In order to improve the thermal efficiency of power station, it is necessary to elevate the operation temperature and pressure of boilers. This condition usually requires the usage of a large quantity of stainless steels and superalloys, which brings in an increase in construction cost and consequently in per kW power cost. Thus, it is important to develop a new kind of steel which is economical and can be a substitute for austenitic stainless steel tubing. In the present study, the effects of molybdenium, vanadium, titanium, niobium, wolfram, cupper and boron on the creep rupture strength of medium chromium steels were investigated, and a new steel with excellent creep rupture strength, workability, and weldability was developed. The chemical composition of the new steel is low carbon-9%Cr-1%Mo-0.2%V-0.3%Nb-B, and its 100000hr creep-rupture strength is 8.0kg/mm2 at 600°C.
Logarithmic plastic strain rate (log εp) versus logarithmic residual stress (logσr) curves of stress relaxation data show an inflection of a convex nature in a certain stress range. Although the low stress range of the curve has been called the secondary relaxation and considered to correspond to the steady state creep, no experimental verification has been given. In this paper, the correspondence of the secondary relaxation to the steady state creep is examined using long-time stress relaxation data up to 10000hr on 1Cr-0.5Mo-0.25V, 12Cr-1Mo-1W-0.25V and 19Cr-9Ni-Mo-W-Nb high temperature bolting steels. Good correlation could not be obtained between the residual stress at the point of inflection on log σrvs. log εp curve and the stress estimated graphically from a trace of plastic strain of stress relaxation on the transient point of creep curves using the strain-hardening theory. The comparison of the time at the point of inflection showed no good correlation between stress relaxation and creep. There was no good agreement between the dependence of the plastic strain rate on residual stress at secondary relaxation and that of the steady state creep on creep stress. The apparent activation energy of relaxation was different between at higher stresses (primary relaxation) and at lower stresses (secondary relaxation). A good linear relation between the time at the point of inflection on log σrvs. log εp curves and that at the starting-point of structural changes was observed. It was concluded that the secondary relaxation does not correspond to the steady state creep, but corresponds to the metallurgical structural changes during the course of relaxation.
As a method of estimating the creep damage of alloys in service, the void formation in the microstructure of centrifugal cast alloys (HK 40 and HP) was investigated. The results obtained are summarized as follows: (1) A quantitative indication of creep void formation can be made by taking the void area percent based on the point count method (JIS G 0555). (2) The relationship between the void area percentage and the creep damage percentage is linear throughout the secondary and tertiary creep stages. (3) In the case where a tube is subjected to bending stress, the creep damage in the tube wall cross-section can be related to the distribution of the creep void area percentage. (4) The examination of actual alloys in service for creep damage by the present creep void formation method gave almost the same results as those obtained by the usual creep rupture test.
In order to estimate the creep and rupture strengths of candidate alloys for the intermediate heat exchanger of VHTR, creep and stress rupture tests in impure helium were conducted on Hastelloy X, Inconel 617, Inconel 625, Incoloy 800 and Incoloy 807 at 900°C. The results were discussed in comparison with those in air and the alloys were examind from the point of view of the elevated temperature structural design. The main results obtained are summarized as follows: (1) No appreciable decrease in creep and rupture strengths in helium as compared with those in air is observed on Hastelloy X and Inconel 625. On the contrary, the creep and rupture strengths of Inconel 617 in helium decrease slightly as compared with those in air. Meanwhile, the creep and rupture strengths of Incoloy 800 in helium are remarkably lower than those in air. In the case of Incoloy 807, the creep strength to cause 1 percent total strain and that to initiate secondary creep increase remarkably in helium as compared with those in air. However, the creep strength to cause initiation of tertiary creep and the rupture strength in helium remarkably decrease as compared with those in air. (2) The order of magnitude of the So value for each material in helium is as follows; Hastelloy X>Inconel 617>Incoloy 807>Inconel 625>Incoloy 800 Meanwhile, that of the St value in helium is; Inconel 617>Hastelloy X>Incoloy 807>Inconel 625>Incoloy 800.
The effect of environment on high temperature fatigue properties of 304 stainless steel was studied. Strain-controlled low-cycle fatigue tests were conducted with a symmetrical and an asymmetrical strain-wave forms at 700° and 800°C in air and vacuum. The results were analyzed and the partitioned strain-life relations Δεij-Nij were determined by the strain-range partitioning approach. The main results obtained in this study are as follows: (1) Δεpp-Npp and Δεcc-Ncc properties are sensitive to environmental effects, while not Δεpc-Npc and Δεcp-Ncp properties. (2) Each of Δεij-Nij properties in air and vacuum is insensitive to the test temperature. (3) Δεpp-Npp and Δεcc-Ncc properties in vacuum can be estimated by the Manson-Coffin equations as follows: Δεpp=Dp0.6Npp-0.6(Dp=-ln(1-ψp)), Δεcc=Dc0.6Ncc-0.6(Dc=-ln(1-ψc)) where ψp and ψc are the values of the reduction in area during tensile and creep rupture test, respectively. (4) When Δεpp-or Δεcc-type of strain is cycled, a crack tends to initiate at the specimen surface and grow to the final failure. This fact is probably the main reason of the remarkable effect of environment on partitioned fatigue life Npp or Ncc. (5) On the other hand, the reason of no environmental effects on Δεpc-Npc and Δεcp-Ncp is probably due to the fact that the specimen failure is caused by the tensile failure mode when Δεpc-type of strain is cycled and by the coalescence or connection of grain-boundary cracks inside the specimen when Δεcp-type of strain is cycled.
Low-cycle fatigue tests were carried out on a Type 403 martensitic stainless steel at room temperature, 400, 500, and 600°C. The strain rates of triangular wave forms used were 40, 4 and 0.4%/min. Cyclic softening behavior was observed at each test condition. The effect of strain rate on low-cycle fatigue lives was scarcely observed. The temperature dependence of low-cycle fatigue life for a given plastic strain range coincides qualitatively with that of reduction of area in tensile test. However, it cannot be explained quantitatively by the Manson's equation. The fatigue lives estimated by Tomkins' analysis coincide with experimental values when Δεp(Δσ/σu)2 is used as a mechanical parameter.
The combined creep-fatigue tests, in which the stress-controlled creep loading and the strain-controlled fatigue loading are repeated alternately, were carried out on SUS 304 austenitic steel to investigate the effect of creep stress on the creep-fatigue interaction. The creep period under constant loading was 10hr in a combined cycle, the total strain range in fatigue loading was 1%, and the numbers of imposed fatigue-cycles (N) were 2 and 20. In the test condition with large creep damage (N=2), the total rupture time depended on creep stress. When large fatigue damage was imposed (N=20), little dependency of creep stress was observed. The fracture mode was intergranular for the test condition of N=2. The voids were observed at the grain boundaries perpendicular to the stress axis, and the cracks of a few grain-facet size were observed in the fractured specimen. The fracture seemed to result from the propagation and linkage of grain-facet cracks. For N=20, the fracture mode was transgranular. The fracture caused by the propagation of surface cracks. The linear damage rule and the strain range partitioning method were used to evaluate the results obtained from the tests. For N=20, the actual life agreed with the life predicted from both method. For N=2, the actual life was shorter than the predicted life with lowering creep stress, that is, with increasing life.
The influences of cyclic load and temperature upon the creep crack growth rate of 2024 aluminum alloy have been studied. The main results obtained are summarized as follows. (1) The creep crack growth rate increases with increasing testing temperature. The apparent activation energy of creep crack growth decreases in the range of 26∼32kcal/mol with decreasing stress intensity factor. (2) The crack growth rate in the range of creep fatigue interaction can be estimated on the basis of linear damage rule.
Based on the satisfactory results of the recent studies on a J-integral approach to the creep crack propagation at elevated temperatures and the fatigue crack propagation at room temperature, the applicability was studied of the creep J-integral to the time dependent fatigue crack and of the cyclic J-integral to the cycle dependent fatigue crack under combined creep-fatigue conditions. A simple method for experimental estimation of the creep J-integral range and the cyclic J-integral was proposed for a center cracked plate (CCP) specimen with a rather shallow crack. Using a 0.16% carbon steel (S15C), the crack propagation test was carried out at 400°C under various frequencies. The correlation between time dependent fatigue crack propagation rate and creep J-integral was found at relatively low frequencies, and that between cycle dependent fatigue crack propagation rate and cyclic J-integral was exhibited at comparatively high frequencies. The creep-fatigue interaction between the two characteristic crack propagations was scarcely recognized, which led to the conclusion that the crack propagation under combined creep and fatigue condition was regarded as being controlled by more dominant one. Also found was the fact that the transition of the crack propagation rate from cycle dependent fatigue to time dependent creep or vice versa did not take place due to the increase in J-integral value during crack propagation.
Thermal shock experiments were conducted on nuclear pressure vessel steel A533 Grade B Class 1. Elastic-plastic fracture toughness tests were carried out within the same high temperature range of the thermal shock experiment and the relation between stretched zone width, SZW and J-integral was clarified. An elastic-plastic thermal shock fracture toughness value, Jtsc was evaluated from a critical value of stretched zone width, SZWtsc at the initiation of thermal shock fracture by using the relation between SZW and J. The Jtsc value was compared with elastic-plastic fracture toughness values, JIc, and the deference between the Jtsc and JIc values was discussed. The results obtained are summarized as follows; (1) The relation between SZW and J before the initiation of stable crack growth in fracture toughness test at a high temperature can be expressed by the following equation regardless of test temperature, SZW=95(J/E), where E is Young's modulus. (2) Elevated temperature fracture toughness values ranging from room temperature to 400°C are nearly constant regardless of test temperature. It is confirmed that upper shelf fracture toughness exists. (3) Thermal shock fracture toughness is smaller than elevated temperature fracture toughness within the same high temperature range of thermal shock experiment.
The thermal fatigue strength of cast steel for turbine casings was examined. The thin-walled cylindrical type specimens with a through-hole were used for thermal fatigue tests. We observed the propagating crack from the hole by means of a travelling microscope, and took the photographs of the ruptured surface by a scanning electron microscope. Basing on those results, we studied the factors affecting the rule of crack propagation rate. We also examined the propagation of a part-through crack, which is important in practical use. Both results obtained above were compared with each other and discussed. The conclusions obtained are as follows; (1) When striations were almost dominant on the ruptured surface, the rate of thermal fatigue crack propagation was expressed by dl/dn∝(Δεin)αl where Δεin, l and α are the inelastic strain range, half crack length and material constant, respectively. (2) The ruptured surfaces consisted of three types of rupture mode, that is, intergranular fracture surface, striation pattern and dimple pattern, and the striation spacing agreed with the macro-rate of thermal fatigue crack propagation. As the dimple pattern began to get dominant on the ruptured surface, under the condition of out-of-phase, many surface cracks began to grow on the specimen's surface, and the main crack propagated coalesing with them rapidly. (3) In thermal fatigue fracture, the rate of part-through crack propagation was about half of that of through crack.
In order to investigate the properties of thermal fatigue strength under creep-fatigue interaction, in-phase and out-of-phase thermal fatigue tests at the temperature ranges of 200-550 and 300-600°C as well as isothermal low-cycle fatigue tests at 550 and 600°C were carried out on three kinds of 304 stainless steel. Based on the relation between the fatigue life and the failure mode, the time-dependent effect on the fatigue life was discussed. Also, an attempt was made to apply the strain range partitioning method to the thermal fatigue life prediction. As a result, it seemed difficult to evaluate the thermal fatigue life at high temperatures simply from the isothermal fatigue life under the same strain condition. It was also found that an unbalanced creep strain during tensile loading, which increased the number of intergranular cracks, gave the largest damage to the material. By the strain range partitioning method, it was possible to predict the isothermal fatigue and the thermal fatigue at the low temperature range within a factor of 1.5. On the other hand, the thermal fatigue life at the high temperature range could be predicted within a factor of 3. However, further detailed investigations are required on the technique of partitioning the inelastic strain range and the effects of dynamic strain aging and recovery during strain holding for a better prediction.
In order to clarify the thermal fatigue life of welded austenitic (SUS 316) ferritic (2.25 Cr-1 Mo) transition joints, the stress and strain behaviors near the weld interface of the joints were analysed by numerical inelastic analysis. The fundamental life relationships of creep rupture and fatigue strength were obtained by using uniaxial test specimens prepared from the joints. Then the lives of the joints were estimated on the basis of the creep-fatigue linear damage rule. The results are summarized as follows. (1) The single bevel grooved butt joint with austenitic (JIS D309) weld metal has a slightly shorter life than the single-Vee grooved butt joint with the same weld metal. (2) The single-Vee grooved butt joint with ferritic buttered layer and austenitic weld metal has a life 1.5 times that of the same joint with only austenitic weld metal. (3) The single-Vee grooved butt joint with Incone type (JIS DNiCrFe) weld metal has a life 7 times that of the same joint with austenitic weld metal.