Transcript Document
Heat treatment 1. Introduction The key to improve the material property is to change its structure at atomic level. This can be achieved through (i) alloying and (ii) controlled heat and cooling basically – heat treatment The basic steps of heat treatment are: Heat -> Soaking -> Cooling 1 Heat treatment Heat -> Soaking -> Cooling Temperature Time of soaking Rate of cooling Medium of cooling - Different combinations of the above parameters - Different compositions of materials and initial phases of materials Different heat treatments 2 Heat treatment 2. Purpose of heat treatment (1) Soften the metal prior (before) to shaping; (2) Relieve (ease) the effects of strain hardening that occurs during cold forming; (3) Achieve the final strength and hardness required in the finished product as one of the end manufacturing processes. 3 Heat treatment Body heat treatment Heat treatment Surface heat treatment 3. Classification of heat treatment processes (for basics of engineering technology) (1) Annealing (2) Martensite formation in steel (3) Surface hardening 4 Heat treatment 4. Annealing Heat -> Soaking -> Cooling -Reduce hardness and brittleness -Alter microstructure for desired mechanical properties -Soften metals to improve formability -Recrystalize cold worked (strain hardened) metals -Relieve stress from shaping 5 Heat treatment 4. Annealing • Full annealing (heating alloy into austenite region and slow cooling in furnace to form coarse pearlite) • Normalizing (cooling at faster rates, cool in air resulted in fine pearlite, high strength and hardness) • Process Anneal (to allow further cold working of part) - Re-crystallization anneal (full recovery of cold worked metal) - Recovery Anneal (partial return of grain structure) • Stress-relief annealing (reduce distortion and dimensional variations) 6 Heat treatment 5. TTT curve – principle for Martensite Formation - eutectoid composition - preheat or heat up alloy to austenite. - austenite to various phases : cooling rate. Pearlite, P; Bainite, B: alternative forms of ferrite-carbide mixtures Martensite, M 8 Metals – Phase diagram for Iron and Carbon Eutectic Eutectoid Fe3C Handouts 2 9 5. TTT curve – principle for Martensite Formation 10 Time ----- x- axis; temperature --- y-axis Curve interpreted by starting at 0 in austenite region above A1=1333F Proceeding downwards & to the right along a trajectory This is for specific composition of C (0.80%) Trajectory proceeds through region indicating transformation into pearlite or bainite TTT diagram shows both start s and finish f lines Pearlite (ferrite + Carbide) --- shows slow cooling from austenite-- -- it passes through Ps above nose of TTT curve Bainite (ferrite + Carbide) ---- rapid cooling to somewhat above Ms ----- Nose of TTT curve is avoided----- Much slower cooling to pass through Bs and into ferrite- carbide region If cooling occurs at much faster rate, austenite ----- martensite BCC to BCT 11 Heat treatment 5. TTT curve – principle for Martensite Formation Martensite: hard and brittle BCT + carbon Tetragonal Ms means the temperature M starts to form. Ms depends on alloyed element; some are lower than room temperature 12 Heat treatment 5. Heat treatment to form Martensite phase Austenitizing: heat up to a certain temperature to form Quenching: cooling media: Brine: the fastest; Air: the slowest Tempering: Heat up to temperature below eutectoid soak for one hour & slow cooling BCT to BCC 13 5. Heat treatment to form Martensite phase Austenitizing – quenching - tempering 14 Heat treatment 5. Hardenability - The relative capacity of a steel to be hardened by transformation to martensite; hardness penetration - Hardenability = Hardness ? - alloying elements increase the hardenability: to make TTT curve right: to increase the time to start the transformation for Austenite to Martensite 15 6. Surface heat treatment 1) Thermo chemical treatment 2) Composition of part surface altered by addition of other elements 3) Adding of carbon, nitrogen, or other elements Carburizing Nitriding Carbonitriding 16 Surface heat treatment - Carburizing Pack carburizing- Pack carbonaceous materials (charcoal); Very thick hard outer layer Gas carburizing- Diffuse Hydrocarbon fuels (propane in a furnace); thin hard outer layer - Low hardness - Ductile - Capable of withstanding stress Liquid carburizing- Diffuse molten salt bath containing sodium cyanide, barium chloride, and other compounds; medium sized hard outer layer 17 Surface heat treatment Carburizing -> HRC 60, Thickness: 0.025 – 4 mm Nitriding -> HRC 70, Thickness: 0.025 – 0.05 mm Carbonitriding -> HRC 70, Thickness: 0.07-0.5 mm Chromizing and Boronizing -> HRC 70 18 7. Heat treatment methods and facilities 1. Heat treatment furnaces 2. Surface hardening methods - flame hardening - induction heating - Laser beam heating 19