Modified 9Cr-1Mo steels (P91) have been used for high temperature steam pipes in ultra super critical power plants not only in Japan but also in other countries. Since weld joints of P91 indicate lower creep damage resistance than base metal, it is necessary to establish damage assessment method for weld joints of P91 to maintain reliable operation. In this study, creep tests have been conducted on a P91 to identify creep rupture life property of weld joints, and creep damage process in heat affect zone (HAZ) has been clarified by interrupting the creep tests. As a result, it was found that creep rupture time of the weld joints reduced to about 1/5 of the base metal and creep voids have already initiated at 20% of creep life in the HAZ. Size and number of voids in the HAZ increased with increasing creep damage. Stress distribution within the weld joint specimen was calculated by a finite element analysis of a weld joint model consisting of base metal, weld metal and HAZ. It was indicated that creep strain accumulated in the HAZ preferentially resulting in Type IV failure of the weld joint specimen. Creep rupture time of the weld joint can be predicted by creep strain rate in the HAZ and “limited creep strain” defined in this study. Authors proposed the creep void growth simulation method in the previous study based on void kinetics. This method applied to creep void growth simulation in the HAZ of the P91 weld joint. It was confirmed that quantitative void growth behavior could be predicted by the void growth simulation method.
Mod.9Cr-1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to Type-IV creep cracking formed in heat affected zone (HAZ) at high temperatures. The present paper aims to clarify the damage process and microstructural degradation of the welded joint. Long-term creep tests for base metal, simulated HAZ and welded joints were conducted at 550, 600 and 650°C. Furthermore, creep tests of thick welded joint specimens were interrupted at several time steps for 600°C, 90MPa, and the damage distribution was measured. It is found that creep voids initiate at early stage of creep life (t/tr = 0.2), and coalesce into the crack at the later stage of life (t/tr = 0.9). Creep damages concentrate mostly at a quarter depths of the plate thickness in the fine-grained HAZ. The experimental creep damage distribution was compared with the computed one by using the damage mechanics analysis.
Creep strength of high Cr steel welds is decreased for long-term services at high temperatures due to Type-IV failure occurs in fine-grained HAZ. Aiming at improving the creep strength of HAZ, we have investigated the effect of boron and nitrogen content on the microstructures and creep strength of the 9Cr and 12Cr heat resisting steel welds. It was found that the creep strength of HAZ of these steels could be improved by addition of about 0.01% boron with low nitrogen. For the 9Cr steel, while formation of the fine-grained HAZ structures was inevitable independent of boron and nitrogen contents, the creep strength of HAZ and welded joint could be improved by utilizing the grain boundary strengthening effect of boron. For the 12Cr steel with 0.01% boron and low nitrogen, formation of the fine-grained microstructures during weld thermal cycle was suppressed and the creep strength of HAZ and welded joint was considerably improved.
Structural materials experience various multiaxial stress states and their integrity under such conditions needs to be evaluated in design and life management. Especially creep rupture behavior is known to be quite sensitive to the stress multiaxiality. To systematically evaluate the multiaxial effect on creep rupture behavior of modified 9Cr-1Mo steel, a number of creep tests were conducted on round-bar specimens with circumferential notches. Strong effects of temperature and inelastic strain rate on rupture strain were observed and their synergetic effect was modeled by a simple expression. Then crack growth in compact tension specimens was simulated by finite element analysis incorporating primary, secondary and tertiary creep strains to derive ductility under higher stress triaxiality. Finally, true rupture strain was expressed as a function of temperature, inelastic strain rate and triaxiality factor and its validity was demonstrated through finite element analyses on notched bar and compact tension specimens employing it as a local ductility criterion.
For the purpose of residual life prediction of high-temperature components after long-term service, both stress-strain behavior for evaluating fracture mechanics parameters and crack growth behavior were examined using CrMoV cast steel taken from a main steam valve served for 155,000 hours at 811K under 216 start-stop cycles. Firstly the influence of cyclic softening and thermal aging history on the stress-strain curve was examined. As a result of comparison between virgin and long-term served material, a possibility of predicting the stress-strain behavior of the long-term-served material from that of virgin material was discussed. Secondly creep and fatigue crack growth behavior of long-term served material were experimented and compared with those of virgin material. Finally creep-fatigue crack growth tests under displacement-controlled condition were carried out with structural model specimens made of the long-term served material. The crack growth behavior of the tests was simulated using the method proposed by the authors on the basis of the monotonic stress strain curve and the stress strain curve of half-fatigue life of the material. Comparison with the test results showed that the simulation using the monotonic stress-strain curve of the material gave the conservative prediction of the crack growth behavior.
In order to evaluate creep deformation mechanism of heat resistant steels, stress change tests were conducted during creep tests. In this study, it was confirmed that the dislocation behavior during the creep tests was in viscous manner, because no instantaneous plastic strain was observed at stress increments. Transient behavior was observed after stress changes for all kinds of steels in this work. Mobility of dislocation was evaluated by the observed backward creep behavior after stress reduction. Internal stress was evaluated by the change of creep rate in stress increment, and mobile dislocation density was evaluated with the estimated mobility of dislocation and the change of creep rate in stress increment. It was found that the variation of mobile dislocation density during creep deformation showed the same tendency as the variation of creep rate. Therefore mobile dislocation density is the dominant factor that influences the creep rate variation in creep deformation of heat resistant steels investigated in this work. The mobility of dislocation showed a good correlation with 1/T and it is related with the amount of solute Mo that is a solution strengthening element. Microstructures of crept specimens were observed by TEM to discuss the validation of these results.
We studied the evolution of microstructure in a Martensitic Stainless Steel (JIS-SUS403) during creep by monitoring ultrasonic attenuation. After a series of creep samples with various strains under a tensile stress of 120MPa at 873K was obtained, small samples were removed from the creep samples and free vibration resonance frequencies and attenuation coefficients were measured with electromagnetic acoustic resonance (EMAR). EMAR is a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We measured changes in ultrasonic attenuation of the small creep sample by free vibration resonance with EMAR through a magnetostrictive mechanism. The attenuation measurement is inherently free from any energy loss, resulting in pure attenuation in a metal sample. Furthermore, the evolution of microstructure was observed with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The result from the small samples showed the same trend as our previous result from larger sample. We propose a non-destructive method using EMAR to evaluate creep damage in small specimens sampled from structural metals in-service.
This paper investigates the applicability of the procedure proposed by one of the authors for estimating the nonproportional loading effect on PP test lives to the estimation of the creep-fatigue lives of SUS304 austenitic stainless steel subjected to nonproportionally combined push-pull and cyclic torsion under CC type strain waveform. Fully reversed strain-controlled CC tests were conducted on thin-walled tubular specimens of SUS304 steel at 973K in air under in-phase and 90deg out-of-phase straining conditions. The partitioned inelastic strain range versus life equations, Δεcc versus Ncc relationships, obtained from three different proportional loading conditions, such as push-pull, cyclic torsion, and in-phase combined push-pull and cyclic torsion, are found to be correlated with one another by the multiaxiality factor. By using the multiaxiality modified Δεpp versus Npp and Δεcc versus Ncc relationships, the 90deg out-of-phase straining CC test lives were estimated by the proposed procedure, where the strain range partitioning was done by superposing the hysteresis loops of PP tests on those of CC tests. As the results, the estimated lives are found to agree well with the experimental lives in adopting the superposition method rather than the rapid loading method.
This paper studies on the tension-torsion multiaxial low cycle fatigue of YH61 nickel base single crystal superalloy at high temperature. Strain controlled tension-torsion low cycle fatigue tests were carried out at 1173K using YH61 hollow cylinder specimens of which specimen axis was <001> crystallographic direction. Mises equivalent total strain range gave a large scatter in the correlation of multiaxial low cycle fatigue lives. On the other hand Mises equivalent plastic strain and Mises equivalent stress ranges correlated the multiaxial low cycle fatigue lives with a small scatter. The deformation behavior of the single crystal superalloy showed strong anisotropy under multiaxial loading due to the crystallographic texture, and the unsatisfactory life correlation with Mises total strain range resulted from the anisotropic deformation behavior of the specimen. A new strain parameter, taking account of the anisotropy of elastic constants into account was proposed, and the parameter estimated the multiaxial low cycle fatigue lives within a factor of 2 scatter band.
This paper describes low cycle fatigue lives of Sn-37Pb and Sn-3.5Ag solders at low temperatures. Push-pull low cycle fatigue tests were carried out at 253K and 273K to study the temperature effect on low cycle fatigue life. Plastic strain range was a suitable parameter for correlating the low cycle fatigue lives for the Sn-37Pb solder at the four temperatures but it was not for the Sn-3.5Ag solder. Strain energy parameter which is defined as the product of strain and stress ranges was a suitable parameter for the Sn-3.5Ag solder. Application of universal slope method was also discussed for predicting the low cycle fatigue lives of Sn-37Pb and Sn-3.5Ag solders.
This paper describes development of a thermal mechanical fatigue testing machine for solders. Several researches studying on creep, low cycle fatigue and creep-fatigue for solders at constant temperature have been reported recently, but no or small number of studies on thermal mechanical fatigue of solders have been reported. It might be resulted from that suitable thermal mechanical fatigue testing machines, which can perform the test at temperatures applicable to solders, have not been developed. In this study, the thermal mechanical fatigue testing machine which can cover the test temperatures between 233K (–40°C) and 423K (150°C) is developed. Using the developed testing machine, thermal mechanical fatigue test for Sn-3.5Ag lead-free solder was carried out and an applicability of the testing machine is investigated. This study also discusses the obtained thermal mechanical fatigue life in comparing with data in low cycle fatigue test at constant temperature.
In this work, the thermal cycle fatigue behavior of an air plasma sprayed thermal barrier coatings was investigated in order to improve the thermal fatigue properties. In addition, the residual interfacial strength after the thermal cycle fatigue was also evaluated by means of the modified 4-point bending test. From the measurement of the AE signals during the thermal cycle fatigue tests, micro-cracking occurred in the cooling stage of each thermal cycle. Therefore, such damage depends on the number of thermal cycle. In addition, the thermal grown oxides (TGO) grew at the interface with the exposed time at elevated temperature, i.e. it is the time dependent damage. Thermal barrier coating undergoes both of the time dependent damage and the cycle dependent one under the thermal fatigue condition. The failure mode by the thermal cycle fatigue depended on the thermal cycle wave form ; in thermal fatigue without a hold, the spallation failure occurred in the top-coating near the interface by the coalescence of the microcracks, on the other hand, in thermal fatigue with a hold, delamination occurred through the microcracks and/or along the top-coating/TGO interface. Based on these experimental results and discussions, the damage equation, which was formed by summation of the time-dependent damage and the cycle-dependent damage, was proposed to evaluate the thermal cycle fatigue life of the thermal barrier coatings.
Isotherm curves of wood samples heated at lower temperatures 150, 175, and 200°C for various periods were examined on the basis of the characterization theories of adsorption, Hailwood and Horrobin theory, Dubinin-Radushkevich theory, and Neimark theory. The adsorption properties were characterized by the parameters of their theories. Their properties without the fractal dimensionality were presented by weight change regardless of heating temperature and had the transition point at weight change = –0.2.