Norikazu Yamamoto

Effect of helium implantation at 533–573 K on tensile properties of 9Cr martensitic steels

Abstract The effects of helium implantation (300 appm) at 533–573 K on the tensile properties of 9Cr-1MoVNb (modified 9Cr-1Mo) and low-activation 9Cr-2W martensitic steels were investigated. Tensile tests were carried out with miniature specimens from room temperature to 973 K. The helium implantation caused an increase in strength at all the test temperatures. Implantation produced defects, but defect clusters were not found in as-implanted specimens and in specimens tested below 773 K. Small bubbles were observed above 873 K. While ductility loss was greater at higher test temperatures, the fracture mode was found to be ductile and transgranular.

The effect of MC and MN stabilizer additions on the creep rupture properties of helium implanted Fe-25% Ni-15% Cr austenitic alloy

Abstract Helium embrittlement resistance of Fe-25% Ni-15% Cr austenitic alloys with various MX (M = V, Ti, Nb, Zr; X = C, N) stabilizers was compared through post helium implantation creep testing at 923 K. While significant deterioration by helium in terms of creep rupture time and elongation occurred for all materials investigated, the suppression of the deterioration, especially in rupture time, was discerned for the materials in which semi-coherent MC (M = Ti, Ti + Nb, V + Ti) particles were distributed at high density. The material which contains the incoherent M 23 C 6 as predominant precipitates seems to be less degraded by helium than those containing the MXs (M = Zr, V; X = C, N), if compared at the same number density of precipitates. Therefore, it is suggested that the high density dispersion of incoherent M 23 C 6 as well as semi-coherent Ti containing MC particles would be beneficial in reducing the detrimental helium influences on mechanical properties.

Effects of helium implantation on creep rupture properties of low activation ferritic steel F82H IEA heat

Thin plate specimens of a low activation ferritic steel, F82H IEA, were cyclotron-implanted with helium at 823 K to concentrations of 100 and 300 appm. Creep rupture properties were subsequently measured at the same temperature and were compared with those from unimplanted controls. No meaningful deterioration by helium was discerned in terms of both creep rupture time and elongation. In addition, the fracture surface remained transgranular and ductile after helium implantation, and no indication of grain boundary failure induced by helium was detected. These results would suggest good resistance of this material toward helium embrittlement.

In-beam creep rupture properties of cold-worked DIN 1.4970 and AISI 316 L at 873 K

Abstract To compare the helium embrittlement resistance of cold-worked DIN 1.4970 SS and AISI 316 L SS, in-beam creep rupture tests, i.e. in-situ creep rupture tests during helium implantation, were carried out at 873 K with helium concentrations up to 3500 appm. Significant reductions in creep rupture time and in rupture strain by helium were recognized in both materials. AISI 316 L SS was more severely embrittled. SEM fractography showed an increasing tendency for intergranular fracture with increasing helium concentration. Especially, a fairly high fraction of intergranular failure on the rupture surface was observed at low helium concentration in AISI 316 L SS. Quantitative TEM observation revealed that smaller helium bubbles were distributed both in grains and at grain boundaries for DIN 1.4970 SS. The better helium embrittlement resistance of this material can probably be explained by the finer bubble microstructure.

Helium bubble microstructure in SS 316L, 20% c.w. and its correlation to the in-beam mechanical properties

Abstract The helium bubble microstructure of in-beam creep ruptured specimens of AISI 316L, 20% c.w. was investigated in detail in order to identify the reasons for the catastrophic embrittlement of this material at 873 K compared to other materials. The main reason seems to be the inhomogeneous precipitate structure in the grain boundaries leading to a bimodal bubble size distribution. Using the modal size of the larger bubbles instead of the mean radius in comparison with the critical radius explains the higher susceptibility of this alloy for grain boundary embrittlement. In contrast to higher temperatures for which gas driven growth of the grain boundary bubbles up to the critical radius was the mechanism controlling the rupture time, it is shown from microstructural data that at 873 K this mechanism does not apply. Instead the microstructural data are in agreement with the assumption that the creep constraint growth of the grain boundary bubbles determines the rupture time. This is also deduced from from the stress dependence of the in-beam rupture times compared with that of the unimplanted controls.

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 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.

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