What is Case hardening?
Involves a change of surface properties to produce a hard, wear resistant shell. Tough, fracture resistant cure
- can be tested by electromagnetic tests and checked by destructive methods. Cutting the object, etching the cut surface. Checking cut forth with a microscope
Carburizing
-Implies and increase in carbon
- performed on low carbon steels containing .70% to 1.20% carbon
- temps between 850-930 Celsius
- may be supplied from a gas, liquid or solid environment
Flame hardening
Does not require a change of composition in the surface of the material
- can only be used on steels that contain sufficient carbon to be harden-able by standard heat treating procedures
Liquid and vapor baths
- most wildly used cleaning methods make use of a cleaning medium in liquid or vapor form
- greases and piles can be removed with solvents that dissolve the grease and oil
- petroleum based used but can be a fire risk
Pickling
Water containing sulfuric acid in concentrations from about 100% to 25% and at a temp of 65 Celsius (149 degrees F)
- used for the removal of surface oxides or scale on Iron or steel
Water additives
-many of the common cleaning liquids are made up of approximately 95 percent water containing alkaline cleaners such as caustic soda, sodium carbonate,silicates, phosphates and bolates.
Application dependent
- degreasing down on small parts frequently with a vapor bath
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Blasting
- velanongetjkd in which the cleaning method is accelerated to high velocity and impinged to the surface to be cleaned
- operator saftey must be considered
Shootpeening
-Blasting tends to increase the surface area this sets up compressive stresses that may warp thin sections up. May be beneficial by reducing the likelihood of fatigue failure
Liquid honing
-water containing rust inhibiting chemicals may carry in suspension fine chrosive particles that may provide grinding cutting type action for cleaning and finishing improvements
Thermal spraying
Metalizing: metal wire or powder fed into oxyacetalyne flame and then after melting impinged against the work surface. Small droplets adhere and bond to the surface, bond largely mechanical.
Anodic material spraying
- zinc, aluminum, And cadmium which are anodic to steel are usually sprayed in thin layers (.010) as protective coatings
- tend to be porous, coatings of two or more times the thickness are used for cathodic materials like tin, lead and nickel
Vaccum metalizing
Some metal can be deposited in very thin films, usually for reflective or decorative purposes as a deposit
- aluminum is the most used metal for deposits for this method and is used frequently for decorating or producing a mirror surface on plastics
Electroplating
-to provide protection against corossion, to improve appearance, establish wear and abrasion resistant surfaces, add material for dimensional increases, intermediate step of multiple coatings majority for corrosion appearance
- thickness low (.0001 to .001 inch)
- chromium for appearance 1/10 this amount but wear resistance as much as .010
- commonly applied are nickel, chromium calcium
Chromate coatings
Corrosion of zinc can be substantialy solved by the production of chromium salts on its surface
- magnesium alloys can be increased by immersion or anodic treatment in acid baths containing dichromates
- chromatic treatment of both zinc and magnesium improves corrosion resistance but also adhesion of part
Phosphate coatings
Used mostly on steels result from a chemical reaction of phosphoric acid with the metal to form a non metallic coating
- may be used for corrosion resistance but most common application is as a base for paint coatings
- called parkerizing and bonderizing
Chemical oxide coatings
Used mainly on steels produce attractive black oxide coatings
- involve the immersing of steel in a caustic soda solution heated at 300 degrees and made strongly oxidants by adding nitrates
- base for paint finish
Organic coatings
— paint type materials are applied by dip, brush and spray
- min labor by dipping
- brushing costly
- greatest amount of organic coatings are applied industrially by spray
- heat often used to speed dry
Electrostatic spraying
Paint sprayed though a high voltage electrostatic field
- uniform coatings by electrostatic spraying more effient that regular spraying
Paint
Mixture of pigment in a drying oil
- color and opacity supplied by pigment
- oil as carrier as well as rough continuous film as it dries
Varnish
- combination of synthetic resin and drying oils, sometimes volatile oils
- dried normally clear
- without the drying oil called sprits or shellac varnish
Enamel
Mixture of pigment in varnish
- better than paints
Lacquers
Finishes consisting of thermoplastic materials dissolved in fast drying solvents
- majority used as color lacquer enamels
Anodizing
Treated electrically in a suitable electrolyte to produce a corrosion resistant oxide coating
- metal being treated is connected to the anode in the circuit
- produces a porous coating (PT no go)
- aluminum, magnesium and zinc can be treated this way
