A Study on Primary Water Stress Corrosion Cracking Behavior of Forged Alloy 600

Dislocation density analysis of (a) as received, (b) 10 year thermally aged specimen and. c) 20 year thermally aged specimen. Representative DCPD curves observed during SSRT with smooth sample, (a) as received, (b) 10 year thermally aged, and (c) 20 year thermally aged sample. Results of surface analysis of SCC-initiated samples after SSRT with smooth samples. a) as received, (b) 10 year thermally aged specimen and (c) 20 year thermally aged specimen.

Surface analysis results of SCC initiated samples after SSRT with notched samples,. a) as received, (b) 10 year thermally aged and (c) 20 year thermally aged sample.

INTRODUCTION

Background and motivation of Research

Many studies have been conducted to investigate the effects of long-term thermal aging on various nickel-based alloy weld metals, and significant effects on material properties have been observed during long-term thermal aging at low temperatures (~450 °C. However, these are expected to is improved when long-term thermal aging and triaxial stress are properly considered.As shown in the equation below, the material properties in terms of microstructure most affected by long-term thermal aging are treated as a single parameter 𝜆𝑒.

This may not be sufficient to consider the complex microstructure change during long-term thermal aging.

Objective

Experimental Conditions

Used material and process of thermal aging simulation

Then, heat treatment was performed in Ar environment to simulate the effect of long-term thermal aging on PWR. Three different heat treatment durations were simulated; 0 years for the accepted condition, 10 and 20 years of thermal aging.

Specimen design and triaxial stress

Experimental procedure

To measure the hardness as a function of the penetration depth, the continuous stiffness measurement (CSM) technique was used with the Nano Indenter XP head. To normalize the tip before the experiment, the calibration procedure was performed with a fused silica reference material. And for Poisson's ratio, 0.3 was used, which is a commonly used value when testing stainless steels or nickel-based alloys.

Then polishing was done with diamond pastes from particle size 6 μm to 1 μm, then with 0.05 μm colloidal alumina to obtain a flatter surface. Then the final polishing was done with vibrating polisher to remove any residual stress induced during surface treatment. Microstructure analysis was performed with sample prepared by the above procedures and then etched to reveal microstructure with 20% HCl in HNO3 etching solution in accordance with ASTM E407-07 standards. Then electron microscopy was observed with scanning electron microscope (SEM) attached Quanta 3D Dual-Beam Focused Ion Beam (FIB).

Energy-dispersive X-ray spectroscopy (EDS) coupled with SEM and TEM was used to investigate the chemical composition. Plunge corrosion testing was performed to analyze susceptibility to intergranular attack in accordance with ASTM A262 Practice C. Three electrodes were used in the electrodynamic cell; a platinized platinum mesh was used as the counter electrode, a saturated calomel electrode was used as the reference electrode, which has 0.241 V vs.

The strain rate of smooth and notched samples was kept the same by controlling the displacement rate calculated by the finite element method with the equipment shown in Figure 2-10. The water chemistry was monitored as shown in Figure 2-11, the load and displacement were also monitored as shown in Figure 2-12. The Direct Current Potential Drop (DCPD) method was used to monitor the SCC initiation of each sample online, as shown in Figure 2-13.

Stress analysis Results of notch center element and surface element of the models shown in figure 2-5.

Table 2-1. Chemical composition of Alloy 600 used in this study [34].

Results

Mechanical properties

Microstructure

It was performed using the same instrument with FIB at 10 kV acceleration voltage, 4 nA current, 70° tilt angle and 17 mm working distance. Since the grain size was about 20 μm, the step size was chosen to be 0.5 μm, which can measure several points in a grain. Scanned area was set as the maximum value at the magnification x μm so that it covers more number of grains to get statistically accurate results of grain and grain boundary misorientation.

Corrosion resistance

Morphology of precipitates was investigated in as received and thermally aged sample as shown in Figures 3-4. Detailed microstructural features were analyzed with image analyzer. Meanwhile, the number of grain boundary precipitates was dramatically changed during thermal aging. As a brief summary, it is conceivable that precipitates were formed over 10 years of thermal aging and growth or combined.

Precipitates were formed in the early stage of thermal aging, also in this period hardening was observed. And over time it has simply been disturbed or migrated to the thermodynamically stable position. The chemical composition was investigated with SEM attached to EDS and is shown in Figure 3-5. a), (d) and (g) show the region where the EDS analysis was performed. From the two results, the precipitates were recognized as chromium carbides, specifically Cr7C3 from the analysis of the diffraction pattern shown in Figure 3-6 (d-f).

As shown in Figure 3-9, Cr7C3 and grain matrix have different crystallographic structure; Cr7C3 has a trigonal structure, while the matrix has a cubic structure. Therefore, the precipitates did not grow in the matrix direction, but in the grain boundary direction, as shown in the figure. Also as shown in the figure, carbides were mainly formed at the high-angle grain boundary, while they were not formed at the CSL boundaries.

The cross section of the specimen after the immersion corrosion test according to Practice C ASTM A262 is shown in Figure 3-10. Intergranular corrosion was observed in the as-received and heat-aged specimen and was more severe in the 10-year heat-aged specimen. From these results, it can be thought that the 10-year thermally aged specimens have the most significant galvanic effect between the carbide and the grain boundary or matrix as no sign of chromium depletion was observed during EDS analysis in any specimen [12], [64 ].

A slight active-passive region transition was observed in the 20-year thermally aged sample, while this phenomenon was not observed in the as-received and 10-year thermally aged sample.

SCC susceptibility

Thermal aging temperature or changes induced by thermal aging may have little effect on oxidation in the primary circuit environment. Small graph shows enlarged scale at an early stage of the stress-strain curve. Only representative graph is shown in the figure to suggest the shape of the graph. Fracture surfaces by test result at room temperature, (a) as received, (b) 10 year thermally aged specimen and (c) 20 year thermally aged specimen.

Detailed results are summarized in Table 3-3. Nanoindentation results of each sample. a) shows the entire displacement curve in surface area and hardness. Detailed information is summarized in Table 3-2. d, e, f) are magnified images of each sample. a, d, g) present the location of the line where the EDS analysis was performed. EBSD analysis results of (a, d) as received, (b, e) 10 years thermally aged sample and (c, f) 20 years thermally aged sample.

Random site lattice boundaries are marked in red, low-angle grain boundaries are marked in green, and high-angle grain boundaries are marked in blue in (a), (b) and ( c). Results of high-resolution EBSD analysis. a), (d) and (g) show the inverse pole image map of the obtained specimen, 10 years thermally aged and 20 years thermally aged, respectively. And the crystal orientation of the precipitate and close to the grains are marked in (b) as-received, (e) 10 years thermally aged, and (h) 20 years thermally aged.

And (c), (f) and (i) show the average kernel misorientation of the obtained specimen, 10 years thermally aged and 20 years thermally aged, respectively. Cross section of sample after immersion corrosion test according to ASTM A262 C practice, (a) as received, (b) 10 years thermally aged and (c) 20 years thermally aged sample. Oxide layer of (a) as-received, (b) 10-year thermally aged, and (c) 20-year thermally aged specimen in simulated aqueous PWR environment for 750 h.

Representative DCPD curves observed during SSRT with notched sample, (a) as received, (b) 10 year thermally aged and (c) 20 year thermally aged sample.

Table 3-1. Mechanical properties achieved from tensile tests of thermally aged specimens

Discussion

• Hardening and softening behavior
• PWSCC initiation of smooth specimen
• PWSCC initiation of notched specimen
• PWSCC initiation model

In addition, intergranular cracking was observed in the surface of PWSCC-initiated sample, the same result observed in previous studies; grain boundaries are affected by precipitation and become brittle or susceptible to PWSCC initiation [25], [80]. It was found that the PWSCC susceptibility is inversely proportional to the PWSCC initiation time measured in this study as shown in Figure 4-2. From this observation, it was conceivable that the PWSCC initiation behavior under triaxial tension could be controlled by the shear stress component.

This observation clearly shows the effect of shear stress induced by triaxial stress on PWSCC initiation susceptibility of nickel-based alloy. To find out the correct model parameter for PWSCC initiation behavior of notched specimen, critical resolved shear stress (CRSS) was calculated with the material factor obtained in this study. Based on the parameters described in section 4, modification was attempted based on the PWSCC initiation model of Garud [42].

And widely used as important PWSCC initiation model in many papers regarding PWSCC initiation of nickel-based alloys in mechanical point of view. To parameters were fitted; area fraction of precipitates and distance between precipitates, as both the PWSCC initiation time for smooth and notched samples are affected by both parameters. Based on the parameters calculated from the fit, the calculation was performed according to the PWSCC initiation model proposed in this study as shown in Figures 4-9.

Similar trends were observed in the calculation results: for the monoaxial stress state where the triaxial state is 0.33 (smooth specimen), the onset time of PWSCC decreases in the early stage of thermal aging and increases in the late stage of thermal aging, while the onset time of PWSCC decreases gradually when the triaxiality is 0.5 (notched specimen). To validate the model, the initial time of PWSCC was calculated and plotted as shown in Figure 4-9 and Figure 4-9.

Figure 4-1. Comparison of yield strength with calculation results according to Orowan mechanism

Conclusion

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