In this work, the austenite grain growth behavior of an ultra-low carbon cobalt-free ultra-high strength stainless steel is studied in the temperature range of 820 °C–1020 °C and the holding time of 0–150 min. The experimental results indicate that in the temperature range below 880 °C, there are a large number of Fe₂Mo-type Laves phases in the steel. The austenite grain size shows no significant change (∼23 μm) upon increasing the isothermal holding temperature or extending the holding time below 880 °C, which is attributed to the pinning effect of the Laves phases. At holding temperatures of 900 °C–960 °C, partial dissolution of the Laves phases reduces their pinning efficacy, triggering incipient grain growth. When the temperature is further increased to 980 °C or above, the Laves phase completely dissolves during the initial heating stage. The grains undergo accelerated coarsening with increasing isothermal holding temperature and prolonged holding time. Mathematical models for austenite grain growth in ultra-low-carbon cobalt-free ultra-high strength stainless steel are established in the temperature ranges of 900 °C–960 °C and 980 °C–1020 °C, with the optimal grain growth exponents (n) determined as 1.518175 and 1.614925, respectively. This study clarifies the effects of heating temperature, holding time, and initial grain size on austenite grain growth. The accuracy and reliability of the model are validated by comparing calculated values with experimentally measured data, achieving an average absolute relative error (AARE) of 1.33026% and a coefficient of determination (R²) of 0.99768.

In this work, we aimed to study the austenite grain growth behavior of an ultra-high-strength stainless steel within the temperature range of 900–1150 °C and holding time range of 0–120 min, using a metallographic microscope and metallographic image analysis software to perform a statistical analysis of grain size variation. The undissolved phases of the steel were investigated using a field emission scanning electron microscope (SEM) and transmission electron microscope (TEM). Within the temperature range of 900–950 °C, the grain growth rate of the steel was slow, while within the range of 1000–1150 °C, the grain growth rate was relatively fast. This is attributed to the precipitation of a large number of M₆C-type carbides during the forging and annealing processes. In the temperature range of 900–950 °C, the solid solubility of the M₆C phase was low and the pinning effect was significant, which hindered the growth of austenite grains. Above 950 °C, the carbides were dissolved extensively, weakening the pinning effect on the grain boundaries and accelerating the grain growth rate. A predictive mathematical model for the growth of the original austenite grains was established based on the Arrhenius equation, elucidating the effects of heating temperature, holding time, initial grain size, and number of carbides on the growth of austenite grains, providing a theoretical basis for heat treatment process design in actual production.

High-strength stainless steels are essential materials for critical load-bearing aerospace components, and solution treatment serves as a core process governing their strength–toughness balance. However, in novel multi-element alloy systems, the complex dissolution behavior of precipitates and its underlying mechanisms affecting matrix phase transformations require further investigation. This study systematically explores the thermodynamic evolution and microstructural response of a novel Fe-Ni-Cr-Mo-Al-Ti ultra-high-strength stainless steel during solution treatment. The research highlights how solution temperature drives Laves phase dissolution, controls prior austenite grain growth, redistributes local chemical elements, and dictates retained austenite stability. By establishing the relationship between microstructural features and macroscopic properties, this study aims to provide crucial theoretical guidance for optimizing heat treatment protocols to achieve superior comprehensive mechanical properties in advanced high-strength stainless steels.