and never use compressed air to blow powder off equipment or clothing. Powder blasted off a surface becomes airborne dust, and a blast of air on an operator’s clothing drives that dust straight into their breathing zone. Use a vacuum with appropriate filtration instead.

Noise may be the most overlooked hazard of all. Combustion guns, air jets and compressors generate sustained high levels, and atomising air adds broadband hiss. OSHA’s 29 CFR 1910.95 Table G-16 sets an 8-hour permissible exposure limit of 90 dBA. An 8-hour time-weighted average of 85 dBA is the action level that triggers a hearing conservation programme and audiometric testing; the standard also limits impulsive or impact noise to a 140 dB peak sound pressure level. For reference, many shops’ spray booths measure in the low 90s, which puts an operator over the PEL inside a normal shift.

One closing thought on process choice. If you are weighing whether to spray, weld or use another technique, safety costs belong in the comparison alongside bond strength and speed. The alloy steel welding electrode guide shows how filler and fume considerations shift with the consumable. Flame spray adds a genuinely different exposure profile — ultrafine metal particulate, sustained noise and UV from a combustion flame. Budget the controls, train the operator, and keep the extraction running from the first pass to the last.

Standards, Qualification and the Mistakes That Ruin a Coating

Now comes the paperwork that decides whether a job is a coating or an argument with the client. A sprayed layer nobody specified, tested or signed off on is not a protected surface, it is a hope. A written specification turns that hope into something you can invoice, defend and repeat next year. AWS’s SprayTime feature on thermal spray repair traces how the trade began: technicians hauling wire spray guns and oxygen-acetylene cylinders into mills and factories to put failed machinery back to work.

The Standards Stack

Six documents cover most of what a flame sprayer will meet:

Standard	What it covers
ANSI/AWS C2.18	Guide for the Protection of Steel with Thermal Sprayed Coatings of Aluminum and Zinc and their Alloys and Composites
ANSI/AWS C2.16M	Guide for Thermal-Spray Operator Qualification
ISO 14918	Thermal spray terminology, classification and requirements
ASTM C633	Standard test method for adhesion or cohesive strength of flame sprayed coatings
ASTM D610	Standard practice for evaluating the degree of rusting on painted steel surfaces
NACE 12 / AWS C2.23M / SSPC-CS 23.00	Coating systems for marine and offshore environments

The keystone is ANSI/AWS C2.18, which covers the corrosion side of the trade: zinc, aluminium and zinc-aluminium duplexes over blast-cleaned steel, film thicknesses and the sealer or paint system on top. ASTM C633 proves the bond and ASTM D610 judges corrosion coatings after service, while for marine and offshore work the literature references NACE 12, also published as AWS C2.23M and SSPC-CS 23.00. TWI’s thermal spraying overview covers the systems behind them.

Operator Qualification: the Part Most Shops Skip

The kit is cheap enough for anyone to buy a gun, while the result is entirely operator-dependent. A qualified operator knows the preparation grade, the standoff, the traverse speed, the substrate temperature limit and the film thickness for the material in front of them. An unqualified one knows what the gun feels like. That gap is why ANSI/AWS C2.16M exists.

It is a trade with a future. The NIOSH review of thermal spray hazards notes that thermal spray operators sit within the occupational group of coating and spraying machine setters and operators, with an estimated 146,350 such workers in the United States in 2019 according to the Bureau of Labor Statistics. The review also cites a study of 34 thermal spray workers across six UK worksites, which found chromium, nickel and cobalt exposure highest with plasma spray, and urinary nickel and cobalt tracking airborne concentrations. Learning the process properly pays twice: better work, and better control of what you are breathing. Training routes run from full welding programmes to short thermal spray courses such as this guide to what you learn, what it costs and where it leads.

Ten Mistakes That Ruin a Flame Sprayed Coating

1. Spraying over mill scale, rust or oil. Blast to a clean, near-white metal profile and keep it oil-free. Prep discipline is covered in the guide to preparing metal properly before welding.
2. Wrong standoff or a wandering gun. Set the distance once, hold the angle, use a standoff guide. Too close wastes material and roughs the profile; too far cools the droplets and the coating comes out weak.
3. Traversing too fast. Slow down, then verify with a wet film thickness gauge or witness coupons rather than trusting feel.
4. Overheating the substrate. Monitor interpass temperature and cool the part. Heat builds up on a large shaft and bends it.
5. Contaminating the surface between passes. Re-clean with solvent and clean, dry compressed air before restarting.
6. Wrong consumable, or non-spray-grade wire. Use certified thermal spray wire and matched powder, and check the batch certificate. General-purpose welding wire is no substitute.
7. Skipping the sealer and paint system. Seal or paint over the sprayed layer as specified. A bare thermal spray layer is not a complete corrosion system.
8. Ignoring post-spray fusion on self-fluxing alloys. Spray-fuse them. Without fusion, the boron and silicon never form the glassy flux that closes the porosity.
9. Treating a flame sprayed coating as a structural weld. It is a mechanically bonded coating, not a load path. Load-bearing work belongs to weld overlay, HVOF or detonation.
10. Running without extraction and without monitoring. Start the extraction before you light the gun. OSHA 1910.252 requires fume control to keep airborne contaminants below the limits in 1910.1000, and where controls are missing, OSHA 1910.134 requires a written respiratory protection programme with medical evaluation and fit testing.

First Job Checklist

• Consumable confirmed against the specification, batch certificate checked.
• Substrate material confirmed, and confirmed as sprayable.
• Degrease, then blast to the specified profile, near-white metal for most work.
• Atomizing air dry, oil-free and at pressure.
• Gas train checked: regulators, flashback arrestors, hoses and fuel handling, per air-acetylene torch basics and safety.
• Standoff and traverse set on a witness coupon, then measured.
• Maximum substrate temperature set, with a way to check it.
• Downstream sealer or paint system, and machining allowance, confirmed.

Document It

Coatings that get documented get repeated; the ones that do not get re-learned by failure. Record spray parameters (standoff, traverse speed, atomizing air pressure, substrate temperature limit), consumable batch numbers, operator, pre-coat surface condition and preparation grade, achieved thickness and the ASTM C633 adhesion result, plus date, machine and environmental conditions. Six lines on a job card is the cheapest insurance in the trade.

Conclusion: Choosing Flame Spray Welding Wisely

Flame spray welding is a combustion-driven thermal spray process that rebuilds and protects surfaces rather than joining them. No filler metal is melted into a puddle, and no metallurgical bond forms with the base metal. Instead, a hot flame melts wire or powder, compressed air atomises the droplets, and flattened splats interlock against a grit-blasted anchor profile. Understand that distinction and the process is easy to place on the shop floor.

It earns its place because it is cheap, portable, low in heat input and fast. A combustion gun and two gas cylinders cost a fraction of an HVOF or laser system, they travel to the job instead of the job travelling to the machine, and they put almost no heat into the substrate, so large shafts and thin parts come back without distortion. As TWI notes, thermal spraying can prevent the need to replace a part, suits a wide range of industries, and reduces the costs of maintenance and unexpected downtime.

Three decision rules

1. Choose flame spray when the answer is a sealed, non-structural coating. Field repair, low-volume restoration, dimensional build-up and corrosion protection are its natural homes, especially where the part is no longer manufactured, replacement cost is prohibitive or fabrication lead time is too long. Those are exactly the situations AWS frames thermal spray repair around.
2. Step up to arc spray for large areas and better bond strength. Arc spray runs hotter and faster and produces a denser, better-bonded coating than flame spray, so it suits big structural surfaces and long runs of corrosion protection, if you can live with arc light, ozone and fume.
3. Step up to HVOF, detonation or plasma for wear, density and aerospace performance. Those high-velocity routes give dense, low-porosity coatings that survive hard wear and heat. TWI’s comparison of the thermal spray processes is the right place to confirm which one a part actually needs.

What to carry back to the shop floor

• Blast properly. Degrease, strip scale and old coating, and blast to a clean near-white metal profile. No coating survives a contaminated surface.
• Control substrate temperature. Interpass temperature control is what stops a long shaft from bending.
• Keep the extraction running. Thermal spray fume levels can exceed arc welding levels. Engineering controls first, respirator second.
• Treat it as a coating. Mechanical bond, machineable, and normally sealed or painted before immersion service.
• Use certified spray-grade consumables. Not the same wire you buy for a welding torch. Background on the combustion side sits in air acetylene welding, and gas and equipment choices are covered in the welding equipment guide.

And if the job genuinely needs a structural joint rather than a coating, use a fusion process. That is where stick welding still wins.

A trade that is growing

Thermal spraying is not a dying craft. The NIOSH review of thermal spray hazards notes that the global thermal spray coating market is expected to grow from roughly 6.5 billion dollars to about 17 billion dollars by 2027, citing ASM Thermal Spray Society and Grand View Research data. The same review cites Bureau of Labor Statistics figures putting 146,350 coating and spraying machine setters and operators to work in the United States in 2019. The skill is portable and in demand.

Which brings us back to the ram on the paper mill rolls machine. It could not be welded back without risking distortion and hours of machining. It could be ground down and scrapped, or it could be rebuilt with a combustion gun and a spool of nickel-alloy wire in an afternoon. Shops that know the difference between a coating and a weld will keep getting calls like that, and the people who can do the work will be the ones who get asked.

Affiliate disclosure: some of the supplier and guide links in this article may be affiliate links. Commission does not change which processes or products we recommend.

Frequently Asked Questions

Is flame spray welding actually welding?

No. Despite the name, flame spray welding is a thermal spray (surfacing) process, not a fusion welding process. The consumable is melted and sprayed onto a prepared surface, and the bond to the substrate is primarily mechanical interlocking rather than a metallurgical fusion, which is why coatings are usually sealed, machined or painted rather than treated as structural joints.

How hot does a flame spray flame get?

Combustion flame spray flames can reach peak temperatures in excess of 3,100 °C, which is more than enough to melt most spray wires and powders. The base metal, however, receives far less heat than a weld arc delivers, which is the main reason flame spray produces minimal distortion.

What is the difference between flame spray and arc spray?

Flame spray melts the consumable with an oxy-fuel combustion flame; arc spray melts it with an electric arc between two wires. Arc spray achieves higher bond strength and higher deposition rates, but flame spray is cheaper, simpler, and does not produce the arc light, ozone and fume that can complicate arc spray work.

Why does my flame sprayed coating peel off?

In almost every case the cause is preparation, not the spray. Thermal spray adhesion depends on the surface being clean and roughened by grit blasting or machining before spraying, so rust, mill scale, oil or a too-smooth surface will cause peeling. Blasting to a near-white metal finish, cleaning with solvent and compressed air, and keeping the surface dry will fix most failures.

Is flame spray safe to do without a spray booth?

It is possible but risky. OSHA 1910.252 requires ventilation to keep fumes and dust below the maximum allowable concentrations, and OSHA 1910.134 requires engineering controls first, with respirators used under a written program with fit testing. NIOSH-affiliated research has measured thermal spray emission rates far exceeding those of arc welding, with most particles under 100 nm, so a booth with extraction is strongly preferred.

Can you flame spray aluminium and zinc for corrosion protection?

Yes. Zinc, aluminium and zinc-aluminium alloy (TSA) are among the most common flame sprayed consumables and are used extensively for steel structures, bridges and offshore equipment. They must be applied to a clean, blasted surface and then sealed or painted, because the as-sprayed coating is porous and the porosity is what allows corrosion to travel under the coating.

How thick can a flame sprayed coating be?

Thermal spray coatings are generally deposited in layers from roughly 60 µm up to about 10 mm thick depending on the process and application, but flame sprayed coatings specifically carry higher porosity and oxide content than HVOF, detonation or plasma coatings. For hard, dense build-up, a self-fluxing nickel alloy is sprayed and then fused, which produces a much more weld-like result.

Do I need a welding certificate to do flame spray welding?

You need competence and, for contract work, qualification. ANSI/AWS C2.16M and ISO 14918 both cover thermal spray operator qualification, and AWS C2.18 covers thermal sprayed aluminium and zinc coatings on steel. Note also that OSHA 1910.252 requires eye protection during gas welding operations and hazard communication for filler metals, so the safety paperwork applies here too.

Can flame spray replace a worn shaft instead of welding a new one?

In many cases, yes, and that is exactly what the process was built for. AWS’s SprayTime feature lists worn-part restoration, obsolete replacement parts, prohibitive replacement cost and machining errors among the standard reasons shops choose thermal spray, since spraying builds material back up without the distortion and heat-affected zone of a heavy weld pad.

What respirator do I need for flame spraying stainless steel or nickel alloys?

Engineered controls come first under OSHA 1910.134. Where local exhaust is not available, the guidance in industry safety literature is that a supplied-air positive-pressure respirator or SCBA may be necessary, and where engineering controls and ventilation are in place, a negative-pressure half mask with OV/P-100 filters is the minimum. Any respirator use requires a written program with medical evaluation, fit testing and training.