| Summary: | 碩士 === 國立臺灣海洋大學 === 材料工程研究所 === 93 === The influence of aging treatment on the fatigue crack growth behavior and sulfide stress corrosion cracking (SSCC) of PH 13-8 Mo alloy was investigated in this work. Microstructural observations showed that both the solution-treated alloy and as-welded laser weld revealed lath martensite with a high dislocation density as well as little plate-like retained austenite present at lath boundaries. The obvious increase in specimen’s hardness was associated with the precipitation of ultra-fine precipitates within lath martensite matrix after aging at the temperature range between 426 to 538 ℃. Significant change in austenite quantity and morphology for the specimen aged at the temperature above 593 ℃accounted for the obvious decrease in hardness. Generally, the hardness of the fusion zone was always slightly lower than that of the base metal. Such consequences could be attributed to the segregation of strengthening elements to columnar boundaries during solidification. Regardless of specimens, 482 ℃ aged samples had the highest hardness, whereas the solution-treated alloy or as-welded laser weld had the lowest hardness among the samples.
All aged welds were susceptible to SSCC to various degrees. The high notch tensile strength (NTS) of variously welds aged in the temperature range between 426 ℃ to 538 ℃ was expected to show high susceptibility to SSCC, especially for W900. The extra amount of austenite in the W and W1100 specimens was associated with the resistance to hydrogen embrittlement. Under the same testing condition, the laser weld behaved higher susceptibility to SSCC than that of the counterpart alloy aged at the same temperature.
The FCGR (da/dN) vs. the stress intensity factor range (△K) curves for the PH 13-8 Mo SS after various aging treatments in air showed 426 ℃ and 482 ℃ aged alloy had similar FCGR in air. Moreover, it was apparent that over-aged specimens, especially 593 ℃ aged alloy, showed a higher resistance to crack growth than others. The decrease in FCGR with increasing aging temperature could be attributed to the decline in strength/ brittleness and increase in ductility/fracture toughness of the alloy after aging at high temperature. In case of the alloy tested in gaseous hydrogen, the FCGR was clearly enhanced as compared to the counterpart tested in air. As a whole, the over-aged specimen still showed a higher resistance to hydrogen-accelerated crack growth. Tensile and fatigue fracture appearance for those specimens suffered from hydrogen embrittlement all revealed quasi-cleavage fracture.
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