Effect of Nano-Size Oxide Particle Dispersion and δ-Ferrite Proportion on Creep Strength of 9Cr-ODS Steel

Abstract

The effects of chemical compositions on the microstructure and high-temperature creep strength of 9Cr-ODS steel was discussed in the light of quantitative data of -ferrite proportion and nano-size oxide particle dispersion, which were evaluated by dilatometric analysis and small angle neutron/X-ray scattering analysis, respectively. These quantitative data are well consistent with the conventional data obtained by transmission electron microscope. Both data indicate that the important microstructural feature for creep strength improvement of the 9Cr-ODS steel is the number density of nano-size oxide particles, and ferrite/martensite (F/M) duplex structure is favorable for high population nano-size oxide particle dispersion. Both optimization of excess oxygen concentration and control of the F/M duplex structure are promising technique for nano-structure control of 9Cr-ODS steel. Tungsten solid solution strengthening appears to be small compared with oxide dispersion strengthening enhanced by duplex microstructure formation.

A new method for the quantitative analysis of the scale and composition of nanosized oxide in 9Cr-ODS steel

“…The values of the Dq 2 ratio of the six alloys are in the range 35-45, which identifies the phase of the oxide nanoparticles as Y 2 Ti 2 O 7 . This is in agreement with previous studies [1,4,9,10], a result which suggests that this method can be used for identification of the phase of any candidate material. Although the crystalline structure or mass density values need to be determined by other techniques, quantitative information about concentrations can be obtained by this ACV between SAXS and SANS, even for particles just a few nanometers in diameter embedded in a steel matrix.…”

“…Although the previous TEM results show that crystalline structure of the oxide nanoparticles is that of Y 2 Ti 2 O 7 , the deviation from stoichiometric composition is still under discussion [4,9,10]. In this study, the Dq 2 ratios for Y 2 On increasing Fe substitution, the Dq 2 ratio becomes small only in the case of (3), while it grows large in the other cases.…”

“…The Dq ratio, defined as Dq 2 x =Dq 2 N , is also listed. q, Dq  10 10 However, the application of the ACV method to region II can introduce large errors due to the effect of both regions I and III. Therefore, further characterization of microstructure by TEM and AP is necessary for complete determination of the phase.…”

“…These results indicate that the quantitative evaluation of the microstructural parameter of nanosized oxide is important for understanding a material's mechanical properties. However, other microstructural parameters, such as the volume of remaining ferrite phase, are also important, especially for the understanding of the creep properties discussed in detail in the companion paper [10].…”

“…The microstructural parameters we obtained are compared with the Vickers hardness. Other properties, such as creep strength, are discussed elsewhere in conjunction with the effects of the microstructure at the scale of a few microns as well as the number density of oxide nanoparticles [10].…”