Xiaosheng Zhou

Formation and evolution of precipitates in high Cr ferritic heat-resistant steels

Abstract For high Cr ferritic heat-resistant steels, dispersion hardening has played a vital role in improving creep strength at high temperatures. In this paper, the various types of precipitates in high Cr ferritic steels are reviewed, concerning M 3C, M 23C6, MX, M 2 X, Laves phase and Z phase. The evolution of these precipitates is a complicated process, since the dissolution of one type of precipitate may be overlapped with the precipitation of another type or other types. The composition of some precipitations varies towards the equilibrium composition under certain conditions, which is relevant to the solubility and diffusivity of the alloying element, while the composition of some other precipitates changes slightly. For the precipitates in high Cr ferritic steels, the preferred nucleation sites involve prior austenite grain boundaries, lath boundaries, dislocations and the interfaces between the precipitates and the matrix.

Relationship between austenite stability and martensite formation in modified 9Cr-1Mo steel

Abstract Aimed at investigating the relationship between austenite stability and martensite formation in modified 9Cr-1Mo steel, the fully austenitized steel was cooled to a temperature above or below the start temperature of martensite transformation (Mso, 406 °C), which occurs upon continuous cooling from the austenitizing temperature to room temperature, and held for different times (10 min, 25 min, and 45 min) followed by cooling to room temperature. The concept of primary and secondary martensite is introduced to indicate that two different, sequential, martensites formed in this processing. When cooled to 650 °C or 450 °C for different times, the austenite stability decreased with prolonged holding time, and during the subsequent cooling the primary martensite start temperature (Ms) was raised to a value higher than Mso. In contrast, Ms was reduced to a level lower than Mso when cooled to 420 °C or 410 °C for 45 min. On the other hand, as the steel was isothermally quenched at temperatures below Mso, during the subsequent cooling a secondary martensite reaction could be observed, and the secondary martensite start temperature (Ms′) decreased with holding time. Retained austenite in the form of thin films along martensite lath boundaries was obtained when cooled to 380 °C or below for different times.

Austenite to polygonal-ferrite transformation and carbide precipitation in high strength low alloy steel

Abstract By differential scanning calorimetry measurement and microstructural examination, the effects of cooling rates on the kinetics and microstructure of polygonal ferrite transformation have been investigated in a high strength low alloy steel. The peak temperature of ferrite transformation exhibits an evident dependency on cooling rate. With the cooling rate decreasing, the ferrite fraction corresponding to the maximum transformation rate is decreased, as well as the size of polygonal ferrite grains. More ferrite is nucleated at lower cooling rate. On the basis of the effects of austenite grain size and carbide precipitation on ferrite formation, it is suggested that carbide precipitation plays a more dominant role in the kinetics of ferrite transformation. During austenite–ferrite transformation, the ferrite nucleation still proceeds, and the nucleation would not be saturated by pre-existing nuclei.