Fundamentals of Arc Spray for Reconditioning Components {0679} Caterpillar

Caterpillar Products

All

Introduction

©2019 Caterpillar All Rights Reserved. This guideline is for the use of Cat dealers only. Unauthorized use of this document or the proprietary processes therein without permission may be in violation of intellectual property law.

Information contained in this document is considered Caterpillar: Confidential Yellow.

The purpose of this Reuse and Salvage Guideline is to provide information on the basic process fundamentals of Arc Spray for reconditioning components. Successful utilization of the Arc spray process depends on incorporating other support types of equipment. All processing, equipment, and installation requirements must be understood for performance and safety reasons. Included in this document is information, including cost, on the required tools and equipment necessary to perform the described process.

For technical questions when using this document, work with your Dealer Technical Communicator (TC).

To report suspected errors, inaccuracies, or suggestions regarding the document, submit a form for feedback in the Service Information System (SIS Web) Interface.

This manual either provides or references all the necessary information to achieve an understanding of the Arc spray process.

For specific Arc spray reconditioning procedures, refer to the following list of documents.

For further information and background on the Arc spray process or related preparation and finishing processes, refer to the following publications:

Reference: "Thermal Spray Manual"

Reference: "Thermal Spraying: Practices, Theory, Application, and Safety"

Safety

Illustration 1	g02139237

Arc spraying can be hazardous, due to the use of high-voltage electrical current, hoses under high pressure, potential of irritating or toxic consumables, noise, heat, and UV radiation. Arc spray may involve air contaminated by dust, fumes, and mists.

Arc spraying is a safe process when performed by operators who follow the recommended precautionary measures, utilize care in operation, and understand thermal spray practices, and the equipment.

Note: Refer to operation manuals for Oerlikon Metco and TAFA equipment as primary source of operating and safety information. Other references for operating and safety information are:

Reference: "American National Standards Institute (ANSI) Safety in Welding, Cutting, and Allied Processes"

Reference: "OSHA Code of Federal Regulations, Title 29 Labor, Chapter XVII, Parts 1901.1 to 1910.1450"

Reference: "Thermal Spraying: Practices, Theory, Application, and Safety"

Reference: "Welding Handbook, 8th ed., vol. 3"

Metal Dust

Precautions for Arc Spray are essentially the same as for welding and cutting. Airborne metal dusts, or finely divided solids, should be treated as a possible explosive. Adequate ventilation must be provided to minimize the danger of dust.

Ventilation and Exhaust Systems

A suitable spray booth and an adequate exhaust system are required to avoid the toxic or noxious effects of dust, fumes, and mists, which can be generated by spraying.

U.S. Federal EPA emissions standards require that exhaust systems for the arc spray industry have an efficiency of 99.99 %. The Dry Cartridge Dust Collection System is the most effective method for exhaust collection in the thermal spray industry.

For ordinary thermal spray work, air at a velocity of 90 m/min (300 FPM) should pass into the opening of the spray booth. With the gun at the booth opening, the exhaust system must be able to exhaust 9 cubic meters/min (300 CFM) of air for every square foot of booth opening.

Reduction of Noise Hazards

Operators and personnel close to the arc spray operation must be protected from excessive noise. The spray operation should take place in an approved room with adequate soundproofing to reduce sound levels to a safe working level. Arc spray guns generate noise levels more than 116 dB. Hearing protection must be worn while working inside the spray room.

Eye Protection

Helmets, hand shields, face shields, or goggles are necessary to protect the eyes during all arc spraying, grit blasting, and machining operations.

Wearing a helmet during arc spraying is recommended to protect face, chin, and neck from ultraviolet and infrared radiation. A recommended lens shade number is shade 9 to 12.

Always wear approved safety glasses under any type of hood or face shield.

Show/hide table

Protect yourself and others; read and understand this warning. Fumes and gases can be dangerous to your health. Ultraviolet rays from the weld arc can injure eyes and burn skin. Electric shock can kill.

Electric Power Precautions

• Do not touch live electrical parts.

• Keep clear of moving parts.

• Arc spray wires and all metal parts in contact with the arc spray wires are energized while spraying.

• Arc rays, sparks, and hot surfaces can burn eyes and skin. Noise can damage hearing.

• The arc spray wire may cause puncture wounds.

• Do not energize power source until ready to apply coating to part.

• Do not point gun toward any part of the body or other personnel when threading arc spray wire.

• Keep all hoses and wire guides as straight as possible.

Abrasive Blasting Equipment

Maintain blasting equipment according to specifications set by the manufacturer. Replace worn parts for efficient operation. Replace deteriorated hoses and nozzles.

Show/hide table

Grit blasting produces airborne particulates that can act as respiratory irritants. If inhaled, these particulates can cause serious injury or death. Additionally, the grit from this process can be abrasive to the skin and eyes of the operator and can cause serious injury. If grit blasting is performed manually or outside of a grit blast cabinet, operators must be provided with respiratory protection, face shields, and helmets. Information on face shields and helmets can be found in ANSI/ISEA Z87.1 (American National Standard for Occupational and Educational Eye and Face Protection Devices) and ANSI/ISEA Z89.1 (American National Standard for Industrial Head Protection). Information on the selection, operation, and maintenance of a respiratory device can be found in ANSI/AIHA Z88.2 (Practices for Respiratory Protection) or other local approval authority documentation.

Mechanical Stripping and Finishing Equipment

Maintain lathes, grinding, and finishing equipment according to specifications set by the manufacturer. Replace worn parts for efficient operation. All operators must be instructed to become familiar with the operation of lathe and grinding equipment.

Show/hide table

Serious injury or death can result from contact with rotating parts or pinch points. To avoid injury, rotating parts, such as grinders, superfinishers, and lathes, should only be operated if all guards and protective devices are properly installed and in good working order. All equipment should be operated according to the manufacturer instructions.

Process Fundamentals

Thermal spraying is a term used to describe a group of processes in which fine molten or semi-molten metallic or non-metallic materials are deposited onto a prepared surface. Generally, thermal spraying is considered a relatively "cold" method of salvaging when compared to welding or brazing. Thermal spraying is considered a cold process because the surface temperature of the piece part is kept below 150° - 175° C (300° - 350° F) when a coating is being deposited. Therefore, the part experiences no metallurgical or physical changes-there is no heat affected zone (HAZ) or loss of case depth.

The term “thermal spraying” encompasses six different types of processes:

1. Powder flame spray using an oxygen-fuel gas mixture for the flame and powder as a consumable.

2. Wire flame spraying using an oxygen-fuel gas mixture for the flame and a wire consumable.

3. Arc spraying using electrical (DC) current and wire as the consumable.

4. Plasma using a non-transferred (DC) arc and powder consumable and nitrogen or hydrogen as a fuel.

5. High Velocity Oxygen Fuel (HVOF) uses a high velocity of oxygen fuel gas-air flame plus a very fine powder as a consumable.

6. Plasma Transferred Wire Arc (PTWA) using a plasma arc and wire as a consumable.

The above metal spray processes differ from one another by:

1. Equipment requirements

2. Material utilization

3. Means and methods used for heating and propelling the molten material

4. Metallurgical structure of the deposit

All Caterpillar salvage applications and process orientation in this Guide are for the arc spraying process.

Arc Wire Spray uses two metallic wires as the coating feedstock. The two wires are electrically charged with opposing polarity and are fed into the arc spray gun at matching speeds. When the wires come together at the nozzle, an arc forms between the wire that creates sufficient heat to melt the tips of the wires. Clean dry compressed air is used to atomize the molten material and accelerate the material onto a prepared work-piece surface to form the coating. In arc wire spray, the amount of coating deposited per unit of time is a function of the electrical power (amperage) of the system along with the density and melting point of the wire.

Illustration 2	g06404428

Coating Fundamentals

Through the arc spray process, a fine metallic material is deposited in a molten or semi-molten condition onto a prepared surface. When the molten particles strike the surface, the particles flatten (elongate), rapidly cool, and quench to form micro-thin platelets. The platelets conform to the surface and to each other. The bond between the base material and metal spray particles may be mechanical, metallurgical or a combination of both.

As the sprayed particles continue to impinge onto a surface, particle-by-particle, a layer, or lamellar structure is formed.

Illustration 3	g02723951
Typical coating cross section illustrates lamellar structure (At 500X magnification).

Since multiple particles are required to create a coating, several traverse passes of the gun applying a 0.08 mm (0.003 in) of coating material per pass, is required to produce the desired coating thickness. These multi-gun passes can be performed by hand holding the unit or mechanical (lathe or robot) movement of the spray gun. In general, a coating can be applied to 2.54 mm (0.100 in) per side thickness maximum. The actual thickness of a coating depends on the amount of wear of the part plus an allowance for finishing. The finishing allowance is generally 0.51 mm to 0.64 mm (0.020 to 0.025 in) on a side.

The formed coating is used for "surface" type applications and will not provide any strength to a part. The only function of the coating is to add some desirable characteristics to the surface that is lacking in the worn piece part. Desirable thermal spray characteristics include:

1. Better wear characteristics

2. Dimension restoration

3. Corrosion protection

4. Electrically conductive or electrically resistive

Because of the nature and structure of metal spray coatings, the metal spray coatings should not be used when the coating will be subjected to line or point-to-point contact and severe, sharp, and repeated type of impact.

Surface Finishing and Post-Coating Treatment

Surface Finishing

Finishing techniques for arc sprayed deposits differ from practices followed for standard machined surfaces and for solid materials. Since arc sprayed deposited coatings consist mainly of mechanical bonds, excessive tool pressure or high feed rate results in premature and excessive tool wear and causes the coating to craze, crack, or pit.

Finishing techniques used for an arc sprayed coating depend on the type of material applied, coating hardness, and desired surface finish.

Machine finishes can be achieved with carbide inset type tools. Follow the recommended parameters for RPM, depth of cut and surface feet per minute requirements. Recommended inserts include:

• ISCAR DNMG 432 TF IC507

• Carboloy DNMG 432 MF CP200

• Kennametal DNMS 432 KD100CPD

• Sandvik DNMG 432 PF G4015 or G4025

Wet type grinding is strongly recommended when the coating is to be finished by grinding.

Post-Coating Treatment

All sprayed coatings are considered to be porous. Porosity is the measurement of the density of a coating when compared to the same type of material in solid or cast from. Generally, the porosity of a coating will be found to be beneficial for bearing type applications since the coatings will retain lubrication.

When a lubrication function is not desired or where the coating will be exposed to a corrosive type of environment or where fluid leakage is not desired, the coating should be treated with a sealer. The recommended sealers are: Dichtol 1532, Loctite 290, a wick type air drying sealer, or Metco 185 crystalline wax sealer. These types of sealers are easy and simple to apply.

Support Equipment and Operations

Grit Blasting

Grit blasting for arc spraying should be considered surface texturing. Clean, dry compressed air, and a sharp, coarse-blasting abrasive that is free from contaminants must be used to avoid finishing problems.

Two types of grit blasting systems are found in most shops: pressure systems and suction systems. For surface preparation of a coating, the pressure type system is preferred because of the following considerations:

1. Pressure blast systems can handle both aluminum oxide and steel angular grits.

2. A pressure blast system is more efficient and, therefore, more economical.

3. A pressure blast system provides the best surface texture and depth using either aluminum oxide or steel angular grit.

During grit blasting for surface preparation, coarse type grit of the following materials should be used:

• Aluminum Oxide: Aluminum oxide is an extremely hard and brittle abrasive used primarily on heat-treated steel substrates. It is not used for preparing aluminum or cast type substrates because the fine dust generated during the blasting operation tends to embed itself into the surface of the component. Because this type of grit is light in weight, it can be used at very low pressures. The use of lower operating pressures allows for the use of aluminum oxide grit on materials hard and soft. The recommended blast air pressure is 413 kPa (60 psi). The blast nozzle to work distance should be maintained at a 101 mm to 127 mm (4.0 to 5.0 in) range to obtain a surface profile of 6.0 µm (250 µinch).

• Angular Steel Grit: This grit is recommended for use on softer substrates, up to a hardness of Rc 35, and for those substrates where aluminum oxide cannot be used on aluminum and cast materials. The effective operating pressure for this grit is between 620 to 689 kPa (90 psi to 100 psi). This type of grit cannot be used in a suction type blast system. Steel angular grit must be kept free of any moisture or oil contamination.

• Using higher than recommended pressure results in rapid breakdown of grit and more grit usage.

Note: With any type of grit, visually inspect to ensure grit sharpness. If dull or contaminated grit is found, the grit must be replaced before poor surface preparation results. Contaminants found in the grit will be blasted into the surface of the components.

Masking Aids

Masking is required to protect critical surfaces and holes from the effects of the grit blasting and thermal spraying. Any type of material that can withstand the effects of either operation is considered a suitable masking material.

The mask can be permanent sheet metal or a temporary material. Temporary-type masking materials are:

• Hard Rubber

• High temperature silicon rubber

• High temperature fiber type materials

• Several layers of duct or masking tape

• Cloths

• Liquids

Spray Booths and Rooms

Any thermal spraying process generates some amount of dust and fumes. The arc spray process also generates an extreme amount of noise. The generated noise is above 116 dB.

For environmental control and personnel safety, an enclosed soundproof area is appropriate. Dealer Support, Research, and Oerlikon Metco can recommend various dry cartridge exhaust and soundproof rooms to meet your local safety standards and regulations.

Rotational/Traverse Equipment

Rotating cylindrical components should be accomplished in a lathe or other dedicated rotational device. An example would be a floor-mounted turntable. The rotational device controls should provide variable rotational settings to relatively high revolutions per minute (RPM) capability. Specially designed rotating systems can withstand the dusty spray environment, yet provide the part rotational requirements.

The rotational requirement for specific parts is provided in the individual Arc spray reconditioning procedures documents. The following formulas can be used to calculate either RPM or SFPM (surface feet per minute) for a part.

(3.82 * SFPM) / coating area diameter (inches) = RPM

0.262 * RPM * coating area diameter (inches) = SFPM

For a traverse motion of the arc spray gun, the following guidelines should be used to apply an even coating deposit:

• Flat work (non-rotating): Traverse the arc spray gun at a rate of 15 - 75 cm (0.5 to 2.5 in) per second, making 6.3 mm (0.25 in) gun position increments per pass.

• Round/Rotating Work: Traverse the arc spray gun at 6.3 - 12.6 mm (0.25 - 0.50 in) per revolution.

Equipment

Illustration 4	g06404447
Smart Arc by Oerlikon Metco

Illustration 5	g06404449
Praxair TAFA 8830 MHU

For the Cat dealer program, dealers will have a choice of using either TAFA 8830 MHU, TAFA 8835 MHU, or Smart Arc by Oerlikon Metco technology.

Recommended arc spray operation consists of the following equipment:

• 3.66 x 3.66 m (12.000 x 12.000 ft) arc spray room

• 3.05 x 3.05 m (10.000 x 10.000 ft) grit blast room with either portable pressure blaster or blast system with elevator and separation system, storage hopper, and media recovery

• Exhaust for grit blast and arc spray rooms

• Exhaust hood and ducting

• Either Smart Arc by Oerlikon Metco, TAFA 8830 MHU, or TAFA 8835 MHU wire arc technology

• Floor installed turntable

• Tank wheel positioner

• Head stock/tailstock

• 4AC air cleaner

• Recommended safety devices

• Recommended consumables

Success of arc spray salvage of piece parts is dependent upon the following steps:

Pre-machine

Chamfer edges to a number 2 break [ 0.05mm (0.002 in)] in thickness. Premachining may include machining to make outside/inside diameters concentric.

Degrease

All piece parts must completely grease free to avoid contamination and to prevent bond failure.

Masking

Use of metal masks, rubber plugs, tape, or anti-bond shield areas of a component from being grit blasted or arc sprayed.

Grit Blasting

Blasting is used to prepare the surfaces for the arc spray coating. Grit blasting prepares the surface for the mechanical bond between the coating and substrate.

Arc Spraying

The spray process deposits consumables at a certain lay down rate to provide for a uniform coating at a consistent temperature.

Finishing

This operation either turn/mills or grinds the coating to dimension and tolerance. Finishing is the final step in the salvage process.

Component preparation is important to the success of salvaging via the arc spray process. The grit blast system must be dedicated for thermal spray parts preparation and must not be used for any other shop operation.

For environmental control and personal safety, proper dust and fume exhaust systems are required. EPA rules dictate exhaust systems be 99.9% efficient.

Consult your local environmental laws for exact exhaust regulations.

Materials

Every commercially available arc spray wire type consumable has different coating characteristics of:

• Deposition Efficiency

• Melting Point

• Bond Strength

• Coating Texture

• Finish Texture

• Hardness

• Coating Density

• Coating Weight

• Shrink

• Coefficient of Thermal Expansion

• Chemistry

Therefore, consider the following criteria when selecting a coating material:

• Engineering material composition, base metal, temperature and environmental factors, and piece part geometry

• User reliability, performance requirements, versatility, reproducibility, finishing requirements, equipment limitations, and economics

• Historical application similarities and past application success

Based on these criteria and application development, the materials described in this bulletin are recommended. These materials offer the following benefits when the arc spray coating is applied:

• Allow self-bonding to any clean substrate

• Exhibit and develop low coefficients of thermal expansion so that thick coatings can be applied

• Offer high internal integrity

• Can be used efficiently and economically

Recommended Materials appear in the following list:

• TAFA Arc Spray BONDARC Wire - 75B®

• TAFA Arc Spray Medium Carbon Steel - 30T

• TAFA Arc Spray 13% Chrome Steel Wire - 60T

• TAFA Arc Spray Iron Chrome Aluminum wire - Alcro

• TAFA 90MXC® UltraHard® ARMACOR CW Wire

• TAFA 95MXC® UltraHard® ARMACOR M Wire

• Metco 8222

Links to Equipment Information and Suppliers

Reference: Caterpillar

Reference: Abbott Machine Co.

Reference: Eitel Presses

Reference: Grinding Equipment & Machinery Company, LLC

Reference: IMPCO Machine Tools

Reference: Oerlikon Metco

Reference: Progressive Surface

Reference: Supfina

Reference: TAFA

Reference: 3M

Reference: Torit Products