Thermal spraying processes use electric arc, plasma, and combustion energy sources to produce a high-temperature and high-velocity gas stream. Powder or wire material is introduced to this gas stream. Particles of this material are heated and propelled onto a surface to produce a coating. The noise, heat, dust, fumes, and mechanical operations of the spraying processes create a unique set of safety hazards for the operator and those nearby. Definitions/Process Descriptions According to ANSI/AWS A3.0, Standard Welding Terms and Definitions, thermal spraying is a group of processes that deposit molten metallic or nonmetallic surfacing materials onto a prepared substrate. All thermal spraying processes introduce a feedstock (usually a powder or wire) into a spraying device (combustion or electrical). The spraying device is generally referred to as a thermal spray gun. The material is heated at the gun , blended into a hot gas stream, and sprayed onto a prepared substrate. The heated particles strike the surface, where they flatten and adhere to the surface. As this process continues, a coating is formed from the spray material. The coating process is stopped when the desired thickness of coating is formed. Thermal spray processes include • Combustion processes LVOF: Low-velocity oxyfuel HVOF: High-velocity oxyfuel • Electrical processes: Arc (twin-wire) Plasma arc Typical operating conditions for the various processes are shown in Table 1. Potential Hazards and Hazardous Effects • Dust — The finely divided airborne dust and fume should be treated as an explosion and inhalation hazard. Adequate ventilation, proper electrical system design, and appropriately designed dry collection systems should be provided. • Fumes, Vapors, and Gases — Use ventilation and safe practices according to ANSI Z49.1, the material supplier's Safety Data Sheet (SDS), and AWS Safety and Health Fact Sheet No. 1. Most spray and abrasive blasting operations require the operator to use an approved respirator that complies with requirements of ANSI Z88.2. Also, precautions should be taken to avoid the presence of chlorinated hydrocarbon solvent vapor in the area of the arc or plasma spraying. Hazardous phosgene gas can be produced when hydrocarbon vapors are exposed to ultraviolet radiation from these processes. • Noise — Thermal spray processes generate noise levels that require hearing protection by the operator. Earmuffs and noise control procedures should be provided to conform to the standard limits of OSHA 29 CFR 1910.95. • Radiation — Intense ultraviolet (UV) and infrared (IR) radiation occurs with these processes. They require total protection of the eyes and all exposed skin to avoid eye damage and burns. • Eye shades of No. 3–6 for combustion and 9–12 for electrical processes are recommended. See AWS Safety and Health Fact Sheet No. 2. • Electric Shock — Arc, plasma arc, and plasma induction spraying utilize electrical voltages in excess of 100 V. Take precautionary measures according to ANSI Z49.1 and AWS Safety and Health Fact Sheet No. 5. • Fire — The gas stream from a thermal spray gun is in excess of 3000°F. Use care when handling thermal spray guns during operation to avoid personal injury or fire (see AWS Safety and Health Fact Sheet No. 6). • Mechanical Hazards — The substrate surface preparation, spraying, finishing, and posttreatment operations involved with thermal spraying processes present a variety of mechanical hazards. Consult the equipment manufacturer’s manuals and material supplier’s Material Safety Data Sheets for their recommended safe practices. • Compressed Gases — Use and handle compressed gases as specified in ANSI-Z49.1. Adapted from AWS Fact Sheet No. 20. All of the AWS Safety and Health Facts Sheets are available through the AWS website by visiting aws.org, then clicking on “Standards” at the top of the home page, then “Safety and Health” on the righthand side of the Standards page. WELDING WORKBOOK 68 WELDING JOURNAL / JULY 2O16 WJ DATASHEET 366 AWS disclaims liability for any injury to persons or to property, or other damages of any nature whatsoever, whether special, indirect, consequential or compensatory, directly or indirectly resulting from the publication, use of, or reliance on this information. AWS also makes no guaranty or warranty as to the accuracy or completeness of any information published herein. American Welding Society, 8669 NW 36th St., #130, Miami, FL 33166, email info@aws.org; website aws.org. Thermal Spraying Safety Table 1 — Operating Conditions for the Various Thermal Spray Processes LVOF HVOF Arc Plasma Arc Temperature To 5000°F To 6000°F 4000°–15,000°F 4000°–15,000°F Velocity 200–700 ft/s 2500–4000 ft/s 800–1800 ft/s 800–1800 ft/s (<Mach 1) (to Mach 5) (to Mach 2) (to Mach 2) dBA (Sound Level) 110 150 115 132 Spray Distance 4–10 in. 6–18 in. 21⁄2–6 in. 21⁄2–6 in.
Welding Journal | July 2016
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