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Mechanism of Scattering of Ultrasonic Waves at Passing Very Fine Pearlitic Steel Bernard Kopec Iron Works and Wiremills /ZDB a.s./, Dept. of Quality Assurance, 735 93 Bohumín, CZECH Republic Contact

The work carried out studying the relationship metalurgy - forming - heat treatment - structure - acoustic properties of wheel sets treated by normalizing and quenching showed a strong dependence of the attenuation on ultrasonic waves on the resulting structure of the product. On the basic of the measurements of attenuation in the structure of very fine pearlitic steel., five areas of structural factors influencing the value of the attenuation coefficient were found out, namely: the grain itself, grain boundaries, grain distribution, influence of history of grain origin, and influence of content of the ferritic component. On the basic of the analysis of these influences, the mechanism scattering ultrasonic waves during the pass through the pearlitic steel treated by normalizing and quenching is described.

Keywords: ultrasonic attenuation, railway wheel, pearlitic structure

Techniques and results

For mesuring attenuation in the mass of railway wheels, the pulse echo method and the contact acoustic coupling method were employed. The experiments were carried out using the ultrasonic flaw detector U S I P 11 manufactured by Krautkrämer. The probes having the frequency range from 2 to 15 MHz were used as well. Stable acoustic coupling was secured by gripping the indicator in a fixture which ensured uniform pressure distribution.

For measuring reflexion and attenuation cofficiets samples of the material used for production of railway wheels were prepared 25 mm and 50 mm thick. The chemical composition of the mass used for railway wheels respected the valid regulations inssued by the internationalRailway Union (U I C).

Corformably to the regulations mentioned, the railway wheels are put to normalizing or quenching procedures before cutting and machining. For experimental evidence the heat - treatment procedures were so planned that all feasible sample structures having diverse grain sizes might be obtained representing all major types of structure which were expected to result from the normalising and quenching process applied to railway wheels. Other sets of samples of the railway wheel material got through heat - treatment procedures of different kind in order that samples exhibiting various pearlite dispersity in pearlite might be collected (Figures 1 - 4).

Fig 1: Structure of sample with pearlite dispersity 1.0 mm (x 1000, etched with nital)

Fig 2: Structure of sample with pearlite dispersity 1.3 mm (x 1000, etched with nital)

Fig 3: Structure of sample with pearlite dispersity 1.0 m m(x 1000, etched with nital)

Fig 4: Structure of sample with pearlite dispersity 0.5 m m (x 1000, etched with nital)

The structure of railway wheel material may be considered to be considered to be that of pearlite and ferrite combined with the pearlitic component prevailing (Figures 5 - 8).The clean - cut continuous or interrupted network of ferrite constitutes the grain boundaries or the boundaries of pearlitic blocks.

Fig 5: Structure of sample with grain size 0.125 mm (x 1000, etched with nital)

Fig 6:Structure of sample with grain size 0.060 mm (x 1000, etched with nital)

Fig 7: Structure of sample with grain size 0.022 mm (x 1000, etched with nital)

Fig 8: Structure of sample with grain size 0.015 mm (x 1000, etched with nital)

The attenuation was measured by means of a difference between amplitudes of the first and of the second bottom echo with frequency ranging from 2 to 15 MHz. The attenuation coefficient was calculated from the formula, which takes into account both the lasses caused by the ultrasonic field geometry and the reflection losses.

Discussion

The results gained shovwed direct relationship between the heating temperature and the speed of cooling - down as constituting components of the heat - treatment procedure on the one hand and the level of attenuation of ultrasound on the other. The heathing temperature exert a predominant influence in formation process of the structure of the railway wheel material and determine in this way spreading of ultrasound in steel. With the intention coefficient of ultrasound, the special controlled method for heat - treatment of railway wheels was was set up.

When the influence of pearlitic dispersity should be investigated with heterogenous pearlitic structures, the diverse pearlitic interlamellar spacing combined with the grain size of the structure remaning constant can be acheieved by heathing the whole group of samples to the same austenitizing temperature which eliminates the grain size effect and by subsequent cooling indivivdual samples at different speeds.

The faster the process of cooling proceeds after dwell at annealing temperature has elapsed, the less the interlamellar spacing among cementite lamellas in pearlite, and the more conspicuously the attenuation of ultrasonic waves decreases. On the other hand, the more the pearlitic interlamellar spacing in material that has been cooled slowly increases, the more the value of attenuation coefficient increases, too. The value of attenuation coefficient does not, however, increase absolutely but respecting the average grain size of the structure [1].

In accordance eith previous experients [2], it can be stated that the grain size of the structure is the primary and decisive factor dcontrolling the level of ultrasonic attenuation which has been discivered in the scattering component. In the Rayleigh area the scattering component of attenuation grows proportionally to the cube of the average grain size. Tiny variation of the grain size result in large changes of the attenuation coefficient.

In case the average grain size does not vary, the attenuation in a pearlitic structure depends on the value of pearlitic interlamellar spacing, the increase of which is followed b the rise of the attenuation.

Based on the comparison of railway wheel structure with prevailing pearlitic component the evidence was gained for the assertion that, if the grain size is kept constant, the attenuation increases proportionaly to the ferrite ratio in the structure.

The increase, however, cannot be considered absolute. With railway wheel material the importance of the ferritic component of the structure for ultrasonic wave spreading consits primarily in the fact that the ferritic component occurs as continuous ferritic network surrouding the pearlitic blocks and causing the conpicious increase of attenuation.

Grain boundaries represent anotherstructural factor which affects the level of the ultrasonic attenuation in pearlitic steel. In the pearlitic structure of railway wheels pearlitic block boundaries are distinctly defined by segregates of ferrite assuming the shape of continuous nettwork.

Consequently, the final scattering depens even on the way in which the particular grain size was reached. The scattering component of attenuation is changed by the forming process even faster than one would expect with respect to the grain reduction reached. This phenomenon can be explained by the fact that in the forming process new boundaries arise the properties of which differ in comparison with boundaries separating orginal grains.

Mechanism of scattering of ultrasonic waves at passing very fine pearlitic steel

The structure of the wheel set rim, surface hardened in water, consists of very fine pearlite in the surface layer transiting into fine pearlite towards the depth, medium pearlite up to the medium coarse pearlite in the central part of the body of the wheel rim. The mechanism of the ultrasonic beam pass can be described as follows:
• I. Grains of very fine pearlite are on the surface layer of the wheel rim. Hence the ultrasound enters directly into the grain in the greater part of the surface without meeting the grain boundaries. Described influence of the grain substructure occur during the pass, particulary the pearlite dispersity, and eventually atoms of alloying additives, armount of inclusion, and - to a lesser degree - dislocation, twins and subgrains.

• II. After the passage trough the surface grain, in fact throught its part, it comes to an interaction with the grain boundary, which is, in the case of the steel investigated, formed by ferritic natwork developed in a lesser or greater degree. The beam passing trough a grain. The attenuation of the beam is particulary strongly influenced by the tickness of the ferritic layer. The attenuation grows with increasing thickness of the grain boundary.

• III. The attenuated beam enters into the nex grain with lower energy and the scattering mechanism occurs as described in point I. relating to the whole grain already during the pass until meeting of the next grain boundary.

During the pass through individual layers of the wheel rim, the growth of the attenuation occurs gradually with increasing coarseness of structure. Due to the fact that coarseness is gradual and the character of the pearlitic structure allows the pass of ultrasound very well, even if it is a structure with greater grain diameter, the attenuation does not influence the identification and determination of the eventual internal defect, even when testing by means of a probe the frequency 10 MHz.

Conclusion

The measurement of ultrasound attenuation was carried out by the pulse echo method. The attenuation coefficient depends on the frequency which causes the necessity of working with relatively long impulses with narrow frequency spectrum during the impulse measuring or of analysing the short impulses in the frequency zone. It was found out that structural factors can be analyzed with sufficient accuracy at the frequency of 10 MHz and that even the use of such hight freguency, which is not current in ultrasdound defectoscopy, will not cause difficulties in the detection of macrostructural defects in wheel sets. The mechanism of scattering ultrasonic waves during the pass through the pearlitic steel can be described as three steps.

References

1. Kopec, B. Characterization of very fine pearlitic structure using ultrasonic attenuation technique. In Proc Ultrasonics Inter 87 London, Kensington Town Hall (1987) 383 - 358.
2. Papadakis, E.P. Revised grain - scattering formulae and tables. J Acoust Soc Am (1965) 37 703 - 710.

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