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Neutron fluence effects on the magnetic parameter changes in SA508 Cl.3 forging and weld K.O. Chang, S.H. Chi, B.C. Kim, S.L. Lee and C.M. Sim Korea Atomic Energy Research Institute P.O. BOX 105, Yusong, Taejon 305-600, Korea Contact

Abstract

The neutron fluence effects on the magnetic parameters, i.e., Barkhausen noise amplitude (BNA), saturation magnetization (M_s), susceptibility (c), and coercivity (H_c), were investigated for specimens made of SA 508 Cl. 3 reactor pressure vessel(RPV) forging and weld surveillance specimens in three different conditions, i.e unirradiated, and irradiated at two different neutron fluences(2.2 x 10¹⁹ ncm^-2, 3.8 x 10¹⁹ ncm^-2, E³ 1 MeV, 290^o). The changes in the magnetic parameters due to irradiation in the present study were roughly the same as the reported results : The hysteresis loop appeared to turn clockwise further as the fluence increased, resulting in a further decrease in the BNA and susceptibility, and an increase in the coercivity for both the base and the weld, irrespective of the metal type. Inter-comparison between the changes in the magnetic parameters and yield strength due to irradiation show that , of those five magnetic parameters investigated, the BNA and susceptibility (c) showed a relationship both for the base and the weld, with some differences due to the metal type.

I. Introduction

Changes in magnetic parameters due to irradiation have recently been investigated within the efforts to find reliable nondestructive evaluation (NDE) techniques in predicting, for example, fracture toughness of embrittled reactor pressure vessel (RPV) steels [1][2]. While some results have shown an apparent possibility in applying the magnetic measurement techniques to the evaluation of material states, including mechanical properties, some are not [3][4][5]. Differences in the specimen conditions with the measurement techniques used in each experiment might have contributed to these discrepancies in the previous studies. If a data base is prepared in the future within the continuing efforts of the relevant research group, the apparent magnetic parameter changes in the neutron-irradiated RPV steels may be correlated with changes in the mechanical properties, like the successful application of techniques in the field of heat treatment and stress evaluation [4][6].
In the present study, in view of the need for additional experimental data regarding the magnetic NDE of neutron irradiated reactor structural steels, the neutron fluence effects on the magnetic parameters, i.e., Barkhausen noise amplitude (BNA), saturation magnetization (M_s), susceptibility (c), and coercivity (H_c), and the inter-relationships between the changes in the magnetic parameters and yield strength due to irradiation, were investigated using RPV surveillance specimens with two different neutron exposure (fluence) levels.

II. Experiment

II. 1 Material
The magnetic measurement specimens used in the present study were made of SA 508 Cl. 3 reactor pressure vessel(RPV) forging and weld surveillance specimens in three different conditions, i.e unirradiated, and irradiated at two different neutron fluence levels (2.2 x 10¹⁹ ncm^-2, 3.8 x 10¹⁹ ncm^-2, E³ 1 MeV, 290^o). Table 1 shows the chemical composition and heat treatment conditions of the forging (base) and weld specimens. Each fluence level corresponds to 5.28 and 7.68 EFPY (effective full power year) irradiation. Tensile yield strength data were obtained from the RPV surveillance test results. It is emphasized that all the unirradiated, and irradiated specimens for magnetic measurement were fabricated from untested Charpy specimens.

Forging (Base)	C: 0.16   Mn: 1.28    P: 0.006   S: 0.005   Si: 0.17   Mo: 0.48    Cr: 0.17    Cu: 0.06    Ni: 0.17   Fe: Bal.      1144±287 K, 4 hr., Austenitizing, Water-quenching      936±287 K, 4 hr., Tempering, Air cooling      868 K, 6 hr., Stress relieving, Air cooling
Weld	C: 0.06   Mn: 1.68    P: 0.007    S: 0.008   Si: 0.37   Mo: 0.55    Cr: 0.33    Cu: 0.04   Ni: 0.58   Fe: Bal.      868 K, 6 hr., Post weld stress relief
Table 1: Chemical composition(wt%) and heat treatment conditions for forging and weld metal.

II. 2 Magnetic Measurement
The magnetic hysteresis loops were obtained by a vibrating sample magnetometer (VSM) for the 3 mm diameter disk shape specimens to deviated the demagnetizing effects due to specimen shape and size. Saturation magnetization (Ms) was obtained for 5 kOe, and coercivity (Hc) was measured for 2 kOe. A block diagram of the instrumentation used for the Barkhausen noise amplitude (BNA) is shown in Fig. 1. A U-shaped ferritic magnet core was used to magnetize the specimen with a size of 3.3 x 3.3 x 18.3 mm³. An electromagnet was placed along the length of the specimen with the magnetic-field direction parallel to the length of the specimen. A magnetic field was applied to the specimen by supplying a sinusoidal current (2.3 Hz) to the electromagnet. An encircling sensing coil wound around the specimen was used to measure the magnetic induction in the specimen. The BNA was measured by bandpass filtering (16-18 Hz) the magnetic induction signal.

Fig 1: Block diagram of the Barkhausen noise measurement equipment.

III. Results and Discussion

Figs. 2 and 3 show the hysteresis loop and BNA shape changes due to the fluence increase for base and weld metals. As seen in the figure, the effects of the fluence increase appear as a clock-wise turn in the case of the hysteresis loop, and a decrease in the BNA in the case of the Barkhausen noise measurement for the base and weld metals. Figs. 4-1 ~ 4-3 show the fluence effects on coercivity(Hc), susceptibility(c), and BNA clearly based on the measurements in Figs. 2 and 3. It is seen that the Hc increases linearly with fluence, while c and BNA decrease. The ratio of coercivity to fluence (in unit of 10¹⁹ cm², E ³ 1MeV) appeared at about 3 regardless of the base or weld metals.

Fig 2a:

Fig 2b:

Fig 2: Comparison of the hysteresis loops for unirradiated and irradiated forging and weld metals.

Fig 3a:	Fig 3b:
Fig 3: Change of Barkhausen wave forms for unirradiated and irradiated forging and weld metals for two fluence levels (2.2 x 1019 ncm-2, 3.8 x 1019 ncm-2, E³ 1 MeV)

Fig 4-1: Fluence effect on the change of coercivity

Fig 4-2: The fluence effects on the change of susceptibility(x)

Fig 4-3: Fluence effects on the change of BNA.

Fig 4:

As discussed in a few relevant studies concerning the microstructure - magnetic parameter relationship, understanding of the changes in these three parameters, i.e, c, Hc, and BNA, may resort to the interaction of the domain wall with irradiation-induced and stabilized defects including, but not limited to, precipitates, interstitial or vacancy clusters, and dislocations [7]. Irradiation induced defects are supposed to act as obstacles to the domain movement under the applied magnetic field [1,5,7]. The interaction of the domain wall with irradiation-induced defects will resulted in a decrease in the domain wall energy, increasing coercivity (Hc), while the mean free path of the wall movement will decrease with the decrease in the BNA as the density of the stabilized defects increases with fluence. It is known that the Barkhausen noise signal depends on the number of the domain walls moving at a given instant and the mean free path for the domain wall movement [7]. The change in the hysteresis loop and BNA with an increase in the fluence in the present study shows a prospect in the role of the microstructural changes in that a similar observation, i.e., a clock-wise turn of hysteresis loop and the decrease in BNA, is obtained when the hardness increases due to heat treatment [8]. It is known that, if irradiated by neutrons to the order of 10¹⁹ ncm^-2, most steels show an increase in hardness and yield strength.
No change was observed in the saturation magnetization (Ms). In the previous study on the correlations between the microstructure and magnetic parameters, the parameter saturation magnetization(Ms) has already appeared to be insensitive to microstructural changes in SA 508 Cl. 3 low alloy RPV steel [2].

III. 2 Fluence effects on the changes in the magnetic parameters with yield strength

Table 2 and 3 show a comparison of the changes in yield strength with the magnetic parameters due to fluence for the base and weld metals. Each change was normalized with the value at the fluence of 3.8 x 10¹⁹ ncm^-2.

Fluence (ncm-2)	Unirr.	2.2 x 1019 ncm-2	3.8 x 1019 ncm-2
Yield Strength (ksi)	71.4	76.4 (76%)	78.0 (24%)
BNA(V)	1.50	1.55 (72%)	1.18 (28%)
Susceptibility (emu/g Oe)	1	0.74(70%)	0.63 (30%)
Coercivity (Oe)	8.8	11.9 (50%)	15 (50%)
Table 2-1: Comparison of the changes in the yield strength and magnetic parameters due to fluence. Each change was normalized with the value at the fluence of 3.8 x 1019 ncm-2 (Base)

Fluence	Unirr.	2.2 x 1019 ncm-2	3.8 x 1019 ncm-2
Yield Strength (ksi)	82.9	84.2 (77 %)	84.6 (23 %)
BNA (V)	2.08	1.54 (51 %)	1.01 (49 %)
Susceptibility (emu/g Oe)	0.89	0.69 (69 %)	0.60 (31 %)
Coercivity (Oe)	8.5	12 (54 %)	15 (46 %)
Table 2-2: Comparison of the changes in the yield strength and magnetic parameters due to fluence. Each change was normalized with the change for the fluence of 3.8 x 1019 ncm-2 (weld)

As seen in the table 2-1 and 2-2, both for base and weld metals, most of the changes in YS, BNA and susceptibility(c) occurred in the early stage of irradiation, and each change is quite similar for the two irradiation steps. Thus, the irradiation effects on the changes in BNA and susceptibility(c) appeared quite similar to the changes in YS. The fluence effects on the changes in BNE and coercivity(Hc) appeared differently to YS, BNA and susceptibility(c). Since this observation regarding the coercivity is contrary to the previous observations for unirradiated RPV steels [2], further systematic investigation may be needed with regard to the observations on coercivity made in the current study. It is note worthy that the BNA shows a good correlation with the change in YS for the irradiated condition in the present study, as well as the previous observation for the unirradiated RPV steels [2].

There were no magnetic parameters that showed a similar change to yield strength for weld metal. However, it is worthy to note that the base and weld metals showed similar changes in yield strength for the present two fluences. From these different changes in the magnetic parameters between the base and weld metals, it is believed that the microstructural evolutions due to irradiation in the irradiated base and weld metals are different from each other.

IV. Conclusion:

The fluence effects on the magnetic parameters appeared as further progress in the magnetic parameters with increasing fluence : the hysteresis loop appeared to turn clockwise further as the fluence increased, resulting in a further decrease in the BNA, BNE and the susceptibility, and an increase in coercivity for both the base and weld metals, irrespective of the metal type. An inter-comparison between the changes in the magnetic parameters and yield strength due to irradiation showed that, of those five magnetic parameters investigated, the BNA and susceptibility (c) appeared to show a relationship for both the base and weld metals, with some differences due to the metal type. Further studies are required to accept the observations on coercivity (Hc) made in the present study.

Acknowledgments

This work has been carried out as a part of Nuclear Materials Technology Development Project under the nuclear R&D program by MOST in Korea.

References

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