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LECTURE 13: HEAT TREATMENT OF PLAIN CARBON STEELS - Coggle Diagram
LECTURE 13: HEAT TREATMENT OF PLAIN CARBON STEELS
HEAT TREATING PLAIN CARBON STEEL
Process annealing is annealing around 80 to 170 below the eutectoid temp. Causing recrystallisation of the ferrite
Speroidise annealing used with hypoeutectoid alloys improves ductility for machining, heated 30 degrees below eutectoid line and causes cementite regoins to restructure from layers to spheres
Normalising heats the alloy then cools slowly forming austentite and fine pearlite structure
Fine pearlite helps prevent dislocation slip and good toughness with smaller grains and more uniform properties
Full annealing is when it is heated and then cooled really slowly to allow more time for diffusion
increased carbon content = increased strength usually
HARDENING AND TEMPERING
to harden it is taken into the austentite region of the phase diagram, left there and then cooled very quickly (non equilibrium cooling)
Cooled so quick the proper structure that should be bcc is elongated forming marstensite, this distortion helps prevent dislocation slips (less close packed)
Steel is reheated forming tempered martensite consisting of finely dispersed small Fe3C particles (dispersion strengthening)
Higher temp. Tempering increases toughness but reduces hardness
Increased time spent tempering = decreased hardness
HARDENABILITY
Due to heat transfer through the material the centre may not harden equally with the outside, the exten to uniform hardening is know as hardenability
Good hardenability has equal hardness along the length of the bar after a Jominy end quench test
ALLOY STEELS
Adding other elements allows to improve the hardenability and poor strength of plain carbon steel (can also add other properties)
Numbering system for alloy steels, remember there are different systems
Range of values usually refers to how much it has been worked (eg tempering)
Example: Tool steels require lots of alloying to help provide adequate hardness even when cutting
Stainless steel has nickel and/or chromium to reduce corrosion. As nickel = stabilises austentite and chromium encourages ferrite formation, there are different groups for stainless steel (ferritic, austentitic, martenstitic)
Cast iron has a much lower melting point, good for casting, often brittle with little ductility (carbon exists as graphite here) with four main types (grey, nodular, white and malleable)