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A new method for accurate plotting continuous cooling transformation curves
Guang Xu ⁎, Lun Wan, Shengfu Yu, Li Liu, Feng LuoKey Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan, 430081, China
⁎ Corresponding author. Mail box 131, Faculty of MaUniversity of Science and Technology, Wuhan, 430081,fax: +86 27 86560679.
Continuous cooling transfo Received 16 February 2008Accepted 17 May 2008Available online 22 May 2008
Keywords:Metals and alloysMicrostructureContinuous cooling transformation diagramDilatometerDifferential scanning calorimetry
rmation (CCT) diagrams are usually plotted using dilatometer tests on a hotsimulator and metallographic analysis. However, for some steel grades it is not sufficient to use conventionalmethods to plot CCT diagrams. Therefore, how to accurately plot CCT diagrams for such steel grades is animportant task in the development of new metals and the determination of processing technology. A newmethod of using differential scanning calorimetry in addition to the dilatometer test and microstructureexamination is proposed to accurately plot the CCT diagrams. An example of a CCT diagram for an alloy steelis also shown.
Carbon and alloy steels are widely used engineering materialswhose properties depend on the concentration of carbon, alloyingelements and microstructures. The microstructures of steel for certainchemical compositions mainly depend on the processing technology.The optimal properties of steels can only be obtained after theapplication of suitable heat and thermo-mechanical treatments[1].Optimum technological parameters for the desired properties can beobtained from a transformation diagram. There are two main types oftransformation diagram that are helpful in selecting the optimumsteel and processing route to achieve a given set of properties. Theyare the time–temperature transformation and continuous coolingtransformation (CCT) diagrams. CCT diagrams are generally moreappropriate for engineering applications as they are more in accordwith the practical situation. When plotting CCT diagrams, a sample isaustenitized and then cooled at a predetermined rate and the degreeof transformation is measured by dilatometer. A large number ofexperiments is usually required to build up a complete CCT diagram.In the production of steels, CCT curves are usually required to selectthe optimum processing technology parameters and cooling rate afterprocessing. Thus many research papers have been published aboutCCT diagrams. Some studies have dealt with the methodology of CCTdiagrams [2–4], whereas others have focused on the CCT curves ofsome steel grades using the conventional method of dilatometer testsand metallographic analysis [5–10]. At present, the phase transforma-
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tion points are usually obtained from the inflexion points of thedilatation curves which are measured by dilatometer. However, whenplotting CCT diagrams for alloy steels, it has been found that the phasetransformation points cannot be accurately determined from theinflexion temperatures in dilatation curves because sometimes theinflexion temperatures are not very clear. Thus, it is interesting toaccurately determine the transformation points in order to plotprecise CCT diagrams. A newmethod of accurately plotting CCTcurvesby differential scanning calorimetry (DSC) in addition to dilatometertests and microstructural examinations is therefore proposed in thisstudy.
2. Materials and experimental procedures
2.1. Materials
During the development of software to predict the microstructureand the evolution of the properties of an alloy, it is necessary to obtainthe CCT curve for this alloy steel. The specimens were supplied byBaosteels. Their chemical compositions were as follows (in wt.%):0.35C, 0.85Mn, 0.027P, 0.006S, 0.005Si, 0.032Als, 0.07Nb, and 0.0006Ti.
2.2. Experimental procedures
Both dilatometer tests on a hot simulator and microstructuralanalyses, are needed to plot the CCT diagram of steels. The sampleswere machined to dimensions of 8 mm in diameter and 12 mm inlength and were heated to 1000 °C at a rate of 5 °C/s and were held atthis temperature for 300 s. They were then cooled to 200 °C at coolingrates of 0.1, 0.2, 0.5,1.0, 2.0, 5.0,10.0, and 20.0 °C/s, respectively. Duringthe cooling processes from 1000 °C to 200 °C, the dilatations of thespecimens were recorded on the simulator by the dilatometer. When
Fig. 2. (a) Metallograph of specimen 1 showing the microstructure consisting of ferriteand pearlite; (b) metallograph of specimen 8 showing the microstructure consisting ofpearlite and Widmanstatten structure.
Fig.1.Dilatation curves. (a) Sample1 at cooling rate of0.1 °C/s showingnoobvious vertices;(b) dilatation curve of sample 8 at cooling rate of 20 °C/s showing obvious vertices.
Table 1Microstructures, transformation types and temperatures for all specimens in the tests
3979G. Xu et al. / Materials Letters 62 (2008) 3978–3980
all the specimens had been tested on the simulator, their micro-structures were analyzed to determine their phase compositions.
3. Dilatometer tests
Dilatometer tests were conducted on a Thermecmaster_Z uni-versal-type simulator with laser-type dilatometer as described inSection 2. Two typical dilatation curves are given in Fig. 1, in whichFig.1(b) shows obvious inflection points (a–c), whereas Fig.1(a) showsan uncertain transformation point (b). As shown in Fig. 1(b), theinflection points are clear enough to determine the transformationtemperatures.
However, for the dilatation curve given in Fig. 1(a), it is difficult todetermine all transformation temperatures as the inflection point b isnot very obvious. The transformation temperatures determined bytangent method and vertex method also differ greatly. Therefore, it isnecessary to find a method to accurately determine the transforma-tion temperatures when a CCT diagram is plotted for certain steels. Foran experimental alloy steel, DSC (see Section 5) was used, in additionto dilatometer tests and microstructure examinations, to determinethe transformation temperatures.
4. Microstructural analyses
Except for dilatometer tests, microstructures for all the testedsamples should also be examined and determined to plot CCTdiagrams. Micrography of all specimens tested on the simulator wasdone by Zeiss optical microscope. The microstructures of specimens 1
Fig. 3. DSC result of sample 1 at a cooling rate of 0.1 °C/s showing the transformationtemperatures from austenite to ferrite (points a and b) and from austenite to pearlite(points b and c).
Fig. 4. CCT diagram for tested alloy steel plotted by dilatometer tests, microstructureexaminations and DSC analyses.
3980 G. Xu et al. / Materials Letters 62 (2008) 3978–3980
and 8 are given in Fig. 2. It can be observed that the microstructure ofspecimen 1 consists of ferrite and pearlite. The microstructure ofspecimen 8 is composed of pearlite andWidmanstatten structure. Themetallurgical structures of other samples are given in Table 1 (seeSection 6).
5. DSC tests
For the dilatation curves shown in Fig. 1(b), transformation typesand temperatures can be measured through dilatometer tests andmetallographic examinations. For example, the transformation typesof sample 8 are phase changes from austenite to pearlite and austeniteto Widmanstatten structure according to the dilatometer curves andmetallographs. The initial and finishing temperatures of the phasetransformation from austenite to pearlite are 630 °C and 565 °C, andthe phase transformation points from austenite to Widmanstattenstructure are 565 °C and 489 °C, respectively. However, it is difficult todecide a transformation point, such as the temperature at point b, inthe dilation curves shown in Fig.1(a). During the research and plottingof the CCT curve of an alloy steel, DSC was used to decide the trans-formation temperatures through the change of phase transformation
heat. One DSC result for sample 1 is given in Fig. 3, from which thetransformation temperatures from austenite to ferrite and austenite topearlite are 775 °C, 663 °C and 663 °C, 645 °C, respectively.
6. CCT diagram
The transformation types and temperatures for all specimens weredetermined by combination of dilatometer tests on a hot simulator,metallurgical structure examinations and DSC tests. The results aregiven in Table 1. The CCT diagram for the tested steel (see Fig. 4) wasplotted according to the results in Table 1.
7. Conclusions
For some steel grades, it is difficult to plot the CCT diagrams if onlydilatometer tests and microstructural examinations are available. Anewmethod of adopting DSC analyses in addition to dilatometer testsand metallographic analyses to determine the transformation tem-peratures has been proposed in this paper. The CCT diagram for analloy steel was plotted using this new method.
Acknowledgments
The authors acknowledge with gratitude the financial supportreceived from the Hubei Natural Science Fund (No.2007ABA244) andthe Research Institute of Baosteel.
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