What Happens to Steel During Heat Treatment? Part Two: Cooling Transformations
June 8, 2007
Once we’ve heated a piece of steel to elevated temperature, it must be cooled in order to complete its transformation into a useful engineering material. Understanding cooling transformations is another important responsibility of the heat treater. Let’s learn more.
Types of Cooling Transformation DiagramsTransformation diagrams are simply another form of roadmap to allow us to predict a steel’s response to heat treating. There are two main types of cooling transformation diagrams, and it is important that we understand what each is and how we can use them.
Time-Temperature Transformation (TTT) Diagrams
Also known as isothermal transformation (IT) diagrams, TTT diagrams measure the rate of transformation at a constant (isothermal) temperature (Fig. 1). In other words, once a part is austenitized, it is rapidly cooled to a lower temperature and held at that temperature while the rate of transformation is measured. The different types of microstructures produced (ferrite, pearlite, bainite, martensite) are then indicated on the diagram together with the holding times required for each transformation to begin and end.
Continuous Cooling Transformation (CCT) Diagrams
Also known as cooling transformation (CT) diagrams, CCT diagrams measure the degree of transformation as a function of time for a constantly changing (decreasing) temperature (Fig. 2). In other words, a sample is austenitized and then cooled at a predetermined rate, and the degree of transformation is measured using such techniques as dilatometry, magnetic permeability or other physical methods.
A Closer Look at TTT DiagramsTTT diagrams are useful in planning heat treatments and in determining the critical cooling rate on quenching, which is the cooling rate at which one just avoids the nose of the TTT curve. If the austenite-to-martensite transformation is incomplete, retained austenite usually transforms during tempering into the transformation product indicated on the TTT diagram.
In general, TTT diagrams allow us to gain limited information on the influence of alloying elements on transformations during continuous cooling by comparing the temperatures at which the transformation products occur.
A Closer Look at CCT DiagramsCCT diagrams provide a useful tool for predicting the microstructure achieved during a typical quench after heat treating. We can measure (or calculate) the rates of cooling at any point within a steel part (surface, core, mid-radius).
CCT diagrams have been developed for many steel compositions by using various experimental methods. One such method is the use of Jominy bars with thermocouples attached along the length of the bar. Microstructures are correlated with the cooling rates calculated from the thermocouple readings. A CCT diagram that indicates the transformation products obtained at various cooling rates is then developed from the Jominy information. Thus the hardenability of steel allows us to develop an engineering approach to understanding the effects of quenching in various cooling media (see “Jominy Testing, The Practical Side,” - October 2001).
CCT diagrams show the approximate proportions of the major phases and the hardness of the microstructures obtained. The effect of tempering on hardness levels is often shown as well. The hardenability effect of the steel can be seen directly from the diagram – low-hardenability steels show early transformation, mainly from the upper left-hand side of the diagram (to ferrite and pearlite or bainite). By contrast, high-hardenability steels exhibit curves in the lower right-hand side of the diagram with austenite changing predominately to martensite over a wide range of part thicknesses and quenching rates.
A Comparison of TTT and CCT DiagramsDespite the general similarity in shape between CCT and TTT diagrams for identical steels, the data is presented differently. On CCT diagrams the products of transformation (martensite-bainite-pearlite) are indicated along the bottom of the diagram, whereas they are shown on the right side of the TTT diagram. Phase changes are recorded within the starting and finishing boundaries on CCT diagrams, whereas on TTT diagrams these regions indicate the transformation phase themselves.
Although similar in shape to TTT diagrams, the nose of the CCT diagram is shifted down to the right, indicating that more time is available for martensite transformation than is shown on the corresponding TTT diagram. TTT diagrams actually err by indicating a faster cooling rate than necessary to form 100% martensite on quenching. This error is usually on the conservative side since the goal of most heat-treatment operations is to produce 100% martensite.
Appropriate CautionsCCT diagrams mainly refer only to the center of a bar, but the microstructures at other positions can be inferred. For example, the microstructure produced on cooling at some mid-radius position in a larger diameter often corresponds to that produced at the center of a bar of smaller diameter – a so-called equivalent diameter – with similar microstructures being produced by similar cooling rates.
CCT diagrams usually refer to the average chemical composition. Variations in composition can lead to considerable differences in microstructure and properties. There are also critical thickness ranges where slightly slower or faster cooling rates produce significant changes in the predominant microstructure indicated. Changes in carbon and manganese content can have pronounced effects on these thickness ranges. A major difficulty in constructing CCT diagrams is the interpretation of transformation behavior. Martensite and bainite are affected by compositional changes in the parent austenite that may have resulted from any prior ferrite formation or carbide precipitation at higher temperatures.
Prior heat treatment can affect grain size and hence modify the subsequent transformations on cooling. The austenitizing temperature may affect the austenite composition of steel that contains strong carbide-forming elements. Consequently, undissolved carbides may be present. These considerations should be taken into account when using or adapting CCT diagrams. Heating by applied energy (induction, flame, laser) with rapid heating and short thermal cycle times has a drastic effect on the condition of the austenite and as a result the accuracy of the CCT diagrams. Welding is another process that is very difficult to predict using these types of diagrams.
Another factor is quench severity and the degree of agitation, the effects of which can only be determined experimentally. Air cooling is normally the main criterion for developing these diagrams. Water – not brine quenching – is represented as a standard medium-to-fast quenching medium.
Summing UpFew heat-treatment processes involve isothermal transformation, and most microstructures are produced as a result of continuous cooling operations. If the rate of cooling is slow, the microstructure corresponds more closely to that indicated on the upper portion of a TTT diagram. Faster cooling rates, however, deviate considerably during the transformation process.
While the heat treater should be aware of both types of transformation diagrams, the use of CCT diagrams is often more directly applicable to microstructures that are produced at the center of a heat treated part under real-world conditions. For this reason they are a valuable tool. IH
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