Non-Equilibrium Heat Treatment

Steel Crystal Structures: Ferrite – BCC iron w/ carbon in solid solution (soft, ductile, magnetic) Austenite – FCC iron with carbon in solid solution (soft, moderate strength, non-magnetic) Cementite – Compound of carbon and iron Fe3C (Hard and brittle) Pearlite – alternate layers of ferrite and cementite. Martensite – iron – carbon w/ body centered tetragonal – result of heat treat and quench HT: ferrite then austentite then martensite

Heat Treatment of Steels for Strength: Steel = 0.6% to 1.0% carbon Must have a carbon content of at least 0.6% (ideally) to heat treat. Must heat to austenitic temperature range. Must rapid quench to prevent formation of equilibrium products. Basically crystal structure changes from BCC to FCC at high Temp. The FCC can hold more carbon in solution and on rapid cooling the crystal structure wants to return to its BCC structure. It cannot due to trapped carbon atoms. The net result is a distorted crystal structure called body centered tetragonal called martensite. Almost always followed by tempering.

Final step: Temper!

Heat treating for strength!

Direct Hardening Austenitizing and quench: Austenitizing – again taking a steel with 0.6% carbon or greater and heating to the austenite region. Rapid quench to trap the carbon in the crystal structure – called martensite (BCT) Quench requirements determined from isothermal transformation diagram (IT diagram). Get “Through” Hardness!!!

Heat to austenite range. Want to be close to transformation temperature to get fine grain structure. Austenitizing:

Quenching: Depending on how fast steel must be quenched (from IT diagram), the heat treater will determine type of quenching required: –Water (most severe) –Oil –Molten Salt –Gas/ Air (least severe) –Many phases in between!!! Ex: add water/polymer to water reduces quench time! Adding 10% sodium hydroxide or salt will have twice the cooling rate!

Temper Almost always done following heat treat as part of the austenitizing process! Because of lack of adequate toughness and ductility after heat treat, high carbon martensite is not a useful material despite its great strength (too brittle). Tempering imparts a desired amount of toughness and ductility (at the expense of strength)

The Heat Treat Processes

CASE HARDENING Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy. Case hardening is usually done after the part in question has been formed into its final shape

Case Hardening - Processes Flame/Induction Hardening Carburizing Nitriding Cyaniding Carbo-Nitriding Nitro-Carburizing

Same requirements as austenitizing: –Must have sufficient carbon levels (>0.4%) –Heat to austenite region and quench Why do? –When only desire a select region to be hardened:Knives, gears, etc. –Object to big to heat in furnace! Large casting w/ wear surface Types: –Flame hardening, induction hardening, laser beam hardening Flame/Induction Hardening

Flame Hardening:

Induction Hardening

Diffusion Hardening (Case Hardening) Why do? –Carbon content to low to through harden with previous processes. –Desire hardness only in select area –More controlled versus flame hardening and induction hardening. –Can get VERY hard local areas (i.e. HRC of 60 or greater) –Interstitial diffusion when tiny solute atoms diffuse into spaces of host atoms –Substitiutional diffusion when diffusion atoms to big to occupy interstitial sites – then must occupy vacancies

Diffusion Hardening: Requirements: –High temp (> 480 C) –Host metal must have low concentration of the diffusing species –Must be atomic suitability between diffusing species and host metal