Abstract ▼ AI-Helper

Conventional test methods for measuring tensile strength and fracture toughness require the removal of large material samples from in-service component on the fossil power plant. However, it is difficult to extract multiple specimens for the tests without damaging the component. Thus, new test metho...

Conventional test methods for measuring tensile strength and fracture toughness require the removal of large material samples from in-service component on the fossil power plant. However, it is difficult to extract multiple specimens for the tests without damaging the component. Thus, new test methods, which use comparatively smaller specimen than that required in conventional materials tests, haye been tried for the micromechanics test. There are several methods, such as small-specimen creep test, the continuous indentation test, and small punch test. Among them, the small punch test method has been applied to many evaluation fields such as ductile-brittle transition temperature(DBTT), stress corrosion cracking(SCC), and fracture properties of advanced materials like metal metrix composites. In this study the small punch test was used to evaluate the mechanical properties at high/low temperature from -196℃ to 650℃ and the material degradation for virgin and aged materials of 9Cr-lMo steel which has been recently developed. The specimens of the small punch test were made miniaturized by 10mm×10mm×0.5mm. The results obtained in this study are as follows : 1. The precipitates of the aged materials appear more than that of the virgin material in the 9Cr-lMo steel. It also shows the decomposition of lath martensite phase, growth of carbides, and appearance of sub-grain boundary with increasing of aging time. 2. The maximum load(P_max) of 9Cr-lMo steel decreases according to the increase of test temperature in both the virgin material and the aged materials in the high temperature test. It shows clearly the blue shortness at 400℃. 3. The maximum displacement(δ_max) of 9Cr-lMo steel decreases up to 400℃ and then recovers the toughness at over 400℃ in high test temperature. 4. The parameter ΔP/Δδ defined a slope in plastic membrane stretching region of SP load-displacement curve decreases according to the increase of test temperature, and that of the aged materials is higher than the virgin material in all the test temperatures. 5. The ductile-brittle transition temperature of the virgin material and the aged materials with 630℃/500hrs, 1000hrs, 2000hrs, 3000hrs and 4000hrs are -156℃, -120℃, -118℃, -130℃, -147℃ and -140℃ respectively. 6. The material degradation degrees(difference of ductile-brittle transition temperature, ΔDBTT_SP) of the aged materials with 630℃/500hrs, 1000hrs, 2000hrs, 3000hrs and 4000hrs are 36℃, 38℃, 26℃, 9℃ and 16℃ respectively. The material degradation degrees of the aged materials with 630℃/500hrs, 1000hrs are large. This is because they show the brittle fracture behavior, which is due to the growth of carbides and migration toward grain boundary. However, those of the aged materials with 630℃/2000hrs, 3000hrs, 4000hrs are small. It is why they show the recovery of toughness by decreasing of dislocation density and partial recrystallization with increasing of aging time. 7. Micromechanical properties(ΔP/Δδ, SP-energy and ΔDBTI_sp etc.) obtained by small punch test in this study can be used to evaluate the material degradation degree of the 9Cr-lMo steel.

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