CHROME PLATING. A Guide for Selecting the Type of Chrome Plating for Use in Contact with BAL Seals in Rotary and Reciprocating Service

Transcription

1 CHROME PLATING A Guide for Selecting the Type of Chrome Plating for Use in Contact with BAL Seals in Rotary and Reciprocating Service Technical Report TR-14 (Rev. F) Pauling Foothill Ranch, CA Tel (949) Fax (949) sales@balseal.com Spinozastraat HD Amsterdam The Netherlands Tel Fax sales@balseal.nl

2 Page 2 of 10 TABLE OF CONTENTS 1.0 Summary 2.0 Purpose of chrome plating 3.0 Types of chrome plating 3.1 Hard chrome plating 3.2 Thin dense chrome plating 3.3 Electrolyzing process using thin dense chrome alloy 4.0 Cost considerations 5.0 Specifications for chrome plating 5.1 Hard chrome plating Specification 320 Class 2E 5.2 Thin dense chrome plating per MIL-C Class Base metal hardness and surface finish prior to plating 7.0 Operating conditions affecting the selection of chrome plating methods 8.0 Effects of plating on BAL Seal performance 9.0 Lubrication of mating surfaces 10.0 Sources of supply 11.0 References The information, descriptions, recommendations and opinions set forth herein are offered solely for your consideration, inquiry, and verification and are not, in part or in whole, to be construed as constituting a warranty, expressed or implied, nor shall they form or be a part of the basis of any bargain with Bal Seal Engineering. If any sample or model was shown to or provided by Buyer/User, such sample or model was used merely to illustrate the general description and type of goods. Such use is not to be construed as a warranty that the goods will conform to the sample or model. Furthermore, THE IMPLIED WARRANTY OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE AND ALL OTHER WARRANTIES, IMPLIED OR EXPRESSED, ARE EXCLUDED AND SHALL NOT APPLY. This document provides product options for further investigation by Buyers/Users having technical expertise. The Buyer/User, through its own analysis and testing, is solely responsible for making the final selection of the products and for assuming that all performance, safety and warning requirements for the application are met. It is recommended that Buyers/Users run evaluation testing under actual service conditions to determine whether proposed Bal Seal products are suitable for the intended purpose. Nothing contained herein or in any of our literature shall be considered a license or recommendation for any use that may infringe patent rights. (LE-17) PATENTS: The items described in this report include products that are the subject of the following issued United States patents: 5,979,904; 5,994,856; 6,050,572; 5,984,316; 6,161,838 and others, as well as foreign patents or products where patents are pending. (LE-88G) Copyright 2001, Bal Seal Engineering Co., Inc. U.S.A TR-14Rev. F; (100-48, , ).

3 Page 3 of SUMMARY This report outlines the types of chrome plating available, their advantages, disadvantages, and effects on BAL Seal performance. Thin dense chrome plating provides superior properties over hard chrome plating for use in contact with BAL Seal in rotary and reciprocating services. Some plating vendors are listed in Section PURPOSE OF CHROME PLATING Chrome plating is generally used for one of two purposes: for decorative uses or for engineering purposes. This report deals with the engineering uses of chrome plating and its effect on BAL Seal performance in rotary and reciprocating services. Chrome plating is used to provide a very high degree of hardness on the surface of a metal to enhance wear resistance, reduce friction, provide anti-galling properties, and, in some cases, improve corrosion resistance. Chrome plating is an electrolytic process that can be applied to regular steel, stainless steel, aluminum, and other materials. This report details the application of chrome plating to various steel surfaces. 3.0 TYPES OF CHROME PLATING There are two basic types of chrome plating: hard chrome plating and thin dense chrome plating. Hard chrome plating leaves a layer of chrome from to inches thick (0.020 to mm) on the surface of the metal, while thin dense chrome plating generally has a chrome thickness from to inches (0.005 to mm). The thickness of chrome plating will vary depending on the application. Chrome plating can be used over a wide temperature range from -70 F to 800 F (-57 to 427 C) and can withstand pressures up to 30,000 psi (2068 bar). The temperature and pressure limits depend on the type of chrome plating, material substrate, and operating conditions. In most cases, thin dense chrome plating will provide better performance because of its greater fatigue strength.

4 Page 4 of 10 Jagged Structure Nodular Structure in. ( mm) (After Grinding / Honing) Magnified Sectional (50X) View of a Hard Chrome Plated Surface per QQC-320B Class 2E Showing Jagged Structure Figure in. ( mm) (After Plating Subsequent Grinding / Honing May Not Be Required)) Magnified Sectional (50X) View of athin Dense Chrome Plated Surface Showing Nodular Structure Figure 2 Shown in Figures 1 and 2 are typical magnified surface cross sections of hard chrome plating and thin dense chrome plating. 2.1 Hard chrome plating Hard chrome plating is generally used in those applications where parts will be subjected to a very high degree of wear and thus a thicker layer of chrome is required. The added thickness induces cracks and greater porosity on the chrome surface. The cracks will increase with an increase in chrome thickness. Further machining, such as grinding, polishing or honing, is necessary to improve the hard chrome plated surface so it can be used in contact with BAL Seals. A coarse surface finish will substantially decrease the wear resistance of the BAL Seal. Hard chrome plating is used in approximately 95% of all applications with the remaining uses being thin dense chrome plating (See chart No. 1 for description). Hard chrome plating is generally applied to exterior surfaces where subsequent machining is easier. BAL Seals operate best against a hard chrome plated surface when used with a lubricant, which promotes a film between the plated surface and the seal surface. The lubricant can be wet, dry or a combination of both. At low speeds under 500 ft/min (152 meters/min), pressures under 50 psi (3.2 bar), and temperatures under 70 F (21 C), wet lubricants may need to be applied on the chrome plated surface to fill in the surface irregularities in order to reduce seal abrasion. 2.2 Thin dense chrome plating Thin dense chrome plating is a process, which results in improved fatigue life, smoother surface finish, and higher corrosion resistance. Since the layer is very thin, from to in. (0.005 to mm), and very dense, it does not have the porosity, large crevices or openings that are found in

5 Page 5 of 10 conventional hard chrome plating, and therefore thin dense chrome plating has improved corrosion resistance. Thin dense chrome plating is generally applied to interior surfaces and inside diameters. It allows for closer tolerances, better surface finishes, better uniformity, and tends to reduce or eliminate subsequent grinding or honing of the plated surfaces Thin dense chrome plating is applied very slowly, as compared to hard chrome plating. It has substantially greater density and higher hardness; approximately Rockwell C-70 versus Rockwell C-65 for hard chrome plating. Prior to plating, the surface should be as smooth as possible and should not exceed 10 microinch RMS (0.229 microns Ra). After plating, stress relief may be necessary to reduce stress cracking. Thin dense chrome plating is recommended over hard chrome plating when in contact with BAL Seals, but it has the limitation of a very small thickness that may tend to wear through at high speeds. Wet lubrication is recommended as a means to provide reduced friction and improved wear resistance of the mating surfaces. Dry lubrication is not recommended in thin dense chrome plating, because there is no positive means for locking the dry lubricant to the plated surface since the plating is very smooth. 2.3 Electrolyzing process using thin dense chrome alloy A proprietary chrome plating process, developed by the Electrolyzing Company, deposits an extremely hard, thin, dense, and non-magnetic alloy on the surface of the base metal. The metal deposited is a high chromium alloy which provides very high wear resistance, low coefficient of friction, high corrosion resistance, excellent anti-galling properties, high hardness (70 to 72 Rockwell C) and an extremely smooth surface finish. Of all the chrome plating methods, this process seems to provide the highest hardness. The surface finish is smooth with a nodular structure as compared to the roughjagged finish of conventional hard chrome plating. These two factors, hardness and surface finish, lead to a great improvement in wear resistance. The lack of any cracks or crevices in the plating gives it a high corrosion resistance. Electrolizing Company reports coefficients of friction from 0.04 to 0.09 with this thin dense plating alloy. 3.0 COST CONSIDERATIONS Generally, thin dense chrome plating is higher in cost than hard chrome plating. This is because thin dense plating uses a bath with a fluoride catalyst, which can attack unmasked metal parts. The high cost is dependent on the amount of masking and setup requires a minimum amount of masking of metal parts. Hard chrome plating may cost anywhere from equal to half the cost of thin dense chrome plating.

6 Page 6 of SPECIFICATIONS FOR CHROME PLATING Whenever possible, chrome plating should be processed to these specifications to assure greater quality control. 4.1 Hard chrome plating Specification QQC-320, Class 2E This is the most widely used specification for hard chrome plating for dynamic services and it is recommended for use in contact with BAL Seals whenever the application involves long term service. This specification has the following general requirements: 1. Proper cleaning of parts prior to plating. 2. Shot peening to assure better bonding with greater fatigue strength. 3. Baking at 375 F (190 C) immediately after plating to reduce hydrogen embrittlement, increase fatigue strength and reduce cracks. 4.2 Thin dense chrome plating Specification MIL-C-23422, Class 2 This specification has some restrictions by the Department of Defense and generally may not be available. 1. Proper cleaning of the parts prior to plating. 2. Surface finish from 4 to 10 microinch RMS (0.071 to microns Ra) prior to plating, with no visible nicks, marks, or occlusions. 3. Surfaces after plating should be homogeneous, uniform in color, free of blisters, pinholes, pits, nodules and under 150X magnification should exhibit no crack. 4. Stress relieving may be deleted if parts are not subjected to stress concentrations to provide better wear resistance. 5.0 BASE METAL HARDNESS AND SURFACE FINISH PRIOR TO PLATING Chrome plating can be applied to either a soft or a hard surface. The harder the surface the greater the adherence and the better the plating. Generally, it is recommended that the surfaces have a hardness of RC 40 or higher to comply with Specification QQC-320 Class 2E. If a softer surface must be used then it should be plated to Specification QQC-320 Class 2E. With conventional hard chrome plating, a rough surface from 16 to 31 RMS is required prior to plating. Thin dense chrome plating requires a smooth surface finish before plating because the amount of chrome to be applied is very small. The better the surface finish prior to plating the smoother the surface finish after plating. In all forms of chrome plating, the material must be clean. Small amounts of contaminants or hard particles, such as from grinding operations, left imbedded in the metal surface will affect the fatigue life of the plated part.

7 Page 7 of OPERATING CONDITOINS AFFECTING THE SELECTION OF CHROME PLATING METHODS When selecting the proper type and class of chrome plating to be used, one must consider the following conditions under which the parts will be used and the required design specifications: 1. Check for compatibility of the base metal with the method of plating to be used. 2. How much friction and abrasion will be experienced? 3. At what speed, temperatures and pressures will the parts operate? 4. How much corrosion resistance is required? 5. What lubrication will be available? 6. How important are anti-galling characteristics? Though the cost of thin dense plating is substantially higher than conventional hard chrome plating, it should be considered whether or not thin dense plating is necessary for proper operation of the design. Cost considerations become secondary to safety considerations in critical applications such as the aircraft and medical industries and thin dense chrome plating is usually specified. Chart No. 1 shows the main forms of chrome plating, the properties of each, and the base metal requirements, such as hardness, surface finish and surface preparations. 7.0 EFFECTS OF PLATING ON BAL SEAL PERFORMANCE The life of BAL Seals in contact with chrome plated rotary or reciprocating shafts is affected by the hardness and surface finish of the shaft material. By chrome plating a shaft to improve the hardness, the seal life is greatly extended. This is due to the fact that increased hardness reduces adhesion between the metal part and the seal, therefore increasing BAL Seal wear resistance. 8.0 LUBRICATION OF MATING SURFACES Lubrication is quite important, especially under conditions of high-speed and high pressures where lubricant breakdown may occur. BAL Seals are self-lubricating and the requirement for lubrication is substantially reduced. However, wet lubricants will provide a lubricating film, which will tend to carry away heat and reduce friction. Dry lubricants are very desirable in hard chrome surfaces to fill the cracks and surface irregularities normally present, in order to reduce seal abrasion.

8 Page 8 of ELECTROLESS NICKEL PLATING AS AN ALTERNATIVE TO CHROME PLATING Electroless nickel plating is generally used for housing/bore or parts with intricate shapes. This type of plating puts a uniform coating of approximately to inch (0,003 to 0,015 mm) radially. The hardness of electroless nickel plating is 48 to 52 Rockwell C for non-heat treated and 58 to 674 Rockwell C for heat-treated to 750 F (400 C) for one hour. After plating, it just requires polishing the surface. Electroless nickel plating provides better corrosion resistance than chrome plating.

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10 Page 10 of SOURCES OF SUPPLY Customer must verify quality and workmanship. Hard Chrome Plating: DIXON HARD CHROME, INC., Pendleton St., Sun Valley, CA Tel: 818/ MULTICHROME/MICROPLATE CO., 1013 W. Hillcrest Ave., Inglewood, CA Tel: 310/ ROLL TECHNOLOGY CORPORATION, 4412 White Horse Road, Greenville, SC Tel: 800/ METAL SURFACES, INC., 6060 Shull St., Bell Gardens, CA Tel 562/ FRANKE PLATING WORKS, INC., 2109 East Washington Blvd., Fort Wayne, IN Tel. 219/ Dense Chrome Plating: ELECTROLIZING INC., 1947 Hooper Ave., Los Angeles, CA Tel. 213/ ELECTROLIZING CORP. OF OHIO, 1325 E. 152 nd, Cleveland, OH Tel. 216/ ELECTRO-COATINGS OF IOWA, INC., 911 Shaver Rd., NE, Cedar Rapids, IA Tel. 800/ PRECISION INDUSTRIES, INC., 1700 Old Clyattville Rd., Valdosta, GA Tel. 800/ (912) ARMOLOGY Corporation, 114 Simonds Avenue, Dekalb, IL 60115, Tel REFERENCE 1. Federal Specification QQC-320B, Military Specification MIL-C B. C. Inwood etal, Electroplated Coatings for Wear Resistance, Tribology, Specification DPS 9.84 Thin Dense Chromium Plate Requirements. 3. Electrolizing Inc Hooper Ave., Los Angeles, CA Quality Assurance Approved Process Services, Q.A. Report 1780, AiResearch Mfg. Co., Phoenix, AZ 5. Approved Process Suppliers list, , Parker Hannifin Corp., Irvine, Calif. 6. Society of Automotive Engineer s Specification AMS 2438.