Genta Takano

Microstructural changes and fracture behavior of CF8M duplex stainless steels after long-term aging

Microstructural changes and fracture behavior in cast CF8M duplex stainless steel after aging at 300-450°C for 300-10000 h have been investigated. Both, optical microscopical and transmission electron microscopical analyses, hardness and ferrite content measurements have been carried out in this study. Strengthening and aging phenomena of the ferrite phase have been identified by hardness measurements. Spinodal decomposition and heterogeneous precipitation of G-phase were found to be responsible for strengthening of the ferrite phase after aging with a temper parameter [P = log(t) + 0.4343 O/R(1/673.2-1/4)] (see Appendix A) in the range ca. 1.8-4.5. Three different fracture modes, dimples, cleavage and α/γ grain boundary separation, have been observed for Charpy V-notch and CT test specimens fractured at 20°C.

Effects of a double sided heating method and its controlling parameters: Development of a residual stress control method for butt-welded pipe joints (1st Report)

Abstract This paper describes a new stress improvement method to reduce tensile residual stress on the inner and outer surfaces near butt-welded pipe joints. Its procedure is as follows. An annular region on both sides of the weld line is heated from the outside at the same time to generate a temperature differential between the weld line and region being heated. The region being heated and the welded joint are not actively cooled after completion of heating. That is to say, the proposed method involves the whole circumference on both sides of the weld line simply being left to cool after heating and includes no water cooling or movement of heating position as required by the low-temperature stress relaxation method. A tensile strain is generated on the inner surface near the welded joint to produce plastic deformation by tensile stress when the annular region is heated. The tensile stress after cooling is reduced by this plastic deformation. The two sided heating method can therefore be expected to exert...

Conditions for execution of the two sided heating method and its control procedure : Development of a residual stress control method for pipe butt-welded joints (2nd Report)

Summary The previously reported two sided heating method proposed by the authors is an effective residual stress control method for pipe butt-welded joints. This paper describes the non-dimensional parameters controlling this method and the conditions to be fulfilled to ensure sufficient effectiveness in actual applications. The results obtained may be summarised as follows: The previously reported elastic analysis solution is used to investigate the effect of the two sided heating method as well as the effect of the temperature distribution during two sided heating on the stress generated near the weld line. The results show that there are three parameters - the non-dimensional stress K, non-dimensional heating position βl, and non-dimensional heating width βW - that control the residual stress effect of the two sided heating method. The relationship between βl, βW, and the non-dimensional stress K is defined and plotted graphically. By use of the diagrams presented, it is possible to select the combinat...