EVALUATION METHODS OF DILATOMETER CURVES OF PHASE TRANSFORMATIONS

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1 EVALUATION METHODS OF DILATOMETER CURVES OF PHASE TRANSFORMATIONS Petr MOTYČKA 1, Michal KÖVÉR 1,2 1 COMTES FHT a.s., Průmyslová 995, Dobřany, Czech Republic, 2 Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology in Trnava, Abstract Paulínska 16, Trnava, Slovakia petr.motycka@comtesfht.cz, michal.kover@comtesfht.cz The paper deals with the evaluation of dilatometer curves for the construction of CCT diagrams of steels. Information value of determined starting and finishing transformation temperatures due to evaluation methods and technological purposes of its measurement is examined. Results of methods using tangents intersections, relative change in volume and derivatives are compared. The influences of other physical and chemical processes accompanying transformations are discussed. In conclusion, recommendations of evaluation methods for the specific purpose of the analysis are given. Key words: Dilatometer, phase transformation, CCT diagram 1. INTRODUCTION The dilatometry is a popular method of phase transformation analysis of steel. There were released many extensive atlases of CCT diagrams based on dilatometer data [1]. These diagrams serve excellently as maps for the basic procedures of heat treatment of steels. For special applications, the material often behaves differently than would be expected according the diagram, which was not measured in our own laboratory. And this is true even for working with the material of comparable chemical composition, austenite grain size and homogeneity of austenite. Therefore it is necessary to think about the choice of proper method and the influence of physical and chemical processes that precede, accompany or follow the phase transformation, in determination of the beginning and end of transformation. 2. EVALUATION METHOD EFFECT Transformation temperatures are the boundary points of an interval of a step change in length, which is caused by the transformation. Smaller or larger differences in the transformation temperatures determination may be caused by the choice of the evaluation method. To compare the results of various methods we come out of the specific volume change according to Fig. 1. Transformation temperatures then may be evaluated as 1% and 99% or 5% and 95% of length jump. Finding intersections of three tangents interleaved with linear sections below and above transformation and with inflexion point during transformation is the simplest way, common in practice to determine the transformation temperatures ( Fig. 1). Finer way of evaluation is to find minimal given deviation of dilatometer curve from linear course (Fig. 2). Determination of transformation interval with expansion derivative is very sensitive as well (Fig. 3).

2 Tab. 1 Results of various dilatometer curve evaluation methods in illustrative comparison. method specific volume change three tangents minimal derivative 1 a 99 % 5 a 95% deviation AC1 [ C] AC3 [ C] Fig. 1 Left - lever rule application for the evaluation of dilatometer curve (solid line). At the temperature indicated dot-and-dash point A corresponds to extrapolated expansion of fully austenitic sample, point C corresponds to extrapolated expansion of the sample with initial phase composition, point B corresponds to real thermal expansion and point D corresponds to the progress of specific volume change (it is located on the dashed curve running from 0% before start to 100% after finish of transformation, % BC AC Right there is method of three tangents shown in use. ). tr / Fig. 2 Transformation temperature as the intersection of linear section and tangent at point of 10 deflection.

3 Fig. 3 Transformation temperatures are shown as points of decrease of expansion first derivative below the minimum value found in the section where the derivative is approximately constant. Derivatives were calculated as ratios of elongation and temperature differentials in the temperature area of 1, 2 and 5 C width. After rounding released AC1=749 C and AC3=780 C regardless of the various derivative smoothing. Tab. 1 and Fig. 1 to Fig. 3 simply illustrate the advantages and weaknesses of each method. Method of three tangents is simple in terms of data processing, it defines the interval of temperatures, where the transformation takes place certainly in a substantial part of a sample volume. Points of deviation from the linear shape and use of derivatives specified wider transformation temperature intervals. These evaluation procedures are more appropriate for searching of conditions under which the transformation has not yet started or has been already finished. Evaluation based on the progress of change in specific volume provides transformation temperatures with chosen sensitivity, moreover it allows observe quantitatively the dynamics of transformation. 3. INFLUENCE OF SAMPLE AND MEASUREMENT CONDITIONS Accuracy of derivative method was already mentioned in chapter above. The problems in this method (and all others) may arise if the curve is too noisy (Fig. 4, Fig. 5). If the transformation is overlapped with some other processes (Fig. 5, Fig. 6) such as recovery and recrystallization, retained austenite transformation or carbide dissolution / formation, this method should be couple to another one. The heterogeneity of the sample composition, slow kinetics of phase transformation leading to very slight length change and sampling frequency should be considered as well (Fig. 6). For the noisy curve, the problem can be sometimes partially solved by smoothing of the raw curve (Fig. 7).

4 Fig. 4 Example of determination of Ms temperature from the derivative curve - uncertainties due to noise and nonlinearity of the dilatometric curve [4].

5 110 Thermal expansion [mm], Alpha [10-6 K -1 ] alpha-physical dilatation A C1 A C Temperature [ C] Fig 5 Determination of the Ac1 and Ac3 temperatures from dilatometric curve and Alpha-phys curve - uncertainty due to transformation overlap and noise. In Fig. 3, more processes seem to be overlapped (two peaks at the beginning of the phase transformation) and the determined Ac3 temperatures are slightly lower / higher in comparison to already known values from literature [5]. Fig. 6 Secondary information - power and temperature change, and smoothing can be helpful tools for the evaluation.

6 In some cases, the secondary information can be very useful. This can be seen in Fig. 6 for determination of the Curie transformation peak plots of power and comparison of setup temperature to real temperature during heating were very helpful. Fig. 7 Raw data variation of sampling frequency, noise can be seen. With lower cooling rate the noise decreases. 110 Thermal expansion [mm], Alpha [10-6 K -1 ] C alpha-physical dilatation curve C alpha-physical (smoothed) Temperature [ C] C mm Fig. 8 Dilatometric curve evaluation - Bs and Ms temperatures determination by various techniques.

7 The different evaluation methods may be used simultaneously, to get better results (Fig. 8). The Bs temperature cannot be determined from the Alpha-phys curve, however it can be clearly seen in the dilatometric curve. Therefore Bs temperature is determined from the dilatometric curve. Ms temperature was determined from both dilatation and Alpha -phys curve. Since Ms in Alpha -phys curve can be observed already at higher temperature, value from Alpha-phys curve was used for Ms temperature determination. 4. CONCLUSION Accuracy of CCT curves depends on measured dilatometric curves and accuracy of determination of transformation points. Results of dilatometric measurements can be influenced by method used for evaluation, material properties and measurement conditions. This influence should be considered by evaluation of the dilatometric curves. According to wide range of materials, their thermal properties and therefore curve shapes, none of the mentioned methods can be used as absolute. By evaluation of dilatometric curves individual approach should be used. In some cases the evaluation by more than one method can lead to acquirement of better results and higher accuracy. During the evaluation all known information should be used- such as composition, initial microstructure, microstructure after heat treatment and knowledge from scientific articles. This could be considered to be the way leading to higher accuracy of CCT diagrams. The verification by comparing to other CCT diagrams can be very useful as well. ACKNOWLEDGEMENTS The results presented in this paper arose under the project West-Bohemian Centre of Materials and Metallurgy CZ.1.05/2.1.00/ co-funded by European Regional Development Fund. REFERENCES [1] VOORT, G. F., Atlas of time-temperature diagrams for irons and steels, ASM International 1991, ISBN [2] Boyer, H. (Ed.), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, ASM 1977, ISBN [3] Atkins, M., Atlas of continuous cooling transformation diagrams for engineering steels, ASM 1977, ISBN X [4] Yang, H.-S., Bhadeshia, H. K. D. H., Materials Science and Technology, 2007, VOL 23, NO 5, p [5] Šmátralová, M., et al., MICROSTRUCTURE PROPERTY RELATIONSHIP IN A 15NiCuMoNb5 STRUCTURAL STEEL FOR BOILER DRUMS AND,In METAL 2004 Hradec nad Moravicí. Proceedings of the 13 th International Metalurgical & Materials Conference, ISBN X