Transcript CHE 333 Class 7

CHE 333 Class 9
Non Equilibrium Heat Treatment
of Steels.
Non equilibrium conditions
Non equilibrium is when a reaction is not allowed to go to completion. An example is “quenching”
when a hot object is plunged into a bath of cool liquid, such as iced brine or oil.
Under these conditions an expected phase change will be suppressed, that is, it will not happen.
Several results are possible.
•
The high temperature phase is retained at low temperature – age hardening.
•
A new phase is produced that is not predicted by the phase diagram.
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If the new phase is stable at room temperature it is termed a “Metastable” phase.
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Example of metastable phase formation by non equilibrium heat treatment is the formation of
“Martensite” in steels.
Martensite Formation.
Martensite does not appear on the phase
diagram. It is a non equilibrium phase.
It is produced by a non equilibrium heat treatment.
Consider a eutectoid steel. If it is heated above 727C
it transforms to g , FCC austenite. When a steel is held
at a temperature in the austentite region to transform
it to single phase g, it is called “austenitizing”. Another
name is a “solution heat treatment”. After a sufficient
time in the g range, 850C for one hour, the part is
rapidly placed in an oil bath at room temperature.
This is the quenching operation.
Phase Changes During Quench
Under equilibrium conditions, the eutectoid reaction will occur:g > a + Fe3C
FCC > BCC + Orthorhombic
The quench suppresses this reaction, and a new phase is formed which is
metastable:g > Martensite
FCC > Body Centered Tetragonal
(BCT)
For each steel composition there is a temperature threshold called the Ms
temperature below which martensite is produced.
If the steel is not quenched to below this temperature, no martensite will be
formed. Martensite can only be formed from the g phase.
Use Temperature Time Transformation Curves or TTT curves – these are
Isothermal. To get 100% martensite, quench finish temperature
is below 0C.
TTT Curve
Isothermally based so hold at a
constant temperature for a time and
measure the amount of transformation.
Ms – start of martensite transformation
Mf – finish of transformation below 0C.
g unstable – g present at the temp and
time but will transform, so unstable.
a+ carbide – stable as in phase diagram.
Any g unstable will transform to martensite
If quenched to below the Ms.
In between the g unstable and the a + carbide
Linear proportion of each, i.e. in middle of
Zone 50% g unstable and 50% a + carbide
This TTT curve is for the eutectoid
Composition as there is no a or Fe3C
Zone above the eutectoid temperature
50% Transformation
Bainite
Formation of Martensite.
FCC to BCT Relationship
Body Centered Tetragonal
Structure
C to a ratio increases with carbon
Martensite Structures
Martensite is a strong but brittle material
Acicular Martensite
Lath martensite
Martensite Amounts
In the top figure, the sample was held until 1% transformation
to Pearlite, 99% retained austenite, which on quenching
Transformed to martensite.
In the middle figure, the sample was held for 25%
transformation to pearlite The 75% retained austenite
transformed to Martensite upon quenching below the Mf
In the bottom figure, the sample was held to
50% transformation
Hypoeutectoid TTT Curve.
Note the region above the eutectoid temperature
of 727C, which is ferrite and austenite.
TTT - CCC
TTT for an alloy steel 4340
0.42%C, 0.78 Mn, 1.79% Ni 0.8% Cr.
Austenitized at 1550F. Note the slower times for the
nose and the splittin due to alloy additions.
Continuous cooling curve for the 4340 steel. Dashed lines
are cooling rate in c/sec.
Homework
1. Upon quenching a 0.4wt% carbon steel from 950C to -50C, what
phases will be present and what will be the composition?
1. If an optimum age hardening treatment is 150C for 22 hours, what is
the effect of raising the temperature to 180C. What is the effect of
lengthening the time at 150C to 5 days or decreasing the time to 12
hours. Indicate the effects by diagrams?
1. What conditions need to be met for a material to be age hardenable?