Introduction
The operating conditions of a supercritical water-cooled reactor (SCWR) system, mainly characterized by high system pressure (≥25 MPa) and a high operating temperature (500°C-650°C) [
1,
2], are different from those of a light water reactor (LWR). The selection of an appropriate material to be used as cladding material, assembly tube, and other structures for a SCWR has been an important issue in SCWR design owing to the harsher environment in the reactor. Zirconium alloy, the widely used cladding material in conventional LWRs cannot be used in a SCWR due to its low resistance to heat and corrosion. Ferritic/Martensitic (F/M) steels, austenitic steels, and nickel alloys were considered as candidate structure materials for the in-core components and cladding tube in a SCWR [
3]. F/M steels have advantages in void swelling resistance, while austenitic steels and nickel alloys have advantages in elevated temperature performance and corrosion resistance. Since 316 austenitic stainless steels have been applied as cladding tube in a SFR (Sodium Fast Reactor), emphasis is therefore put on austenitic steels for use as cladding in a SCWR.
The AL-6XN alloy [
4], a superaustenitic stainless steel, exhibits far greater resistance to chloride pitting, crevice corrosion, and stress corrosion cracking than the standard 300 series stainless steels and is less costly than the traditional nickel-base corrosion-resistant alloys. The chemical composition of AL-6XN is shown in Table 1. With the addition of microalloy elements such as zirconium and titanium, modified AL-6XN (Mod AL-6XN) steel was fabricated. This modified steel is expected to have excellent integrated mechanical properties and intergranular corrosion resistance because of the forming of (Ti, Zr)(C,N), which prevents recrystallizing grains from coarsening during thermo-mechanical treatment and leads to a decrease of Cr
23C
6along the grain boundary.
Thermo-mechanical treatment
Alloys were melted in a vacuum induction furnace and poured into a 100 kg ingot in the protection of an argon atmosphere. According to Thermo-Calc calculation (see Fig. 1), holding or cooling at relatively slow rates between 400 and 1080°C increases the potential for precipitating an excess phase such as sigma, which generally detracts from the corrosion resistance of the material. Therefore, hot working should be performed in the temperature range of 1100 to 1250°C. To ensure maximum corrosion resistance after hot working, Mod AL-6XN should be given a full anneal between 1100 and 1250°C plus quench heat treatment.
Microstructure and mechanical properties
To examine the microstructure, a specimen was sectioned along the rolling direction from a full annealing plate at 1150°C, and then polished and etched. The etching reagent was aqua regia: 30 ml HCl and 10 ml HNO3. The specimen was immersed in the etching reagent for one hour. Figure 2 shows the microstructure of Mod AL-6XN steel. The average grain size was about 50 μm. Twins could be observed in this initial material, possibly because of the pre-thermal-mechanical treatment.
A tensile test was performed using a MTS809 test machine located in a hot cell. The test machine was equipped with a high-temperature furnace capable of testing up to 800°C in vacuum. The testing was performed following GB/T 4338-2006 and GB 228-2002. Cylindrical tensile specimens, with dimensions of Ф5 mm×25 mm (d×L0), were machined from a 15 mm-thick plate and subjected to tensile tests between room temperature (RT) and 600°C. The strain rate was 2 mm/min, and the specimens tested at elevated temperature were held for 15 minutes.
Mod AL-6XN steel exhibited a higher yield (Rp0.2) and ultimate tensile strength (Rm) at room temperature and elevated temperature than was typical for austenitic stainless steels. The ultimate tensile strength was still above 550 MPa at 600°C. As shown in Fig. 3, the curves of the ultimate and yield strength vs temperature between Mod AL-6XN and commercial AL-6XN steel matched very well. However, Mod AL-6XN exhibited superior plasticity. The total elongation of Mod AL-6XN was always above 53% in the temperature range from RT to 600°C compared to below 45% for commercial steel from 100°C.
The Charpy impact test was conducted for the V-notch specimens with dimensions of 55 mm×10 mm×10 mm, following GB/T229-1994 eqv. ISO14821983, between -100°C and RT. The V-notch was in the T-L orientation. All the specimens were tested by using an instrumented Charpy impact testing machine with an electrically controlled hydraulic system and an impact energy capacity of 300 J. The specimens tested below 0°C were kept in a mixture of liquid nitrogen and ethanol for 20 minutes at testing temperature.
Mod AL-6XN steel exhibited excellent toughness at RT and sub-zero temperatures compared with commercial steel. The absorbed energy at RT could be beyond 300 J for Mod AL-6XN and 190 J for commercial steel. Even at -100°C, the energy could be up to 230 J for the modified steel despite a little lower energy in the transverse orientation (see Fig. 4).
Effect of aging behavior on corrosion resistance and impact toughness
An aging treatment was conducted to examine the microstructure stability of Mod Al-6XN steel. As shown in Fig. 5, needle-like precipitates could be observed in the matrix, while flaky precipitates could be observed along the grain boundary after aging at 600°C for 26 hours and 50 hours separately. These precipitates may be the sigma phase according to Fig. 1. The EDS results (see Fig. 6) showed that the precipitates along the grain boundary was a molybdenum-, silicon-, and chromium-rich phase that would lead to poor intergranular corrosion resistance. The following work would determine the nature of these precipitates.
A noticeable drop in corrosion resistance could be observed in the aged material at 600°C. The polished specimens from different heat treatments were immersed in aqua regia for one hour. As shown in Fig. 7, the morphology changes exhibited that the aged specimens were of poor corrosion resistance, especially of intergranular corrosion resistance in a corrosive agent. Severe corrosion could be observed in the matrix and along the grain boundaries for the aged specimens compared to just slight corrosion along the grain boundaries for the unaged specimen.
The aging treatment also degraded impact toughness. A 200 J decrease in absorbed energy could be observed in the temperature range from -100°C to RT for the 50-hour aged material (see Fig. 4).
The aging treatment at 600°C deteriorated the corrosion resistance and impact toughness of Mod AL-6XN steel, possibly due to the changes in the microstructure. Based on the science of precipitate thermodynamics, the addition of microalloy elements such as titanium and zirconium could only bring MX precipitates, which formed during the hot working temperature region but would not do harm to the corrosion resistance and impact toughness. In other words, if aged at 600°C, AL-6XN steel would show similar microstructure changes to that of Mod AL-6XN steel. The microstructure changes would also lead to deterioration in corrosion resistance and impact toughness, which should be considered carefully when AL-6XN steel is considered as one of the candidate materials for the cladding tube and in-core components, since the operating temperature in a SCWR generally ranges from 500°C to 650°C.
Conclusions
Mod AL-6XN steel was patterned after AL-6XN steel by adding microcontent titanium and zirconium. By optimizing the thermo-mechanical treatment, Mod AL-6XN steel exhibited comparable strength, and superior plasticity and impact toughness to commercial AL-6XN steel. An aging treatment at 600°C would degrade the corrosion resistance and impact toughness of AL-6XN and Mod AL-6XN steels, which is unfavorable for the application of the steels as cladding tube and in-core components in a SCWR.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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