Effect of microstructure on attenuation mechanism of ultrasonic waves ...

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sound velocity and the microstructure. Keywords: Carbon steels, Microstructure, Ultrasonics. Introduction. The study of the ultrasonic wave propagation in metals.
Effect of microstructure on attenuation mechanism of ultrasonic waves in carbon steels P. Behjati*1, H. Vahid Dastjerdi2, R. Mahdavi2 and D. Rasouli1 In this paper, fundamental concepts of ultrasonics and characteristics of distinctive microstructures have been used to simply explain the effect of microstructure on the attenuation mechanism of ultrasonic waves in carbon steels. In addition, it has been shown that application of the second medium hardness instead of the bulk hardness is more appropriate to correlate the sound velocity and the microstructure. Keywords: Carbon steels, Microstructure, Ultrasonics

Introduction The study of the ultrasonic wave propagation in metals and alloys provides valuable information about the microstructures,1,2 mechanical properties,3 thermomechanical history of the material,4–7 etc. When an ultrasonic wave travelling through one medium impinges on the boundary of a second medium, a portion of the incident acoustic energy is reflected back from the boundary, while the remaining energy is transmitted into the second medium. Intensity and travel time of the reflected beam (sound pulse) is used in non-destructive ultrasonic testing (UT) to measure the thickness of mill products and to estimate the type, size and position of discontinuities. Therefore, determination of the factors affecting the intensity of the sound pulse within a material is a challenging task for the industrial applications. Characteristics that determine the amount of reflection are the acoustic impedance difference of the two materials on either side of the boundary and angle of incidence of the ultrasonic wave at the boundary. The acoustic impedance for a longitudinal wave (used in this work) Zl, given in grams per square centimetre second, is defined as the product of material density r, given in grams per cubic centimetre, and longitudinal wave velocity Vl, given in centimetres per second8 Zl ~rVl

(1)

Sound velocity depends on the elastic properties and the density of the medium through which it is propagating. If the impedances of the two materials are equal, there will be no reflection; if the impedances differ greatly (as between a metal and air, for example), there will be virtually complete reflection.

The intensity of the ultrasonic beam that is sensed by a receiving transducer is considerably less than that of the initial transmission. Factors which are primarily responsible for the loss in beam intensity within a material (attenuation) can be classified as transmission losses, interference effects and beam spreading. Beam spreading losses involve mainly a transition from plane waves to either spherical or cylindrical waves, depending on the shape of the transducer–element face. Beam spreading is not dependent on material microstructure and therefore is not discussed in this paper. Transmission losses include absorption, scattering and acoustic impedance effects at interfaces. Interference effects include diffraction and other effects that create wave fringes, phase shift or frequency shift. Among the above factors, transmission losses have been shown to be strongly dependent on the microstructural features. Formation of a new microstructural constituent creates some acoustic boundaries. Interference of these boundaries with a travelling wave depends on the nature of boundary. Earlier studies reveal that there is a direct relationship between attenuation and bulk hardness of the material.2–4,9,10 In addition, an increase in grain boundary area, which means a decrease in grain size, results in large scattering of ultrasonic waves.11,12 Although several studies have been performed to correlate the attenuation of ultrasonic waves with the microstructure, the existing relationship between the microstructure and the attenuation mechanism has not been discussed so far. In the present paper, fundamental concepts of the ultrasonics and the characteristics of different microstructures in carbon steels are used to explain the origin of attenuation.

Experimental 1

Department of Materials Science and Engineering, Sharif University of Technology, Tehran 11365-8639, Iran 2 Department of Materials Engineering, Isfahan University of Technology, Isfahan 14156-2752, Iran *Corresponding author, email [email protected]

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ß 2010 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 30 March 2009; accepted 1 July 2009 DOI 10.1179/026708309X12495548508509

In this work, Ck45 and X210Cr12 steels were used to investigate the effect of microstructure on the sound velocity in carbon steels respectively. Composition, grain size, hardness and austenitisation condition of

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the raw materials are given in Table 1. The raw materials in the form of hot rolled bars of 100 mm diameter were cut perpendicular to the rolling direction to 10 mm thick samples. Then, they were austenitised and quenched in water, oil and air media, separately, to produce different microstructures. After that, the heat treated samples were ground to obtain suitable surface roughness (0?8 min) and parallelism (¡0?02 mm). On each sample, sound velocity was measured on five different positions, and the average value was reported. Before measuring the sound velocity, exact thickness of the sample in the position of the probe was determined using a micrometer. Then, the couplant, ZG-F gel from Krautkramer Co., was applied on the surface of the sample. The pulse echo technique was used to determine the sound velocity in the samples. Ultrasonic wave was produced using Quantum TE UT equipment and a 5 MHz normal transmitter–receiver probe with a 4 mm effective diameter. The measured thickness was given to the equipment as the input, and the sound velocity was calculated using equation (2) as the output V ~S=t

(2)

where V is the sound velocity, S the sound path (two times thickness) and t the flight time of the wave. The specimens for optical microscopy were prepared from the longitudinal cross-sections of the samples. On each specimen, 10 HV values were taken, and then a mean value for each group was determined.

Results and discussion Since characteristics of distinctive microstructures differ considerably in carbon steels, a variety of mechanisms may be applicable to explain the origin of ultrasonic wave attenuation in this group of alloys. Conventional microstructures of carbon steels include mainly ferrite, ferrite–pearlite, bainite, lath martensite and plate martensite.

Ferrite and ferrite–pearlite Absorption of ultrasonic energy in a substance occurs mainly due to the atoms’ elastic motion resisting the sound wave that is trying to move them from their rest position. Increase in the elastic motions promotes conversion of the wave mechanical energy into heat. Reitinger et al.13 observed that elastic properties of steels decrease with the increase in hardness. This suggests that absorption effect is more pronounced in the soft and ductile microstructures (ferrite and pearlite) than the hard and brittle microstructures (bainite and martensite). Therefore, absorption effect is expected to be greater in ferrite–coarse pearlite microstructure than ferrite–fine pearlite microstructure, which is confirmed by the works of Gu¨r and Tuncer1 and Reitinger et al.13 It should be noted that, although this effect is generally

1 a microstructure of water quenched sample of CK45 and b sound velocity measurements versus bulk hardness for different microstructures of CK45

less in metals than in rubbers and plastics, absorption losses increase directly with frequency of the wave.

Lath martensite The predominant type of martensite in Fe–C alloys with up to about 0?6%C is the lath type. In these alloys, martensite consists of domains which have groups of roughly parallel lathes. Figure 1a shows the martensitic microstructure of the water quenched sample of CK45, which consists predominantly of lath martensite. The structure within the martensite laths is highly distorted, consisting of the regions with high densities of dislocation tangles.14 Figure 1b indicates the wave velocity measured in different microstructures of CK45. It is clearly observed that the martensitic microstructure has the highest hardness and the minimum velocity. It has been reported that sound velocity decreases due to the increase in dislocation density, lattice distortion and residual stress.15,16 This means that acoustic impedance of the lath martensite is less than that of the surrounding matrix, and thereby, the lath surface can reflect (acoustic

Table 1 Composition, grain size, hardness and austenitisation condition of CK45 and X210Cr12 steels used in this work Composition, wt-%

Austenitisation

Steel

C

Mn

Si

P

S

Cr

MozNi

Fe

HVN

Grain size, mm

Temperature, uC

Time, min

CK45 X210Cr12

0.45 2.6

0.7 0.71

0.25 0.47

0.005 0.02

0.012 0.009

… 9.9

… 0.22

Bal. Bal.

220 248

32 …

860 980

25 40

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the scattering and mode conversion of an obliquely incident beam on the same interface. With increase in temperature, dislocation density, lattice distortion and residual stress decrease, resulting in the increase in sound velocity. In other words, increase in temperature decreases the acoustic impedance difference. Therefore, attenuation is expected to decrease after tempering or stress relieving processes, being consistent with the earlier observations.2,6,17

Bainite Upper bainite is comprised of lath bundles or sheaves, while lower bainite is in the form of individual plates. The common feature of the different bainite structures is that they all contain dislocation rich ferrite. In addition, the bainite transformation causes a relief effect similar to the martensite transformation.14 Owing to higher formation temperature, internal structure of the bainite has less dislocation density and distortion than the martensite, and thereby, less acoustic impedance difference is created. In fact, attenuation mechanism of the bainite is virtually identical to the tempered lath martensite. Gu¨r and Tuncer1 reported such a similarity in the attenuation measurements of the bainite and the tempered martensite microstructures of 1040 and 4140 carbon steels.

Plate martensite

2 Effect of martensite on ultrasonic wave beam after a normal incidence and b oblique incidence where IL is longitudinal incident beam, IL longitudinal deflected beam, It transverse deflected beam, IT and I9L longitudinal transmitted beam, I9t transverse transmitted beam, IR longitudinal reflected beam, Z1 acoustic impedance of matrix medium and Zm acoustic impedance of martensite

impedance effect) and deflect (scattering effect) some portions of the ultrasonic beam. In addition, mode conversion of the incident beam tends to occur at the lath surface due to the slight difference in acoustic impedance across the interface. Figure 2a schematically shows the partial reflection of a normally incident beam from the martensite–matrix interface. Figure 2b shows

Above 1%C, the microstructure of Fe–C alloys is found to be exclusively consisted of plate martensite (black regions in Fig. 3c–e) and retained austenite (white regions in Fig. 3c–e). The effect of the plate martensite on transmission losses is expected to be more intensive than the lath martensite, since: (i) owing to higher carbon content and required cooling rate, lattice of the plate martensite is more distorted than that of the lath martensite (ii) formation of the plate martensite is accompanied by higher amount of residual stress in comparison to the lath martensite (iii) internal structure of the martensite plates consists of very fine parallel twins. It has been stated that twin boundaries reduce the effective grain size18 (iv) in contrast to the lath martensite, plate martensites grow along high index habit planes in the austenite ({225}, {259}), leading to a large number of possible orientations within a same grain. The above discussions are briefly presented in Table 2. Considering the microstructural constituents other than austenite as the second medium, it is observed that the acoustic impedance difference DZ increases with the hardness of the second medium as a result of the decrease in sound velocity. For low and medium carbon

Table 2 Effect of microstructure on attenuation mechanism Microstructure

Attenuation mechanism

Hardness

VL

DZ

Mainly ferrite Ferrite–pearlite Bainite

Absorption effect Absorption effect Acoustic impedance and scattering effect

Min.

Max.

Min.

Max.

Min.

Max.

Lath martensite Plate martensite

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3 a sound velocity measurements versus bulk hardness of X210Cr12 after quenching in different media, b effect of retained austenite on bulk hardness of carbon steels and c, d and e microstructures of X210Cr12 after quenching in air, oil and water respectively

steels, bulk hardness of the material increases directly with the hardness of the second medium. The inverse relationship between the sound velocity and the bulk hardness has been observed in the earlier investigations.1,4,5 However, in the case of high carbon steels due to the considerable amount of the retained austenite, this relationship does not exist. Figure 3a evidently shows that the water quenched sample of X210Cr12 has the minimum velocity but not the maximum bulk hardness. Although the increase in carbon content and cooling rate increases the hardness of the martensite plates, it also promotes the amount of retained austenite. The presence of the retained austenite decreases the bulk hardness of the material (Fig. 3b).

As shown in Fig. 3c–e, oil quenched sample has the minimum retained austenite and the maximum bulk hardness. Therefore, application of the second medium hardness instead of the bulk hardness is more appropriate to correlate the sound velocity and the microstructure.

Conclusion In this paper, fundamental concepts of ultrasonics were correlated with the characteristics of distinctive microstructures in carbon steels. It was observed that, by understanding this correlation, one can simply explain the effect of microstructure on the attenuation mechanism of ultrasonic waves.

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