Pub Health 4310 Health Hazards in Industry

Pub Health 4310Health Hazards in Industry John Flores Lecture 16 Metal Fabrication

Lecture 16:Metals Fabrication Chapters 7-12 • Metals Fabrication • Forging • Foundry Operations • Metal Machining • Welding • Heat Treating • Nondestructive Testing PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Introduction: • Heat treating is designed to strengthen and harden both ferrous and non-ferrous alloys, • We will focus on heat treatment of steel, since it is the most common application • Heat treatment facilities may be stand alone or part of a manufacturing process • Comprehensive studies are lacking in assessing the health status of these workers • In general, anything that changes the crystal lattice of the steel will harden it • There are 2 general methods used today: • If a workpiece made from medium carbon steel is heated above a critical temperature, it will increase in strength and hardness • The addition of carbon or nitrogen to a low carbon metal surface of a workpiece will undergo hardening (done in an atmospheric furnace) • Common processes only harden the surface of the metal, not the entire mass • Softening the metal is sometimes required, and like hardening is completed in a bath or furnace in an annealing process • Hardening and annealing processes define the final properties of the metal PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Case Hardening • The production of a hard surface or case on the workpiece that is normally accomplished through the diffusion of carbon or nitrogen into the surface of the metal workpiece • Can take from 1 to 20 hrs to complete and can be at a thickness ranging from 0.1 to 0.25 inches depending on the process, desired case thickness, and the metal • Carburizing • Gas Carburizing • Is used to add carbon to the surface of the steel • The workpiece is held in a furnace containing high concentrations of carbon monoxide at temperatures of 870-980 ºC (1600-1800 ºF) • The heat treating atmospheres are classified as 100, 200, 300, …, 600 depending on the generation technique and the composition sought • gas carburizing uses class 100, 300, 400, and 500 • After processing the carbon concentration of mild steel can go from 0.1% to 1.2% PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Case Hardening (cont.) • Carburizing (cont.) • Pack Carburizing • In this process the workpiece is placed in a metal box covered with an organic carburizing compound, is sealed with a gas tight top, then processed through a furnace causing the organic material to degrade and release CO which diffuses into the metal workpiece • The parts must then be quenched to complete the hardening process • Liquid Carburizing • Occurs by immersing the workpiece in a molten salt bath containing sodium or potassium cyanide (or cyanate) and barium chloride • Since this process adds some nitrogen to the surface of the workpiece, this is not a true carburizing process • Health Hazards • The principal hazard concern is exposure to CO • Concentrations at the furnace can be as high as 40% (400,000 ppm) enabling small leaks to become significant workroom exposures • Common for air concentrations to have 100 ppm of CO in the work area • Volatile materials left on workpieces can also be driven off in the furnace and enter the work area through furnace leaks PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Case Hardening (cont.) • Gas Nitriding • Common case hardening technique which adds nitrogen to the metal surface • The technique uses a Class 600 furnace atmosphere of ammonia operating at 510-570 ºC (950-1050 ºF) • Anhydrous ammonia passes over a catalyst in a cracking unit, the ammonia dissociates, releasing 25% nitrogen and 75% hydrogen which forms the furnace atmosphere • This technique is somewhat slow, taking 10-20 hrs to complete • Next the workpiece is loaded in a vacuum vessel containing a low pressure nitrogen environment and high-voltage dc field with the vessel wall as the anode and the workpiece as the cathode • Nitrogen ions accelerate towards the cathode (workpiece) forming a nitride hardened case on the metal • Hazards • Handling or using ammonia presents potential fire, explosion, and toxicity hazards • Uses • Nitriding is often used to harden gears and other automotive parts PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Case Hardening (cont.) • Cyaniding • Liquid cyaniding adds carbon and nitrogen to the workpiece surface by immersing it into a cyanide salt bath and subsequent quenching • The workpiece is immersed into a bath containing sodium and potassium cyanide with sodium chloride (salt) as the carrier for about 30-60 min., the bath is operated at a temperatures above 870 ºC (1600 ºF) • Carbon Nitriding • This furnace process uses both nitrogen and carbon to harden the workpiece • Uses an endogas generator (produces endothermic gas) with the addition of 5% natural gas as the carbon source and 5% ammonia as the nitrogen source • Other Processes • Oxyacetylene or Methylacetylene–Propadiene Torch: • Used on small specialty parts to harden the metal by impinging the flame directly onto select areas • Induction Heating: • Used for high production of small parts with well defined surface geometry; a current is induced through the part heating it to a desired temperature for hardening • Laser Heat Treating: • This hardening technique requires the use of coatings to enhance the absorption of light into the metal • General laser hazards are associated with this process, but depending on the coating used, air contaminants could be created by the degradation of the coating PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Annealing • The general term used to cover many cooling and heating cycles designed to modify the metallurgical properties of the workpiece • Annealing process uses salt baths, slow cooling, and high temperature neutral baths • For steel • Low temp salt baths operate in the range of 538 ºC (1000 ºF) and contain a blend of potassium nitrate and sodium nitrate • High temp neutral baths contain sodium and potassium chloride and barium nitrate • Baths are considered neutral if they do not create a chemical reaction with the workpiece • For stainless steel and nickel chrome alloys • Similar low temp baths are used and the high temp baths are operated at much higher temperatures as is used for steel, which range from 844-1177 ºC (1550-2150 ºF) • Rigorous storage and handling precautions are needed for nitrate salt baths due to their powerful oxidizing capabilities • Nitrate salts will decompose at 400 ºC (750 ºF), and at 650 ºC (1200 ºF) the breakdown can be violent, with the release of nitrogen oxides (nitric oxide is highly flammable, while nitrogen dioxide can be highly toxic) • Explosion/fire potential if hot nitrate salts contact organic materials such as carbon or grease PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Quenching • Controlled cooling or “quenching” is required after furnace and salt bath processes • Quench baths may be water, oil, molten salt, liquid air, or brine • Commercial quenching oils are based on refined mineral oils, animal or vegetable fats • Emulsifiers, accelerators, and antioxidants are added to the oils • Agitators are used to keep the baths at uniform temperatures for even cooling • Water or water-brine quench tanks are used with proprietary additives which include nitrates, nitrites, hydroxides, and corrosion inhibitors • Aqueous polymer quenchants have been used as replacements for oil based quenchants to eliminate fire hazards • Water soluble organic polymers used in quenching are polyvinyl alcohols, polyvinyl pyrrolidones, acrylates, and polyalkylene glycol (most commonly used) • Some quenching activities require enclosures with inert circulating gases (i.e., helium, argon, and nitrogen) • Gas quenching is used for workpieces that require slow cooling rates PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Quenching (cont.) • Patenting • Is a special quenching operation that uses molten lead baths for thin cross-sectional parts such as wire • Hazards • Oil quench tanks • may contaminate workplace air through thermal and mechanically generated mists and thermal decomposition of the oils, and present fire hazards due to their low flash points • Local exhaust ventilation is needed for production oil quench baths • Water or water-brine quench tanks • Endotoxins may be present in quench water from bacteria which may grow in the tanks and can become airborne in the water mist • Inert quenching • Creates the same hazards as those associated with inert gas environments, such as confined space and oxygen displacement • Patenting • Can expose worker to airborne lead PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Control of Health Hazards • The principle concerns in heat treating operations are due to the special furnace environments, especially CO emissions, and the hazards from handling cyanide and nitrate bath materials • To controls fugitive emissions from carburizing operations • Combustion processes must be closely controlled, • Furnaces maintained in tight conditions, and equipped with flame curtains at any doors • Dilution ventilation to remove fugitive leaks • Use SCBA for repair operations when normal breathing air cannot be maintained • Salt Baths • Temperature controls have auto shutoffs to prevent over heating • Before bringing bath down to room temperature, rods should be added to maintain vent holes as the baths harden, to prevent explosions or blowouts during reheating • Make sure workpieces are clean and dry before immersion into the nitrate baths since residual grease, oil, or paint may create an explosive atmosphere PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Heat Treating Control of Health Hazards (cont.) • Lead quenching • Local exhaust ventilation is needed to prevent lead inhalation exposures when removing dross or surface debris • Oil quench tanks • At a minimum, general exhaust ventilation is needed to remove smoke that is generated • General hazards with heat treating operations include heat stress, noise, IR radiation, and burns • Control of these hazards include local and general exhaust ventilation, hearing conservation program, goggles, gloves, face shields, fixed and portable screens, and flameproof garments PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Non-Destructive Testing Introduction • Testing of the quality of the metalworking product has given rise to non-destructive testing which allows for extensive testing without damaging the product Industrial Radiography • Radiography is widely used to examine metal fabrications such as weldments, castings, and forgings • There are about 40-50 thousand technicians throughout the US • Radiography can be performed in the shop, off-site, aboard ships, and on pipelines for example • The process consists of exposing the metal object to x-rays or gamma rays from one side and measuring the amount that transmits through the object • This measurement is usually done with a film or fluoroscopic film to provide a 2 dimensional picture of the radiation distribution which will show any defects in the metal workpiece • Defects show up because some of the radiation is absorbed while some passes through creating a darker image on the film in places where the density or thickness in the object is less than the other areas • The principle hazard of industrial radiography is the potential exposure to ionizing radiation PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Non-Destructive Testing Industrial Radiography (cont.) • X-ray Sources • X-rays used in radiography are produced electrically and therefore fall into the category of “electronic product radiation.” • The conventional tool used is the “X-ray Generator” • The device consists of an evacuated tube in which electrons are accelerated through a high potential difference from the cathode to the anode • The anode contains a target material of relatively high atomic number (usually Tungsten), when electrons impinge on the target, they rapidly decelerate causing bremstrahlung (braking) radiation which is in the form of x-rays • X-ray energies generated can be in the range of 40 to 420 keV • X-ray generator tubes contain shielding (lead) to limit the radiation intensities except in the direction of interest • Tube can often be activated from a remote location to limit worker exposure • Because of the size, weight, and service requirements, X-ray generators are placed in fixed locations such as shielded exposure rooms or specially designed cabinets • Higher energy X-rays are generated by Van de Graaf accelerators (few MeV), linear accelerators (up to 10 MeV), and betatrons (up to 25 MeV) • These devices create radiation from bremstrahlung and through the acceleration of electrons PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Non-Destructive Testing Industrial Radiography (cont.) • X-ray Sources (cont.) • Regulatory Standards • The design and manufacture of X-ray generators are regulated by the Center for Devices and Radiological Health (CDRH) of the Food and Drug Administration (FDA) • ANSI has developed standards for the design and manufacture of these devices • ANSI specifies maximum allowable radiation intensities outside of the useful beam, and • requires control panel and tube head warning lights to indicate when the X-rays are being generated • OSHA regulates the industrial use of X-ray generators, with some states having additional regulatory requirements, • OSHA references or uses the ANSI and CDRH information within their standards • Radiation signage is required for areas in which radiography work is performed • Access to radiation areas must be secured to prevent unauthorized entry • Radiographic operators are required to wear personal monitoring devices to measure radiation exposures • Operators must be trained in the use of radiation survey instruments to monitor radiation levels to which they are exposed and to check that X-ray sources are turned off at the end of a testing operation • Among the hazards of radiation from X-ray generators, there are also high voltage electrical hazards associated with the use of this equipment PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Non-Destructive Testing Industrial Radiography (cont.) • Gamma Ray Sources • The source of gamma rays used in industrial radiography are a result of the decay of radioactive materials • The principle radioisotopes used are Iridium-192 and Cobalt-60 • Ytterbium-169 and Thulium-170 use is uncommon, but there is some limited applicability • Radioisotope produce gamma rays which decrease exponentially with time, unlike X-ray generators which produce a continuous spectrum of energy • Typical radiographic sources contain up to 200 Ci of Iridium-192, and 1000 Ci of Cobalt-60 • An advantage of radioisotope sources over X-ray generators are that they do not require external energy sources making them useful for remote and off-site usage • A disadvantage is that these sources cannot be turned off and continuously emit gamma rays, so require additional safety precautions beyond what’s needed for X-ray generators • The source is sealed inside a source capsule that is usually made from stainless steel, which is then placed inside a shielded container or “pig” when not in use to limit unwanted radiation • The “pig” containers weigh up to 50-lbs for Ir-192 sources, and up to 300-lbs for Co-60 • A flex tube and source stop are connected to the shielded container to allow transfer of the source • The flex tube allows the operator to move the source to the source stop (irradiation point) and back to the source container from a safe distance, thus preventing radiation exposures PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Non-Destructive Testing Industrial Radiography (cont.) • Gamma Ray Sources (cont.) • Regulatory Requirements • Design, manufacture, and use of radioisotope sources and exposure devices are regulated by the US NRC, and 26 “Agreement States” which regulate usage within their jurisdictions • Organizations that perform radioisotope radiography must be licensed by either the NRC or the “Agreement State” in which the work is being conducted • The NRC and Agreement States require: • Radiographic operators to receive formal radiation safety training, company safety requirements, and OJT under the direct supervision of a qualified radiographer • Radiation warning signage must be posted in areas in which radiography is being performed • Secured access to the area, and limited only to authorized personnel • Operators must wear both a direct reading pocket dosimeter and a film badge or thermoluminescent dosimeter, and use a calibrated radiation survey meter during all radiographic operations • At completion of a test, the operator must survey the entire exposure device and the entire tube and source stop to ensure that the source has been properly shielded for storage • Radiation incidents do occur and usually result from the operator failing to properly return the source to a shielded position and then approaching the source stop without a radiation survey • The NRC provides an excellent summation of the licensee responsibilities for a radiation program, and additional licensee requirements if work is contracted to an independent firm PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Non-Destructive Testing Magnetic Particle Inspection • Used for detecting surface discontinuities, especially cracks in magnetic materials • Since the procedure is simple and low cost, its widely used in metal fabrication plants • The ferromagnetic particles are either applied to the surface of the metal part by an air powder gun or the part is dipped into a bath that contains the particles suspended in a light petroleum oil or water • The metal part is then subjected to an induced magnetic field by a low voltage, high current power supply which causes the magnetic particles to gather at the area of discontinuity • Sometimes the magnetic particles are designed to fluoresce and are identified by UV irradiation using a mercury vapor lamp which has a filter that only allows UV-A to pass through • Work exposures are minor and consist of • skin contact and minimal air contamination from the suspension fluid • Magnetic field during the inspection process PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Non-Destructive Testing Liquid Penetrant Inspection • This process is complementary to magnetic particle inspections because it can be used on non-magnetic materials to identify surface cracks and weldment failures • Either fluorescent or visible dies are suspended in a liquid carrier and applied to the workpiece by brush, dip, or spray • Die is absorbed into any imperfections through capillary action, the excess die is drained off, then the workpiece is rinsed clear, and a dry absorbent powder is either dusted or dipped onto the workpiece. • Any die remaining in the workpiece is drawn out and into the powder which then becomes visible or shows up under a UV lamp if the die fluoresces • Hazards • Diverse materials are used as dies and carriers, with some dies being “taken up” in a low volatility petroleum oil, but exposure are minimal and seem non-hazardous • Absorbent powder is considered by manufactures as “nuisance dust” with “low” to “no” toxicity PH 4310 - Health Hazards in Industry, Lct 16

Metals Fabrication – Non-Destructive Testing Ultrasonic Inspection • Pulse-echo and transmission-type ultrasonic inspection have a wide range of application for flaw detection and structural analysis • Can detect voids much smaller than all other testing methods • The process works by placing a transducer on the part to pass ultrasound waves through it which in turn reflect the pulse back to the transducer as imperfections are found • The process is often done by immersing the workpiece into a fluid to improve coupling between the ultrasound transmitter/receiver and the workpiece • Hazards • Exposures to ultrasound either through the air or by direct contact with the workpiece or the coupling fluid can occur, but no adverse effect of this exposure have been reported PH 4310 - Health Hazards in Industry, Lct 16

Pub Health 4310 Health Hazards in Industry