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The transient behavior of the fuel cladding of a sodium-cooled fast reactor (SFR) is one of the main design issues concerning the safety of a reactor. The safety margin of the cladding during transient events has been widely studied in the past. However, previously, the transient testing was conducted using as-received specimens (and not aged specimens). For ensuring safety margin of the fuel rod of an SFR, the effect of aging on the transient behavior of the fuel cladding should be considered. This study examines this effect on the transient behavior of HT9 cladding by correlating between the UTS and the result of the ramp test and proposes a modified Dorn parameter that reflects the aging effect. At 100 MPa of the hoop stress, the modified Dorn parameter, which reflects an aging effect, is 7.7266 × 10 −14 and the Dorn parameter of as-received specimen is 1.749 × 10 −12 . This results in the enhanced safety during transient event by considering aging effect.

Precipitates in the two 10%Cr ferritic/martensitic steels in the normalized-and-tempered condition were investigated through transmission electron microscopy with an energy dispersive X-ray system. A large number of precipitates with large difference in size and block-like, or plate/rod-like, or sphere-like morphology have been observed in the two steels. Dominant precipitate phases in the two steels were Nb-rich carbide NbC and Cr-rich carbide M23C6. Z-phases have been found in the two steels. Two types of modified Z-phases, Cr(V,Nb)(C,N), with similar chemical composition and totally different crystal structures, body-centered tetragonal and NaCl-type cubic lattices, as well as original Z-phase, CrNb(C,N), were identified in the two steels. The modified and original Z-phases were observed to coexist with a clear orientation relationship within the same precipitate particles of the steel. The formation of Z-phase in the steels has also been discussed.

Oxidation characteristics of Alloy 617 and Haynes 230 at 900 °C in simulated helium environment, hot steam environment containing H 2 as well as in air and pure helium conditions were investigated. Compared to air condition, the oxidation rate of Alloy 617 was not significantly affected in helium and hot steam environments, while Haynes 230 showed lower oxidation rate in helium environment. On the other hand, the oxide morphology and structure of Alloy 617 were strongly affected by the environments, but those of Haynes 230 were less dependent on the environments. For Haynes 230, a Cr 2 O 3 inner layer and a protective MnCr 2 O 4 outer layer were formed in all environments, which contributed to the better oxidation resistance. As the mechanical properties, such as creep and tensile properties, were significantly affected by the oxidation behaviors, surface treatment methods to enhance oxidation resistance of these alloys should be developed.

Tensile and low cycle fatigue tests were conducted at RT-600 °C and cyclic softening behavior during low cycle fatigue was evaluated with temperature, strain range, heat treatment, aging, and welding. Yield stress decreased with temperature and was the highest for two step heat treatment and almost the same with welding. Elongation decreased to 400 °C, was minimum at 400 °C, and increased above 400 °C. Elongation was the lowest for91B of two step heat treatment. Cyclic softening was not much different with temperature to 500 °C but was high at 600 °C. Cyclic softening was almost the same with strain range at RT but increased with strain range at 300-600 °C. Cyclic softening decreased with welding and was not much different with aging. Amount of cyclic softening increased with temperature, was the lowest for weldment, and increased with yield stress. The decrease of dislocation density by cell structure formation and the increase of lath widthwas important for cyclic softening to 500 °C and recovery was important for cyclic softening at 600 °C.

The high temperature oxidation behavior of alloy 617 and Haynes 230 have been investigated for VHTR intermediate heat exchanger applications. Oxidation tests were carried out for up to 500 h at 900 °C and 1000 °C in impure helium environments containing H2, H2O, CO, CO2, and CH4. The oxidation kinetics of the alloys followed a parabolic rate law in all cases. In the impure helium environments with very low oxygen, the external oxides of alloy 617 were composed of a Cr2O3 layer, TiO2 ridges on the grain boundaries, and isolated MnCr2O4 grains on top of the Cr2O3 layer. On the other hand, those of Haynes 230 consisted of a Cr2O3 inner layer and a protective MnCr2O4 outer layer, which increased the oxidation resistance. The effect of small amounts of CH4 and H2 on the oxidation kinetics of the alloys was insignificant. Irregular oxide morphology, such as cellular Cr2O3 oxides for alloy 617 and MnCr2O4 platelets for Haynes 230, was formed in the impure helium environment at 900 °C. For Haynes 230, along with platelets, whiskers were frequently found at the tip of the MnCr2O4 oxide crystals.

The precipitate phases in a 11Cr ferritic/martensitic steel normalized at 1050 °C for 1 h followed by tempered at 750 °C for 2 h have been studied through electron micro-diffraction combined with EDX analysis. Two niobium-rich carbonitride phases, (Nb 0.75 V 0.15 Cr 0.1 )(C,N) and (Nb 0.5 V 0.4 Cr 0.1 )(C,N), with the same f.c.c. structure were identified. Three vanadium-rich carbonitride phases were identified to be (V 0.65 Nb 0.15 Cr 0.2 )(C,N), (V 0.45 Nb 0.45 Cr 0.1 )(C,N) and (V 0.55 Nb 0.25 Cr 0.2 )(C,N) with the same f.c.c. structure. Chromium-rich carbonitride M 2 (C,N) and chromium-rich carbide M 23 C 6 phases with a hexagonal and a f.c.c. crystal structure as well as a typical chemical formula of (Cr 0.8 V 0.2 ) 2 (C,N) and (Cr 0.7 Fe 0.25 W 0.05 ) 23 C 6 , respectively, were also identified in the steel sample. The size of each kind of precipitate phase was examined. The morphologies and precipitation sites along with the amount of observed precipitate phases were described and discussed.

The effect of tempering temperature on the precipitate phases in a 11%Cr ferritic/martensitic steel normalized at 1050 °C for 1 h and tempered for 2 h at temperatures ranging from 600 to 780 °C has been investigated using transmission electron microscope and energy-dispersive X-ray spectroscopy. The results show that tempering temperature does not affect the existences of niobium-rich carbonitrides, (Nb 0.7 V 0.2 Cr 0.1 )(C,N) and (Nb 0.55 V 0.35 Cr 0.1 )(C,N), vanadium–niobium-rich carbonitride (V 0.45 Nb 0.45 Cr 0.1 )(C,N), chromium-rich carbonitride (Cr 0.83 V 0.12 W 0.05 ) 2 (C,N) and chromium-rich carbide (Cr 0.7 Fe 0.25 W 0.05 ) 23 -C 6 , whilst the precipitations of vanadium-rich carbonitrides, (V 0.65 Nb 0.2 Cr 0.15 )(C,N) and (V 0.55 Nb 0.25 Cr 0.2 )(C,N) are dependent on tempering temperature, which were detected only at the higher tempering temperatures of 750 and 780 °C. No coarsening was occurred during the temperings for the niobium-rich and spherical vanadium-rich carbonitrides. There was a low coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides with increasing the tempering temperature from 600 to 700 °C and 650 to 780 °C, respectively, and a high coarsening rate for the chromium-rich carbonitrides and chromium-rich carbides at the tempering temperatures 750 through 780 °C and 650 °C, respectively. The compositions show an increase in vanadium and a decrease in niobium and chromium contents for the niobium-rich carbonitrides, and a decrease in niobium and an increase in vanadium and chromium contents for the vanadium–niobium-rich carbonitrides, and an increase in vanadium and a decrease in tungsten contents for the chromium-rich carbonitrides. The chromium-rich carbides show an increase and a decrease in their iron and chromium contents, respectively, with increasing the tempering temperature from 650 to 780 °C.

Low cycle fatigue tests of 9Cr-1Mo steel aged at 600°C for 5000 h and 10000 h were conducted at RT-600°C. Fatigue life was not decreased with aging. Cyclic softening was seen with cycles in both unaged and aged specimens. Aging decreased the amount of softening at RT and 400°C but increased it at 600°C. The precipitate species were not changed with aging upto 5000 h but Laves phase was precipitated after 10000 h aging. Fatigue life of 9Cr-1Mo steel aged to 10000 h is dependent on dislocation slip behavior rather than the amount and coarsening of precipitate.

Creep behaviours for Alloy 617, such as creep properties, oxidation and creep fracture morphologies were investigated by a series of creep tests with different stress levels at 1223 K. Creep constants of A, n, m, and C were obtained at 1223 K for Alloy 617. To accurately predict the long-term creep life (or strength) for Alloy 617, a multi constant method with two C in the Larson-Miller (LM) parameter was newly proposed instead of the conventional one with a unique C. The conventional method did not thoroughly match with the creep rupture data, and revealed an overestimation for the prediction of the long-term creep strength. On the other hand, the multi constant method revealed a good agreement with the creep rupture data, and this method was more accurate than the conventional one. The multi constant analysis can be used to accurately predict the long-term creep rupture life of Alloy 617 above 105 h for 1173 K, 1223 K and 1273 K.

This study dealt with how to mathematically obtain creep constants of C, Q/2.3R, T a and t a used in Time-Temperature Parametric (TTP) methods. A number of creep rupture data from 550 °C to 800 °C for Type 316LN stainless steel were collected through a worldwide literature survey and experimental data produced at KAERI. It was found that the creep rupture data revealed a linear relationship when the data were plotted by the semilogarithmic coordinates of log (t r ) versus stress. On the basis of this linear relationship, the creep constants of C, Q/2.3R, T a and t a used in the Larson-Miller (LM), the Orr-Sherby-Dorn (OSD), and the Manson-Haferd (MH) methods were obtained by a new numerical method. This approach can be utilized as a useful method to predict the long-term creep life of Type 316 stainless steel.

One of the reasons that ferritic/martenstic steels have been considered as candidate materials for nuclear power reactors is their superior creep resistance at elevated temperature. The creep rupture strength of 9% chromium steel could be improved by a fine dispersion of secondary precipitate phase. The precipitate phases in extra-low carbon 9% chromium steel with tempered conditions were investigated by transmission electron microscope and energy-dispersive X-ray analysis. The steel specimens were normalized and then tempered at different temperatures. Niobium-rich MN nitrides (Nb0.6V0.3Cr0.1)N, and two kinds of vanadium nitrides, (V0.6Nb0.2Cr0.2)N and (V0.45Nb0.45Cr0.1)N having a f.c.c. crystal structure, were identified in the steel specimens tempered at 600–780 °C, and 750 or 780 °C respectively. Hexagonal chromium-rich M2N precipitate phases with different lattice parameters, a = 2.80 A/c = 4.45 A and a = 7.76 A/c = 4.438 A, were determined in the tempered steel specimens. The M2N phase showed a decrease/an increase in its chromium/vanadium content as the tempering temperature was increased. The influence of precipitates and heat treatment conditions on the high temperature properties of 9% Cr steel was discussed.

To evaluate the oxidation resistance of Alloy 617 and Haynes 230, oxidation tests were performed at 900 °C and 1100 °C in air and helium environments. Scale characterizations were assessed on specimens exposed to air using thin-film XRD, XPS, SEM and EDX. Oxidation resistance was dependent on the stability of the surface oxide layer, which can be affected by minor alloying elements such as Ti and Mn. At 900 °C, for Alloy 617, a mixture of the extensive NiO–Cr2O3 double layer and isolated NiO–NiCr2O4–Cr2O3 triple layer were observed at a steady-state condition. For Haynes 230, a MnCr2O4 layer was formed on top of the Cr2O3 layer, resulting in a lower oxidation rate. At 1100 °C, both alloys showed a double layer consisting of an inner Cr2O3 and outer MnCr2O4 or TiO2. The spallation of outer layer and subsequent volatilization of the Cr2O3 layer produced a rugged surface and interface as well as internal oxidation.

The investigations on the precipitate phases in a 9%Cr ferritic/martensitic steel under different normalization conditions have been made by using a transmission electron microscope and an energy-dispersive X-ray spectroscopy. Hot-rolled steel samples were normalized at 1050–1200 °C for 1–2 h followed by an air cooling to room temperature. MN vanadium nitride precipitates with a plate-like morphology and a chemical formula of about (V0.4Nb0.4Cr0.2)N have been observed at triple junctions, grain boundaries and within matrix in the steel samples normalized at 1050–1150 °C for 1–2 h, but they were dissolved out at 1200 °C within 1 h. Vanadium nitride is a stable phase at 1050 °C according to thermocalc prediction of equilibrium phases in the steel. With increasing normalizing temperature and time, there was no a striking change in the chemical composition of metallic elements in the MN phase, but a considerable increase in the size of the MN precipitate.

M 2 N nitride phases of 9% chromium steels with an extra-low carbon content have been investigated using a transmission electron microscope and an energy-dispersive X-ray (EDX) spectroscopy. The steel samples were normalized for 1 h at 1050 °C and then tempered at 600–780 °C for 30 min to 5 h followed by an air cooling. Through the analyses of the electron micro-diffraction patterns and EDX data for the precipitate particles on the extracted carbon replica, two types of Cr-rich M 2 N nitride phases with the same hexagonal structure but totally different lattice parameters, a = 2.80 A/ c = 4.45 A and a = 7.76 A/ c = 4.438 A, were determined in the steels. Four types of Cr-rich M 2 N phases with different lattice parameters probably existed in the steels. The M 2 N phase revealed a decrease in its Cr content, an increase in its V content as the tempering temperature was increased, and no obvious change in its content for the metal fraction with an increasing tempering time.

Low-cycle fatigue tests with continuous cycling and creep–fatigue tests with 10 min hold times at tensile maximum strain were conducted at 600 °C in air for Type 316L and Type 316LN stainless steels containing nitrogen contents of 0.04% and 0.10%. The creep–fatigue life was less than the fatigue life for both alloys. The fatigue and creep–fatigue life and saturation stress were increased with the addition of nitrogen. The fracture mode was transgranular for fatigue and intergranular for creep-fatigue regardless of the nitrogen content. The dislocation structure was cellular for Type 316L and planar for Type 316LN after fatigue and creep-fatigue tests. Carbides were precipitated at grain boundaries after creep-fatigue tests and nitrogen decreased the precipitation. Creep–fatigue life was well predicted by a model based on cavity nucleation and growth at grain boundaries. The increase of creep–fatigue life with the addition of nitrogen was due to the decrease of precipitation and stress relaxation by the change in dislocation structure.

Low cycle fatigue properties of nitrogen-alloyed 316L stainless steel (SS) has been studied at various temperatures between room temperature and 873 K. Four heats of the alloy with nitrogen contents of 0.042, 0.103, 0.131 and 0.151 wt% were tested in fully reversed loading conditions at a constant strain amplitude of ±0.5% and strain rate of 2 × 10−3 s−1. Using the data, an artificial neural network model was developed to predict fatigue life of nitrogen-alloyed 316L SS. The neural network model could predict fatigue life within a factor of 2.0 of the experimental values over the whole range of test temperature and nitrogen content. The model was expanded to develop a unified model to predict fatigue life of 316 SS grade of stainless steel with and without nitrogen, and with normal and low carbon contents. The fatigue test parameters covered a wide range of temperatures, strain ranges and strain rates. The unified model was found to predict fatigue life at any test condition within a factor of 2.

The precipitation of nano Cr2N particles in high Cr FM steels has been studied. The nitrogen content of the FM steels was changed to form stable Cr2N particles. Tempering temperature was also changed from 500 oC to 800 oC to study the precipitation behavior of the Cr2N particles with the tempering temperature. The Cr2N particles remained as a stable phase at a higher tempering temperature by increasing the nitrogen content. The shape of these particle was a fine needle type which was very similar to V(C,N) particles. The size of some Cr2N particles was increased as the nitrogen content increased. But these precipitates were not dissolved or largely coarsened during a creep deformation at 600°C. So it seems that they may act as an effective obstacle against a dislocation glide during a creep deformation, thus contribute to an increase of the creep rupture strength in high Cr FM steels.

Tensile and fatigue properties were evaluated for base and welded type 316LN stainless steel. Welding methods were GTAW (308L, Ar environment) and GTAWN (316L, Ar + N2 environment). Yield strength of weld joint was higher than that of base metal but elongation of weld joint was lower than that of base metal. UTS of weld joint was slightly lower than that of base metal. Yield strength and elongation with welding method were almost same. Fatigue life of weld joint was lower than that of base metal but fatigue strength of weld joint was higher than that of base metal. Ferrite content was increased with welding. Fatigue life welded by GTAWN was better than that of GTAW at RT and 600°C. This fatigue life behavior was consistent with the behavior of ferrite content.

The effects of thermal aging on microstructural evolution and mechanical properties are important in the understanding of the in-service behavior of ferritic/martensitic steels in advanced nuclear power system. Ferritic/martensitic steels have been aged at 600oC for times up to 20,000 hrs. The change of mechanical properties has been examined for these aged materials. The strength and hardness was hardly changed after the thermal aging at 600oC for 20,000 hrs in all specimens. The impact absorbed energy decreased with the aging time. But the decrease of the impact absorbed energy was larger at the early stage of aging in tungsten added steels. This is attributed to the formation of Laves phase. Nitrogen which is known to increase the creep rupture strength had no effect on the degradation of the microstructure and mechanical properties during thermal aging.

Inconel 617 is a candidate tube material for high temperature gas-cooled reactors (HTGR). The microstructure and mechanical properties of Inconel 617 were studied after exposure at high temperature of 1050oC. The dominant oxide layer was Cr-oxide. The internal oxide and Crdepleted region were observed below the Cr-oxide layer. The major second phases are M23C6 and M6C types of carbides. The composition of M23C6 and M6C were determined to be Cr21Mo2C6 and Mo3Cr2(Ni,Co)1C, respectively, by EDS. These carbides are coarsened during exposure. M6C carbide is more stable than M23C6 at high temperature. There was not much change in mechanical properties after exposure at 1050oC for 1000 h.

There has been a great progress in the development of heat-resistant silicon carbide ceramics, owing to the better understanding of composition-microstructure-properties relations. Based on the progress, it has been possible to fabricate heat-resistant SiC ceramics with improved fracture toughness. In this paper, three rare-earth oxides (Re2O3, Re=Er, Lu, and Sc) in combination with AlN were used as sintering additives for a β-SiC containing 1 vol% α-SiC seeds. The effect of intergranular phase, using Re2O3 and AlN as sintering additives, on the microstructure and mechanical properties of liquid-phasesintered, and subsequently annealed SiC ceramics were investigated. The microstructure and mechanical properties were strongly influenced by the sintering additive composition, which determines the chemistry and structure of IGP. The strength and fracture toughness of the Lu2O3-doped SiC were ∼700 MPa at 1400oC and ∼6 MPa.m1/2 at room temperature, respectively. The beneficial effect of the new additive compositions on high-temperature strength was attributed to the crystallization of the intergranular phase.

The creep properties for the Hastelloy-X alloy which is one of candidate alloys for a high temperature gas-cooled reactor are presented. The creep data was obtained with different stresses at 950oC, and a number of the creep data was collected through literature surveys. All of the creep data were combined together to obtain the creep constants and to predict a long-term creep life. In the Norton’s creep law and the Monkman-Grant relationship, the creep constants, A, n, m, and m’ were obtained. Creep master curves based on the Larson-Miller parameter were presented for the standard deviations of 1σ, 2σ and 3σ. Creep life at each temperature was predicted for a longer-time rupture above 105 hours. Failure probability was also estimated by a statistical process of all the creep rupture data.

Fatigue tests of type 316 and 316LN stainless steel were conducted at RT and 600ı, 0.8~1.5% strain range for low cycle fatigue (LCF), 300~600ı, 0% strain range for thermal fatigue (TF) and 300~600ı, 2% strain range, in-phase or out-of-phase for thermomechanical fatigue (TMF). LCF, TF, and TMF lives were increased but saturation stresses were decreased with the addition of nitrogen. The higher temperature was the lower TF life at a same temperature change. The minimum temperature change for TF failure was more than 100ı. TMF life was higher at inphase condition than at out-of-phase condition. Fracture mode was transgranular for LCF and outof- phase of TMF and almost transgranular and small intergranular for TF and in-phase TMF.

This paper is to evaluate the creep crack growth rate (CCGR) of the type 316SS series: 316SS, 316FR and 316LN, and to apply a creep ductility model. A number of the data are collected through wide literature surveys and experiment, and evaluated by the C* parameter. The results of the CCGR data were nearly matched with a small scattering band regardless of the different applied stresses, temperatures and test specimens configuration. In the CCGR, type 316FR and 316LN steels were slower than type 316SS. Type 316SS showed a better agreement in the application of the creep ductility model than the type 316FR and 316LN steels.

The effect of nitrogen on creep rupture strength and microstructure evolution in 10Cr–N steels has been studied. Creep testing was carried out at 600 and 650 °C under constant load conditions. The prior austenite grain size and martensite lath width were decreased by addition of nitrogen. The Cr 2 X precipitates were stabilized at 750 °C as the nitrogen content increased. The 0.08 wt% N steel had a minimum martensite lath width and finer Cr 2 X precipitates in the as-tempered state. Creep rupture strength increased with increasing nitrogen content. The optimum nitrogen content for creep rupture strength was 0.08 wt% in 10Cr ferritic/martensitic steels, and Cr 2 X precipitates were not dissolved after creep deformation. These fine and stable Cr 2 X precipitates may contribute to the increase of creep rupture strength.

In order to analyze the creep behaviour of Cr-Mo steels, an elasto-viscoplastic constitutive model based on dislocation density considerations is described. A combination of a kinetic equation, which describes the mechanical response of a material at a given microstructure in terms of dislocation glide, and evolution equations for internal variables characterising the microstructure provide the constitutive equations of the model. Microstructural features of the material are implemented in the constitutive equation. The internal variables are associated with the total dislocation density. The model has a modular structure and can be adjusted to describe a particular type of materials behaviour and metal forming processes. In this paper, the predicted creep behaviour of Cr-Mo steels is compared with the experimental results.

Tensile, strain-controlled low cycle fatigue (LCF), and crack propagation tests were conducted at RT and 600 °C in air atmosphere for type 316L(N) stainless steels containing 0.04% N and 0.10% N. Nitrogen increased tensile strength without the reduction of elongation. LCF life increased with nitrogen content and the magnitude of increase was higher at 600 °C than at RT. Fatigue crack propagation rate increased with temperature but decreased with the addition of nitrogen. Nitrogen retarded dynamic strain aging (DSA) and decreased crack propagation. The larger increase of LCF life at 600 °C than that observed at RT with the addition of nitrogen was attributed to the retardation of DSA by the addition of nitrogen.

The Monkman-Grant (M-G) and its modified parameters were evaluated for type 316LN and modified 9Cr-Mo stainless steels prepared with minor element variations. Several sets of creep data for the two alloy systems were obtained by constant-load creep tests in 550-650°C temperature range. The M-G parameters,m, m’,C, andC’ were proposed and discussed for the two alloy systems. Them value of the M-G relation was 0.90 in type 316LN steel and 0.84 in modified 9Cr-Mo steel. Them’, value of the modified relation was 0.94 in type 316LN steel and 0.89 in 9Cr-Mo steel. Although creep fracture modes and creep properties between type 316LN and modified 9Cr-Mo steels showed a basic difference, the M-G and its modified relations demonstrated linearity quite well. Them’ of modified relation almost overlapped regardless of the creep testing conditions and chemical variations in the two alloy systems, and the parameterm’ was closer to unity than that of the M-G relation.

The Kachanov and Rabotnov (K-R) creep damage model was interpreted and applied to type 316LN and HT-9 stainless steels. Seven creep constants of the model,A, B. k, m, λ, γ, andq were determined for type 316LN stainless steel. In order to quantify a damage parameter, the cavity was interruptedly traced during creep for measuring cavity area to be reflected into the damage equation. For type 316LN stainless steel, λ=e R /e* and λ f =e/e R were 3.1 and increased with creep strain. The creep curve with λ=3.1 depicted well the experimental data to the full lifetime and its damage curve showed a good agreement whenr=24. However for the HT-9 stainless steel, the values of A and A/ were different as λ=6.2 and λ f =8.5, and their K-R creep curves did not agree with the experimental data. This mismatch in the HT-9 steel was due to the ductile fracture by softening of materials rather than the brittle fracture by cavity growth. The differences of the values in the above steels were attributed to creep ductilities at the secondary and the tertiary creep stages.

The effect of W and N on the creep properties and microstructural degradation in 10Cr steels was studied. Creep testing was performed to determine the creep rupture strength and minimum creep rate. Transmission electron microscopy was used to observe the microstructural degradation during the creep deformation. W and N which were added to the 10Cr steel increased the creep rupture strength and decreased the minimum creep rate. As W and N were added, the thermal stability of the subgrain and carbide was improved, thus the growth of the subgrain and carbide during creep deformation was restricted. In W added steel, the Laves phase played an important role in increasing creep rupture strength. But the impact toughness was rapidly degraded by the addition of W after aging at 600°C for 5000 hours. So one must evaluate more accurately the effect of the Laves phase on long term creep and impact properties. In N added steel, V(C, N) was precipitated in the lath boundary and within the lath. The size of the precipitates was 20–50 nm. The increase of creep rupture strength in N added steel may be due to the precipitate of the V(C, N). Future tests are required to clarify the effect of N on creep and impact properties.

The effect of grain size on creep properties of type 316LN stainless steel has been investigated at 600°C under different stresses. The initial strain at the beginning of creep tests decreased with the decrease of grain size. This was confirmed by the Hall-Petch relationship. The steady state creep rate decreased to a minimum value at the intermediate grain size (dm=80–130 μm) and then increased with the further increase of grain size. This result agreed with Garofalo's model stating that grain boundaries act simultaneously as both dislocation sources and barriers to dislocation movement. The rupture elongation at the intermediate grain size was minimal due to the cavity formed easily by carbide precipitates in the grain boundaries.

The effects of irradiation on fracture toughness of modified 9Cr–1Mo steel in the transition region were investigated. Half size precracked Charpy specimens were irradiated up to 1.2 × 10 21 n/cm 2 ( E > 0.1 MeV) at 340 °C and 400 °C in the Korean research reactor. The irradiation induced transition temperature shift for a modified 9Cr–1Mo was evaluated by using the Master Curve methodology. The T 0 temperature for the unirradiated specimens were measured as −67.7 °C and −72.4 °C from the tests with standard PCVN (precracked charpy V-notch) and half sized PCVN specimens, respectively. The T 0 shifts of specimens after irradiation at 340 °C and 400 °C were 70.7 °C and 66.1 °C, respectively. The Weibull slopes for the fracture toughness data obtained from the unirradiated and irradiated modified 9Cr–1Mo steels were determined to confirm the applicability of master curve methodology to modified 9Cr–1Mo steel.

To provide the design data for the Hastelloy-X alloy, the tensile and creep test data was obtained experimentally, and a number of creep data was also collected through literature surveys. All of the experimental and collected data was combined to determine a minimum design value for a tensile stress and to predict a long-term life for a creep rupture time. The design yield and ultimate tensile stress curves for the Hastelloy-X alloy were determined by a best fitting all of the data up to 1000 °C. Creep master curves based on the Larson–Miller parameter were obtained for the standard deviations of 1σ, 2σ and 3σ. The creep-rupture life for up to 105 h was predicted for the Hastelloy-X alloy.

The precipitates in nitrogen-added type 316L stainless steels (SS) were investigated by transmission electron microscopy (TEM) after thermal aging. Besides carbides (M23C6 and M6C) and intermetallic phases, an unknown phase of an Fe–Mo–Cr(–Si) system n a filmlike morphology precipitated at grain boundaries. In spite of the similarity in ts chemical composition to that of the Laves phase, the phase of the Fe–Mo–Cr(–Si) system exhibited five-, three-, and twofold symmetries, which are generally observed in quasicrystals having icosahedral symmetry. This phase was formed from the intergrowth of small crystalline clusters of the Laves phase. Decreasing the nitrogen content to that of commercial type 316L grade suppressed the formation of the filmlike fivefold phase. This was attributed to the dissipation of small Laves clusters by M23C6 carbides, which increased as a result of the decreased nitrogen content.

Strain-controlled low cycle fatigue (LCF) tests were conducted in the temperature range of RT–600°C and air atmosphere to investigate the nitrogen effect on LCF behavior of type 316L stainless steels with different nitrogen contents (0.04–0.15%). The waveform of LCF was a symmetrical triangle with a strain amplitude of ±0.5% and a constant strain rate of 2×10−3/s was employed for most tests. Cyclic stress response of the alloys exhibited a gradual cyclic softening at RT, but a cyclic hardening at an early stage of fatigue life at 300–600°C. The hardening at high temperature was attributed to dynamic strain aging (DSA). Nitrogen addition decreased hardening magnitude (maximum cyclic stress — first cyclic stress) because nitrogen retarded DSA for these conditions. The dislocation structures were changed from cell to planar structure with increasing temperature and nitrogen addition by DSA and short range order (SRO). Fatigue life was a maximum at 0.1% nitrogen content, which was attributed to the balance between DSA and SRO.

The dynamic strain ageing (DSA) behaviours of type 316L stainless steels containing different nitrogen contents (0.01–0.15 wt% N) were studied in tension under varying strain rates (1 × 10−2−2 × 10−4s−1) and the test temperatures (R.T.–1023 K). The temperature range for DSA was moved to higher temperature for increasing nitrogen contents. The critical strain, ec for the onset of serration increased with nitrogen content at 773 K and then became almost constant at 873 K. Type A and B serrations were observed at 873 K with the value of the strain required to effect the transition from type A to type B serration increasing for nitrogen contents upto 0.1 wt% and then becoming saturated. The activation energy for DSA was 23.4–26.2 kcal mol−1 (97.8–109.5 kJ mol−1) at the onset and 65.0–76.6 kcal mol−1 (271–320.2 kJ mol−1) at the end of serration. The lower activation energy was related to vacancy diffusion and the higher activation energy was attributed to the diffusion of chromium to dislocations. The activation energy for DSA was slightly increased with nitrogen addition. DSA was retarded by an increase in the nitrogen content since nitrogen reduced the chromium diffusion to dislocations due to a strong interaction between the nitrogen and chromium.

Type 316 L stainless steel specimens were creep rupture tested at 620 °C in a vacuum. The FeO type wustite phase with the lattice parameter of about 4.056 A was identified on the creep cavity wall. The {200} planes of the oxide phase have shown the epitaxial growth on the {111} planes of the matrix phase. As the oxide film forms on the creep cavity wall, the surface energy term of the cavitation could decrease, making the critical nucleation radius of the creep cavity smaller and the growth rate faster.

Creep behavior and degradation of creep properties of high-temperature materials often limit the lives of components and structures designed to operate for extended period under stress at elevated temperatures. A nickel-base superalloy, Alloy 617 in particular which is considered as a prospective material for hot gas duct and intermediate heat exchanger in very high temperature gas cooled reactor, was studied for creep properties. Creep tests were carried out under various sustained tensile loadings in air and helium environments at temperature of 800°C, 900°C, and 1000°C. Times for 1% creep strain and creep rupture were taken from the short-term creep tests within 1000 hours. Effect of creep-environment interaction on creep strain and changes in viscous deformation properties by dynamic recrystallization were discussed.Copyright © 2008 by ASME

Since very high temperature gas-cooled reactor (VHTR) components for a hydrogen production are designed for a lifetime of 30 to 60 years at a high-temperature of 950°C, a material selection is important, and also one of the most important properties in these metallic components is their longterm creep behavior. In this study, for selecting a suitable material for the intermediate heat exchanger (IHX), the hightemperature strengths of several candidate alloys approved for use by ASME VIII (boiler and pressure vessel code) were reviewed. From this review, alloy 617 was immediately selected as one of the best materials. In addition, to provide design data for a long duration for alloy 617, lots of creep data was collected through world-wide literature surveys of European Alloy-DB and available literature surveys. Using these data, a long-term creep life for alloy 617 was predicted for up to 106 hours based on the Larson-Miller (LM) parameter. Also, the creep rupture data was analyzed by a statistical treatment. Creep master curves for the creep-life prediction were presented for various standard deviations; 1σ, 1.65σ, 2σ and 3σ. A minimum design value for the creep-rupture time was found to be 10 times lower than the average value. Failure probability of the creep rupture data followed a lognormal distribution well.Copyright © 2007 by ASME

Short-term high temperature degradation test was conducted on Hastelloy X, a candidate tube material for high temperature gas-cooled reactors (HTGR), to evaluate the variation of microstructure and mechanical property in air at $1050^{\circ}C$ during 2000 h. The dominant oxide layer was Cr-oxide a...

Iodine-induced stress corrosion cracking (I-SCC) experiments on Zircaloy-4 tube were undertaken in the iodine concentration range of 0–4 mg/cm2, the temperature range of 330–400 ° C and a nominal hoop stress of 475 MPa, using the internal pressurization method. The time-to-failure, failure strain and strain rate were measured as a function of the iodine concentration. An apparent activation energy for I-SCC was calculated from the temperature dependence of I-SCC. A fractographic interpretation was also made through scanning electron microscopic examination. The results suggest that the iodine concentration has an influence on the crack propagation step to increase the propagation rate of I-SCC, promoting the I-SCC susceptibility. It is found that the critical iodine concentration means the phenomenological value to show the iodine concentration dependence of I-SCC susceptibility, not the critical value to determine the occurrence of I-SCC. The I-SCC behavior of Zircaloy is discussed from the viewpoint of the crack initiation and propagation process.