Pub Health 4310 Health Hazards in Industry

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Pub Health 4310Health Hazards in Industry

• John Flores
• Lecture 16
• Metal Fabrication

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Lecture 16Metals Fabrication

• Chapters 7-12
• Metals Fabrication
• Forging
• Foundry Operations
• Metal Machining
• Welding
• Heat Treating
• Nondestructive Testing

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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

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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

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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

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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

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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 MethylacetylenePropadiene 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

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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

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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

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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

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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

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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

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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

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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

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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

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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
  whats 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

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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

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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

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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

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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