Influence of the Evolution of 9CrMoCoB Steel Precipitates on the Microstructure and Mechanical Properties during High-Temperature Aging

In this study, the short-term aging was carried out to reveal the evolution of precipitates and mechanical properties of heat resistant 9CrMoCoB steel during the early creep, replacing the conventional creeping. The tempered martensite lath structure (TMLS) and precipitates were observed in the as-aged 9CrMoCoB steel. TMLS in the matrix underwent a transition to the polygonal ferrite after aging only for 300 h. In comparison, the mean diameter of the precipitates increased from 183 to 267 nm after aging at 650 °C for 300 h. Also, the mean diameter of the precipitates increased from 183 to 302 nm at 700 °C. The room-temperature and high-temperature strength of 9CrMoCoB steel decreased after high-temperature aging, which may be mainly due to precipitates coarsening. Many M23C6 phases precipitate in the prior austenite grain boundary (PAGB) and lath boundary. After aging 100 h, TMLS transformed into polygonal ferrite, and the size of the precipitate at the subgrain boundary was about 100 nm, while after 300 h of high-temperature aging, large precipitates appear (400 nm) in the matrix. After 200 h of high-temperature aging, the obvious growth of precipitates on the PAGB and lath boundary weakens the pinning effect on the PAGB and martensite lath boundary and accelerates the transformation of microstructure and mechanical properties.

Microstructure Evolution of the as Tempered P92 Steel at High Temperature Exposure

The microstructure evolution during aging at high temperatures is usually used to thermodynamically simulate those cases of aging at low temperatures but for a very long time for P92 steel, because high temperature can accelerate the microstructure process. Therefore, in the present research, in order to comprehensively understand the microstructure evolution mechanisms during aging at especially high temperatures, the as-tempered P92 steel was exposed at 790 °C. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the microstructures. The dominant mechanisms at the four stages in microstructure evolution process during thermal exposure are (I) dislocation annihilation, (II) lath broadening and equiaxed subgrain nucleation, (III) equiaxed subgrain growth, (IV) recrystallization nucleation and growth. The martensitic lath broadening is dominated by both the motion of “Y”-type lath boundary and the combination of parallel lath boundary. The subgrain growth is by virtue of both the combination of the equiaxed subgrain and the bowing out of subgrain boundary.

Temperature Dependence of Austenite Nucleation Site on Reversion of Lath Martensite

The temperature dependence of austenite nucleation behavior within lath martensitic structure was investigated in an ultralow carbon 13%Cr-6%Ni martensitic stainless steel partially reversed at (austenite + ferrite) two phase region. The shape and nucleation site of the reversed austenite grains were varied depending on the reversion temperature; fine acicular austenite grains frequently formed along the lath boundaries at a temperature lower than 915 K, while the granular ones tended to nucleate mainly on the prior austenite grain boundaries at a higher temperature. In order to explain the temperature dependence of nucleation site transition, the difference in energetics of austenite nucleation between the lath boundary and the prior austenite grain boundary was discussed on the basis of the classical nucleation theory and FEM analysis. The calculation of the changes in interfacial energy and elastic strain for austenite nucleation suggested that the lath boundary acts as more preferential nucleation sites for austenite rather than the prior austenite grain boundary to reduce the increment of elastic strain when the reversion temperature is low.

Effect of Vanadium and Niobium on Creep Strength in 10% Chromium Steel Analyzed by STEM-EDS

The effect of vanadium and niobium on creep strengthening was studied in 10% chromium steels. Eleven kinds of samples which varied the additive amounts of vanadium and niobium were prepared. From the creep deformation behavior of them, the threshold stresses were estimated. The distribution maps of precipitates were obtained by STEM-EDS. In a steel with added 0.06% vanadium where no particles were observed in the lath, creep strength was slightly increased, indicating that it was not strengthened by a particle hardening effect in the lath. Lump-shaped precipitates and film-shaped precipitates including chromium and vanadium, which were observed on lath boundaries in the steel with added vanadium, are expected to inhibit the lath boundaries from migrating. This inhibition is the mechanism of the improvement in the steel with added vanadium. In the case of steels with added niobium, creep strength was found to be increased by dispersion hardening due to fine precipitates in the lath. The threshold stress was quantitatively estimated depending on the particle spacing. The estimated threshold stress corresponded to the one obtained by the creep deformation behavior. These results revealed that vanadium and niobium each have a role in improving creep strength. In steels with added both vanadium and niobium, the effect on creep strength was expressed by the sum of the effects due to each element, which were the retardation of the lath boundary migrations and the pinning of the dislocations in the lath.