Metal Enhancement

METALS PPT 2

work / cold hardening

method
internal changes
properties that change
product applications

deformation of a metal below its recrystallisation temperature. done through mechanical processes such as cold rolling wire rolling, wire drawing stamping, bending or hammering

regular crystal grains of the metal are physically crushed, flattened or elongated. crystal lattice has microscopic defects - dislocations causing it to harden and fracture. becomes cold worked and work hardened.

hardness increases and becomes resistant to scratching and indentation, tensile strength increases, ductility and malleability decreases (brittle)

structural steel components, electrical wiring, fasteners

grain size properties of a metal

fine grain size -

high strength and hardness
high toughness and impact resistance
low ductility and malleability
excellent machinability & finish
quenching / cold working

coarse grain size -

low strength and hardness
brittle
high ductility and malleability
poor machinability or finish
annealing

annealing

method
internal changes
properties that change
product applications

metal is heated in a furnace (bright red for steel, 700-900C), held at this temperature for a set period then cooled as slow as possible by cooling it overnight. (aluminium cools slowly whereas copper needs quenching

works on work hardened metals. recrystallisation - distorted / heavily stresses / fragmented grains caused by cold working are completely obliterated. slow cooling allows atoms to reorganize into large stress free grains. dislocation and lattice distortions are relieved.

ductility and malleability increases, softer, low hardness and tensile strength, internal stresses relieved.

copper plumbing tubes, pressed steel components, silversmith,

normalising

method
internal changes
properties that change
product applications

metal (carbon steel) is heated in a furnace to a temperature above its upper critical point (800-900C). cooked evenly throughout, cooled in still air at room temp.

created uniform fine sized grains across the entire piece but does not allow the grains to grow, homogenisation - no uneven structural zones, stresses, irregularities.

better toughness and impact resistance, higher hardness and tensile strength, better machinability

hammer heads / spanners / crowbars to prevent shattering when they are being struck, structural steel beams, large castings

hardening

method
internal changes
properties that change
product applications

carbon steel or steel alloys is heated to its critical temperature (800-850C) until it grows cherry red. then it is quenched to cool rapidly in water, oil or brine.

steels structure transforms into austenite. rapid quenching drops the temp so fast that the carbon atoms do not have time to diffuse out of the crystal lattice. distorts the grain to create stresses.

increased hardness, increased tensile strength, improved wear resistance, very brittle (must be followed by tempering)

cutting tools, screwdriver tips, blades

tempering

method
internal changes
properties that change
product applications

immediately after hardening. clean with abrasive to reveal a bright, polished surface. reheated a temp below its critical point controlled by watching the metals surface change colour due to oxidation. once temperature is reached, it is cooled in air or water.

stresses during quenching are relaxed

toughness and impact resistance increases, brittleness decreases, ductility improved, hardness and tensile strength reduced.

qorkshop chisels and punches. screwdrivers and allen keys hacksaw blades and springs.

case hardening

method
internal changes
properties that change
product applications

low carbon mild steel is heated to bright red (900C) inside a carbon-rich environment. carbon soaks into the outer layer of the steel. the component is removed and quenched in water or oil.

carbon diffusion into the outer surface structure. high carbon outer skin. core is low carbon.

increased surface hardness, core toughness is same - ductile and shock absorbing, wear resistance, brittleness

padlocks, shafts, gears, self tapping screws.