Published on March 13, 2011
Various HEAT TREATMENT Processes: Various HEAT TREATMENT Processes Presented By:- Rajesh Singh Rathore (OPJIT, 6 th Sem, Metallurgy) What is Heat Treatment?: What is Heat Treatment? Heat treatment is a process of combination of heating and cooling operations, timed and applied to metals and alloys in their solid state to obtain desired properties . Slide 3: Basic stages for all Heat Treatment Processes are:- Heat Treatment Softening Heat Treatment Hardening Heat Treatment CLASSIFICATION OF HEAT TREATMENT PROCESSES: CLASSIFICATION OF HEAT TREATMENT PROCESSES Softening Heat Treatment : Softening Heat Treatment 1. Annealing:- On the basis of Heat Treatment Temp Full annealing Partial annealing Sub critical b) Phase Transformation l. First Order Annealing ll. Second Order Annealing c) Specific Purposes l. Diffusion Annealing ll. Spheroidizing Annealing lll. Full Annealing lV. Recrystallization Annealing Normalising Tempering Austempering Martempring Sub-zero Treatment Patenting Hardening Heat Treatment: Hardening Heat Treatment Through Hardening Surface Hardening (Case Hardening) a) Surface hardening without changing the surface chemistry of steel i ) Induction hardening ii) Flame hardening b) Surface hardening by changing the surface chemistry of steel i ) Carburising * Pack or solid carburising * Liquid carburising * Gas carburising ii) Cyaniding iii) Nitriding PROCESS DESCRIPCTION: PROCESS DESCRIPCTION Slide 8: Annealing:- Heating the metal or alloy to pre-determine temp, holding at this temp and finally cooling at very slow rate(i.e. Furnace cooling) Objectives:- 1) Relieves internal stresses developed during solidification, machining, forging, rolling, welding etc. 2. Improve or restore ductility and toughness 3. To enhanced machinability . 4. Eliminate chemical non-uniformity. 5. To refine grain size and grain structure. 6. Enhance magnetic and electrical properties. 7. Reduce gases content in steel. Full annealing : Full annealing Process Variables:- * Austenitizing temperature: - hypo-eutectoid steel = Ac 3 +(30-50 0 C) Eutectoid steel = A 1 + (30-50 o C) Hyper-eutectoid steel = Acm + 50 o C *Austenitizing or soaking time:- soaked for a sufficient length of time at austenitizing temp to convert ( ferrite+ pearlite ) complete austenite * Cooling :- Furnace cooling Slide 11: Normalising :- Heating the steel to about 40-50 0 c above the upper critical temp ( Ac 3 or Ac m ),holding for proper time and then cooling in still air or slightly agitated air to room temperature. Objectives :- To produce harder & stronger steels compared to fully annealed steel. Grain size refinement of steel. Improve machinability , ductility & toughness. Homogenize chemical composition of cast ingots. Process variable of normalising :- a) Austenitizing temperature Hypo-eutectoid steel = Ac 3 +50 0 c Eutectoid steel = A 1 +50 0 c Hypereutectoid steel = Ac m + 50 0 c b) Austenetizing time Sufficient length of time (i.e - 1hr/1” of thickness) c) Cooling pattern cooling in still air or slightly agitated air Slide 13: Tempering :- Heating the harden steel below the lower critical temperature , followed by cooling in air or at any other desired rate, holding time is 3-5 min/mm of thickness. Structural changes during tempering :- a) Isothermal transformation of retained austenite. b) Ejection of carbon from BCT lattice to martensite. c) Growth & spheroidisation of carbide particle & formation of ferrite carbide mixture. Stages of Tempering: Stages of Tempering STAGES STRUCTURAL CHANGES PROPERTIES GAIN APPLICATION Low temp. tempering (80-250 0 c) High c martensite low c martensite (epsilon carbide Fe 2.4 C ) good strength , toughness excellent wear resistance reduces internal stresses Applicable to high c & low alloy steel. for manufacturing cutting & measuring tools. Medium temp . Tempering (350-500 0 c ) Retained austenite bainite (ferrite+epsilon carbide) ↑ ductility & toughness with ↓ hardness & strength ↑ Elastic property Suited for coil and laminated springs High temp. Tempering (500-680 0 c) Martensite ferrite C +Fe 2.4 C Cementite ↑ tensile , yield & Impact strength Relief internal stresses Applicable to medium and low alloy steel Connecting rod, shaft and gears Austemepering (isothermal Quenching or isothermal Hardening):- : Austemepering (isothermal Quenching or isothermal Hardening):- Advantages:- * Better ductility & high hardness value. * Improve impact & fatigue strength than conventional tempering & hardening process . * Freedom from distortion. Disadvantages :- * Long time required for isothermal transformation. 1. Heating the steel above austenitizing temp about 875 0 C 2. Isothermal quenching in quench bath above Ms point within the bainitic range of temp. 250-525 0 C Austenite Bainite 3. Taken out from bath and cool in air or any other cooling rate Slide 16: Martempering :- Process :- 1) Heating To austenitizing temperature. 2) Isothermal quenching In bath above Ms point within 180-250 0 C Quench till temp of metal is equal to bath temp. 3) Cooling High cooling rate in air and for short holding time. 4) Microstructure Austenite Martensite Advantages :- 1) Obtained stress free material due to step quenching process 2) Provide better mechanical properties than conventional quenched & tempered. Slide 17: Hardening :- Hardening of steel is opposite to softening. Objectives :- To harden the tool steel so as to improve their cutting ability To improve the wear resistance of the steel To develop high strength, better toughness, ductility by hardening and subsequent tempering Process Variables :- Austenitizing Temperature a) Hypo eutectoid steel = Ac 3 +50 0 C b) Eutectoid Steel = A 1 +50 0 C c) Hyper eutectoid steel = Acm + 50 0 C 2. Rapid Cooling (Quenching) High cooling rate that is quenched by water Slide 18: THANKING YOU For your attention !