Creep tests were carried out at uniaxial applied stress of 90∼175MPa at 650℃, using different kinds of hardness of 12%CrMoVNb. Microstructure was investigated by transmission electron microscope and room temperature hardness measurements were performed by Rockwell and Vickers hardness tester.
Specimens were austenitized at 1050℃ for 2h, and then tempered at 700℃ for 2h, at 750℃ for 2h and at 750℃ for 9h. Hardness of specimens was Rc30, Rc25 and Rc 20, respectively. The average grain size of prior austenite was about 50μm.
The results of creep tests which were tested at 650℃ are summarized as follows
As hardness increases from Rc20 to Rc30, minimum strain rate($\dot{\epsilon}_{min}$) decreases and the stress exponent($\eta$) of power law creep decreases from 11.2 to 4.0.
In short term(about $10^2$h), the rupture strength of Rc30 is 170MPa and is 1.4 times larger than that of Rc20. But, in long term(about $10^4$h), the rupture strength of Rc30 is 85MPa and is 1.17 times larger than that of Rc20.
Hardness measurement of crept specimens showed that hardness of Rc20, $t_f$ = 10.5h, drops about 30% to Rc14 and then maintains almost constant hardness. While hardness of Rc30, $t_f$ = 93h, drops about 27% to Rc22 and then maintains almost constant hardness.
Martensitic lath width of as-tempered Rc30 is about 0.40μm. That of Rc20 is about 1.1μm. As the higher hardness is, the finer carbide is precipitated along lath boundary. For Rc30, $t_f=10^3h$, lath boundary is almost annihilated and changes to dislocation cell structure and precipitations are coarsened. This is similar to microstructure of Rc20, $t_f = 10.5h$. Thus, hardness has no distinct effect of creep properties in long term.
