Effect of microstructures on magnetic Barkhausen noise level in the weld HAZ of an RPV steel

~ ELSEVIER

Journal of Magnetism and Magnetic Materials 196-197 (1999) 351-353

Journalof amnadgneUsm magnetic , ~ materials

Effect of microstructures on magnetic Barkhausen noise level in the weld HAZ of an RPV steel Joo-Hag Kim a'*, Duck-Gun Park a, Chi-I1 Ok a, Eui-Pak Yoon b, Jun-Hwa Hong a aReactor Material Department, Korea Atomic Energy Research Institute, Yusong P.O. Box 105, Taejon 305-600, South Korea bDepartment of Metallurgical Engineering, Hanyang University, Seoul 133-791, South Korea

Abstract Microstructural state and magnetic Barkhausen noise (BN) level have been correlated in the weld heat-affected zone (HAZ) of a pressure vessel steel. The BN level increased with increasing size of carbide, and the tempered bainite structure showed higher BN signal than the tempered martensite. The results indicated that heat-treated materials may result in microstructurally different domain wall pinning obstacles at different thermal cycles. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Weld heat-affected zone (HAZ); Barkhausen noise; RPV steel

Microstructural changes induced in a thermal or radiation environment often produce the degradation of the mechanical properties of steels. It is, therefore, necessary to develop a nondestructive evaluation (NDE) technique for microstructural characterization and monitoring degradation in order to assure the integrity of steel components such as reactor pressure vessel (RPV). It has been recently recognised [1,2] that a magnetic NDE method has a great advantage over other NDE methods because of its high susceptibility to the change of various metallurgical factors. The weld heat-affected zone (HAZ) of a heavy section RPV steel has a variety of microstructures such as martensite and bainite, and various sizes of grain and precipitates. Furthermore, the HAZ region has been known to give a steep gradient in mechanical properties since the local temperature peak rises in the temperature range of 650-1350°C [3]. In the present study, an attempt has been made to correlate the microstructures with the magnetic

* Corresponding author. Fax: + 82-42-868-8346. E-mail address: [email protected] (J.H. Kim)

Barkhausen noise level in the weld HAZ of nuclear RPV steel. The material studied was an RPV steel SA 508-3 with a composition of 0.19C, 1.35Mn, 0.82Ni, 0.17Cr, and 0.51Mo. The weld was made by multipass narrow gap submerged arc welding (SAW) process. Each characteristic region in the actual weld HAZ was identified in Fig. 1. Specimens, which have different microstructures corresponding to each HAZ region, were prepared through heat treatments simulating various weld thermal cycles using a thermal/mechanical simulator Gleeble 1500. Some specimens were subjected to the simulated post-weld heat treatment (PWHT) at 620°C for 7 h. Microstructural examinations by OM, SEM and TEM were performed. The magnetic Barkhausen noise amplitude (BNA) [4] was measured on the 1 x 2 x 10 mm bar sectioned from each simulated HAZ specimen. The macrostructure and specimen identification on the actual weld HAZ region was specified in Fig. 1. The typical optical micrographs of the various HAZ regions including the base metal are presented in Fig. 2. The detailed microstructural characterization is reported elsewhere I-5]. Fig. 3 shows the variations of BNA in the simulated HAZ specimens before and after PWHT. The samples

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~Lic categorized in two groups prexailing in martensitic and bainitic structmcs. The coalse grained martensite (S1-$3} showed a highcl BNA value than the fine grained bainite ($4-$6) in the as-simulated specimen (before PWHT), bu! lhe trends were reversed due to the temperillg effect a~st~cialed with P W H T . There was a sinai[ decrease m the marlensite structure but a signilicant increase in the bainite structure. The decrease of BNA due to I ' W I I T m the samples of coarse-grained H A Z region was c o n t r a r y to o u r expectalion of an increase iT? /he BNA level. It is k n o w n that the tempering reduces the increase of BNA by' increasing the n u m b e r of d o m a i n walls lnoving at a given instant and the m e a n frec path of d o m a i n wall m o v e m e n t due to reduction in dislocation density [6]. The decrease of BNA due to P W I I T in the specimens S1 and $3 is attributed to the d o m a i n wall pinning dulc to tile precipitation ol carbides. The a b r u p t increase of BNA m the bainite region ($4-$6) due to P W H [ could be related with the

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Fig. 2. Optical microstructures in various HAZ regions showing coarse grained HAZs ((a). {hi and (c)). fine grained HAZs lidi, (e)). intercritically reheated HAZ (fl and subcritically reheated HAZ (g), and base metal (h). All of the HAZs were subjected to PWHT.

J.H. Kim et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 351-353

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change of microstructures in the intercritical region. The major precipitates of as-simulated specimen were Fe3C carbides with rod-like or elongated shape, the size of which is over 0.1 gm. The sizes of carbides increase and the volume fraction decreases with increasing tempering time [2], which result in a reversal effect in the domain wall motion. It is known that the large size carbides reduce the mean free path for the movement of the reversal domain walls by making spike domain resulting in the decrease of BNA, but the reduction of the volume fraction results in an increase of BNA by increasing the mean free path [7]. For specimens of $4-$6 having microstructures of fine grained tempered bainite, the carbide morphology was spheroidized by the PWHT. In general, the specimens with spheroidized carbides were always magnetically softer than the specimens with rod-

353

type and needle-like carbides at the same carbon contents [8], which makes the domain wall easy to move, resulting in an increase of BNA. Therefore, the rapid increase of BNA in the intercritical region is attributed to the morphology changes of carbides with decreased dislocation density. The rapid decrease of BNA in the specimen $7 is difficult to explain, but it may be attributed to the large size carbide and lath boundary resulting from the over tempering effects. The present study showed that magnetic Barkhausen noise (BN) is susceptible to the microstructures in the weld HAZ of an RPV steel. The different characteristics of the BN signals were observed between martensite and bainite structures. The size and morphology of carbides are thought to be the major metallurgical factors in the induced BNA level. The difference of BN behavior in a similar microstructure could be explained as a competition of the size and volume fraction of carbides.

References [1] S. Titto, Acta Polytech. Scand. Appl. Phys. Set. 119 (1977) 1. [2] V. Moorthy et al., Mater. Sci. Eng. A 231 (1997) 98. [-3] J.H. Kim, E.P. Yoon, J. Koream, Inst. Met. Mater. 36 (8) (1998) 1329. [4] D.G. Park, S.S. Kim, B.G. Ahn. J.H. Hong, C.G. Kim, H.C. Kim, I.S. Kim, J. phys. IV France 8 (1998) Pr2-797. [-5] J.H. Kim, E.P. Yoon, J. Nucl. Mater. 257 (1998) 303. [6] B. Raj, V. Moorthy, S. Vaidynathn, Mater. Eval. 82 (1997) 55. [-7] C.C.H. Lo et al., IEEE Trans. Magn. 33 (5) (1997) 4035. [8] D.C. Jile, J. Appl. Phys. 63 (1988) 2980.