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SURFACING BYTHERMAL SPRAYING

Dr. M. KAMARAJ

DEPT OF METALLURGICAL AND MATERIALS ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY MADRASCHENNAI 600 036

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OUTLINE

Classification of Surface damages

Industrial Components : problems

Need for Surface Modification

Coating Techniques:

Hardfacing by welding methods

Thermally spray processes

Applications

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LOSS OF USEFULNESS OF MATERIAL OBJECTS

The cause of loss of usefulness of material objects

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LOSS OF USEFULNESS OF MATERIAL OBJECTS

The cause of loss of usefulness of material objects

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CLASSIFICATION OF WEAR

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An accelerated corrosion of metal surfaces

The combined effect of oxidation and reactionswith sulfur compounds and other contaminants,such us chlorides, to form a molten salt on ametal surface that fluxes, destroys, ordisrupts the normal protective oxide.

HIGH TEMPERATURE CORROSION

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HIGH TEMPERATURE CORROSION

Engineering components:

- Gas turbines of aircraft/Marines engines

- Fossil-fired power plants: Boilers, superheaters,

and heat exchangers

-Refineries and Petro-chemical processing

Components: Fluid-catalttic cracking units (FCCU),catalytic steam reformersand Ethylene cracking

- Fluidized-bed coal combustion

performance : materials

Corrosion resistance or

high temperature mechanical properties

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HIGH TEMPERATURE CORROSION DAMAGE

High temperature corrosion may have variousDetrimental effects:

i. Due to the loss of materials

ii. Corrosion reduces the load-bearing capability ofconstructoin materials

iii. Significant amount of material may be lost or wastedin hot working of metals

iv. Reaction chambers to which a particular reaction orprocess is confined may break down etc.

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MODES OF HIGH PEMPERATURE CORROSION

Principal modes of high –temperature corrosion in industrialenvironments, as well as the interaction between oxygen activity andprincipal corrodent activity

Applications above 650 C

-More than one mechanismacts to degrade the material

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SURFACE

TREATMENTMETHODS

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INTRODUCTION

Surface Engineering :

- is a discipline that combines chemistry, physics,Mechanical engg with Metallurgy and Materialsscience

- provides the weapon for deterioration processes

product SE Enhanced Product

Raw materials Thermal Energy Product

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PERFORMANCE OF COMPONENTS

Chemical : acids, alkalis, salts, solvents, etc

Thermal : High temperatures, direct flames,oxidising gases, etc

Atmospheric: humidity, rain, snow, sunlight, sea water,erosion, etc

Mechanical : Abrasion, vibration, adhesion, galling, etc

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SIGNIFICANCE OF SURFACE ENGINEERING

Main purpose may be :

i. To minimize corrosionii. Reduce frictional energy lossesiii. Reduce weariv. Act as a diffusion barrierv. Provide thermal insulationvi. Exclude certain wave length of radiationvii. Promote radiation electronic interactionsviii. Electrically insulateix. Improve asthetic appearance for the surface

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SURFACE TREATMENT METHODS

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THERMAL SPRAYING

- A group of processes in which a finely divided material (metallicor nonmetallic) is heated rapidly in a hot gaseous medium &simultaneously projected at high velocity on to a preparedsubstrate surface where it builds up the desired coating”

Definition of Thermal Spraying: ASMDefinition of Thermal Spraying: ASM

- powder, rod, cord or wire

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PRICIPLE OF TS

1 - Spraying particles transport2 - Impact on the surface3 - Thermal transfer from particles to substrate4 - Particles Solidification and5 - Mechanical bond6 - Local Fusion

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THERMAL SPRAY METHODS

Air/Vacuum/Low pressure plasma

D - Gun

High velocity oxygen fuelHigh velocity flameHigh velocity air fuel

Electrical

Oxyfuel

Oxyfuel

High energy processa)Plasma spraying

b)Detonation flame spraying

c)High velocity oxyfuelspraying

Oxyfuel gas-powder sprayingOxyfuel gas-wire sprayingMetallising

Electrical arc sprayingTwin arc sprayingMetallising

Oxyfuel

Electrical

Low energy processa)Flame spraying

b)Arc spraying

Other names for the processEnergy sourceProcess

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CLASSIFICATION

Thermal spray classificationThermal spray classification

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FLAME SPRAYING

Flame spraying is the oldest of the thermal sprayingprocesses

Flame spray uses the chemical energy :

- combustion of gases to generate heat

Oxyacetylene torches are the most common :

- acetylene as the main fuel in combination ofoxygen to generate the highest combustiontemperature

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FS process : Powder, wires or rods are

introduced axially through the

rear of the nozzle into the flame

at the nozzle exit

- the feedstock materials are melted and the

particles/droplets accelerated toward the substrate

surface by the expanding gas flow and air jets

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OXY FUEL GAS WIRE SPRAYING

Schematic diagram of Oxy-fuel gas wire spraying

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MATERIALS USED

• Zn & Al anti-corrosion cathodic coatings on

steel.

• Ni/Al composite wire bond coats, heat

& oxidation resistance.

• Mo bond coats, excellent resistance to

adhesive wear.

• High Chromium steel hard and wear resistantcoating.

• SS, Ni and monel anti-corrosion and wear.

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WIRE ARC SPRAYING

Schematic diagram of Wire Arc Spraying

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Detonation Thermal Spraying Process

Schematic diagram of D-Gun wire spraying

Barrel 1-1.5m long and 20-30 mm Internal dia into which thegas mixture is injected and ignited by a spark plug

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ADVANTAGES

• Dense microstructure with 0.1-2 % porosity.

• Smooth surfaces finish (1-4 Ra).

• Better impact wear /fretting wear /erosion/corrosion/resistance.

• Controlled residual compressive stress

• Negligible thermal degradation of powder butalso to preserve bulk microstructure

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Materials and Applications

• WC/Co coatings :

- D-Gun coatings have higher degree of retainedcarbides

(Due to the reducing atmosphere of the confined

combustion zone in the barrel and the shorter dwell

time)

- D-Gun coatings : densest and hardest

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HIGH VELOCITY OXYFUEL SPRAYING

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11-Feb-10 11111 30HVOF spraying Tungsten Carbide / Cobalt Chromium Coating(WC/10Co4Cr) onto Roll for the Paper Manufacturing Industry

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Materials Used

(i) Cr3C2- NiCr

(ii) NiCrMo (wire process)

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SURFAC MODIFICATION BY

PLASMA PROCESSES

Non-transferred arc :Plasma spraying

- An arc is established between theelectrode and the constrictingorifice

: working piece is essentially keptout of the electrical circuit

The heat imparted to the job isobtained from the plasma jet only

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PLASMA SPRAYING (APS)

Schematic diagram of Plasma Spraying

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PLASMA SPRAYING (APS)

Schematic diagram of Plasma Spraying

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AIR PLASMA SPRAYING

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SPRAYING MATERIALS

Pure metal Mo,Ni,Ta,Al ,Zn.

Alloys NiCr, NiCrAlY, FeCrBSiC ,steels ,Bronzes.

Pseudo alloys CuW, AlMo.

Ceramics/Carbides Al203 ,Cr203 ,Ti02 ,Zr02 ,WC.

Cermets Cr3C2/NiCr ,WC/Co ,Zr02/NiAl

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COLD SPRAY PROCESS

A coating technology: in which spray particles ranging in size from1 to 50m in dia, in the solid state are accelerated to high velocity(above 700-1200m/s, supersonic velocity) and subsequently develop adeposit or coating on a substrate by an impaction process

Various terms: Kinetic energy metallization,Kinetic spraying,High velocity powder deposition &Cold gas-dynamic spray method

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COLD SPRAY PROCESS

)

Schematic diagram of Cold spraying

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ADVANTAGES

TS may be used to deposit non-weldable coating materialssuch as plastics or ceramics

No distortion

No post treatment Reduced cost

Low Heat Input -No HAZ.

Versatility: Almost any M, C or P.

Thickness Range: 0.001 inch to more than 1 inch thick,depending on the material and spray system.

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PROTECTIVE COATINGS FOR HIGHTEMPERATURE APPLICATIONS

Superalloys : Designed - to operate at high temp - good high temp mechanical, - heat resistance, & - good corrosion resistance

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- aircraft gas turbine engines

- Industrial land-based turbines

SUPERALLOYS: APPLICATIONS

The result of 2500 h low altitude sea flightservice on an uncoated and NiAl coated bladeturbine blade

Two general types of environmentaleffects:

i. Oxidation ii. Hot corrosion

iii. Combination of these effects

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COATING PROCESSES

Air-plasma sprayed coatingcontain porosity & micro-cracks :- help to redistribute thermal stresses but providecorrosion paths through the coating

Low-pressure plasma spray coating :

-Provide high coating purity and essentially eliminatesoxides and porosity

HVOF, EBPVD

Or combinations methods oxides and porosity

11-Feb-10 11111 43Typical coating thickness/depth of penetration for various coatingand surface hardening processes

COATING THICKNESS/DEPTH OF PENETRATION