EVALUATION METHODS OF DILATOMETER CURVES...

21. - 22. 11. 2012, Plzeň, Czech Republic, EU

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,

Paulínska 16, 91724 Trnava, Slovakia

[email protected], [email protected]

Abstract

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).


Tab. 1 Results of various dilatometer curve evaluation methods in illustrative comparison.

method specific volume change three tangents minimal

deviation derivative

1 a 99 % 5 a 95%

AC1 [°C] 749 753 756 748 749

AC3 [°C] 781 778 778 782 780

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, ACBCtr /% ).

Right – there is method of three tangents shown in use.

Fig. 2 Transformation temperature as the intersection of linear section and tangent at point of 10° deflection.


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).


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


80

85

90

95

100

105

110

675 700 725 750 775 800 825 850 875

Temperature [°C]

Th

erm

al

ex

pa

ns

ion

[m

m],

Alp

ha

[1

0-6

K-1

]

AC1

AC3

dilatation

alpha-physical

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.


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.

80

85

90

95

100

105

110

50 150 250 350 450 550 650

Temperature [°C]

Th

erm

al

ex

pa

ns

ion

[m

m],

Alp

ha

[1

0-6

K-1

]

212.1°C

445.9°C 16.493mmdilatation curve

alpha-physical

alpha-physical (smoothed)218.7°C

Fig. 8 Dilatometric curve evaluation - Bs and Ms temperatures determination by various techniques.


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.1.05/2.1.00/03.0077 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 0-87170-415-3

[2] Boyer, H. (Ed.), Atlas of Isothermal Transformation and Cooling Transformation Diagrams, ASM 1977, ISBN 0-

87170-043-3

[3] Atkins, M., Atlas of continuous cooling transformation diagrams for engineering steels, ASM 1977, ISBN 0-87170-

093-X

[4] Yang, H.-S., Bhadeshia, H. K. D. H., Materials Science and Technology, 2007, VOL 23, NO 5, p.556-560

[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 13th International Metalurgical & Materials Conference, ISBN 80-85988-95-X

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