Continuous temperature-transformation diagrams

Temperature-Transformation Diagram (TTT Diagram), Continuous-temperature-Transformation Diagram (CTT Diagram) and Critical Cooling Rate... 

For a number of applications, particularly those associated with conditions of continuous cooling or heating, equilibrium is clearly never approached and calculations must be modified to take kinetic factors into account. For example, solidification rarely occurs via equilibrium, amorphous phases are formed by a variety of non-equilibrium processing routes and in solid-state transformations in low-alloy steels much work is done to understand time-temperature-transformation diagrams which are non-equilibrium in nature. The next chapter shows how CALPHAD methods can be extended to such cases. 

Fig. 18. Continuous-cooling transformation diagram for a Type 4340 alloy steel, with superimposed cooling curves illustrating the manner in which transformation behavior during continuous cooling governs final microstmcture (1). Ae is critical temperature at equiHbrium. Ae is lower critical...

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

It is important to note that the treatments relating to the kinetics of phase transformations in Section 10.3 are constrained to the condition of constant temperature. By way of contrast, the discussion of this section pertains to phase transformations that occm with changing temperatirre.This same distinction exists between Sections 10.5 (Isothermal Transformatiorr Diagrams) and 10.6 (Continuous-Cooling Transformation Diagrams). 

With regard to the representation of the martensitic transformation, the M(start), M(50%), and M(90%) lines occur at identical temperatures for both isothermal and continuous-cooling transformation diagrams. This may be verified for an iron-carbon alloy of eutectoid composition by comparison of Figures 10.22 and 10.25. 

In summary, isothermal and continuous-cooling transformation diagrams are, in a sense, phase diagrams in which the parameter of time is introduced. Each is experimentally determined for an alloy of specified composition, the variables being temperature and time. These diagrams allow prediction of the microstructure after some time period for constant-temperature and continuous-cooling heat treatments, respectively. 

Stmctures that form as a function of temperature and time on cooling for a steel of a given composition are usually represented graphically by continuous-cooling and isothermal-transformation diagrams. Another constituent that sometimes forms at temperatures below that for peadite is bainite, which consists of ferrite and Fe C, but in a less well-defined arrangement than peadite. There is not sufficient temperature and time for carbon atoms to diffuse long distances, and a rather poody defined acicular or feathery stmcture results. 

Important concepts in any analysis of the cure of thermosetting polymers are the possibility of reaching vitrification at particular locations in the part and the need to provide heat from an external source to devitrify the material and continue the cure. The analysis of vitrification cannot be avoided when the cure is performed at room temperature using different types of radiation (e.g., UV, EB, microwave) to initiate the polymerization. A temperature vs conversion transformation diagram (Chapter 5) may be useful in the design of cure cycles. 

Fig. 4.15 Characteristic free-energy temperature diagram (a) and DSC traces (b) for the enantiotropic relationship between polymorphs. The Gi and Gu curves cross at the transition temperature 7[ n below their melting points mpi, and mpn all indicated on the temperature axis. DSC trace A at the transition temperature modification I undergoes an endothermic transition to modification II, and the heat absorbed is A/fi n for that transition. Modification II then melts at mpn, with the accompanying AHfu. DSC trace B Modification I melts at mpi with A//n followed by crystaUization of II with A//ni at the intermediate temperature. Modification II then melts with details as above. DSC trace C modification II, metastable at room temperature, transforms exothermically to modification I with A/fn i at that transition temperature. Continued heating leads to the events in trace A. DSC trace D modification II exists at room temperature and no transition takes place prior to melting at mpn, with the appropriate A//ni- (After Giron 1995, with permission.)...

D. Time-Temperature-Transformation and Continuous-Heating-Transformation Curing Diagrams of PF Resins in situ in the Joint... 

VI. TEMPERATURE-TIME-TRANSFORMATION AND CONTINUOUS-HEATING-TRANSFORMATION CURING DIAGRAMS OF MUF RESINS WHEN ALONE AND HARDENING IN A WOOD JOINT (OR OTHER INTERACTIVE SUBSTRATE)... 

Fig. 3.1-113 Continuous-cooling-transformation (CCT) diagram for a 4130 grade low-alloy steel. Acs and Ac signify the temperatures of the y/ y+a) and eutectoid reation, respectively. A - austenite, F - ferrite, B - bainite, P - pearlite, M - martensite. The cooling rate is measured at 705 °C. The calculated critical cooling rate is 143 K/s [1.80]...

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). 

Chapter 1 (Phase Equilibria in Binary and Ternary Hydro-thermal Systems, V. M. Valyashko, Russia) contains a description of the general trends of sub- and supercritical phase behavioin in binary and ternary systems taking into accoimt both stable and metastable equilibria. A presentation of the various types of phase diagrams aims to show the possible versions of phase transitions under hydrothermal conditions and to help the reader with the determination of where the phase equilibrium occurs in p-T-X space, and what happens to this equilibrium if the parameters of state are changed. Special attention is paid to continuous phase transformations taking place with variations of temperature. 

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]. 

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