Fundamentals of Flame Spray for Reconditioning Components {0100, 0599, 0679} Caterpillar

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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 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 Flame Spray for reconditioning components. Successful utilization of the Flame 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.

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

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

For further information and background on the flame 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

Cat dealers are requested to read the safety sections of equipment operation manuals cited in this guide.

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

Reference: "Flame Spraying Handbook Vol. II, Powder Process"

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

Reference: "Welding Handbook, 7th ed., Vol. 3"

Like any process that generates dust and fumes, the thermal spray process involves a potential hazard to personnel. All personnel involved in this process must become familiar with operating practices and regulations. Local, state, and federal health regulations (OSHA standards) must be understood for compliance.

The following information is derived from the sources mentioned above.

Fire Prevention and Protection

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

U.S. Federal EPA emission standards require that exhaust systems for the thermal spray industry have an efficiency of 99.99%.

Federal, state, and local exhaust emission requirements in North America have become more demanding.

The Dry Cartridge Dust Collection System is the most effective method for exhaust collection in the flame spray industry.

A wet collector system may meet requirements but must also meet local requirements.

Good housekeeping must be practiced to avoid the accumulation of metal dust. Paper, wood, oily rags, and flammable solvents must be removed from the spraying area.

Operating Considerations

Like welding, the thermal spray process involves gas cylinders. Improper storage, handling, and use of gas cylinders can create a safety hazard.

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Do not use oil or grease on oxygen equipment. Never over-tighten or force a connection. A fire hazard can result and cause injury to the operator. Only the hoses recommended by the manufacturer should be used with the spraying system. Acetylene pressure must never exceed 103 kPa (15.0 psi). Only use recommended lubricants as specified by the manufacturer. Use proper tools to connect regulators to cylinders.

For further detail on these operating hazards, refer to the following.

Reference: "Safety in Welding and Cutting"

Thermal Spray Equipment

Maintain and operate thermal spray equipment only to standards set by the equipment manufacturer. All operators must be instructed to become familiar with the operation of the spray equipment.

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Never use an open flame to light the thermal spray gun. Serious burns or other personal injury could result. Equip the gun with an electric ignition system or use a spark lighter.

If a thermal spray gun malfunctions or backfires, the cause of the trouble must be investigated. When thermal spraying is completed, the system must be properly "shut down" as follows:

1. Close all gas cylinder valves.

2. Open the gun valves to release hose gases.

3. Back out all regulator screws until free.

4. Close gun valve.

5. Check all (first stage) regulators to ensure that no pressure buildup is taking place. Pressure buildup would indicate that a bottle valve has not been tightly closed.

Abrasive Blasting Equipment

Maintain blasting equipment according to manufacturers specifications. Replace worn parts for efficient operation. Replace deteriorated hoses and nozzles.

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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.

Process Fundamentals

Thermal spraying is a term used to describe a group of processes in which fine molten or semi-molten metallic or nonmetallic materials are deposited onto a prepared surface. Flame spraying is considered to be a relatively "cold" method of part 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. As a result, the part experiences no metallurgical or physical changes.

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 the consumable.

2. Wire flame spraying using an oxygen-fuel gas mixture for the flame and wire as the 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) using a very high velocity of oxygen-fuel gas-air flame and a very fine powder as the consumable.

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

These thermal spray methods differ from one another due to the following considerations:

1. Equipment requirements

2. Material utilization capabilities/limitations

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 flame (powder combustion) spray process. Applications detailed in this guide assume the use of Oerlikon Metco 6P II spraying equipment and materials.

Illustration 2	g06406177
Cross section of a powder flame spray gun

Note: Distance between nozzle and substrate depends on type of powder and equipment used. Consult equipment manufacturers operating manual for further information.

There are two types of thermal spraying systems:

• Gravity feed

• Pressure feed

Both systems use the same type of powder material but differ in methods of powder delivery.

Gravity Feed

With the gravity feed system, a powder canister is attached directly to the body of the gun. Powder is then gravity fed into the flame. Since the material flow is solely related to gravity, this type of system is susceptible to inconsistent material feeding because of the following conditions:

• Movement of the spray gun and/or orientation to the work

• Wear and/or fouling of the internal feed mechanism of the gun

• Variation in material classification (particle size)

• Powder condition (moisture content)

Pressure Feed

In a pressurized feeding system, powder is transported and delivered to the gun by an inert gas (pre-purified nitrogen or argon) at a constant rate. The delivery is independent of the orientation or movement of the spray gun. In addition, the material feed rate (measured in kg/hour (lb/hour)) can easily be set, monitored, and controlled by the operator. As a result, pressurized feeding offers more process control and deposits high quality, repeatable coatings.

Coating Fundamentals

Through the flame-spraying process, fine metallic or non-metallic materials are deposited in a molten or semi-molten condition onto a prepared surface. When molten particles strike a 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 flame spray particles may be mechanical, metallurgical, chemical, or a combination in nature.

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 needed to create a coating, several transverse passes of the gun, applying 0.10 - 0.15 mm (0.004 - 0.006 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) movement of the spray gun. In general, a coating can be applied greater than 2.54 mm (0.100 in) per side in thickness. 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 - 0.64 mm (0.020 - 0.025 in) on a side.

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

1. Better wear characteristics

2. Dimension restoration

3. Corrosion protection

4. Electrical conductivity or electrical resistivity

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

Surface Finishing and Post-Coating Treatment

Surface Finishing

Finishing techniques for flame sprayed deposits differ from practices followed for standard machined surfaces and for solid materials. Since spray deposited coatings consist mainly of mechanical bonds, excessive tool pressure or a high feed rate can result in premature and excessive tool wear. The stress from excessive tool wear will cause the coating to crack, craze, or pit.

The technique used for finishing a spray coating depends on the type of material applied, hardness, and the desired surface finish.

Excellent machine finishes can be achieved with C-2 grade carbide insert type tools. Follow recommended parameters for RPM, depth of cut, and surface feet per minute requirements.

Although dry grinding can be performed for a coating, wet-type grinding is recommended. When grinding a coating, selecting the proper wheel is critical. Open structured "green" grit (46-60) wheels are preferred over "white" wheels because the grit wheel structure constantly breaks down and exposes clean, sharp wheel abrasive. This breakdown prevents wheel loading tendencies.

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 form. 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 flame 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 and suction systems. For surface preparation of a coating, the pressure type system is preferred because:

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. Pressure blast provides the best surface texture and depth using either aluminum oxide or steel angular grit.

In grit blasting for surface preparation, a coarse type grit of the following materials should be used (G40 through G16).

• Aluminum Oxide: Aluminum oxide is a hard and brittle abrasive used primarily on 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 into the surface pores. Because this type of grit is light in weight, it can be used at low air pressures [between 276 - 413 kPa (40.0 - 60.0 psi)]. When used in a suction blaster system, the minimum air pressure must be 517 kPa (75.0 psi). Using higher than recommended pressures results in rapid breakdown of grit and more grit usage.

• Steel Angular Grit: This grit is recommended for use on softer steel materials, up to a hardness of Rc35, 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 - 689 kPa (90.0 - 100.0 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.

Note: With either type of grit, visually inspect to ensure grit sharpness. Dull or contaminated grit must be replaced because poor surface preparation results and contaminants in the grit will be blasted into the surface and will lower bond strength.

Masking Aids

Masking is required to protect critical surfaces and holes from the effects of the grit blasting and 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

• Several layers of duct or masking tape

• High temperature fiber type materials

• Liquids

Spray Booths and Rooms

Any flame spraying process generates some amount of dust and fumes. For environmental control and personnel safety, an enclosed or segregated area is appropriate. Oerlikon Metco can provide exhaust equipment and rooms for dust and fume removal. The following standard units are available from Oerlikon Metco:

• Type SME 12-35. Exhaust equipment is available from Torit as in the 2DF-16 3500 CFM Dry Cartridge Dust Collection System

• Type 4BWH Flame Spray Booth. The booth has a front opening that measures 1.27 x 0.79 x 1.07 m (50.0 x 31.0 x 42.0 in). The booth is drilled and bracketed for lights. Rotational equipment is optional.

• Type 3BL Lathe Exhaust Unit. This unit consists of two parts: an exhaust hood, designed to fasten onto the lathe carriage which will travel with the carriage, and a 25 cm (10.0 in) diameter, 244 cm (8.0 ft) long flexible duct which connects to the Dry Cartridge Dust Collection System.

Flame spray rooms are available to fit your needs either from Oerlikon Metco or from Industrial Acoustics Company.

Rotational/Transverse Equipment

Rotating cylindrical parts can be sprayed manually by hand or fixtured onto a transverse mechanism. Some type of rotational device, dedicated solely to the flame spraying process, is strongly recommended. Rotational equipment may be found to have limited capabilities, but must have some type of variable rotational control to provide relatively high rpm capability. Specially designed rotating systems manufactured by Oerlikon Metco and other companies can withstand the dusty spray environment and provide the part rotational requirements.

The rotational requirement of a part is provided in the salvage guideline specific to that part. The following formulas can be used to calculate rotations per minute (rpm), surface meters per minute (SMPM), or surface feet per minute (SFPM) for a part.

31.8 x SMPM ÷ diameter (mm) = rpm

3.82 x SFPM ÷ diameter (inch) = rpm

For example, if a cylindrical part is coated at 300 SFPM and has a diameter of 2 inch, then the rpm is (3.82 x 300 ÷ 2) = 573 rpm.

The formulas for calculating SMPM and SFPM are:

0.00314 x rpm x diameter (mm) = SMPM

0.262 x rpm x diameter (inch) = SFPM

Example 2: a cylindrical part with an 86 mm diameter and a rotation speed of 600 rpm would be (0.00314 x 600 x 86) = 162 SMPM

Example 1: a cylindrical part with a 3 inch diameter and a rotation speed of 600 rpm would be (0.262 x 600 x 3) = 472 SFPM.

For traverse motion of the flame spraying gun, the following guidelines should be used to apply an even coating deposit:

• Flat Work (Non-rotating): Traverse at a rate of 15 - 75 cm (0.5 - 2.5 ft) per second, making 6.3 mm (0.25 in) gun increments per pass.

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

Equipment

Illustration 4	g06406341
4MP Powder Feeder by Oerlikon Metco

Illustration 5	g06406351
Type 6P II Gun by Oerlikon Metco

For the Cat dealer flame spray program, all salvage applications will use the oxygen-acetylene 6P II Thermal Spray System by Oerlikon Metco. The 6P II system consists of the following basic components:

• 6P II Thermospray Gun

• 4MP Powder Feeder

• 4MP 211 Air Regulator

• 2GF or 3GF Gas Flowmeter Unit

• 3G Gas Control Unit (acetylene and oxygen)

• 5A Air Control Unit

• 7GNR Nitrogen Regulator Unit

• 8H 4 ft Hose Unit

• 2AJ Air Blast Cooler Unit

• 6P 330 Air Jet Assembly

• 4 AC Air Cleaner Unit

The following accessories are optional or part of the 6P II package:

• 6P 7C - "B" Nozzle, Acetylene

• 6P 7C - "K" Nozzle

• 6P 7C - "N" Nozzle Concentrating

• 6P 600 Siphon Plug Assembly - two jets

• 6P 645 Siphon Plug

• 4MP 400 Pedestal

• 40 Surface Pyrometer

Component preparation is important to the success of salvage via flame spray. A grit blast system is used to prepare surfaces. The grit blast system must be dedicated for flame spray parts preparation and must not be used for any other shop operation.

Recommended equipment includes:

• Empire Blast Cabinet Model 4848PRC-6 PBV-05U-30L-CP Portable Vacuum Recovery

• Pressure Blaster 914.40 x 1219.20 mm (36.000 x 48.000 in) Optional Cabinet for above portable blaster

For environmental control and personal safety, proper dust and fume exhaust systems are required. New EPA rules dictate exhaust systems be 99.9% efficient. For U.S. and Canadian dealers, the recommended system is the 3500 CFM, Type SME 12-35 Dry Cartridge Dust Collection System

Other types of exhaust systems include:

• Fabricated Hood or Enclosure with Capture Plenum for Lathe or Turntable.

• 8BH Spray Booth

• Sound Proof Rooms either from Oerlikon Metco or from Industrial Acoustics Company (IAC)

Consult your local environmental laws for exact exhaust regulations.

Materials

Every commercially available powder material has different coating characteristics of:

• Shrink

• Wearability

• Particle Size Distribution

• Composition

• Chemistry

• Flow

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 Oerlikon Metco composite type materials described in this bulletin are recommended. These materials offer the following benefits when thermal spray is applied:

• Allow self-bonding to any clean substrate

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

• Are technique independent

• Offer high internal integrity

• Can be used efficiently and economically

Links to Equipment Information and Suppliers

Reference: Caterpillar

Reference: 3M

Reference: Torit Products

Reference: Oerlikon Metco

Reference: TAFA

Reference: Eitel Presses

Reference: Progressive Surface

Reference: Abbott Machine Co.

Reference: Supfina

Reference: IMPCO Machine Tools

Reference: Grinding Equipment & Machinery Company, LLC

Reference: Industrial Acoustics Company (IAC)