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For the construction of ultra super critical (USC) power plant, 9Cr-3W base ferritic heat-resistant steels with relatively high B and no N have been investigated. Authors have been revealed in the previous report that the addition of 139ppm B significantly improves creep strength of the steels, whereas most of added B forms unidentified borides, which are deemed almost ineffective to creep strength. The effect of improved heat treatment on creep strength and distribution of B in precipitates is investigated to effectively utilize and decrease added B. As a result of the analysis of the extracted residue and characterization of precipitates using field emission Auger electron spectroscopy (FE-AES), most of added B still forms borides in the 92ppm B added steel. These composites are almost dissolved and the B content in M 23 C 6 carbides is significantly increased by normalizing at 1423K. It is also found by FE-AES analysis that B content in M 23 C 6 carbides near prior-austenite grain boundaries is relatively higher than that inside grains. Creep strength at 923K for the 92ppm B added steel normalized at 1423K is not improved at short times, but it is remarkably improved to almost the same level as the 139ppm B added steel at long times. This excellent creep strength is achieved resulting in improving microstructural stability through the effective utilization of added B by high-temperature normalizing.

Microstructure of C-free martensitic alloys at elevated temperatures was studied for understanding the origin of their superior creep properties over 923 K. These alloys initially have martensitic structure and the heterogeneous creep deformation seems to be suppressed even under low stress region in distinction from the conventional heat resisting steels because there is no rapid growth of carbides during creep at high temperatures. Inter-metallic compounds such as Laves or μ phase were dominantly observed over 923 K and these precipitation had been followed by the low temperature reaction that greatly contributed to the typical aging behavior like in the usual maraging steels. It was found that the density of the precipitation was still kept high even at 973 K because its gathering and coalescence rate was small comparing with that of high Cr steels. Furthermore, the reversely transformed austenite phase certainly appeared by the shearing mechanism in these alloys when the materials were rapidly heated up to the temperature for creep testing, because their As points were measured to be at around 873 K. This austenite phase represented terrace like feature in SEM observations. Fortunately, this characteristic structure was resulted from a selective electropolishing in specimen preparation due to the compositional fluctuation. However, it was also confirmed that partitioning of alloying element was not completed in the short-term aged materials. It was concluded that the characteristic microstructure of these alloys over 923 K was continuously formed as follows. Firstly, uniform fine precipitation of inter-metallic compounds occurred at early stage and then the reversely transformed austenite was broken out without diffusion process. Finally, dissolution of precipitation in the retained martensitic region was assisted by long-term keeping at high temperatures. Consequently, it was expected that specific creep resistance of these alloys over 923 K be achieved by existence of large amount of austenitic phase strengthened by fine precipitation.

Precipitation strengthening is important to improve creep strength of heat resisting steels at elevated temperatures. Especially, in the high-Cr ferritic steels recently developed for Ultra Super Critical Power Plants, precipitation behavior is complicated and should be clarified because the correlation between various strengthening factors are still not well understood. In this study, we focus on the change in precipitation behavior of MX type carbonitrides in model steels with different heat treatment conditions. It is found that dominant precipitation behavior has been changed with cooling rate from normalization temperature before tempering and better creep properties have been obtained in the steel quenched rather than in the one air-cooled. It is confirmed that precipitation of M 23 C 6 is suppressed and total fraction of MX is relatively increased by the quenching procedure. It is thought that better creep resistance is mainly due to fine distribution of MX that is achieved by suppressing typical co-precipitation of MX such as Nb(C, N)-VN during cooling after normalization and tempering.

Creep deformation and the corresponding microstructural evolution in high-Cr ferritic steels has been studied using model steels with the initial microstructures consisting of the various combination of a, MX and M 23 C 6 to clarify the role of each precipitate on creep deformation and the resultant creep strength of the steels. A carbon free steel strengthened by the a phase has exhibited high creep resistance at the initial stage of transient creep region but a quick transition to the acceleration creep at a small strain, due to less precipitation along boundaries such as lath, block, packet and the prior austenite grain boundary. Precipitation of M 23 C 6 along these boundaries has been found to delay the transition to the acceleration creep, which gives a larger offset creep strain. Fine dispersion of MX has reduced much the creep rate in the transient creep region but enhances the heterogeneous creep deformation in the acceleration creep region. It is thus concluded that an optimum microstructure consists of fine dispersion of the a phase and MX carbonitride inside lath grain decorated with M 23 C 6 along lath, block, packet and the prior austenite grain boundary.

The effect of trace elements such as Mn and B on creep properties of 0.06C-2.25Cr-1.6W-0.1Mo-0.25V-0.05Nb has been investigated from the standpoints of a long-term microstructural stability. The chemical analysis of extracted residues and TEM observation show that the M23C6 carbide and/or M7C3 carbides, precipitated by tempering are replaced by M6C with a concentration of W; i.e., the amount of W in solution reduces during creep. On the other hand, MX type carbides such as VC and NbC are very stable during long-term aging and contribute to the creep strength by obstructing the dislocation annihilation. One of the most significant results is that a reduction in Mn-content lowers the minimum creep rate, resulting in an increase in the creep rupture time. The nucleation and/or growth of M6C are retarded with reducing Mn-content, thereby the increase in dissolved W seems to enhance the resistance to creep deformation. Another significant result is that an increase in B-content has delayed the transition from the primary creep to a tertiary creep stage. The principal roles of B are stabilizing M23C6 or M23(C, B)6 on former austenite grain boundaries and retarding the dynamic recrystallization during creep. In addition, in the specimens with higher amount of B, the bainite lath interface is covered by MX and some filmy precipitates with high density, thereby the softening resistance is enhanced.