Heat Treatment ISAT 430

Module 6 Spring 2001Dr. Ken Lewis ISAT Heat Treatment Three reasons for heat treatment To soften before shaping To relieve the effects of strain hardening To acquire the desired strength and toughness in the finished product.

Module 6 Spring 2001Dr. Ken Lewis ISAT Heat Treatment Principal heat treatments Annealing Martensite formation in steel Precipitation hardening Surface hardening

Module 6 Spring 2001Dr. Ken Lewis ISAT Annealing Process Heat the metal to a temperature Hold at that temperature Slowly cool Purpose Reduce hardness and brittleness Alter the microstructure for a special property Soften the metal for better machinability Recrystallize cold worked (strain hardened) metals Relieve induced residual stresses

Module 6 Spring 2001Dr. Ken Lewis ISAT The Iron Carbon System Steels, ferrous alloys, cast irons, cast steels Versatile and ductile Cheap Irons (< 0.008% C) Steels (< 2.11% C) Cast irons (<6.67% [mostly <4.5%]C) The material properties are more than composition – they are dependent on how the material has been treated.

Module 6 Spring 2001Dr. Ken Lewis ISAT The Phase Diagram

Module 6 Spring 2001Dr. Ken Lewis ISAT Fe - C Iron melts at 1538°C As it cools, it forms in sequence Delta ferrite Austenite Alpha ferrite Solid solution of BCC iron Maximum C solubility of 0.022% at 727°C Soft and ductile Magnetic up to the Curie temperature of 768°C

Module 6 Spring 2001Dr. Ken Lewis ISAT Fe - C Delta ferrite exists only at high temperatures and is of little engineering consequence. Note that little carbon can be actually interstitially dissolved in BCC iron Significant amounts of Chromium (Cr), Manganese (Mn), Nickel (Ni), Molybdenum (Mb), Tungsten (W), and Silicon (Si) can be contained in iron in solid solution.

Module 6 Spring 2001Dr. Ken Lewis ISAT Fe - C Austenite (gamma  iron) Between 1394 and 912°C iron transforms from the BCC to the FCC crystal structure. It can accept carbon in its interstices up to 2.11% Denser than ferrite, and the FCC phase is much more formable at high temperatures. Large amounts of Ni and Mn can be dissolved into this phase The phase is non-magnetic.

Module 6 Spring 2001Dr. Ken Lewis ISAT Fe - C Cementite 100% iron carbide Fe 3 C Very hard Very brittle Pearlite Mixture of ferrite and cementite Formed in thin parallel plates Bainite Alternate mixture of the same phases Needle like cementite regions Formed by quick cooling

Module 6 Spring 2001Dr. Ken Lewis ISAT Martensite formation in Steel The diagram at left assumes slow equilibrium cooling. Each phase is allowed to form Time is not a variable.

Module 6 Spring 2001Dr. Ken Lewis ISAT Martensite formation in Steel However If cooling is rapid enough that the equilibrium reactions do not occur Austenite transforms into a non-equilibrium phase Called Martensite.

Module 6 Spring 2001Dr. Ken Lewis ISAT

Module 6 Spring 2001Dr. Ken Lewis ISAT Fe - C Martensite Hard brittle phase Iron carbon solution whose composition is the same as austenite from which it was derived But the FCC structure has been transformed into a body center tetragonal (BCT) The extreme hardness comes from the lattice strain created by carbon atoms trapped in the BCT

Module 6 Spring 2001Dr. Ken Lewis ISAT The Time – Temperature – Transformation Curve (TTT)

Module 6 Spring 2001Dr. Ken Lewis ISAT The Time – Temperature – Transformation Curve (TTT) Composition Specific Here 0.8% carbon At different compositions, shape is different

Module 6 Spring 2001Dr. Ken Lewis ISAT C

Module 6 Spring 2001Dr. Ken Lewis ISAT The Time – Temperature – Transformation Curve (TTT) At slow cooling rates the trajectory can pass through the Pearlite and Bainite regions Pearlite is formed by slow cooling Trajectory passes through Ps above the nose of the TTT curve Bainite Produced by rapid cooling to a temperature above Ms Nose of cooling curve avoided.

Module 6 Spring 2001Dr. Ken Lewis ISAT The Time – Temperature – Transformation Curve (TTT) If cooling is rapid enough austenite is transformed into Martensite. FCC > BCT Time dependent diffusion separation of ferrite and iron carbide is not necessary Transformation begins at M s and ends at M f. If cooling stopped it will transition into bainite and Martensite.

Module 6 Spring 2001Dr. Ken Lewis ISAT Martensite hardness The extreme hardness comes from the lattice strain created by carbon atoms trapped in the BCT

Module 6 Spring 2001Dr. Ken Lewis ISAT Tempered Martensite Step 1 -- Quench in the martensitic phase Step 2 – soak Fine carbide particles precipitate from the iron – carbon solution Gradually the structure goes BCT > BCC

Module 6 Spring 2001Dr. Ken Lewis ISAT Quenching Media The fluid used for quenching the heated alloy effects the hardenability. Each fluid has its own thermal properties Thermal conductivity Specific heat Heat of vaporization These cause rate of cooling differences

Module 6 Spring 2001Dr. Ken Lewis ISAT Quenching Media 2 Cooling capacities of typical quench media are Agitated brine5. Still water1. Still oil0.3 Cold gas0.1 Still air0.02

Module 6 Spring 2001Dr. Ken Lewis ISAT Other quenching concerns Fluid agitation Renews the fluid presented to the part Surface area to volume ratio Vapor blankets insulation Environmental concerns Fumes Part corrosion

Module 6 Spring 2001Dr. Ken Lewis ISAT Surface Hardening Refers to a “thermo chemical” treatment whereby the surface is altered by the addition of carbon, nitrogen, or other elements. Sometimes called CASE HARDENING. Commonly applied to low carbon steels Get a hard wear resistant shell Tough inner core

Module 6 Spring 2001Dr. Ken Lewis ISAT Surface Hardening 2 The common procedures are: Carburizing Nitriding Carbonnitriding Chromizing and boronizing

Module 6 Spring 2001Dr. Ken Lewis ISAT Carburizing Heating a low carbon steel in the presence of carbon rich environment at temperature ~ 900°C Carbon diffuses into the surface End up with a high carbon steel surface. Pack parts in a compartment with coke or charcoal Gas carburizing Uses propane (C 3 H 8 ) in a sealed furnace Liquid carburizing Used NaCN, BaCl 2 Thickness in. to in.

Module 6 Spring 2001Dr. Ken Lewis ISAT Nitriding Nitrogen is diffused in the surface of special alloy steels at temperatures around ~510°C. Steel must contain elements that will form nitride compounds. Aluminum Chromium Forms a thin hard case without quenching Thicknesses in – in.

Module 6 Spring 2001Dr. Ken Lewis ISAT Chromizing Diffuse chromium into the surface – in. Pack the parts in Cr rich powders or dip in a molten salt bath containing Cr salts.

Module 6 Spring 2001Dr. Ken Lewis ISAT Boronizing Performed on tool steels, nickel and cobalt based alloy steels. When used on low carbon steels, corrosion resistance is improved.