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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.

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che 333 class 9

CHE 333 Class 9

Non Equilibrium Heat Treatment

of Steels.

non equilibrium conditions
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.
  • 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 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
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
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 id for the eutectoid

Composition as there is no a or Fe3C

Zone above the eutectoid temperature

50% Transformation

Bainite

formation of martensite
Formation of Martensite.

FCC to BCT Relationship

Body Centered Tetragonal

Structure

C to a ratio increases with carbon

martensite structures
Martensite Structures

Martensite is a strong but brittle material

Acicular Martensite

Lath martensite

martensite amounts
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
Hypoeutectoid TTT Curve.

Note the region above the eutectoid temperature

of 727C, which is ferrite and austenite.

ttt ccc
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
Homework
  • For a 1040 steel, what are the compositions and amounts of pro-eutectiod ferrite and pearlite. What are the amounts of eutectoid ferrite and cementite?
  • For a eutectoid steel, after quenching to 520C from 800C, and then holding for the following times, 0.3, 1.5 and 30 secs, what phases and amounts would be present on further quenching to -200C, for each of the 520 C hold times.
  • What is the effect of alloy additions to plain carbon steels in regard to martensite formation?