| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 91 === The study investigates the mechanical properties and fracture behavior of the plasma arc welded 304L stainless steel under different temperature and strain rate conditions. Annealed 304L stainless steel is welded by continuous current (CC) and pulsed current (PC) plasma arc welding, then machined as rectangular tensile specimens which will be tested at strain rate of 10-3s-1, 10-2s-1 and 10-1s-1, and at temperature of -100℃, -50℃, 25℃, 300℃ and 500℃ with a MTS machine. A ferritescope is used to determine the amount of magnetic martensite after plastic deformation. The experimental results show that temperature and strain rate have an obvious effect on the mechanical properties and fracture behavior of the material. Flow stress increases with increasing strain rate, but decreases with increasing temperature. The work hardening rate, strain rate sensitivity, activation volume and fracture elongation change with strain, strain rate and temperature levels. Microscopic observations show that martensitic transformation increases with decreasing temperature and strain rate. The formation of martensite during plastic deformation causes an increase in flow stress, work hardening rate, and fracture elongation. And its effect is especially strong at temperature of -100℃and -50℃. The flow stress, work hardening rate, strain rate sensitivity of PC weldments are higher than those of CC weldments in the same condition, but activation volume, fracture elongation, and the formation of martensite are lower. All tested specimens rupture at weldment between 25℃and -100℃, and at base metal above 25℃. SEM observations of the ruptured specimen surfaces reveal dimple morphology appears, the dimple size increases with increasing temperature and strain rate. Compared to the dimple size of CC and PC weldments indicate that CC weldments exist smaller dimple in the same condition. The Zerrilli-Armstrong constitutive equation with the experimentally determined specific material parameters successfully describes the flow stress of the tested weldments for the range of test conditions.
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