(898) ISIJ Activity Report. Committee,

Transcription

1 Method of Ultrasonic Angle Beam Examination for Welds of Ferritic Steels with Acoustic Anisotropy* -Activity Report of Non-destructive Inspection Subcommittee, Quality Control The Joint Research Society, ISIJ- Committee, By Keiji IBA** Synopsis Angle beam ultrasonic testing has been applied to the steel weld based on isotropy of steel. However, in the test of textured steel, the problems arise sometimes in detectability and location estimation of flaws. These problems are considered to be a result of acoustical anisotropy of material. Many researchers have investigated the ultrasonic characteristics in the anisotropic media. The present work reports the acoustical anisotropy of textured steel plates, and ultrasonic characteristics of the anisotropic steel plates. The effects of anisotropy on the angle beam ultrasonic test are discussed. As a result, an effective ultrasonic testing method of textured steel was developed and applied to weld specimens of anisotropic steel plates. It was made clear that the method was effective for detection of weld flaws in steels. Key words: ultrasonic testing; ferritic steel; weld; manual testing; anisotropy; thermomechanical controlled process; angle beam method; quality assurance; offshore structure; standard group study. I. Introduction Steels produced through the thermo-mechanical controlled process (TMCP) have excellent low-temperature toughness and satisfactory weldability even in the as-rolled plates; hence it is used for ship-building and offshore structure construction.' To guarantee the weld soundness of these steel, non-destructive examinations such as ultrasonic angle beam examination and radiographic examination are widely used. For offshore structures, including oil rigs, ultrasonic angle beam examination is frequently the final quality assurance means because of their complicated structures. In some cases of ultrasonic angle beam examination of TMCP steels, it has become evident in recent years that, reduction of the sensitivity occurs in some propagation directions of ultrasonic waves, and the detection of the flaws becomes impossible by the conventional method such as specified by JIS Z 3060 and Thus the differences in the ultrasonic propagation characteristics between TMCP steels and conventional steels are pointed out. Ultrasonic angle beam examination of steel welds has been standardized and executed on the basis of elastic isotropy, because the conventional steels are rolled above 900 C and texture is randomized in the phase transition of r to a. However, some types of TMCP steel plates show elastic anisotropy in dependence on the TMCP condition. Hereinafter these steel products are referred to as " anisotropic steels ". Problems for ultrasonic angle beam examination of these steel welds arise, if elastic anisotropy of these steel plates exceeds a certain limit.2,3~ Therefore, it is necessary to establish an ultrasonic angle beam examination method applicable to the welds of anisotropic steels to maintain a quality assurance level. The Non-destructive Inspection Subcommittee in Quality Control Committee of the Joint Research Society of The Iron and Steel Institute of Japan started a Working Group (WG) for Investigating Ultrasonic Angle Beam Examination Method of TMCP Steels to solve this problem. The WG consists of the members from five Japanese steel mills : Nippon Steel Corp.; Nippon Kokan K.K.; Kawasaki Steel Corp.; Sumitomo Metal Ind., Ltd; and Kobe Steel, Ltd. Propagation characteristics of the ultrasonic waves in anisotropic steels were reported in the past.4-8) Activities of the WG were to clarify the propagation characteristics in the anisotropic steels and to propose an ultrasonic angle beam examination procedure applicable to the welds of these steels. II. Propagation Characteristics of Ultrasonic Waves in Anisotropie Steels 1. Difference due to Propagation Direction of Ultrasonic Waves In ultrasonic angle beam examination, no variation arises in refraction angle or echo height with the propagation direction for isotropic steels, but the influence of the propagation direction of ultrasonic waves is significant for anisotropic steels. Figure 1 shows an example of the propagation direction dependence of echo height from a side drilled hole (c2.4 mm) in a highly anisotropic steel specimen examined by nominal 60 and 70 probes. The actual refraction angle varies with the propagation direction; the actual refraction angle is larger than the nominal angle in the rolling direction (L-direction), while it is smaller in the transverse direction (C-direction). The echo height with the nominal 60 probe is nearly equal in the L-direction and in the C-direction, whereas the echo height with the nominal 70 probe is much lower in the L-direction, and the position of its maximum amplitude is unclear. Transmitted pulse amplitude and actual refraction angle of various types of anisotropic steels were investigated by the V-path method, shown in Fig. 2, with various nominal refraction angle probes. The results * ** Originally published in T etsu-to-hagane, 73 (1987), 616, in Japanese. English version received on April 24, ISIJ Chairman, Non-destructive Inspection Subcommittee, Quality Control Committee, The Joint Research Society, ISIJ; Sheet and Strip Technology Department, Kawasaki Steel Corporation, Uchisaiwai-cho, Chiyoda-ku, Tokyo 100. (898)

2 Transactions ISIJ, Vol. 27, 1987 (899) Fig. 2. Measurement of refraction by V-path method. angle Fig. 1. MA scopes for anisotropic steel plates. Fig. 3. Transmitted pulse amplitude in each testing direction relative to the C-direction in anisotropic steels. are shown in Figs. 3 and 4. Figure 3 shows the ratio (4 V) of transmitted pulse amplitude in each propagation direction relative to the C-direction. At refraction angles of 45 and 60, almost all specimens show nearly an identical transmitted pulse amplitude in all directions. However at a refraction angle of 70, the transmitted pulse amplitude become lower in many steel specimens as the direction deviates from the C- direction. Figure 4 shows the difference of refraction angles measured with isotropic steel specimens (standard refraction angle) and measured with anisotropic steel specimens (actual refraction angles). The deviation of actual refraction angle from the standard is negative in the L-direction at an angle of 45, whereas it is positive in the L- and negative in the C-direction above 60. Figure 5 shows examples of the MA scope measured by the V-path method; at a test frequency of 5 MHz for the upper figure and at 2 MHz for the lower. The straight lines downward to the right in both figures indicate the distance amplitude curves (DACs)

3 (900) Transactions Is", Vol. 27, 1987 Fig. 5. Results of the V-path anisotropic steel with angle beam probes. method for an 5 and 2 MHz Fig. 4. Deviation of actual refraction angle angle for anisotropic steels (5 MHz). from nominal of isotropic steel specimens. Even at a test frequency of 2 MHz, the transmitted pulse amplitude in the L- direction is low, and conversely the ultrasonic beam diverges to diffuse the position of maximum amplitude. In general, the transmitted pulse amplitude attenuates by scattering at grain boundary and it is recommended to lower the test frequency when an extensive attenuation is anticipated. Since TMCP is designed to refine the grain sizes, it is expected that attenuation arising from scattering at grain boundary will be low. Figure 6 shows examples of the DAC of side drilled holes in the isotropic and the anisotropic specimens. The echo height in the G-direction of anisotropic steel specimen is slightly higher than that of isotropic steel specimen, but the gradients of both DACs are same. On the other hand, the echo height in the L-direction of anisotropic steel specimen is lower than that of isotropic steel specimen but the gradient of both DACs are still the same. This evidence suggests that the reduction in echo height is not caused by the attenuation due to scattering at grain boundary, but by the Fig. 6. Distance amplitude curves of anisotropic steel (L-direction and C-direction) and isotropic steel. reduced generation of shear waves by refraction. 2. Influence of Acoustic Anisotropy on Examination 1. Accuracy of Flaw Location Estimation The error in flaw location estimation in anisotropic steel occurs when the standard refraction angle measured by IIW block is used, because the actual refraction angle in each propagating direction through the anisotropic steel differs from the standard refraction angle obtained with IIW block, as previously stated. Figure 7 shows the influence of the difference in refraction angle on the accuracy of flaw location estimation when anisotropic and isotropic steels with side drilled holes were examined. In this investigation,

4 Transactions ISIJ, Vol. 27, 1987 (901) the actual refraction angle was measured by the V- path method shown in Fig. 2. When the standard refraction angle was used to determine the flaw location for the anisotropic steel, the accuracy of estimated flaw depth was poor and the estimated flaw depth fluctuated significantly, particularly in the L-direction. These phenomena are considered to be caused by the difference in a degree of anisotropy and deviation of the actual refraction angle in each plate. On the other hand, Fig. 7 shows that the accuracy of flaw location estimation by using the actual refraction angle measured in each anisotropic plate is nearly same as that in the isotropic steel. 2. A Reference Block for Calibration of Sensitivity In order to investigate the influence of propagating direction in the anisotropic steel on sensitivity, reference blocks cut in the L-, Q and C-directions from anisotropic steel were compared with the isotropic reference block. For nominal refraction angles of 45, 60 and 70, the difference in echo height between those reference blocks, RB-4 (with a 03.2 mm side drilled hole), and STB-A2 (with a 04.0 X 4.0 mm vertical drilled hole specified in JIS Z 2348) is shown in Table 1. In the case of 45 probe, the sensitivity in each propagating direction is almost same between the anisotropic steel and the isotropic steel. In the cases of 60 and 65 probes, the sensitivity of RB-4 is almost equal between the isotropic steel and the anisotropic steels along the L- or Q-direction; however, it is 3 to 4 db lower in the anisotropic steel along C- direction than the isotropic steel. Therefore, when the anisotropic steels are examined the reference block to adjust the scanning sensitivity may be taken from any direction in either isotropic steel or anisotropic steel. However, when a block is taken from the C-direction of an anisotropic steel, it may be necessary to make a certain correction in the sensitivity. 3. Detectability of Artificial Flaws By using several isotropic and anisotropic steels, the variations of the echo height from artificial flaws were studied with the 60 and 70 probes. The welded specimens had the weld line perpendicular to the L-, Q; and C-directions and had X3.2 mm side drilled holes and 10 % square notch (N 10 notch specified in API RP-2X) in the welds as shown in Fig. 8. Table 2 compares the echo height from the side drilled hole or notch in each specimen with 04 X 4 mm vertical hole in STB-A2 in the use of 60 and 70 probes. Those results represent the detectability of the side drilled hole and notch when the scanning sensitivity was calibrated by ~b4 X 4 mm in STB-A2. For the 60 probe, each flaw in the anisotropic steel is detected with higher sensitivity than that in the isotropic steel. Table 3 shows the difference in echo height between the side drilled hole or notch in each specimen with reference reflector in RB-4 with the 60 and 70 probes. The results also show the detectability of the Fig. 8. Artificial flaw in welds (submerged arc welding). Table 1. Echo height difference between c3.2 mm side drilled hole and STB-A2 (~b4 X 4 mm) of each test specimen. ISIJ Activity Report

5 (902) Transactions ISIJ, Vol. 27, 1987 Table 2. Echo height difference between side d ri lled hole or notch and STB-A2 (~b4 x 4 mm) of each test specimen. Table 3. Echo height difference between side drilled hole or notch and RB-4 (c/i3.2 mm) of each test specimen. side drilled hole and notch when the scanning sensitivity was calibrated by X3.2 mm in RB-4. It is probable that the planar flaw perpendicular to a scanning surface such as square notch cause a mode conversion and a loss of reflection amplitude with the nominal 60 probe. The result shown in Table 3 shows that the echo height from a square notch in the anisotropic specimen is 3 db difference by the maximum in directions of the anisotropic specimen. However, this difference is within a dispersion of echo height that was encountered in the examination of isotropic steels. When a square notch is examined by the nominal 60 probe, the loss of reflection amplitude is smaller in the anisotropic specimen than the isotropic specimen. This may be due to the deviation of the actual refraction angle from the nominal refraction angle in the anisotropic steels. On the other hand, for the nominal 70 probe the echo height of the L-direction is small as shown in Table 3. Therefore the influence of the anisotropy on detectability is significant in the L-direction with the 70 probe. In other cases, the detectability comparable to isotropic steel is ensured provided the deviation of the refraction angle in the propagating direction is adequately taken into account. 4. Reduction of Undetectable Zone In the direct scan, the undetectable zone extends in thin plates with a relatively small refraction angle such as 60, because of the limited approach between the probe and weld. Therefore it is desirable to use a probe with the refraction angle probe as large as possible, but the 70 probe is not applicable to anisotropic steels. There remains the applicability of the nominal 65 probe to anisotropic steels. Actual Fig. 9. angle of the nominal 65 probe may be near 70 in the L-direction in anisotropic steel. Figure 9 shows an MA-scope by the V-path method for anisotropic steel with the nominal 65 and 70 angle probes. A straight line iii the figure indicates the DAC of the isotropic steel. In the L-direction of the anisotropic steel, the transmitted pulse amplitude is very small and the amplitude peak on MA-scope is indistinct with the 70 probe. However, for the 65 probe, the transmitted pulse amplitude is sufficiently high and the peak on MA-scope is sharp. Therefore it is possible to use the nominal 65 probe by making a certain correction of sensitivity. Furthermore the 65 probe is useful to reduce the undetectable zone in relatively thin plates. MA scopes for anisotropic steels (V-path method). III. Ultrasonic Propagation in Anisotropic Steel Plates The ultrasonic propagation in anisotropic steel

6 Transactions ISIJ, Vol. 27, 1987 (903) plates are analyzed by using a texture model. 1. Texture Model for Analysis The texture of anisotropic steels consisted of the preferred orientations of a partial <011 > fiber such as {100}(011>, {211}011>, and {311}<Oll>, as analyzed from X-ray diffraction. Therefore, a discrete textured model, where the < 110> axis of crystals coincides with RD (y) of the rolled plate and the <001> axis of the crystals deviated by 0 in angle from ND (z) of the plate was adopted. The relation between the axes fixed to the plate (x, y, z) and those fixed to the crystal (X, Y, Z) is shown in Fig. 10. The plate texture was considered to consist of the above discrete oriented crystals existing in the probability P and random oriented crystals existing in the probability (1-P). Because of the orthorhombic symmetry of rolled texture, the stiffness tensor C'3 of the oriented cyrstals with reference to x, y, z axis shows the same stiffness tensor as orthorhombic system. The stiffness tensor Ci3 is calculated by using stiffness elements of steel crystals. Voigt's mean (C~~) of the textured plate is calculated from the following equation, C2; = PC~3+(1-P)C2~...(1) where, C!~ : the stiffness element of isotropic steel. Reuss's mean (Si3) of textured plate is also calculated in the same way. Then the resultant stiffness values are obtained from Hill's mean (C?3) which is the average of C21 and SX Analysis of Sound Propagation with the Textured Model The plane wave propagating in a textured model is described as8~ (u, v, w) = (Ax, Ay, Az) exp {2iri(lx+mv+nzVt)/A}...(2) where, (u, v, w) : the displacement components in the coordinates (x, y, z) (Ax, Ay, Az) : the displacement amplitude vector of plane wave (l, m, n) : the direction cosines indicating propagating direction of plane wave V: the velocity A : the wave length. Replacement of C)3 with C2;, Hooke's law and the Fig. 10. Representation of crystallite orientation. equation of motion give the following equation. Al-pV2 ala2 ala3 Ax ala2 A2-pV2 a2a3 Ay = 0 ala3 a2a3 A3 - p V 2 Az... (3) where, Al = C1112+C66m2+C55n2 A2 = C C22m2+ C44n2 A3 = C5512+C44m2+C33n2 a2a3 = (C23 + C44) mn al.a3 = (C13+C55)nl a1a2 = (C12+C66)lm To satisfy the condition of (Ax, Ay, A)0, the eigenvalues of matrix in Eq. (3) is calculated to obtain three solutions of V2 correspond to a quasi-longitudinal wave and two quasi-shear waves. In the case of the wave propagating in the Z-L (z y) plane in the direction of angle o to z axis, the direction cosines of wave normal are 1=0, m = sin o and n = cos o, and three solutions are obtained as follows. pv2 = A1...(5) pv2 = {A2+A3+ ~' (A2-A3)2+4(ala2)2 }/2... (6) Equation (5) represents the shear wave with polarization normal to the Z-L plane, and Eq. (6) the longitudinal wave (the higher velocity) and the shear wave (the lower velocity) with polarization parallel to the plane. In the same way, the velocities of three waves propagating in the Z-C plane are calculated. It is known that the energy flow direction of sound wave in anisotropic steel is different from the normal direction of wave. The energy flow direction is normal to the curvature of slowness suface 5.2) In the case of waves in the Z-L plane the difference angle 4 of the directions between wave normal and energy flow is given by the following equation. tan 4 =1 /S. ds/d o...(7) In refraction from isotropic material to anisotropic material, the relation of wave normals of incident wave and refracted wave is given by Snell's law. Therefore, the wave normal of refracted wave at the interface of the material and angle beam probe is calculated by Snell's law : Fig. 11 shows examples of energy flow direction of wave refracted from a 60 probe to anisotropic steels. Then the energy flow direction is calculated by using Eq. (7). 3. Analysis Result The calculated velocity of shear wave with polarization parallel to the Z-L and Z-C planes on the basis of the texture model described above are shown in Figs. 12 and 13. Figure 12 shows the sound velocity changes in the Z-L and Z-C planes, which correspond to the examinations in the L- and G directions, respectively. The velocity changes in the planes

7 (904) Transactions ISIJ, Vol. 27, 1987 Fig. 11. Sound refraction in anisotropic steel (in the case of nominal refraction angle 60 ). Fig. 12. Calculated results of dependence of velocity on refraction angle. sound Fig. 13. Calculation of the flow direction from angle. deviations nominal of energy refraction qualitatively agree with the experimental results. Figure 13 shows the relationship between the energy flow direction and the nominal refraction angle calculated from the velocity curve on Fig. 12. In the L-direction examination, the actual refraction angle (energy flow direction) is larger than the nominal refraction angle in the higher range of refraction angles, on the contrary, it is smaller in the C-direction examination. These results well explain the phenomena observed in anisotropic steels. Figure 13 also shows that the beam diverges in a higher refraction angle in the L-direction examination, and the Rayleigh wave may occur at the nominal refraction angle of 70. These phenomena are assumed to be the cause of amplitude reduction of refracted wave with 70 probe. Iv. Method of Ultrasonic Examination for Anisotropie Steels From the above result, the method of ultrasonic examination for anisotropic steels should be as follows. (1) A 60 probe should be used instead of a 70 probe. (When a 60 probe is not applicable from geometrical limitation, a 65 probe is usefull.) (2) Scanning sensitivity should be adjusted with a reference block (RB-4) taken from the specimen or the material with similar acoustic characteristics. (3) The actual refraction angle in each propagation direction measured by the V-path method should be used for estimating flaw location. Degree of anisotropy can be estimated by d B measured by the V-path method shown in Fig. 2 (d8 means the absolute difference of the refraction angle between the L-direction el and in the C-direction Bc. ) or by the sound velocity ratio, V L f V c (V Ll V c means the ratio of the shear wave velocity which propagates to the thickness direction with polarization in the L- direction VL to that in the C-direction VC as shown in Fig. 14.) The relations between 40 and V L' V c are shown in Fig. 15. ISIJ Activity Report

8 Transactions ISIJ, Vol. 27, 1987 (905) Fig. 14. Measurement of ultrasonic wave velocity ratio by the shear wave method. Fig. 16. Relation between the estimated flaw in isotropic steel and anisotropic steel. echo heights Fig. 15. Relation between wave velocity ratio and refrac- tion angle deviation. V. Ultrasonic Examination for Sample with Natural Flaws Butt weld samples and mock up samples of offshore structure with natural and artificial flaws were prepared to confirm the detectability of these flaws by the above method. Fig. 17. Relation between the in isotropic steel and estimated flaw echo heights anisotropic steel. 1. Ultrasonic Examination for Butt Weld Samples Butt weld specimens of anisotropic steels with natural flaws were examined with a 60 probe, and normalized to reduce the degree of anisotropy, for further examination with a 70 probe. Figures 16 to 19 show the relation of flaw echo height and estimated flaw length with both examinations. Although there is a little difference in echo height between both results, the flaw detectability can be regarded as comparable, since the difference of incident angle to flaws may affect. Both results in estimated flaw length are also comparable. Furthermore, the deviation (4y and 4d) shown in Fig. 20 between the flaw location estimated by the above method and the actual flaw location measured Fig. 18. Correlation of flaw length measured steel and anisotropic steel. in isotropic

9 ( 906 ) Transactions ISIJ, Vol. 27, 1987 on macro section are shown in Figs. 21 to 23. Since the deviations in flaw location obtained from these methods were almost same, this method was proved to be able to estimate flaw location with the same accuracy in anisotropic samples as in isotropic sampies. Figure 24 shows a macro section of natural weld flaws. From the above results the flaw detectability and the accuracy of flaw location for anisotropic samples by using the method described in Chap. I V were same as those for isotropic samples by using the conventional method. 2. Ultrasonic Examination for Mock Up Samples of Offshore Structure Detectability of flaws in mock up samples of offshore structure was investigated. As shown in Table Fig. 19. Correlation of flaw length steel and anisotropic steel. measured in isotropic Fig. 20. Evaluation of the deviation between flaw location and actual flaw location. estimated Fig. 21. Deviation of estimated flaw location from actual flaw location (60 ). Fig. 22. Deviation of estimated flaw location (65 ). flaw location from actual Fig. 23. Deviation of estimated flaw location from actual flaw location (70 ) Activity Report

10 Transactions ISIJ, Vol. 27, 1987 (907) Fig. 24. Examples of natural flaw on cross section. Fig. 25. Mock up sample of offshore structure. 4, plates used for brace were 19 mm in thickness and the anisotropy of plates was low, moderate and high. A plate used for a chord was 30 mm in thickness and the anisotropy was moderate. All plates were formed into pipes so that the directions of their pipe axis agreed with the L-direction of plates. Six mock up samples were fabricated and contained natural flaws, incomplete penetration (IP) and lack of fusion (LF) at three positions of A, B and C, as shown in Fig. 25. Evaluation of flaws was done in accordance with JIS Z , JIS Z , API RP-2X-80 and ASME Sec. V Art. 5. Table 5 shows the results of flaw detection. Except for an IP at location A in a moderately anisotropic brace member and an IP at location B in a highly anisotropic brace member, the natural flaws in mock up samples were clearly detected regardless of the anisotropy of brace member, when the 45, 60 and 65 probes were used. Especially, the use of 65 probe was effective to detect an LF at location A. Difference in the echo height of an identical flaw between two testing groups was below 4 db. Table 6 compares the flaw length estimated according to the various specifications with the flaw length measured Table 4. Steel plates used for mock offshore structure. by radiograph. Figure 26 is a typical example of radiograph of an IP. Table 6 shows that a flaw length estimated by 25 % DAC of a ~b4 X 4 mm vertical drilled hole (STB- A2) is comparatively long but that one estimated by the 25 % DAC of a 2.4 mm side drilled hole (RB-4) or the 6 db down method of API is comparatively short. The same relation obtained from butt weld samples of anisotropic steel plates described in the Chap. I V is also shown in Table 6, from which the accuracy of flaw length estimated in the mock up samples is same as that in the butt welds samples. On the other hand the difference in estimated flaw length between two testing groups was below 18 mm

11 (908) Transactions ISIJ, Vol. 27, 1987 Table 5. Detectability of flaws. Table 6. Accuracy of flaw length applied specification. in accordance with Table 7. Accuracy of flaw location. Fig. 26. Radiograph of incomplete penetration at location B in low anisotropy leg pipe. for the flaws at locations A and B, and below 8 mm for the flaws at location C (cf. Fig. 25). The deviation between flaw location estimated with ultrasonic examination and the actual flaw location determined by macro section is shown in Table 7, and Fig. 27 is macro section. The flaws at locations A and B were estimated by using the actual refraction angle of L- direction, and the flaw location at location C with the actual refraction angle of Q -direction. It became clear that flaw location could be deteramined accurately by the use of the actual refraction angle. Difference in estimated flaw locations between two testing groups was below 2 mm in both d- and y- directions. VI. Conclusion The Non-destructive Inspection Subcommittee in Quality Control Committee of the Joint Research Fig. 27. Cross section of incomplete penetration at location A in moderate anisotropy leg pipe. Society of The Iron and Steel Institute of Japan has investigated to solve the problems of anisotropy which is encountered in the ultrasonic angle beam examination of some TMCP steel welds. The activities expanded to many areas. The data are collected for many steels rolled by various mill conditions. The conclusions are summarized as follows. (1) Acoustic characteristics in anisotropic steel: The actual refraction angle in anisotropic steel changes due to the degree of anisotropy and the propagation direction of wave. Moreover, when the examination is carried out in L-direction by the nominal 70 probe, considerable decrease of sensitivity is observed in some anisotropic steels. (2) Analysis of the phenomenon : The propagation characteristics of sound waves in anisotropic steels has been explained qualitatively by using an orthorhombic elastic anisotropy. (3) Evaluation of anisotropy: A part of TMCP

12 Transactions ISIJ, Vol. 27, 1987 (909) steels can be examined by the conventional method. Applicability of the conventional method is judged from the measurement of the actual refraction angles or the sound velocity ratio of shear waves polarized in L- and C-directions. When the difference between actual refraction angles measured L- and C-directions with nominal 60 probes exceeds 2, or sound velocity ratio exceeds 1.02, the steel shall be considered anisotropic. Such steels should be examined by the following method. (4) Examination method : The conventional examination method can be applied to anisotropic steels by limiting the nominal refraction angle up to 65. The location of flaws, however, shall be calculated from the actual refraction angle. (5) Verification of recommended examination method : The examination of natural flaws in butt weld samples and mock up samples of offshore structure was carried out to confirm the recommended method. As a result, the detectability and the accuracy of flaw location have been verified to be equivalent to those of the conventional method. The results of these activities were used to establish the standard of Japanese Society for Non-destructive Inspection, NDIS , and will also be used to the revison of JIS Z 3060 which is now under discussion. REFERENCES 1) Y. Moriyama: Symposium of Application of Newly Processed Steel (TMCP) to the Welding Structure, Soc. Naval Architects of Japan, Tokyo, (1984), 1. 2) M. G. Silk: Met. Sci.,15 (1981), ) A. Singh, J. Lankford, D. Davidson and Y. Hasegawa : Review of Progress in Quantitative Nondestructive Evaluation, 1, Plenum Press, New York, (1982), ) Y. Inagaki, K. Nishifuji, F. Oda and N. Takeda: Tetsuto-Hagane, 68 (1982), ) H. Kitagawa, H. Maruyama and F. Ichikawa: J. Soc. Mater. Sci., Jpn., 30 (1981), ) D. E. Brag and D. M. Egle: Met. Sci.,15 (1981), ) M. G. Silk: Research Techniques in Non-destructive Testing, IV, Academic Press, London, (1980), ) M. J.P. Musgrave : Crystal Acoustics, Holden-Day, San Francisco, (1970).