Hirosada Irie

Microstructural evolution and change in hardness in type 304H stainless steel during long-term creep

Abstract The microstructural evolution and the change in hardness have been investigated for 18Cr–8Ni (type 304H) stainless steel during long-term creep. Creep and creep-rupture tests were carried out at temperatures between 550 (823) and 750°C (1023 K) for up to 180 000 h. The hardening behavior during creep depends on the stress level, as well as the precipitation of M 23 C 6 carbides and σ phase. At a high stress of 177 MPa, the hardening during creep is much larger than the age hardening, indicating that the hardening during creep is mainly caused by the strain hardening due to creep deformation. At a later stage of creep, the softening occurs due to the recovery of excess dislocations, which becomes more significant with decreasing stress and increasing test duration. The strain hardening disappears with decreasing stress level and increasing test duration. At a low stress of 61 MPa or less, the hardening during creep can be approximately given by the age hardening under no stress, except for the final stage of creep.

Alloying to improve the properties of welded molybdenum

Abstract Tensile tests at 203 K were performed on electron-beam-welded molybdenum and molybdenum-base alloys. The effects of alloying elements, carbon, zirconium, niobium, vanadium (with or without boron) and rhenium were determined on the low-temperature ductility of weld joints. The results are related to the microstructural effects. The effects of alloying are as follows: (1) Carbon enhanced the fracture strength and improved the ductility due to the grain boundary strengthening. (2) Zr, Nb or V with boron enhanced both the yield and fracture strengths due to the grain refining, but the ductility was only slightly improved. (3) 5% Re induced a high fracture strength due to the refinement of precipitates, an exceptionally low yield strength due to the solution softening, and the ductility was substantially improved.

Characteristics of stably induced laser plasma

A large and stably induced laser plasma has been formed to characterise the plasma structure and the absorption property of the laser beam. It can be attained with a good combination of an arc and the laser beam. The average absorption coefficient was measured using a power probe. The distributions of the electron density and temperature were evaluated by measuring the infrared radiation from the plasma. Maximum values of the electron density and temperature of the plasma are 2.0×l017 cm−3 and 18000 to 19000 K, respectively. The absorption coefficient shows an approximately constant value of 0.6 cm−1 independent of the laser power and the flow rate of the assist gas. On the other hand, the composition of the plasma exerts a great influence on the absorption coefficient. The results are discussed on the basis of inverse Bremsstrahlung.A large and stably induced laser plasma has been formed to characterise the plasma structure and the absorption property of the laser beam. It can be attained with a good combination of an arc and the laser beam. The average absorption coefficient was measured using a power probe. The distributions of the electron density and temperature were evaluated by measuring the infrared radiation from the plasma. Maximum values of the electron density and temperature of the plasma are 2.0×l017 cm−3 and 18000 to 19000 K, respectively. The absorption coefficient shows an approximately constant value of 0.6 cm−1 independent of the laser power and the flow rate of the assist gas. On the other hand, the composition of the plasma exerts a great influence on the absorption coefficient. The results are discussed on the basis of inverse Bremsstrahlung.