heat treatment isat 430. module 6 spring 2001dr. ken lewis isat 430 2 heat treatment three reasons...
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Spring 2001 Dr. Ken Lewis ISAT 430 2Module 6
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.
Spring 2001 Dr. Ken Lewis ISAT 430 3Module 6
Heat Treatment Principal heat treatments
Annealing Martensite formation in steel Precipitation hardening Surface hardening
Spring 2001 Dr. Ken Lewis ISAT 430 4Module 6
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
Spring 2001 Dr. Ken Lewis ISAT 430 5Module 6
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.
Spring 2001 Dr. Ken Lewis ISAT 430 7Module 6
Fe - C Iron melts at 1538°C
As it cools, it forms in sequence Delta ferrite Austenite Alpha ferrite
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
Spring 2001 Dr. Ken Lewis ISAT 430 8Module 6
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.
Spring 2001 Dr. Ken Lewis ISAT 430 9Module 6
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.
Spring 2001 Dr. Ken Lewis ISAT 430 10Module 6
Fe - C Cementite
100% iron carbide Fe3C 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
Spring 2001 Dr. Ken Lewis ISAT 430 11Module 6
Martensite formation in Steel The diagram at left
assumes slow equilibrium cooling.
Each phase is allowed to form
Time is not a variable.
Spring 2001 Dr. Ken Lewis ISAT 430 12Module 6
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.
Spring 2001 Dr. Ken Lewis ISAT 430 14Module 6
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
Spring 2001 Dr. Ken Lewis ISAT 430 16Module 6
The Time – Temperature – Transformation Curve (TTT)
Composition Specific Here 0.8% carbon
At different compositions, shape is different
Spring 2001 Dr. Ken Lewis ISAT 430 18Module 6
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.
Spring 2001 Dr. Ken Lewis ISAT 430 19Module 6
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 Ms and ends at Mf. If cooling stopped it
will transition into bainite and Martensite.
Spring 2001 Dr. Ken Lewis ISAT 430 20Module 6
Martensite hardness The extreme
hardness comes from the lattice strain created by carbon atoms trapped in the BCT
Spring 2001 Dr. Ken Lewis ISAT 430 21Module 6
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
Spring 2001 Dr. Ken Lewis ISAT 430 22Module 6
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
Spring 2001 Dr. Ken Lewis ISAT 430 23Module 6
Quenching Media2
Cooling capacities of typical quench media are
Agitated brine 5. Still water 1. Still oil 0.3 Cold gas 0.1 Still air 0.02
Spring 2001 Dr. Ken Lewis ISAT 430 24Module 6
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
Spring 2001 Dr. Ken Lewis ISAT 430 25Module 6
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
Spring 2001 Dr. Ken Lewis ISAT 430 26Module 6
Surface Hardening2
The common procedures are:
Carburizing
Nitriding
Carbonnitriding
Chromizing and boronizing
Spring 2001 Dr. Ken Lewis ISAT 430 27Module 6
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 (C3H8) in a sealed furnace Liquid carburizing
Used NaCN, BaCl2 Thickness 0.005 in. to 0.030 in.
Spring 2001 Dr. Ken Lewis ISAT 430 28Module 6
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 0.001 in – 0.020 in.
Spring 2001 Dr. Ken Lewis ISAT 430 29Module 6
Chromizing Diffuse chromium into the surface 0.001 –
0.002 in. Pack the parts in Cr rich powders or dip in a
molten salt bath containing Cr salts.