HVOF Identification and Polishing (Superfinishing){0674, 0679, 1000, 7000, 7562} Caterpillar

HVOF Identification and Polishing (Superfinishing){0674, 0679, 1000, 7000, 7562}


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Table 1
Revision     Summary of Changes in SEBF2101    
03     Changed name of supplier from Sulzer Metco to Oerlikon Metco, and added Expanded Mining Product.    
02     Added confidentiality statement and safety information. Updated surface analyzer information.    
01     Change reference from HVOF Spray Guide CD, SERF8705 to Fundamentals of High Velocity Oxygen Fuel (HVOF) Spray for Reconditioning Components, SEBF9236.    

© 2014 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.

This Reuse and Salvage Guideline contains the necessary information to identify and polish (superfinish) high velocity oxygen fuel (HVOF) spray coating on a rod. For complete details of the entire process as recommended by Caterpillar Inc., refer to Fundamentals of High Velocity Oxygen Fuel (HVOF) Spray for Reconditioning Components, SEBF9236.

For questions or additional information concerning this guideline, submit a form for feedback in the Service Information System. In order to address an urgent issue, use the following resources in order to communicate your request to Caterpillar Repair Process Engineering:

  • Caterpillar Dealer Technical Communicator

  • Dealer Solution Network

  • Caterpillar Technical Representative

  • Knowledge Network


Illustration 1g02139237

This bulletin is a reference to information provided in equipment operation manuals. Cat dealers are requested to read the safety sections of operation manuals cited in this bulletin.

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

Table 2
Safety References    
Publication Title     Publisher    
"Flame Spray Handbook Vol. II - Powder Process"     Oerlikon Metco    
"Welding Handbook, 8th ed., vol. 3"     American Welding Society    
"Thermal Spraying: Practices, Theory, Applications and Safety."     American Welding Society    

Like any process that generates dust and fumes, the HVOF spray process involves a potential hazard to personnel. All personnel involved in this process must understand and become familiar with operating practices and regulations [OSHA standards (US) or equivalent] for compliance.

Information in this bulletin is derived from the sources mentioned, but not all of the safety points are covered here.

Environmental Health & Safety Advisory

High Velocity Oxygen Fuel (HVOF) Thermal Spray is a process that is an alternative to hard chrome electroplating on parts used in Caterpillar Products. Chrome electroplating uses the hexavalent form of Chromium, which is recognized by the international Agency for Research on Cancer (IARC) as a human carcinogen. Many countries have Occupational Exposure Limits (OEL'S) for Hexavalent Chromium, which range from 0.005 mg/m3 (3.12x10-10 lb/ft3) to 0.05 mg/m3 (3.12x10-9 lb/ft3) for an 8-hour Time Weighted Average (TWA) exposure. The electroplating process requires extensive EHS controls because of the hazards posed to employees and the environment by corrosive acid solutions of Hexavalent Chromium. The HVOF process is a dry process that eliminates the need for employee handling of corrosive chromic acid solutions. The HVOF process can be operated manually or robotically in a controlled booth environment. While the HVOF process can facilitate compliance with OEL'S and comprehensive standards like the OSHA Hexavalent Chromium Standard, the HVOF process has inherent safety and health hazards, which must be addressed. The metal powders used for chrome coating can contain Chromium and other nickel. In order to protect employees and comply with OEL'S for Hexavalent Chromium and other materials, implementing the following actions is required:

Fire Prevention and Protection

Precautions for HVOF spray are essentially the same as for welding and cutting. Airborne metal dust, or finely divided solids, should be treated as a possible explosive as well as a breathing hazard. Adequate ventilation must be provided to minimize the danger of dust. Metal Powders used to deposit Chromium can also contain Nickel. The HVOF process will create dust particles, which can contain Nickel and some Hexavalent Chromium. Measure employee exposure time to document employee exposure to Hexavalent Chromium and Nickel.

U.S. Federal EPA emission standards require that exhaust systems for the HVOF spray industry have an efficiency of 99.99%. Federal, state, and local exhaust emission requirements in North America have become more demanding.

In the flame spray industry, the Dry Cartridge Dust Collection System is the most effective method for cleaning exhaust gasses.

Enclosed operations would also need local exhaust ventilation systems equipped with dust collectors. Employees performing maintenance on the dust collection systems will need to use respiratory protection, disposable coveralls, and protective gloves to minimize inhalation and skin exposure to Hexavalent Chromium and Nickel.

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.

Safety Material

Compliance with OSHA Hazard Communication Standard requires current Material Safety Data Sheet (MSDS) and employee training.

Operating Considerations

HVOF differs from welding and cutting processes in sound generation. The high exhaust gas velocity generates noise levels of over 110 dB. Chronic exposure will likely result in hearing loss. A combination of booth sound insulation and personal equipment (earplugs and earmuffs) should be used, to reduce this noise to a safe working level.

Like welding, the HVOF process involves gas cylinders or bulk gas containers. Improper storage, handling, and/or use of gas cylinders can create serious safety hazards. For example, if a valve is broken off a full gas container during a fall, the container will become a missile. If an oil-contaminated regulator or gauge is installed in an oxygen system, the assembly will explode.

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

Table 3
Operating Considerations References    
Publication Title     Publisher    
"Safety in Welding, Cutting, and Allied Processes" ANSI/ASC Code Z49     American National Standards Institute and American Welding Society    

HVOF Spray Equipment

Maintain and operate HVOF spray equipment exclusively to standards set by the equipment manufacturer. All operators must be instructed to become familiar with the operation of the spray equipment. If an HVOF spray gun malfunctions or backfires, the cause of the trouble must be investigated.

When HVOF spraying is completed, the system must be properly "shut down" as follows:

  1. Close all gas cylinder valves.

  1. Open the gun valves to release hose gases, with room ventilation on.

  1. Back out all regulator screws until free.

  1. Close the gun valve.

  1. 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.

------ WARNING! ------

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.


Abrasive Blasting Equipment

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

------ WARNING! ------

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 the manufacturer specifications. Replace worn parts for efficient operation. All operators must be instructed to become familiar with the lathe and grinding equipment operations.

------ WARNING! ------

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.


Other Safety Hazards

Other safety hazards include compressed gases, thermal burns, high voltage, and mechanical pinch points from robots and turntables. These are potential ergonomic hazards involved with handling parts. The American Society of Metals Thermal Spray Society (TSS) has made two Safety Guidelines available to help provide control of the process: "SG001-02 Safety Guidelines for the Handling and Use of Gases in Thermal Spraying", and, "SG003-03 Thermal Spray Booth Design Guidelines". The guidelines are available free from TSS web site or Corporate EHS.

HVOF Identification

Before a repair option can be decided upon, identify the material on the rod. The difference between HVOF and chrome is not obvious at a glance. One way to identify HVOF is by visual inspection. HVOF will be present in the weld area between the rod and the eye as in Illustration 2. The HVOF in this area will not be polished and will be dull gray in color and rough to the touch. If the weld area is chrome plated, the appearance will be shiny and smooth to the touch.

Illustration 2g02624696

The visual description above also applies to suspension cylinders, struts, and hoist cylinders. As seen in Illustration 3, the HVOF will be present near the bottom of a strut where the diameter tapers down. The HVOF will be present on the end of a hoist cylinder where the piston bolts on.

Illustration 3g02624717

On this rod, the dull gray band between the two delineative dashed lines has been sprayed with HVOF but has not been surface finished.

Illustration 4g02700917

The freckle-like aggregate surface seen in HVOF compared to chrome surface. Surfaces viewed through 8S-2257 Eye Loupe As of 10x magnification and a 5x zoom camera lens.

Illustration 5g02701836

The freckle-like aggregate surface on HVOF can be seen with the unaided eye. Most noticeable though, is the reflection of white light on HVOF compared to the reflection of dispersed red and green light on chrome.

Another method of visually inspecting rods for HVOF coating involves using a brightly focused, white light shown lengthways down the rod. When using an eye loupe, small freckle-like aggregate surface can be seen in the HVOF coating (Illustrations 4 and 5) that will not be seen in chrome. Conversely microfissures may be found in the chrome that should not appear in HVOF. If the rod is HVOF coated, then the aggregate surface is not detrimental to the finish or the function of the rod. By shining the light at an angle to the rod in the long direction, chrome rods will glint light back prismatically everywhere the light reflects off a microfissure meaning a full spectrum of color will be seen. When HVOF reflects light off the edges of the contours, the light will appear white and undispersed. These reflections appear as sparkles and are more easily identified by color under slight magnification such as an eye loupe or magnifying glass. Generally HVOF finishes are more dull than chrome but this comparison is hard to recognize unless the two finishes are viewed side by side.

Additionally, surface roughness is a good indicator between chrome and HVOF hydraulic rods. HVOF finishes generally have a higher roughness reading when measured with a profilometer on the Ra (roughness average) scale. The most simple practice for obtaining a surface roughness value is by use of a stylus profilometer. Portable industrial-use profilometer units are the most practical and facilitate measuring the surface roughness of rods that are still assembled on the machine being serviced. However, stationary units can serve the same purpose for disassembled rods and may provide more accurate results or additional information about the rod surface.

The recommended practice for using a profilometer is to steadily support the unit on the hydraulic rod being examined and record a minimum of four measurements using a 17.5 mm (0.69 in) trace length and a 0.8 mm (0.03 inch) cutoff at each end of the measurement in varying locations on the rod. It is recommended to use a 5 µm (200 µin) diamond stylus for measuring. It is important to clean the surface of the rod with 70% isopropyl alcohol before taking any surface measurements, as contamination on the rod will skew the measurement. Also, keep in mind that external vibrations can affect the results.

Generally, the roughness of HVOF coated rods will be greater than 0.10 µm (3.94 µin) on the Ra scale and the roughness of chrome coated rods will be less than 0.12 µm (4.72 µin) on the Ra scale as shown in Illustration 6. It is important to note that this test alone is not definitive especially if the recorded roughness of a rod is included in the overlap of chrome and HVOF roughness values. Be sure to use this test in addition to the other practices listed in this document before making a conclusion on the hydraulic rod coating material in question. The surface roughness measurement can also be taken using the Rz scale. A new HVOF coated rod will measure 0.7 µm (27.56 µin) Rz or less.

Illustration 6g02731719

Ra values of chrome and HVOF as measured with a stylus profilometer.

The following surface analyzers are available through the Caterpillar parts system:

Illustration 7g03507589

448-3697 MarSurf M300 and Drive Unit MarSurf RD 18

Illustration 8g03507627

448-3698 MarSurf PS1

The OmniSurf software package is available for use with the MarSurf M300, and will provide the following features:

  • Processing of measured or created profiles

  • Graphical presentation of the processed profile in the working area

  • Graphical presentation of the loaded or recorded profile in the overview area

  • Roughness evaluation of the zoomed profile sections

  • Measurement/evaluation of interrupted profiles, excluding profile sections, for example, bores

  • Presentation of the aligned profile as well as the waviness and roughness profiles in separate areas

  • Presentation of the altered profile as a result profile

  • Storage of the processed profile, i.e. the result, in a file


Using a grinding or abrasive wheel for spark testing on rod finishes is potentially destructive. Reduce the chance of ruining the surface by taking care to use very light pressure. Rough surfaces that result from spark testing can ruin hydraulic seals and allow contamination into the system if left unfinished when installed.

A potentially destructive method to use is to lightly touch a grinding wheel to the surface of the rod. Care must be taken not to apply too much pressure when performing this test. This test should be performed in an area that will not be in contact with the seals. Briefly touch the grinding wheel to the surface finish and identify the color of the spark produced. A white spark indicates the coating is chrome, a yellow spark indicates the base material, and no spark indicates an HVOF coating.

HVOF Polishing Equipment

A mechanical method will be used to polish all HVOF components. The choices of superfinishing equipment that will attach to a lathe are:

Supfina (www.supfina.com)

Technology Center Wolfach
Germany Supfina Grieshaber GmbH & Co. KG
77709 Wolfach/Germany
49 (7834) 866-0info@supfina.com

Technology Center North Kingstown
USA Supfina Machine Company, Inc.
181 Circuit Drive
North Kingstown, RI 02852/USA
1 (401) 294-6600info@supfina.com

Supfina Beijing-Office
China Beijing Representative Office
DiYang Tower, 1202B
Chaoyang District
Beijing 100027/P.R. China
86 (10) 84536-669supfina@126.com

Illustration 9g02611597

Supfina 210 Super Finisher

IMPCO Machine Tools (www.impco.com)

3417 West St. Joseph Street
Lansing, Michigan USA 48917
1 (517) 484-9411sales@impco.com

Illustration 10g02714578

IMPCO Microstar Attachment

Grinding Equipment & Machinery Company, LLC. (www.gem-usa.com)

15 South Worthington Street
Youngstown, OH 44502
1 (330) 747-2313

Illustration 11g02714579

GEM 04150-S Microfinisher

For information on a lathe contact:

Abbott Machine Co. (www.abbottmachineco.com)

700 W. Broadway
Alton, IL 62002
1 (618) 465-18981 (800) 262-6478

Illustration 12g02611476

Lathe with grinder attachment

Equipment Specifications

  • Variable Speed Controller

  • 14.9 kW (20.00 hp) motor

  • Digital Amp load readout

Illustration 13g02611678

SG-6 control panel

HVOF Super Finishing Abrasives

All applications will use 3M Diamond Microfinishing film 675L (20.0 µm (0.79 µin)) to finish HVOF.

3M (www.3m.com)

3M Global Headquarters
St. Paul, MN 55144-1000
1 (888) 364-3577

Illustration 14g02714576

3M Diamond Microfinishing Film 675L

Super Finishing:

Illustration 15g02611617

Finisher's Tech SF-4 Super Finisher

101.6 mm (4.00 inch) wide 3M diamond strips will be required for the Finisher's Tech SF-4 Super Finishing Machine and 203.2 mm (8.00 inch) wide 3M strips will be required for the optional equipment (Finisher's Tech SF-8 Super Finisher Machine). The 3M strips can be ordered in various lengths.1219.2 mm (48.00 inch ) with a 609.6 mm (24.00 inch) leader and 609.6 mm (24.00 inch) tail is a standard length. The leader and tail portion is required to feed the strips through the super finisher.

A good measure is to keep a few of the plastic spools the 3M strips are shipped on. This practice can reduce the machine setup time when switching 3M strip grades.

HVOF Super Finishing Mechanically

Use the super finisher to lightly grind the component to the proper size and surface finish.

Although finishing components to a specified size and surface finish using only a grinder is possible, a super finisher has three important purposes. First, the process will remove any blemishes or "striping" (barber poling) the belt grinder leaves on the component. This step will eliminate variations in finish due to lathe conditions and operator experience. Second, the process produces a slight crosshatch in the coating similar to the fine cracks which occur in plated surfaces. This finish is desirable to help lubricate seals and enhance seal life. The third purpose is to achieve a more polished surface appearance, which is similar to the finish produced by plating. A 20.0 µm (0.79 µin) film should be used for superfinishing.

Note: Care must be taken to monitor final surface finish as belts wear.

The super finisher should be mounted to the cross slide of a lathe. To save floor space and reduce capital investment, the super finisher can be combined with a rear-mounted belt grinder setup on one lathe. This arrangement will allow both operations to be performed on the same machine, avoiding excess material handling.

HVOF coatings can be super finished at a rate of 12.7-38.1 µm (0.50-1.50 µin) per pass at a traverse rate of 0.64 m/min (25.0 in/min).

Coolant with a rust and bacteria inhibitor is required for the HVOF super finishing operations. Hard water may require treatment, since minerals can interfere with rust inhibitors and other coolant additives. The coolant should be flooded into the contact area between the abrasive and the component. As contact pressures increase, the pressure of the coolant should be enough to assure that the abrasive and component are being cooled properly. To improve diamond film life, a coolant formulated for machining high nickel content materials should be used. The following coolants meet this requirement:

  • Castrol WE3-046A

  • Castrol Syntello 9954

  • Cincinnati Milacron CimPerial 1070

  • Bosse Grind Rust Bar 750 (Star Metal Fluids)

  • Milicron 46C

Since lathe coolant systems are only designed to let coarse chips collect in the sump, the system must be modified to provide adequate filtrations. The lathe pump should pipe the coolant to an auxiliary filtering system which is at least 50% efficient for 10 µm (0.39 µin) particles. This process is necessary to remove abrasive "mud" which can damage rod finish and also the lathe. The modifications should also include coolant plumbing to wash abrasive material from the machine ways to prevent premature wear. A filter tank/pump setup using an automatically advancing paper roll element has several advantages. These systems have far more capacity than canister filters and can be sized to handle several machines simultaneously. Filter fixture suppliers include:

  • Oberlin Filter Company

  • Filtra Systems Hydromation Division

  • J.R. Schneider Co., Inc

  • Hoffman Air and Filtration Systems

Illustration 16g02611696

Oberlin Coolant Filtration System

Oberlin Fitler Company can be contacted by the following means:

Oberlin Filter Company
404 Pilot Ct.
Waukesha, WI 53188

Filter paper suppliers include:

  • Lydall Manning

  • National Filter Media Corp.

  • Industrial Filter Fabrics

The products and manufacturers listed here are not intended as the only acceptable choices. For information on these or other suppliers, questions, or to address an urgent issue, use the following resources to communicate your request to Caterpillar Repair Process Engineering: Cat Dealer Technical Communicator, Dealer Solutions Network, Caterpillar Technical Representative, or Knowledge Network.

Check with your local environmental authorities on the handling of waste coolant and filter material.

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