HARD – FACING WITH THERMAL SPRAY
- Thermal spray technology is used to apply coatings which can enhance wear resistance , resistance to corrosive environments , prevent high temperature oxidation , apply special electrical insulation or enhance conduction characteristics , deposit thermal barrier coatings , build up to restore dimensions besides many more applications . Our discussion will be limited to wear resistance only .
- Thermal spraying techniques are a line of sight coating process in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating material) is heated by electrical (plasma or arc) or chemical means (combustion flame). The feedstock is in the form of wire , rod or powder .
- The "feedstock " is fed throuch the flame / arc where it is heated and propelled at a high velocity on to the base metal ( workpiece ) Coating materials available for thermal spraying include metals, alloys, ceramics, plastics and composites. They are fed in powder or wire form, heated to a molten or semi molten state and accelerated towards substrates in the form of micrometer-size particles.The molten material flattens and forms a mechanical bond with the substrate . Resulting coatings are made by the accumulation of numerous sprayed particles. In general the substrate temperature can be below 200 C eliminating the possibility of metallurgical change in the substrate . Since the surface may not heat up significantly, it allows the coating of flammable substances. Combustion or electrical arc discharge is usually used as the source of energy for thermal spraying.
- Thermal spraying can provide thick coatings (approx. thickness range is 20 micrometers to several mm, depending on the process and feedstock ) , over a large area at high deposition rate as compared to other coating processes such as electroplating, physical and chemical vapor deposition.
- Coating quality is usually assessed by measuring its porosity, oxide content, macro and micro-hardness, bond strength and surface roughness. Generally, the coating quality increases with increasing particle velocities.
- Thermal spray can be done both on site or off site . The oxy fuel process is a portable and cheap process with minimum eqpt .
THERMAL SPRAY PROCESSES
- Oxy fuel wire spray
- Oxyacetylene Powder Welding.
- Atmospheric Plasma Spraying (APS)
- Low Pressure Plasma Spraying (LPPS ,VPS)
- Detonation Gun Spraying (D Gun)
- Wire Arc Spraying
- Plasma Transferred Arc Process (PTA)
- Ceramic Rod Process (ROKIDE Process)
- Flame Spraying
- High Velocity Oxyfuel Coating Spraying (HVOF)
- Warm Spraying
- Cold Spraying
- Laser Cladding
In classical (developed
between 1910 and 1920) but still widely used processes such as flame spraying
and wire arc spraying, the particle velocities are generally low (< 150
m/s), and raw materials must be molten to be deposited. Plasma spraying, developed
in the 1970's, uses a high-temperature plasma jet generated by arc discharge
with typical temperatures >15000 K, which makes it possible to spray
refractory materials such as oxides, molybdenum, etc.
Oxyfuel wire spray :
This is one of the oldest process where the consumable in wire form is fed into a oxy acetylene flame .The wire is melted by the flame , atomized into small particles by a jet of compressed air and propelled on to the work piece . This process can be manual or automated . It is used to reclaim worn out parts as well as can be used to coat new components , It is suitable for outdoor work and is used for corrosion protection of bridges , large storage tanks etc.Low cast and high productivity is its hallmark .wide variety of consumables are available making it a good choice for all purpose spraying
Oxyacetylene Powder welding:
Oxyfuel wire spray :
This is one of the oldest process where the consumable in wire form is fed into a oxy acetylene flame .The wire is melted by the flame , atomized into small particles by a jet of compressed air and propelled on to the work piece . This process can be manual or automated . It is used to reclaim worn out parts as well as can be used to coat new components , It is suitable for outdoor work and is used for corrosion protection of bridges , large storage tanks etc.Low cast and high productivity is its hallmark .wide variety of consumables are available making it a good choice for all purpose spraying
Oxyacetylene Powder welding:
- Employs a standard oxyacetylene torch, with the powder fed into the flame from an attached hopper. Typically used for glass moulds, smaller parts and repairs. Particularly suitable for the repair of cast iron and machined parts. Powder welding gives a smooth, dense coating with diffusion bond to the base material. Deposition rates are from 05-2.5 kg/hour, and surfacing thickness from 0.2-12 mm.
Atmospheric Plasma spraying:
- Employs a technique essentially similar to flame spraying. It differs in that the flame comprises an electrically excited plasma of high velocity and temperature ("'15,0000K). This permits a denser coating (95-98%). Deposition rates are from 2-8 kg/hour, and surfacing thickness from 0.1-2.5 mm.
Low Pressure Plasma
spraying (LPPS):
- LPPS coating is a low-velocity, high-temperature spraying process that is performed in a near-vacuum argon atmosphere.
Detonation Thermal Spraying:
- The detonation gun consists of a long water-cooled barrel with inlet valves for gases and powder. Oxygen and fuel (acetylene most common) is fed into the barrel along with a charge of powder. A spark is used to ignite the gas mixture and the resulting detonation heats and accelerates the powder to supersonic velocity through the barrel. A pulse of nitrogen is used to purge the barrel after each detonation. This process is repeated many times a second. The high kinetic energy of the hot powder particles on impact with the substrate results in a buildup of a very dense and strong coating.
Wire Arc Spray Process:
- Wire arc spray is a form of thermal spraying where two consumable metal wires are fed independently into the spray gun. These wires are then charged and an arc is generated between them. The heat from this arc melts the incoming wire, which is then entrained in an air jet from the gun. This entrained molten feedstock is then deposited onto a substrate. This process is commonly used for metallic, heavy coatings.
PTA (Plasma Transferred Arc)
welding:
- Is a process which can be highly automated . It utilizes a combined arc/plasma stream to form a limited melt on the work piece surface. A low base metal dilution with a small heat affected zone and a dense uniform coating is achieved. A wide range of user-specific consumables are possible.Because the hardfacing materials arc in powder form .it is possible to produce overlays from many different materials and combinations of materials with a wide range of hardness and other properties. This process has found extensive use in high-volume automated applications such as the thermal surfacing of exhaust valves. Deposition rates up to 12 kg/ hour are possible and surfacing thickness from 1-6 mm.
Ceramic Rod process
(ROKIDE):
- The ROKIDE Spray System is unique because it utilizes ceramic oxide rods. The rods are melted in a patented spray unit which projects the fully molten particles onto the substrate. The ROKIDE particles cannot leave the spray unit until fully molten. These particles have high kinetic energy and high thermal mass, so they remain molten until they reach the substrate.
Flame spraying:
- Where powder is fed into a fixed oxy-acetylene or oxy-hydrogen flame, and projected towards the base material the semi-molten material forms a mechanical bond which, when fused to the work piece, creates a metallurgical bond. Flame spraying is ideal for coating cylindrical parts. High-hardness alloys and tungsten carbide mixtures can be used with advantage. Deposition rates are from 1-9 kg/hour, and surfacing thickness from 0.1-3 mm.
HVOF (High- Velocity
Oxy-Fuel) spraying:
- Combines velocities up to 700 m/s with moderate temperatures. This process provides a very dense coating (> 97%). Deposition rates are up to 9 kg/hour, and surfacing thickness from 0.05-25 mm.
Warm Spraying:
- Warm spraying is a novel modification of high velocity oxy-fuel spraying, in which the temperature of combustion gas is lowered by mixing nitrogen with the combustion gas, thus bringing the process closer to the cold spraying. The resulting gas contains much water vapor, unreacted hydrocarbons and oxygen, and thus is dirtier than the cold spraying. However, the coating efficiency is higher. On the other hand, lower temperatures of warm spraying reduce melting and chemical reactions of the feed powder, as compared to HVOF. These advantages are especially important for such coating materials as Ti, plastics, and metallic glasses, which rapidly oxidize or deteriorate at high temperatures.
Cold Spraying:
- In the 1990s, cold spraying (often called gas dynamic cold spray) was introduced. The method was originally developed in Russia, with the accidental observation of the rapid formation of coatings. This occurred while experimenting with particle erosion of a target exposed to a high velocity flow loaded with fine powder in a wind tunnel. In cold spraying, particles are accelerated to very high speeds by the carrier gas forced through a converging–diverging de Laval type nozzle. Upon impact, solid particles with sufficient kinetic energy deform plastically and bond mechanically to the substrate to form a coating. The critical velocity needed to form bonding depends on the material's properties, powder size and temperature. Metals, polymers, ceramics, composite materials and nanocrystalline powders can be deposited using cold spraying.[5] Soft metals such as Cu and Al are best suited for cold spraying, but coating of other materials (W, Ta, Ti, MCrAlY, WC–Co, etc.) by cold spraying has been reported.
- The deposition efficiency is typically low for alloy powders, and the window of process parameters and suitable powder sizes is narrow. To accelerate powders to higher velocity, finer powders (<20 micrometers) are used. It is possible to accelerate powder particles to much higher velocity using a processing gas having high speed of sound (helium instead of nitrogen). However, helium is costly and its flow rate, and thus consumption, is higher. To improve acceleration capability, nitrogen gas is heated up to about 900 C. As a result, deposition efficiency and tensile strength of deposits increase.
Laser cladding:
- Focuses and controls heat and weld depth, offering a clean metallic bond with minimal dilution, a small heat affected zone and fine grain structure. Deposition rates are up to 8 kg/hour, and surfacing thickness from 0.5 to more than 4 mm.
HOW TO CHOOSE THE RIGHT
POWDER .
Thermal spray powders are manufactured by the water atomization , gas atomization or sol-gel process . The performance and consistency of any Thermal spray process is dependent on the quality of the powders . Chemical composition
and porosity determines the abrasion, heat, impact and corrosion resistance of
the thermal surface. Particle size distribution can effect weld shape and is
often equipment specific. These are the main factors used to determine the
right powder for a specific application. This diagram and the tables on the
following pages, will help you to focus your search. The hardness of the
deposit is strongly dependent on spray performance.
Resistance properties for Different Materials:
- Cobalt based alloys has high abrasion and heat resistance where the impact resistance is lower.
- Nickel CrMo has high heat and impact resistance where the abrasion resistance is lower.
- Nickel CrSiB has medium resistance to impact and abrasion but has a good heat resistance property.
- Iron CrMoWVC HSS has good heat and abrasion resistance but the impact resistance is low.
- Iron CrNi Austenitic SS has a high impact resistance, medium heat resistance and a very low abrasion resistance.
- Iron Cr high C has a very high abrasion resistance, medium heat resistance and low impact resistance.
- Iron Cr Martensitic SS has a very low heat resistance and it has a medium impact and abrasion resistance
Material
|
Abrasion Resistance
|
Heat Resistance
|
Impact resistance
|
Cobalt based alloys
|
High
|
High
|
Low
|
Nickel CrMo
|
Low
|
High
|
High
|
Nickel CrSiB
|
Medium
|
High
|
Medium
|
Iron CrMo WVC HSS
|
Good
|
Good
|
Low
|
Iron CrNi Austenitic SS
|
Low
|
Medium
|
High
|
Iron Cr high C
|
High
|
Medium
|
Low
|
Iron Cr Martensitic SS
|
Medium
|
Very low
|
Medium
|
The Feedstock ( Powders ) characteristics play a very important role in the performance of the thermal spray coating. The graphs below explain why .
Size Range of Powders available:
Effect of the particle size on weld bead shape:
Characteristics of
Flame, HVOF and Plasma Spraying:
Characteristics
|
Flame Spraying
|
HVOF Spraying
|
Plasma Spraying
|
Gas Temperature (°C)
|
3000
|
2600 – 3000
|
12000 – 16000
|
Spray rate (Kg/h)
|
1 – 9
|
1 – 9
|
2 – 8
|
Particle velocity (M/s)
|
>50
|
>700
|
>450
|
Bond Strength (MPa)
|
7 – 83 + Fused
|
48 – 80
|
14 – 48
|
Coating thickness
|
0.1 – 3
|
0.05 – 2.5
|
0.1 – 2.5
|
Hardness (HRC)
|
20 – 60
|
20 – 60
|
20 – 60
|
Porosity (%) cold spray
|
10 – 15
|
>3
|
2 – 5
|
Porosity (%) Fused
|
~1 – 2
|
<3
|
~1 – 2
|
APPLICATIONS
Hardfacing technology has wide spread use in the following Industries
- Aerospace
- Oil & Gas
- Automotive
- Rubber
- Power Generation
- Steel
- Timber
- Plastics
- Glass
- Forging
- Dental
- Earthmoving
- Mining
- Food Processing
- Valves & Fittings
Hardfacing technology has wide spread use in the following Industries
- Aerospace
- Oil & Gas
- Automotive
- Rubber
- Power Generation
- Steel
- Timber
- Plastics
- Glass
- Forging
- Dental
- Earthmoving
- Mining
- Food Processing
- Valves & Fittings
Email us: salesarcraft@gmail.com Visit us : www.arcraftplasma.com

Disclaimer
REFERENCES:
- "Hoganas thermal spray catalog"
No comments:
Post a Comment