Isothermal time-temperature diagrams

The effects of quench rate on IGC for Al-Gu, Al-Gu-Mg, and Al-Cu-Mg-Mn alloys as well as for austenitic stainless steels is considered to be well-understood [43, 74, 75, 106]. Integration of the effects of precipitation and solute depletion at each temperature during a quench (i.e., quench factor analysis) can be compared to isothermal time-temperature-sensitization diagrams in order to predict the quench rate required to avoid IGC [43, 74]. Alloys... 

This critical cooling rate ( q ) has been estimated by use of isothermal time-temperature transformation ( IT T) diagrams (Uhhnann 1972) or continuous cooling transformation (CT) curves Onorato and Uhlmann 1976). 

For hardening and subsequent tempering operations, the heat treatment contractor should be guided by the recommendations of the steel maker given in the data sheets for the material concerned. These heat treatment recommendations are derived from the chemical composition of the steel used. Material-specific, continuous and isothermal Time-Temperature-Transformation (TTT) diagrams, as shown in Figure 4.80, describe the transformation behaviour of the steel s microstructure [3]. 

FiGURE 4.80 Continuous and isothermal time-temperature-transformation diagram of the steel 1.2343 (X37CrMoV5-1) [4]... 

The phase diagram tells us only what phases can exist in equilibrimn. The final micro-structure is controlled by the rate of transformation. A sketch of a typical isothermal time-temperature-transformation (often called a T-T-T diagram) map for the austenitic to ferritic transformation is shown in Figure 14.3. 

Time-temperature-transformation (T-T-T) diagrams are used to present the structure of steels after isothermal transformation at different temperatures for varying times. The T-T-T diagram for a commercial eutectoid steel is shown in Fig. 20.48a. Also shown on the curves are the points at which the microstructures illustrated in Figs. 20.46 and 20.47 are observed, and the thermal treatments producing these structures. When a steel partially transformed to, say, pearlite, is quenched from point a in Fig. 20.48a to below nif, the untransformed austenite transforms to martensite. 

At hi temperatures thermal degradation becomes important, and may prevent full cure from being achieved Two degradation events have been noted in relation to the TTT diagram devitrification followed by elastomer formation and vitrification followed by char formation. The devitrification event corresponds to a decrease in Tg from above to below the isothermal cure temperature the time to this event may be considered to be the lifetime of the material since it marks the limit in time for the material to support a substantial load. The second event is an elastomer-to-glass transformation, accompanied by an increase in Tg and rigidity, and is presumably due to the onset of char formation... 

Figure 3. ATS time-temperature-transformation diagram for isothermal cures in N2.

Isothermal Transformation Diagram. To separate the effects of transformation temperature from those of heat flow, it is essential to understand the nature of the transformation of austenite at a given, preselected temperature below the A. Information needed includes the starting time, the amount transformed as a function of time, and the time for complete transformation. A convenient way to accomplish this is to form austenite in specimens so thin (usually about 1-mm thick) that heat flow is not an issue, rapidly transfer the specimens to a Hquid bath at the desired temperature, and foUow the transformation with time. The experiment is repeated at several other transformation temperatures. On the same specimens, the microstmcture and properties of the transformation products can be assessed. These data can be summarized on a single graph of transformation temperature versus time known as an isothermal transformation (IT) diagram or, more usually, a time—temperature—transformation (ITT) diagram. A log scale is used for... 

Figure 2.8. A time-temperature-transformation (TTT) isothermal cure diagram for a thermosetting system. Adapted from Figure 1 (GilUiam, 1986).

Experimental time-temperature-transformation (TXT) diagram for Ti-Mo. Xhe start and finish times of the isothermal precipitation reaction vary with temperature as a result of the temperature dependence of the nucleation and growth processes. Precipitation is complete, at any temperature, when the equilibrium fraction of a is established in accordance with the lever rule. Xhe solid horizontal line represents the athermal (or nonthermally activated) martensitic transformation that occurs when the p phase is quenched. 

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