Surface Effect on Void Swelling Behavior of Stainless Steel

The nucleation and growth of voids have been observed successively in different thickness specimens of Type 316 stainless steel electron-irradiated at 550°C in a high voltage electron microscope. In a bulk representative 1.5 μm thick specimen, the void number density increases rapidly and saturates during the initial stage of irradiation and then decreases with following dose by void coalescence. The swelling increases proportionally with (dpa)^1-5 up to about 30 dpa. In a thin specimen, of 0.4 μm thickness, on the other hand, the void number density increases continuously with dose up to about 25 dpa. The swelling of the thin specimen showed a tendency to saturation due to the disappearance of voids at the specimen surfaces. The difference in swelling behavior between the 1.5 and 0.4;μm thick specimens can be ascribed to the different effects of the specimen surfaces, which serve as a dominant sink for both radiation-produced point defects and dislocations.     

Void Swelling of Solution-Annealed Type 316 Stainless Steel under Long Time Proton Irradiation

Solution-annealed type 316 stainless steel was irradiated by 150 keV proton to a dose of about 6 dpa at the irradiation temperature ranging 450–700°C. To examine the effect of aging during irradiation, the present proton irradiation was carried out for about 25 h at a low dose rate of 7×10–^?5dpa/s. The specimens without He preinjection showed much smaller void swelling than those preinjected with He to the content of 10 at.ppm. Similarly to the case of neutron irradiations, the void swelling in the He preinjected specimens showed the temperature dependence with double peaks, and the peak swelling temperatures were about 550 and 650°C. In these specimens with He preinjection. void number density decreased and average void diameter increased with the increase of irradiation temperature in the range of 450–600°C, but these trends were reversed between 600 and 650°C. The volume of the grain boudary M₂₃C6 precipitates increased with the increase of irradiation temperature from 600 to 700°C, and it was concluded that the decrease of soluble carbon due to the precipitation of M₂₃C6 caused the second swelling peak at 650°C.     

Effect of High Heat Flux Irradiation by High Power CO2 Laser on Fatigue Characteristics and Fracture Behavior of Stainless Steel

High power CO₂ laser beam was used to simulate high heat flux on a plasma disruption. Fatigue strength and fracture toughness were examined in order to estimate the extent of degradation due to the melted layer, which was the region melted and resolidified in the course of transient high heat flux. The materials were austenitic stainless steels SUS316 and SUS304. Fatigue strength of the irradiated specimen was notably reduced because of the macrodefects on and just below the top surface of the melted layer. However, fracture toughness of the irradiated specimen was not reduced at all. It is considered that melting and resolidification in SUS316 and SUS304 due to high power CO₂ laser beam did not necessarily bring about the degradation of their material strength in the internal part of the melted layer.     

Irradiation Test of Stainless Steel by High Power CO2 Laser

Electron beam, plasma arc and ion beam are often employed to simulate the high heat flux applied to the first wall or the divertor plate in a fusion reactor. In this study, an irradiation test with high heat flux was carried out under atmospheric condition by using high power CO₂ laser. The test material is SUS316 and the temperature change and the melting amount were measured. A thermal analysis code to take melting and evaporation behavior into account was developed. The laser absorption coefficient can be raised up to 95% before melting if special paint is coated on specimen surface. After melting, this coefficient is estimated to be 60% by thermal analysis. However, it was revealed that a precise modification of this model was indispensable. Although the effect of irradiation environment or heat source on material damage was also examined, there is no significant difference one another. In conclusion, it is found that CO₂ laser is quite suitable for use as a heat source to simulate a high heat flux.     

Temperature and Dose Dependence of Swelling in 10% and 20% Cold-Worked Type 316 Stainless Steels

Void swelling in 10% cold-worked (10% CW) and 20% cold-worked (20% CW) type 316 stainless steels was investigated by 200 keV C⁺ ion irradiation and transmission electron microscope observation. Both 10% CW and 20% CW 316 steels show the swelling maximum at 923 K. Swelling in 10% CW 316 is much higher than that in 20% CW 316. The voids in the former material are larger and fewer than those in the latter material. The bilinear equation is applicable to describe swelling dose relation for both materials, except 10% CW 3 16 at higher doses than 50 dpa, where sharp swelling increase is observed. Heat to heat variability seems to exist in incubation dose, though it is not large. With regard to swelling rate, all three heats examined show good coincidence for both 10% CW and 20% CW 316 steels. Comparison of 20% CW 316 swelling rate for various irradiation projectiles indicates that the swelling rate is described as a simple function of the projectile mass, and there may exist a scaling law between the different projectile data.