Presentation on theme: "Non Equilibrium Heat Treatment of Steels."— Presentation transcript:
1 Non Equilibrium Heat Treatment of Steels. CHE 333 Class 8Non Equilibrium Heat Treatmentof Steels.
2 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.
3 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 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.
4 Phase Changes During Quench Under equilibrium conditions, the eutectoid reaction will occur:-g > a + Fe3CFCC > BCC + OrthorhombicThe quench suppresses this reaction, and a new phase is formed which is metastable:-g > MartensiteFCC > 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 temperatureis below 0C.
5 TTT Curve Isothermally based so hold at a constant 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 andtime 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 + carbideThis TTT curve is for the eutectoidComposition as there is no a or Fe3CZone above the eutectoid temperature50% TransformationBainite
6 Formation of Martensite. FCC to BCT RelationshipBody Centered TetragonalStructureC to a ratio increases with carbon
7 Martensite Structures Martensite is a strong but brittle materialAcicular MartensiteLath martensite
8 Martensite AmountsIn the top figure, the sample was held until 1% transformationto Pearlite, 99% retained austenite, which on quenchingTransformed to martensite.In the middle figure, the sample was held for 25%transformation to pearlite The 75% retained austenitetransformed to Martensite upon quenching below the MfIn the bottom figure, the sample was held to50% transformation
9 Hypoeutectoid TTT Curve. Note the region above the eutectoid temperatureof 727C, which is ferrite and austenite.
10 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.
11 HomeworkUpon quenching a 0.4wt% carbon steel from 950C to -50C, what phases will be present and what will be the composition?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?What conditions need to be met for a material to be age hardenable?