Experimental Investigation Of Fracture Toughness And Very High Cycle Fatigue Behavior Of Welded Joints Of Cr-Ni-Mo-V Steel

Fracture toughness and very high cycle fatigue behavior is the key content of strength design and safety evaluation of steam turbine welded rotor, especially welded joints for size limitation, via non-standard geometry specimen to assess fracture toughness growing up to the new issues of engineering development. The fracture and fatigue behavior of a turbine rotor Cr-Ni-Mo-V steel welds was investigated, which was highlighted by applying small specimen test technology for fracture toughness, investigating long artificial aging effect on fracture toughness and temperature effect on fatigue behavior, as well as the fatigue crack initiation behavior of micro-pore.According to these mainly aspects, the primary ideals of this article are as follows:1)Using miniature three-point bending specimens room temperature fracture toughness tests of a Cr-Ni-Mo-V steel were conducted, and compared with that from traditional standard specimens to analyze the specimen size dependence of the fracture toughness. Results show that the fracture toughness of miniaturized specimens is lower with higher degree of scattering, hence indicating remarkable size effect. It is indicated that the dominated plane stress state of miniature specimens reduces the ability of deformation and stable crack growth near crack tip. The feasibilities of the fracture toughness testing of miniature specimens based on standard specimen are discussed, and concluded that the notched micro-three-point bending specimen in our experiments can provide a good support for further research.2) A fracture toughness comparison test research between the 470℃×3000h aged and which non-aging Cr-Ni-Mo-V rotor steel weld joint was investigated by this article. Under room temperature, it turns out that after little stable fracture crack propagation specimen will brittle fracture, moreover the non-aging weld-joint has limited J-Δa crack propagation ability, whilst aged has higher degree of scattering with lower fracture toughness. After carefully researched the micro-structure and the morphology of the fractured crack plane between aged and non-aging weldment, it shows that the effect of age improved the strength of weld joint but reduced the ability to resist fracture crack propagation. Combined with ductile-brittle transition temperature, a further discussion on the fracture toughness test method about aged weld-joint is conducted.3) Axially push-pull fatigue tests of Cr-Ni-Mo-V steel welded joint were conducted up to very high cycle fatigue regime at room temperature and 370℃. The S-N curve at room temperature shows a duplex shape while at 370℃ is continuously decreasing with lower fatigue strength. At 370℃, there is equal distribution of failures among different parts of the welds. Further taking internal fatigue initiation and micro-defect location into consideration, a new life-controlling model is recommended.4) Under SEM circumstance, through the in situ loading platform, the fatigue initiation and propagation behavior of a micro-pore of Cr-Ni-Mo-V steel is investigated, moreover the fatigue crack initiation process and regular pattern of the micro-pore are systematically investigated. It showed fatigue initiation along the loading direction45°, and the micro-pore sheared failure. Furthermore combining with Digital Image Correlation (DIC), the critical strain of micro-pore fatigue initiation was measured.