In this study, mechanical tests and microstructural analyses including TEM analyses with EDX of precipitates in modified 9Cr-1Mo steel were carried out to determine the cause of embrittlement observed after heat-treatment, which limits the usage of the alloy for power plants. Mod. 9Cr-1Mo steel specimens at austenite temperature were quenched to the molten salt baths at 760˚C and 700˚C, in which the specimens were kept for 10 min ~ 10 hr with subsequent air-cooling. Impact tests showed that the impact value dropped abruptly when the specimens were kept longer than 30 min at ~760˚C reaching to minima in about 1 hr, and then increasing at further retention. The tensile strength of the specimens reached the minimum value without much change afterward, whereas the values of elongation showed the same trend as that of the impact value. The isothermally heat-treated steel at 700˚C also showed a minimum impact value in about 1 hr. These results suggest that the isothermal heattreatment at 760 and 700˚C for about 1 hr induces temporal embrittlement in Mod. 9Cr-1Mo steel. The microstructural examination of all the specimens with extraction replica of the carbides revealed that the specimens with temporal embrittlement had Cr2C, indicating that the cause of the embrittlement was the precipitation of the Cr2C. In addition, TEM/EDX results showed that the Fe/Cr ratio was 0.033 to 0.055 for Cr2C, whereas it was 0.48 to 0.75 for Cr23C6, making the distinction of the Cr2C and Cr23C6 possible even without direct electron diffraction analyses.
It is well known that modified 9Cr-1Mo steel has a low thermal expansion and high thermal conductivity with excellent high temperature properties compared to austenitic stainless steel. For these advantages, the steel is very popular for the boiler tube of thermal power plants. Normalizing is commonly utilized to obtain martensite in this steel, which shows an unusual toughness for martensite. However, some accidents related to this steel have been reported recently, opening the necessity for further study. As a particular behavior of the steel, an abrupt drop of the impact value has been identified upon tempering at 750˚C for about 1 hour. It is well known that Fe3C forms during autotempering and turns to Cr2C at an early stage and then transforms to Cr23C6. In this study, the cause of the abrupt drop of the impact value was investigated with an impact test, microstructural observation, nanodiffraction and phase analyses using instruments such as optical and transmission electron microscopes (TEM) with an extraction carbon replica of the carbides. The analyses revealed that the M2C that formed when retained for about 1 hour at 750˚C causes a drastic decrease in the mechanical properties. The sharp drop in mechanical properties, however, disappeared as the M2C transformed into M23C6 with longer retention.
Ni기 초내열합금 GTD 111의 미세조직의 변화와 크리프 파단특성에 대해 연구하였다. 조직관찰을 통해 본 합금의 응고거동과 주조 후 응고과정에서 석출거동을 분석하였다. MC탄화물의 생성위치가 γ/γ' 상 보다 수지상 중심에서 가까운 것으로 MC탄화물이 γ/γ'공정상보다 먼저 응고된 것을 확인할 수 있었다. η상은 Ti가 많은 γ'상에서 변태되어 형성되었으며, γ/γ'공정상에서 η상으로의 변태에 따라 η상 근처에 PFZ가 형성되고 PFZ 내부에 TaC가 석출됨을 확인하였다. 871˚C이상의 온도에서 크리프 파단은 결정입계를 따라 진행되는 것이 명확하였으며, 표면에서 형성된 균열과 내부에서 생성된 균열이 전파, 조합되어 최종파단을 초래하였다. 결정입계 균열의 생성은 최종응시 형성된 미세공동과 밀접한 관계가 있는 것으로 분석되었으며, η상과 PFZ는 균열 생성에 큰 영향을 주지 않았다.