Yoshiharu Murase

Fatigue behavior of 20% cold-worked 316 stainless steel under in situ irradiation with 17 MeV protons at 60 °C

Abstract Load-controlled fatigue tests in the mode of tension–tension were performed for the side-notched 20% cold-worked 316 stainless steel under in situ irradiation and following irradiation with 17 MeV protons at 60 °C. In comparison with the unirradiation tests, fatigue life was substantially prolonged for in situ irradiation tests, while a slight increase of fatigue life was detected in post-irradiation condition. The SEM measurements of spacing of fatigue striations on fracture surface suggested higher resistance to fatigue fracture in the in situ irradiation specimens. Some essential differences in the irradiation effects between in situ and post-irradiation conditions were summarized in this paper.

Irradiation creep at 60°C in SUS 316 and its impact on fatigue fracture

Structural materials in fusion reactors will be subjected to irradiation by energetic particles at temperatures widely ranging from liquid He to above 1000°C. Hence, the very large irradiation creep strain at 60°C previously reported in the ORR/ORNL pressurized tube experiment to 8 dpa is important. Computer calculations for the 20% cold-worked SUS 316 demonstrated the transient nature of this radiation-induced creep, caused by the overwhelming flux of excess interstitial atoms lasting nearly one year at 60°C where the diffusivity of vacancies is very low. In order to confirm such a transient nature, continuous creep measurement under irradiation is necessary and was carried out using 17 MeV protons. Development of very significant creep strain, much larger than that at 300°C, and steadily decreasing creep rate were observed at 60°C as the calculation predicted. A significant influence of the dynamic irradiation effect at 60°C on fatigue fracture was also observed.

Creep behavior of reduced activation martensitic steel F82H injected with a large amount of helium

Creep response against DEMO reactor level helium was examined on F82H steel, a candidate structural material for advanced fusion systems. Helium was injected into the material at 823 K to a concentration of about 1000 appm utilizing α-particle irradiation with a cyclotron. Post-injection creep rupture tests were conducted at the same temperature. It has been demonstrated that helium brought about no significant effect on a variety of creep properties (lifetime, rupture elongation and minimum creep rate). In parallel with this, it did not cause any influence on fracture appearance. Both helium implanted and unimplanted samples were failed in a completely transcrystalline and ductile fashion. No symptom of helium induced grain boundary separation was thereby observed even after high concentration helium introduction. These facts hint a fairly good resistance of this material toward high temperature helium embrittlement even for long-time service in fusion reactors.

Void swelling in Fe–15Cr–xNi ternary alloys under proton irradiation

Abstract Void swelling and microstructure in Fe–15Cr– x Ni ( x =20, 25, 30 wt%) ternary alloys irradiated with 180 keV protons at 723–873 K (450–600°C) were examined by TEM. The displacement damage levels were 5–20 dpa for the alloys preinjected with 10 appm He at room temperature, and 5 dpa for the alloys without He. The introduced H seems to play the same role as He in enhancing void nucleation at the lower temperatures (723, 773 K). Although the preinjected He can suppress the Ni-dependent incubation dose at all the irradiation temperatures examined, the Ni-dependent swelling rate may be responsible for the Ni influence on swelling at the higher temperatures (823, 873 K). The turning point of the Ni influence on swelling lies around 823 K in the present proton irradiation.

Microstructural observation of helium implanted and creep ruptured Fe–25%Ni–15%Cr alloys containing various MC and MN formers

Abstract Transmission electron microscopic observations have been carried out on Fe–25%Ni–15%Cr austenitic alloys with various MX (M=V, Ti, Nb, Zr; X=C, N) stabilizers after helium implantation and creep rupture at 923 K. It is shown that suppression of helium embrittlement can be achieved through a higher dispersion density of incoherent precipitates because of their high capability of bubble entrapment. A good agreement between the average distance of grain boundary bubbles exceeding the minimum critical size and the spacing of cavity traces on intergranularly fractured surfaces is obtained. This suggests that the enhancement of grain boundary decohesion by helium is a result of unstable growth of super-critical helium bubbles.

Effect of cold work on void swelling in proton irradiated Fe–15Cr–20Ni ternary alloys

Abstract Effect of cold work on swelling and void microstructures was investigated for Fe–15Cr–20Ni alloy irradiated with 180 keV protons to 10 dpa at 773–873 K (500–600°C). The He preinjection level was 10 appm at room temperature. The introduced dislocations in Cold-worked (CW) specimen recovered thoroughly during the irradiation for all the irradiation temperatures. Although there was no significant difference in void microstructure between Solution-annealed (SA) and CW specimens at 773 K, larger and fewer voids were observed in CW specimens at temperatures above 823 K. The amount of swelling was similar in both SA and CW specimens at all the irradiation temperatures. Cold work treatment seems to be less effective in controlling swelling of the He preinjected Fe–15Cr–20Ni alloy in the present proton irradiation.

Effect of combined addition of Ti and P on creep rupture properties of helium implanted Fe–25%Ni–15%Cr alloy

Abstract Creep rupture tests after hot helium implantation (60–70 appm) at 923 K were conducted on thermomechanically treated Fe–25%Ni–15%Cr alloys with and without combined addition of Ti and P. The results obtained were compared with those of single additions in order to gain systematic information concerning the effects of Ti and/or P modifications on the austenite matrix. In terms of creep rupture time Ti-bearing alloys could withstand deleterious helium effects. In contrast, the alloy of no addition suffered from the most pronounced degradation by helium. On the other hand, considerable decrease in rupture elongation was discerned in all alloys after the implantation though the degree of embrittlement varied from alloy to alloy. The largest degradation was again observed for the alloy with no addition. These results suggest that helium induced mechanical degradation of austenitic alloys could be suppressed by proper additions of above mentioned elements and appropriate microstructure arrangements.

Helium embrittlement of Ti and P added austenitic alloys crept at 923 K

Abstract The helium embrittlement response of Ti and P modified and thermomechanically treated Fe–25% Ni–15% Cr type austenitic alloys was studied through post helium implantation creep testing. Helium was introduced into the specimen at 923 K with a concentration of about 60 appm by helium–3 ion irradiation with a cyclotron. Subsequent creep tests were carried out at the same temperature. In terms of creep rupture time, no significant degradation by helium was appreciated in the Ti added alloy, although the P modified one suffered from about a 40% reduction. On the other hand, a helium-related decrease in the rupture elongation was perceived in both alloys to a considerable extent. The beneficial result obtained from the Ti modified alloy suggests that the mechanical property deterioration induced by helium could be suppressed by proper Ti addition and/or microstructure controlling even in alloys with intermediate nickel contents (20–25%), like in Type 316 stainless steels of less nickel content.

Irradiation-Induced Deformation in Cold-Worked SUS 304 Stainless Steel

SUS 304 stainless steel has been used in the light-water reactors constructed in earlier days, in which irradiation-assisted stress corrosion cracking has drawn increasing attention and tensile residual stress is believed to be one of the major causes. It is, therefore, essential to assess its stress relaxation behavior under irradiation, which can be evaluated from the irradiation creep data, and the effect of cold work on it. Creep experiments under 17 MeV proton irradiation (2x10−7 dpa/s) at 288°C were conducted for SUS 304 with 5% and 25% cold work (CW). Irradiation creep rate of 5%CW was only slightly larger than that of 25%CW. Stress dependence was almost quadratic in both specimens, in contrast with the linear dependence in cold-worked SUS 316L reported earlier. Stress relaxation under irradiation was found to reflect this quadratic dependence. Martensite is induced by cold-working in SUS 304, not in SUS 316L, and marked difference in its amount was found between 5%CW and 25%CW, despite the small difference in irradiation creep behavior. Thus, the observed quadratic dependence appears to result not directly from the induced martensite itself but from a climb-enabled glide of the tangled dislocations densely formed in the vicinity of martensite phase boundaries.

EFFECT OF DISLOCATION ON THE IRRADIATION CREEP OF SUS 316L

Creep deformation under 17 MeV proton irradiation was examined at 288°C for two materials with different levels of cold working (5% and 25%). Tests for 25%CW material were carried out with applied stresses of 170, 270, 370, 470, 570 MPa, and for 5%CW material with 70, 95, 120, 170 MPa. Throughout the stresses examined, creep rate of 5%CW material was higher than that of 25%CW. At lower stresses, experimental results of the two materials showed stress dependence of irradiation creep rate, that corresponds well with the results of simulation calculation. At higher stresses, stronger stress dependence of the creep rate was observed for both 5%CW and 25%CW materials. Acceleration of thermal creep mechanism by irradiation appears to be one of the causes for the increase of stress dependence especially at high stresses