·Table of Contents
Neutron fluence effects on the magnetic parameter changes in SA508 Cl.3 forging and weldK.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
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, 290o). 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.|
|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.|
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 . 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 . 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 . 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 .
|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 , 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 .
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.
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