Cold-work has been associated with the occurrence of intergranular cracking of stainless steels e... more Cold-work has been associated with the occurrence of intergranular cracking of stainless steels employed in light water reactors. This study examined the deformation behavior of AISI 304, AISI 347 and a higher stacking fault energy model alloy subjected to bulk cold-work and (for 347) surface deformation. Deformation microstructures of the materials were examined and correlated with their particular mechanical response
ABSTRACT 13th International Conference on Environmental Degradation of Materials in Nuclear Power... more ABSTRACT 13th International Conference on Environmental Degradation of Materials in Nuclear Power Systems. Whistler, B.C., Canada, August 19 - 23, 2007, 411 - 422
VTT has been hosting the Finnish national hot laboratory infrastructure since the first nuclear p... more VTT has been hosting the Finnish national hot laboratory infrastructure since the first nuclear power plants were constructed in Finland in the 1970’s. Historically the principle radioactive materials handling has been for the testing of reactor pressure vessel steels, but over time the activities have broadened to outgrow both the capacity and capabilities of the existing facilities. As such, a decision was made in 2011 to build a whole new facility, with the additional goal of gathering most of the VTT Nuclear Safety research personnel currently scattered around the Otaniemi campus, into a single, compact facility called the VTT Centre for Nuclear Safety (CNS). The CNS is comprised of an office wing and a laboratory wing, and the laboratory wing is comprised of C-, Band A-class radiological laboratories. Although the site is atop an underground parking garage, special attention was paid to optimizing the basement space, to exploit the natural gamma shielding offered by the granite. The VTT research staff has been integral in the design process, in order to assure that the laboratories meet the current and foreseen future needs. The C-class laboratories include radiochemistry laboratories, cleanroom facilities for trace element analysis, nuclear waste management research, and C-laboratory support facilities like an electronics shop and utensil cleaning and storage. The Bclass radiological laboratories contain the controlled entrance area (also used for accessing the Alaboratory), an iodine filter laboratory, radiochemistry laboratory with nuclear material handling allocation, and a mechanicals workshop. Microscopy facilities include a TEM on the B-laboratory side, and a SEM and light microscopy facilities in the A-class laboratory. The A-laboratory spans a part of the main floor shared with the B-laboratory, as well as the basement facilities. On the main floor it includes the hot cell high bay and the pilot hall, equipped with a 10 ton capacity bridge crane. The basement facilities are mainly for storage and handling of radioactive sources, specimens and waste, but include a liquid waste handling room, autoclave room and double hot cell. Transportation of radioactive materials into and out of the laboratory occurs by way of a separate, covered truck park, through an airlock into the basement. Transfer of radioactive materials within the laboratory is possible with a dedicated facility cask handled by the bridge crane and/or a cart. Overall the design process carried out over the last two years has yielded very satisfactory results and the facilities can be expected to serve the purposes well.
There has been an on-going effort to improve the performance of Co-Cr-Mo biomedical implant alloy... more There has been an on-going effort to improve the performance of Co-Cr-Mo biomedical implant alloys. However, efforts have been thwarted by the alloy's dependence upon carbon and carbide structures for strengthening, which are also associated with lower incipient melting temperatures, and lower ductilities. In this study, the effect of nitrogen additions to a Stellite 21 has been explored. Borrowing technology from its cousin, austenitic stainless-steels, additions were made to the alloy through nitrogen-high pressure melting (N-HPM) at pressures ranging from 1.4 to 50 atm. The ingots were then rolled into plates from which bars were extracted, and then heat-treated. This paper describes the microstructural development in terms of the second phase structure and grain size of heat treated plate materials. The total nitrogen achieved increased with pressure, and ranged from 0.19 to 1.34 wt%. The findings indicate not only that additions of nitrogen increase the second phase content, but also that as the nitrogen content is increased, a Cr 2 N second phase is favored over the traditional Cr 23 C 6 carbide. This phase change apparently leads to the subsequent elevation of the incipient melting temperature to above 1260°C (2300°F).
ABSTRACT Transmission electron microscopy (TEM) observations show that dislocation channel deform... more ABSTRACT Transmission electron microscopy (TEM) observations show that dislocation channel deformation occurs in pre-irradiated austenitic stainless steels, even at low stress levels (similar to 175 MPa, 290 degrees C) in low neutron dose (similar to 0.16 dpa, 185 degrees C) material. The TEM observations are utilized to design finite element (FE) meshes that include one or two "soft" channels (i.e. low critical resolved shear stress (CRSS)) of particular aspect ratio (length divided by thickness) embedded at the free surface of a "hard" matrix (i.e. high CRSS). The CRSS are adjusted using experimental data and physically based models from the literature. For doses leading to hardening saturation, the computed surface slips are as high as 100% for an applied stress close to the yield stress, when the observed channel aspect ratio is used. Surface slips are much higher than the grain boundary slips because of matrix constraint effect. The matrix CRSS and the channel aspect ratio are the most influential model parameters. Predictions based on an analytical formula are compared with surface slips computed by the FE method. Predicted slips, either in surface or bulk channels, agree reasonably well with either atomic force microscopy measures reported in the literature or measures based on our TEM observations. Finally, it is shown that the induced surface slip and grain boundary stress concentrations strongly enhance the kinetics of the damage mechanisms possibly involved in IASCC.
Irradiated AISI 304 stainless steel extracted from the Chooz A center filler assembly has been th... more Irradiated AISI 304 stainless steel extracted from the Chooz A center filler assembly has been the subject of a number of studies. Previously the results of slow strain rate tensile and constant load autoclave tests of 30 dpa material have been reported. They showed an influence of temperature, strain rate and environment on the fracture behavior of the material. The irradiated microstructure and deformation micro structures of those materials following testing have now been examined by TEM. The findings suggest that substantial channel deformation was associated with the purely ductile fracture following SSRT testing in argon, while the intergranular fractures following SSRT in simulated PWR environment and constant load testing in both simulated PWR and argon environments were associated with very localized deformation primarily exhibiting alpha martensite. This possibility is discussed in the light of literature.
This paper describes the methods and results of analytical TEM examinations and autoclave testing... more This paper describes the methods and results of analytical TEM examinations and autoclave testing of two highly-irradiated flux thimble tube materials harvested from a commercial pressurized water reactor. The materials are cold-worked 316L, and accumulated 65 dpa and 100 dpa of radiation dose. To set the baseline for a broader study, the materials were examined in the as-irradiated condition and tested as O-ring specimens at relatively high constant loads in simulated PWR water conditions. Tests were also conducted with elevated hydrogen. For a given load, more rapid cracking was associated with higher radiation dose, and with the elevated hydrogen.
Cold-work has been associated with the occurrence of intergranular cracking of stainless steels e... more Cold-work has been associated with the occurrence of intergranular cracking of stainless steels employed in light water reactors. This study examined the deformation behavior of AISI 304, AISI 347 and a higher stacking fault energy model alloy subjected to bulk cold-work and (for 347) surface deformation. Deformation microstructures of the materials were examined and correlated with their particular mechanical response
ABSTRACT 13th International Conference on Environmental Degradation of Materials in Nuclear Power... more ABSTRACT 13th International Conference on Environmental Degradation of Materials in Nuclear Power Systems. Whistler, B.C., Canada, August 19 - 23, 2007, 411 - 422
VTT has been hosting the Finnish national hot laboratory infrastructure since the first nuclear p... more VTT has been hosting the Finnish national hot laboratory infrastructure since the first nuclear power plants were constructed in Finland in the 1970’s. Historically the principle radioactive materials handling has been for the testing of reactor pressure vessel steels, but over time the activities have broadened to outgrow both the capacity and capabilities of the existing facilities. As such, a decision was made in 2011 to build a whole new facility, with the additional goal of gathering most of the VTT Nuclear Safety research personnel currently scattered around the Otaniemi campus, into a single, compact facility called the VTT Centre for Nuclear Safety (CNS). The CNS is comprised of an office wing and a laboratory wing, and the laboratory wing is comprised of C-, Band A-class radiological laboratories. Although the site is atop an underground parking garage, special attention was paid to optimizing the basement space, to exploit the natural gamma shielding offered by the granite. The VTT research staff has been integral in the design process, in order to assure that the laboratories meet the current and foreseen future needs. The C-class laboratories include radiochemistry laboratories, cleanroom facilities for trace element analysis, nuclear waste management research, and C-laboratory support facilities like an electronics shop and utensil cleaning and storage. The Bclass radiological laboratories contain the controlled entrance area (also used for accessing the Alaboratory), an iodine filter laboratory, radiochemistry laboratory with nuclear material handling allocation, and a mechanicals workshop. Microscopy facilities include a TEM on the B-laboratory side, and a SEM and light microscopy facilities in the A-class laboratory. The A-laboratory spans a part of the main floor shared with the B-laboratory, as well as the basement facilities. On the main floor it includes the hot cell high bay and the pilot hall, equipped with a 10 ton capacity bridge crane. The basement facilities are mainly for storage and handling of radioactive sources, specimens and waste, but include a liquid waste handling room, autoclave room and double hot cell. Transportation of radioactive materials into and out of the laboratory occurs by way of a separate, covered truck park, through an airlock into the basement. Transfer of radioactive materials within the laboratory is possible with a dedicated facility cask handled by the bridge crane and/or a cart. Overall the design process carried out over the last two years has yielded very satisfactory results and the facilities can be expected to serve the purposes well.
There has been an on-going effort to improve the performance of Co-Cr-Mo biomedical implant alloy... more There has been an on-going effort to improve the performance of Co-Cr-Mo biomedical implant alloys. However, efforts have been thwarted by the alloy's dependence upon carbon and carbide structures for strengthening, which are also associated with lower incipient melting temperatures, and lower ductilities. In this study, the effect of nitrogen additions to a Stellite 21 has been explored. Borrowing technology from its cousin, austenitic stainless-steels, additions were made to the alloy through nitrogen-high pressure melting (N-HPM) at pressures ranging from 1.4 to 50 atm. The ingots were then rolled into plates from which bars were extracted, and then heat-treated. This paper describes the microstructural development in terms of the second phase structure and grain size of heat treated plate materials. The total nitrogen achieved increased with pressure, and ranged from 0.19 to 1.34 wt%. The findings indicate not only that additions of nitrogen increase the second phase content, but also that as the nitrogen content is increased, a Cr 2 N second phase is favored over the traditional Cr 23 C 6 carbide. This phase change apparently leads to the subsequent elevation of the incipient melting temperature to above 1260°C (2300°F).
ABSTRACT Transmission electron microscopy (TEM) observations show that dislocation channel deform... more ABSTRACT Transmission electron microscopy (TEM) observations show that dislocation channel deformation occurs in pre-irradiated austenitic stainless steels, even at low stress levels (similar to 175 MPa, 290 degrees C) in low neutron dose (similar to 0.16 dpa, 185 degrees C) material. The TEM observations are utilized to design finite element (FE) meshes that include one or two "soft" channels (i.e. low critical resolved shear stress (CRSS)) of particular aspect ratio (length divided by thickness) embedded at the free surface of a "hard" matrix (i.e. high CRSS). The CRSS are adjusted using experimental data and physically based models from the literature. For doses leading to hardening saturation, the computed surface slips are as high as 100% for an applied stress close to the yield stress, when the observed channel aspect ratio is used. Surface slips are much higher than the grain boundary slips because of matrix constraint effect. The matrix CRSS and the channel aspect ratio are the most influential model parameters. Predictions based on an analytical formula are compared with surface slips computed by the FE method. Predicted slips, either in surface or bulk channels, agree reasonably well with either atomic force microscopy measures reported in the literature or measures based on our TEM observations. Finally, it is shown that the induced surface slip and grain boundary stress concentrations strongly enhance the kinetics of the damage mechanisms possibly involved in IASCC.
Irradiated AISI 304 stainless steel extracted from the Chooz A center filler assembly has been th... more Irradiated AISI 304 stainless steel extracted from the Chooz A center filler assembly has been the subject of a number of studies. Previously the results of slow strain rate tensile and constant load autoclave tests of 30 dpa material have been reported. They showed an influence of temperature, strain rate and environment on the fracture behavior of the material. The irradiated microstructure and deformation micro structures of those materials following testing have now been examined by TEM. The findings suggest that substantial channel deformation was associated with the purely ductile fracture following SSRT testing in argon, while the intergranular fractures following SSRT in simulated PWR environment and constant load testing in both simulated PWR and argon environments were associated with very localized deformation primarily exhibiting alpha martensite. This possibility is discussed in the light of literature.
This paper describes the methods and results of analytical TEM examinations and autoclave testing... more This paper describes the methods and results of analytical TEM examinations and autoclave testing of two highly-irradiated flux thimble tube materials harvested from a commercial pressurized water reactor. The materials are cold-worked 316L, and accumulated 65 dpa and 100 dpa of radiation dose. To set the baseline for a broader study, the materials were examined in the as-irradiated condition and tested as O-ring specimens at relatively high constant loads in simulated PWR water conditions. Tests were also conducted with elevated hydrogen. For a given load, more rapid cracking was associated with higher radiation dose, and with the elevated hydrogen.
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Papers by Wade Karlsen