Isamu Nonaka

Performance of repair welds on aged 2.25Cr–1Mo boiler header welds

Abstract In order to clarify the performance of repair welds on power boiler, thick parts such as header and steam piping, an ex-service aged 2.25Cr–1Mo header was repaired using SMAW with postweld heat treatment and the mechanical properties of the repair welded joints were experimentally evaluated. Creep rupture life of the repair welded joint was almost same as that of service-degraded base metal and heat-affected zones. It was proved that the life reduction would not be caused by repair welding. In creep–fatigue tests with strain holding, some type of repair welded joints was fractured at the heat affected zone caused by repair welding. This may be caused by strain concentration at the heat-affected zone under strain holding. Charpy impact toughness of the simulated heat affected zone due to repair welding was much higher than that of service-degraded base metal. It was proved that the toughness would be restored by repair welding.

Small Crack Behavior and Assessment of High Temperature Fatigue Damage for 2.25Cr-1Mo Steel

To complement the lack of reliable method to estimate the fatigue life for high temperature equipment operating in a power station, strain-controlled fatigue tests were conducted for base metal and HAZ simulation specimens of boiler grade 2.25Cr-1Mo steel, and the prospect of assessing the progress of fatigue on information obtainable from naturally developed fatigue cracks was examined. It was shown that reasonably accurate estimation of fatigue life and assessment of damage, or remaining serviceable life for that matter, was indeed possible, at least on the laboratory level.

Micro-texture and physical properties of the cold-sprayed copper deposit

The cold spray (CS) method is a new deposition technique. In this technique, small solid particles impact and deposit on a substrate without melting. The deposition rate of the CS method is much higher than that of the other deposition processes. Expected applications of this technology are not only coating film production but also direct copper wiring technique for electronic products. This method also can be applied to patterning of thin films without photomasks. For example, an arbitrary geometry thin-film pattern can be formed on a substrate by scanning a spray nozzle. However, mechanical properties of the CS deposit and its micro-texture are found to be quite different from those of bulk copper. The previous researches showed that the CS deposit has high strength and indicates brittle-like fracture. The cold-sprayed copper deposit has a micro-texture which is stacked heavily deformed particles. The difference of the micro-texture between the cold-sprayed copper deposit and bulk copper is the main reason of the different mechanical properties. Thus, there is a possibility that the electrical properties of the cold-sprayed copper deposit are also different from bulk copper. In this study, both micro-texture and the electrical properties of the cold-sprayed copper deposit were measured by using SEM and EBSD technique and a four-point probe method. From the EBSD evaluation results, it was found that the average grain size in the deposited particles was much smaller than that of feed stock powder and bulk copper. The electrical resistivity of cold-sprayed copper deposit was much higher than that of bulk copper. The fine grain texture of the cold-sprayed copper deposit is one of the reasons for this high electrical resistivity.

High Cycle Fatigue Properties of Modified 9Cr-1Mo Steel at Elevated Temperatures

Since high-cycle fatigue loads is applied to the pipes in various energy and chemical plants due to the vibration and frequent temperature change of fluid in the pipes, the high-cycle fatigue behavior of the alloys used for pipes should be understood quantitatively in the structural reliability design of the pipes. The purpose of this study, therefore, is to clarify the high-cycle fatigue strength and fracture mechanism of the modified 9Cr-1Mo steel at temperatures higher than 400°C. This material is one of the effective candidates for the pipes in fast breeder demonstration reactor systems. A rotating bending fatigue test was applied to samples at 50 Hz in air. The stress waveform was sinusoidal and the stress ratio was fixed at −1. The fatigue limit was observed at room temperature and it was about 420 MPa. This value was lower than the 0.2% proof stress of this alloy by about 60 MPa. This decrease can be attributed to the cyclic softening of this material. The limited cycles at knee point was about 8×105 cycles. All fracture was initiated from a single surface crack and no inclusion-induced fracture was observed in the fracture surface by SEM. Thus, the high-cycle fatigue design based on the fatigue limit may be applicable to the modified 9Cr-1Mo steel at room temperature. The fatigue limit of about 350 MPa was also observed at 400°C, and it appeared at about 107 cycles, while it appeared at around 106 cycles at room temperature. Thus, it was confirmed that the fatigue strength of this alloy decrease with temperature. However, the fatigue limit didn’t appear at 550°C up to 108 cycles. The fatigue limit may disappear in this alloy at 550°C. It is very important, therefore, to evaluate the ultra-high cycle fatigue strength of this alloy at temperatures higher than 400°C.Copyright

High Cycle Fatigue Strength of Modified 9Cr-1Mo Steel at Elevated Temperatures

In order to clarify the characteristics of high-cycle fatigue of the modified 9Cr-1Mo steel, a high temperature rotary bending test was carried out. As a result, the fatigue strength of this alloy decreased monotonically at elevated temperatures. It decreased from 440 MPa at room temperature to about 350 MPa at 400°C. This decrease of the fatigue strength was attributed to the temperature dependence of the yielding strength of this alloy. The fatigue limit appeared near 107 cycles at 400°C, whereas it appeared around 106 cycles at room temperature. The most important result is that the fatigue limit disappeared up to 108 cycles at temperatures higher than 500°C. Thus, the number of cycles at which the fatigue limit appeared shifted to higher cycles with increasing the testing temperature. Clear striation was observed in the stable crack growth region on the fracture surface of all the specimen tested at room temperature, 400°C, 500°C, 550°C, and 600°C. Intergranular cracking, which have been observed in creep-fatigue tests, was not observed. Since the estimated operating temperature of FBR is 550°C, it is very important to consider this fatigue strength in the structural and reliability design of the modified 9Cr-1Mo steel.In this study, the change of crystallinity of this alloy under fatigue loading was also analyzed by applying an EBSD method. The image quality (IQ) value obtained from the analysis was used for the quantitative evaluation of the crystallinity in the area where an electron beam of 20 nm in diameter was irradiated. The quality of the atomic alignment was found to degrade under the cyclic loading, and a crack started to occur on the surface of the alloy when the quality of the atomic alignment decreased to a certain critical value.Copyright

Reconfirmation and new discussions on frequency effect of fatigue property of materials based on numerous published data

Ultrasonic fatigue testing is a convenient method to examine the S-N property in long life region, within a definite period. However, a distinct gap of the loading stressing frequency between conventional and ultrasonic testing methods (100–1,000 times larger) urges us to study a potential effect of the loading frequency. The present paper deals with a comparison of results obtained from conventional and ultrasonic tests, in the high cycle fatigue region, from two viewpoints. Firstly, a short direct comparison of fatigue data from conventional and ultrasonic tests, with similar conditions, has been undertaken. Secondly, overview analyses based on a far larger amount of data have been carried out for structural steels and aluminium alloys. Results obtained from this work give us some idea of the extent of differences between fatigue test results obtained at usual and ultrasonic frequencies in the fatigue regime of N f < 10 7 cycles. Particularly, the second study gives a potential 18% difference of fatigue strength for steels, whereas it reveals only a slight difference of fatigue behaviours for aluminium alloys.

Remaining life evaluation of 2.25Cr –1Mo steel boiler elbow used for a long time

Abstract The high temperature parts of a thermal power boiler are designed by allowing for stress based on 105 h creep rupture strength. As a safety factor is involved in the allowable stress, the parts can be operated for more than 105 h. Actually some units have been operated for over 3×105 h. Life evaluation is required for the units operated for such a long time to maintain reliability. Various researches for evaluating residual life have been carried out and some have been applied to actual units. But the results of life evaluation done on actual units have not been verified. In this paper, the verification of life evaluation methods is performed with a weld elbow of steam pipe used for 2.6×105 h in an actual plant.

Creep-Fatigue Properties for Repair Welded Joint

Repair welding has been performed when damage has been detected in aged power boiler piping. Discontinuity of deformation ability occurs between the new weld metal and degraded used material in the repair welded joint. Therefore creep-fatigue properties become important under displacement controlled piping system loading. Partial repair welded joint specimen and full repair weld joint specimen were made using the retired 2.25Cr-1Mo steel main steam header. In order to clarify the creep-fatigue properties for two kinds of repair welded joint specimens, fatigue tests under 0.7% strain range with 60min tensile strain dwell were performed at 600C. Both kinds of specimens were necked and fractured at the heat affected zone of welded joint due to the axial strain concentration and ratcheting at heat affected zone. The creep-fatigue life of partial repair weld joint specimen was about half of that of full repair weld joint specimen. This may be due to the low creep-fatigue resistance of the heat affected zone for partial repair welded joint specimen.Copyright

Full Size Internal Pressure Creep Test for Welded P91 Hot Reheat Piping

In order to establish the life assessment method for the welded modified 9Cr-1Mo steel hot reheat piping, an internal pressure creep test is conducted with a full size test component. As a result, the fracture mode of the component is clarified and the life prediction method is established. Furthermore, the creep damage detection procedures are proposed.Copyright

Creep-Fatigue Damage and Life Evaluation for Boiler Header Stub Weldment

In order to clarify the creep-fatigue damage process and to evaluate the creep-fatigue life for boiler 2.25Cr-1Mo header stub welds, a series of creep-fatigue tests were performed on partial mock-up specimens of actual plant under simulated plant loading conditions. Creep voids and micro-cracks occurred along the weld toes at an early stage of life and grew to form many short cracks. These short cracks grew both on the surface and through the wall of the stub tube and later coalesced to form one crack. It was proved that there was a correlation between the maximum crack depth and life ratio and also that there was a correlation between the maximum crack depth and the maximum crack length on the surface. A life prediction method was proposed based on these two correlations.Copyright