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.

Phase Transformation and Carbide Precipitation of Functional Gradient Semi-solid 9Cr18 Steel

“…The M 23 C 6 carbide is rich in carbon and chromium [25]. The stability of meta-austenite will decrease after carbide precipitation due to the decrease of alloying elements [26]. But, matrix austenite can be retained at this temperature.…”

Phase Transformation Behavior and Microstructural Control of High-Cr Martensitic/Ferritic Heat-resistant Steels for Power and Nuclear Plants: A Review

“…Tamura et al [49] suggested that the untransformed austenite would evolve into a new microstructure during subsequent tempering, which was related to the peaks in hardness and time to rupture of modified 9 wt% Cr steel. To obtain certain amount of retained austenite in 9 wt% Cr steels, after the austenization treatment, the steels can be cooled to a temperature between the martensite start temperature (M s ) and finish temperature (M f ) for different time [50] . At the interrupted cooling stage, the carbon partitions from the transformed martensite into the remaining austenite, increasing the stability of austenite and resulting in the film-like retained austenite between martensite laths.…”

Identification of Retained Austenite in 9Cr-1.4W-0.06Ta-0.12C Reduced Activation Ferritic Martensitic Steel

“…Santella et al [16] report an incomplete austenite transformation in the weld metal of modified 9Cr-1Mo steel during initial cooling from an austenitizing treatment and the transformation of retained austenite to martensite during tempering, and attributed to the effects of microsegregation. The presence of retained austenite can be undesirable in the steel, if it is metastable, and can lead to the formation of untempered martensite or inhomogeneity after tempering [17]. Inhomogeneity in such steel components can have a significant negative impact on the mechanical properties, reducing the service life.…”