Keiji Kubushiro

Novel Fractgraphy of Ni-Based Alloy by SEM/EBSD Method

Alloy 718 superalloy has been used for blades of air craft jet engine and gas turbine. The failures of these parts derive from fatigue and creep, but it is difficult to estimate applied stress from observation of samples from damaged parts. Oxidation and crush on fracture surfaces prevent from the estimation by fractgraphy especially. Therefore, we established the novel observation of the microstructure on the cross section beneath cracks by using EBSD. Tensile test, fatigue test and creep test were performed, and the change in dislocation density and the difference of strain contour of these specimens were specified by the new method. Finally the applied stress could be estimated and the test temperature could be also estimated.

Effect of Cold Work on Creep Rupture Strength of Alloy263

Creep rupture strength and the microstructure change during creep deformation of pre-strained Alloy263 were investigated. Creep rupture tests were conducted at 1023, 1073 K at stress range from 120 to 250 MPa. Creep strength of the pre-strained samples was higher than that of the non-strained samples. However, rupture strain of the pre-strained samples was much lower than that of non-strained samples. In the pre-strained samples, Ni3(Al, Ti)- γ’ and M23C6 inside of the grains precipitated finer than the non-strained samples compared at the same creep time. At grain boundaries, the grain boundary shielding ratio covered by M23C6 carbide showed almost same value among each samples. However, diameter of the M23C6 particle decreased in pre-strained samples. Furthermore, dynamic recrystallization was promoted and precipitation free zone (PFZ) was formed around Ni3Ti-η phase at grain boundary. These observations show that increase in creep-strength of pre-strained sample was due to increase in precipitation strengthening in the grain by fine precipitation of γ’ and M23C6. In addition, resistance against crack propagation at grain boundary increased by the fine precipitation of grain boundary M23C6 even though formation of PFZ and promotion of the dynamic recrystallization. It is estimated that Orowan-stress of pre-strained samples was 1.7 times higher than non-strained samples. It is considered that these strengthening effects overcome the weakening effects in the pre-strained samples.

Microstructure and Creep Property of Long-Term Serviced Mod.9Cr-1Mo Steels After Repair Welding

The microstructure and creep properties of the Mod.9Cr-1Mo steel of repair welds after long-term service for about 80,000 hours at around 600°C were investigated. Two types of heat affected zone (HAZ) were formed after repair welding performed at the interface between base metal and original weld metal. The first type was a HAZ formed in an ex-service original weld metal and another type was formed in an ex-service base metal. Creep rupture life of HAZ formed in an ex-service original weld metal was extremely short and it was about one fourth of that of HAZ formed in an ex-service base metal. From microstructural observation, many fine ferrite grains surrounded by carbides were present on grain boundaries and, therefore, duplex structure of ferrite and martensite was observed in the fine grain HAZ region formed in an ex-service original weld metal. These fine ferrite grains were considered to be formed by diffusion transformation of gamma phase to alpha phase instead of martensitic transformation during cooling of thermal history due to repair welding. It has been concluded that the creep strength of an ex-service weld metal is remarkably reduced by the formation of many fine ferrite grains due to repair welding.Copyright

Change in Microstructure at Grain Boundaries with Creep Deformation of Polycrystalline Nickel-Based Superalloy IN-100 at 1273K

The changes in the morphology of ’ precipitates and the dislocation substructures at the vicinity of grain boundaries with creep deformation were investigated on a polycrystalline nickel-based superalloy, IN-100. The experiments were done at 1273K in the stress range of 70-180MPa with one set of samples deformed until rupture and another interrupted after reaching the minimum creep rate. The cuboidal ’ precipitates were regularly arrayed at the vicinity of grain boundaries and eutectic ’ precipitates and carbides were observed at the grain boundaries on as-heat treated IN-100. However, on both the creep interrupted and ruptured samples at all stress conditions, plate - like shaped ’ precipitates covered the grain boundaries. The width of the plate - like shaped ’ precipitates increased with decreasing the stress and is wider in the creep ruptured than in the creep ruptured samples. There were few dislocations in the grains and in the plate - like shaped ’ precipitates, but a large number of the dislocations were observed around the interface of the plate - like shaped ’ precipitates. TEM observations on the specimen creep ruptured at 180MPa indicated that strong constant was observed at the vicinity of grain boundaries, but weak contrast in the grain interior. These results suggested that drastic plastic deformation occurred only at the vicinity of grain boundaries. Consequently, the plate - like shaped ’ precipitates at the grain boundaries inhibited the dislocations movement and acted as the creep strengthener.

Creep-Fatigue Properties of the Candidate Materials of 700°C-USC Boiler

Creep-fatigue properties of candidate materials of 700°C-USC boiler are investigated. The candidate materials are Alloy 230, Alloy 263, Alloy 617 and HR6W. Creep-fatigue tests were conducted at 700°C and the effect of both strain range and hold time were studied. Experimental results showed that at 1.0% strain range, cycles to failure with 60 min strain holding is about 10% of that without strain holding, but at 0.7% strain range, cycles to failure with 60 min strain holding decreases down to about 1% of without strain holding. It appears that cycles to failure is decreased by increasing strain holding time at all tested strain ranges, and the effect of holding time is emphasized at small strain range. These phenomena depend on the kind of alloys.© 2007 ASME