Steel temperature-time-transformation

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. 

Characteristics and implementation of the treatments depend on the expected results and on the properties of the material considered a variety of processes are employed. In ferrous alloys, in steels, a eutectoid transformation plays a prominent role, and aspects described by time-temperature-transformation diagrams and martensite formation are of relevant interest. See a short presentation of these points in 5.10.4.5. Titanium alloys are an example of the formation of structures in which two phases may be present in comparable quantities. A few remarks about a and (3 Ti alloys and the relevant heat treatments have been made in 5.6.4.1.1. More generally, for the various metals, the existence of different crystal forms, their transformation temperatures, and the extension of solid-solution ranges with other metals are preliminary points in the definition of convenient heat treatments and of their effects. In the evaluation and planning of the treatments, due consideration must be given to the heating and/or cooling rate and to the diffusion processes (in pure metals and in alloys). 

Transformation Point Indicators and Recorders.—Such instruments are designed to indicate the temperature at which a chemical or physical transformation occurs. They are based on the fact that heat is absorbed or given off when such transformations take place. The simplest instrument will give a temperature-time curve of the sample of steel placed in a furnace and heated. The method is not applicable industrially and a modified form has been developed. Burgess has described the use of a neutral body and the differential couple, an arrangement first devised by Sir Roberts-Austen. The differential couple is mounted with one hot junction in the test piece and the other in a neutral body (one with no transformation points). The data obtained are curves of temperature of test piece vs. temperature difference between the test piece and the neutral body. These are mounted side by side in the furnace and heated and cooled as uniformly as possible. 

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. 

Sketch the time-temperature-transformation (TTT) diagram for a plain carbon steel... 

The polymorphism of certain metals, iron the most important, was after centuries of study perceived to be the key to the hardening of steel. In the process of studying iron polymorphism, several decades were devoted to a red herring, as it proved this was the P-iron controversy. P-iron was for a long time regarded as a phase distinct from at-iron (Smith 1965) but eventually found to be merely the ferromagnetic form of ot-iron thus the supposed transition from P to a-iron was simply the Curie temperature, p-iron has disappeared from the iron-carbon phase diagram and all transformations are between a and y. 

Fig. 20.48 Isothermal time temperature transformation curves for (a) a eutectoid steel and...

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. 

This section will begin by looking at how thermodynamic and kinetic modelling has been combined to understand time-temperature-transformation diagrams in steels. The woric, for the most part, is semi-empirical in nature, which is forced upon the topic area by difficulties associated with the diffusional transformations, particularly where nucleation aspects have to be considered. The approaches have considered how best to predict the time/temperature conditions for austenite to... 

To stress relieve or postweld heat treat carbon and low-alloy steels, they are typically heated to 1100°F to 1350°F (595°C to 730°C) for extended time, followed by air coohng. The minimum time is specified by the relevant engineering code, and the temperature must be less than the lower transformation temperature of the steel, which is the lowest temperature at which austenite starts to form, for example, 1333°F (720°C) for plain carbon steels. In order to avoid degrading the required mechanical properties of a heat treated alloy, subsequent fabrication heat treatment temperatures, such as those for stress relief and PWHT, must not exceed the tempering temperature (discussed in the next section). 

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