CHE 333 Class 8

Non Equilibrium Heat Treatment

of Steels.

Non equilibrium conditionsNon 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.

• If the new phase is stable at room temperature it is termed a “Metastable” phase.

• 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 phasediagram. It is a non equilibrium phase.It is produced by a non equilibrium heat treatment.Consider a eutectoid steel. If it is heated above 727Cit transforms to g , FCC austenite. When a steel is heldat a temperature in the austentite region to transformit to single phase g, it is called “austenitizing”. Anothername is a “solution heat treatment”. After a sufficienttime in the g range, 850C for one hour, the part israpidly placed in an oil bath at room temperature.This is the quenching operation.

Phase Changes During QuenchUnder 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 CurveIsothermally based so hold at aconstant temperature for a time andmeasure the amount of transformation.Ms – start of martensite transformationMf – 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 martensiteIf quenched to below the Ms.

In between the g unstable and the a + carbideLinear proportion of each, i.e. in middle ofZone 50% g unstable and 50% a + carbide

This TTT curve is for the eutectoidComposition as there is no a or Fe3CZone above the eutectoid temperature

50% Transformation

Bainite

Formation of Martensite.

Body Centered TetragonalStructure

C to a ratio increases with carbon

FCC to BCT Relationship

Martensite Structures

Martensite is a strong but brittle material

Lath martensite

Acicular Martensite

Martensite AmountsIn 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 thenose and the splittin due to alloy additions.

Continuous cooling curve for the 4340 steel. Dashed linesare 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?

2. 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?

3. What conditions need to be met for a material to be age hardenable?