Heat Treatment ISAT 430. Module 6 Spring 2001Dr. Ken Lewis ISAT 430 2 Heat Treatment Three reasons for heat treatment To soften before shaping To relieve

Heat Treatment

ISAT 430

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.


Heat Treatment Principal heat treatments

Annealing Martensite formation in steel Precipitation hardening Surface hardening


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


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.


ThePhase

Diagram


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


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.


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.


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


Martensite formation in Steel The diagram at left

assumes slow equilibrium cooling.

Each phase is allowed to form

Time is not a variable.


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.



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


The Time – Temperature – Transformation Curve (TTT)



Composition Specific Here 0.8% carbon

At different compositions, shape is different


0.8C



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.



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.


Martensite hardness The extreme

hardness comes from the lattice strain created by carbon atoms trapped in the BCT


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


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


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


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


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


Surface Hardening2

The common procedures are:

Carburizing

Nitriding

Carbonnitriding

Chromizing and boronizing


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.


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.


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.


Boronizing Performed on tool steels, nickel and cobalt

based alloy steels. When used on low carbon steels, corrosion

resistance is improved.

Documents

Heat Treatment ISAT 430. Module 6 Spring 2001Dr. Ken Lewis ISAT 430 2 Heat Treatment Three reasons for heat treatment To soften before shaping To relieve