| Summary: | 碩士 === 國立海洋大學 === 機械與輪機工程學系 === 87 === The purpose of this study focuses on two aspects of low-temperature mechanical properties of steels and its SMAW weldments: (1) the effect of microstructural morphology on the low temperature mechanical properties of steels produced by process, (2) the effect of microalloying addition on the low temperature and hydrogen environment mechanical properties of steels under the same Thermo-mechanically Controlled Processed (TMCP) with accelerated cooling. The results indicate that the hardness of the root in the multipass weld metal was lower than the top bead for these three steels. Furthermore the hardness in the HAZ of middle thickness presented softening. Lower value was ascribed to the post heat treatment of successive passes during multi-pass welding. The ultimate tensile strength and yield strength of experimental steels increased with decreasing temperature, but their ductility almost was remained unchanged. The yield strength and impact toughness of TMCP EH36 steels and its weldments was better than that of the traditional controlled rolled SM490C. It attributes to the fine equiaxed grain size produced by accelerated cooling process. The superior strength and impact toughness of micro-alloy addition of API 5L X65 steels and its weldments to EH36 results from very fine precipitates superimposed with fine equiaxed grain and the solid solution strengthing. Especially, the impact toughness of base metal HAZ was higher than parent metal due to that the brittle grain-coarsened zone was reheated to form better toughness of fine grain ferrite during multipass welding.
The results show that the ultimate tensile strength and yield strength for the base metals and weldments of all three steels (TMCP API 5L X65, TMCP EH36, and SM490C) were independent of increasing hydrogen-charging current density. The superior strength and ductility of TMCP API 5L X65 after hydrogen charging can be attributed to the solid solution strengthening and fine precipitates superimposed on the equiaxed refined grains. The relative hard band structures in SM490C steels and weldments were the major cause that rendered the lower ultimate tensile strength and elongation inevitable.
Hydrogen permeation experiments were conducted using a double electrolytic cell, current density controlled at 10mA/cm2, containing a 0.1N NaOH solution in both compartments. Under the same composition, the hydrogen diffusivity along the through-surface direction of that in the equiaxed grain EH36 steel produced by TMCP processes with accelerated cooling render a lower value in a banded structure of SM490C steel. Whereas, the hydrogen diffusivity and hydrogen permeability of microallyed API 5L X65 steel were lower than EH36 steel with no alloying. The reason could be that the sites of Carbides and Nitrides of Niobium, Vanadium, Copper and Aluminum trapped the hydrogen. Hydrogen microprint technique confirmed the main diffusion path and trapping sites are the grain boundary, carbide/ferrite interface in pearlite at incipiency during hydrogen permeation. The carbide and inclusion-ferrite interface is the main trapping sites and short diffusion path for hydrogen in steels.
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