| Summary: | 碩士 === 國立臺灣海洋大學 === 材料工程研究所 === 102 === The research engaged in the T91 steel which topic is “ The Influence of Microstructures on The Stress Rupture Properties of Simulated Heat-Affected Zones for T91 Weldment”. The first section of the research is using infrared ray to heat the T91 steel rapidly at varied temperatures(820°C,860°C,900°C and 940°C.) to simulate the heat-affected zone(HAZ) of actual weldment , then proceeded post-weld heat treatment(PWHT) at 750°C and aging at 720°C to analyse the variation of microstructure and hardness . The second part is taking some of the simulated weldments to carry out the long term(1000h) stress rupture test at 615°C and applied stress of 80MPa to evaluate the effect of microstructures on stress rupture properties .
The results of HAZ simulated test for T91 weldment showed that the microstructures and hardness of HAZ didn’t change obviously after tempering at 820°C , and some M23C6 carbides which precipated on grain boundaries became coarser after aging . Heating at 860°C which is beyond the Ac1 temperature of T91 that made the substrate become softer lightly than before , and it observed that the newly subgrains grown in the substrate after PWHT , besides the above , the carbides might grow larger and spheroidized during aging treatment , but aging couldn’t induce subgain grew fastly . In the other way , tempering at 900 °C might cause the T91 steel have partially hardened situation , while some M23C6 carbides had been coarser and the substrate got soften after PWHT . By the way , aging treatment could increase the precipates in the substrate that cause the hardness ascended lightly . Tempering at 940°C which is higher than the Ac3 temperature of T91 that had make the steel been completely hardened , and situation after PWHT is almost the same as tempering at 900°C ,such as some M23C6 carbides which precipated on grain boundaries spheroidized and coarsened , the substrate softened etc.. ,while the hardness raised just a little higher after aging treatment .
The results of the stress rupture test showed that recrystalization happened and some grains were shape-changed due to high temperature and stress loaded environment in this long term test . The secondary cracks mainly grew along prior austenite grain boundaries which were observed in lateral sides of stress ruptured specimens , and the ruptured specimen which tempered at 860°C for several seconds then proceeded PWHT and aging showed the weakest stress-rupture properties . In addition to the above, the ruptured surface just observed dimple structures because of over-loaded stress during this test .
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