Non Equilibrium Heat Treatment of Steels. CHE 333 Class 8 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. 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 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 Upon 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?