In this study, the 0.2% yield stress of duplex stainless steel was evaluated using a compound Hall-Petch equation. The compound Hall-Petch equation was derived from four types of duplex stainless steel, which contained 0.2-64.4 wt% δ-ferrite phase, had different chemical compositions and were annealed at different temperatures. Intragranular yield stress was measured with an ultra-microhardness tester and evaluated with the yield stress model proposed by Dao et al. Grain size, volume fraction and texture were monitored by electron backscattering diffraction measurement. The k γ constant in the compound equation for duplex stainless steel agrees well with that for γ -phase SUS316L steel in the temperature range of 1323-1473 K. The derived compound Hall-Petch equation predicts that the yield stress will be in good agreement with the experimental results for the Cr, Mn, Si, Ni and N solid-solution states. We find that the intragranular yield stress of the δ-phase of duplex stainless steel is rather sensitive to the chemical composition and annealing conditions, which is attributed to the size misfit parameter.

No abstract

No abstract

, in the equation is calculated and is found to follow the equation PϭK · exp(0.5/q). Meanwhile, the grain growth exponent, n, for the anisothermally annealed SUS316L steel is also determined and is found to lie between 2.5 and 3.0. On the other hand, EBSD analysis of the evolved microstructure at different heating rates indicates that low heating rate caused partial recrystallization with preferred orientations at the recrystallization finish temperature, while high heating rates above 1 K/s induced the homogeneously nucleated recrystallization microstructure with random orientations and a lognormal type grain size distribution.

There has been a great demand for superior heat resistant steels in order to raise the thermal efficiency of fossil-fuel power plants and to reduce CO 2 emission to the global environment. To this end, by using the d-electrons concept, 9-12% Cr ferritic steels were designed for use of a steam turbine rotor operated in the USC power plants at the steam temperature of 620 to 650°C. The crucial issue for the design is to suppress the deterioration of the long-term creep strength by alloying. First, the Re addition was found to give a beneficial effect on the creep strength of a 10%Cr-4%W steel. Then, the creep tests were performed with the six Re-free and 3.5% W ferritic steels to get an optimum Cr content in the range of 8.5 to 11.5%. As the result, it was found that an excess amount of Cr yielded a detrimental effect on the creep properties, and the 9% Cr steel was the best in view of the very long-term creep strength tested in the condition of 650°C, 98 MPa. Subsequently, a series of creep tests was conducted with the steels by fixing at 9% Cr but by varying the W content from 2 to 4% and the Re content from 0 to 0.5%. From the prolonged creep tests for more than 40,000 h, it was shown that the 9Cr-4 W-0.5Re steel had the longest creep rupture life among all the high Cr ferritic steels so far developed in the world.