Research on Low-Temperature Mechanical Properties and Fracture Behavior of 09MnNiDR Steel Based on Small Punch Test

Abstract To develop a microdamage evaluation method applicable to in-service equipment under low-temperature conditions, this study systematically investigates the mechanical properties and fracture behavior of 09MnNiDR cryogenic steel over a broad temperature range from room temperature to −196 °C. The small punch test (SPT) technique is employed, supplemented by electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) for micromechanism analysis. Results indicate that under cryogenic conditions, dislocation slip is suppressed, leading to a more uniform distribution of plastic strain. Concurrently, the deformation process at low temperatures refines the grains within the plastic zone through mechanisms such as mechanical subdivision. As temperature decreases, the material strength increases linearly, exhibiting a significant cryogenic strengthening effect. The fracture mode transitions from ductile to brittle, with a ductile-to-brittle transition zone identified near −150 °C. An empirical formula based on SPT deformation energy is proposed to predict yield and true tensile strength, with prediction errors below 6%. By introducing a normalized energy parameter, an empirical correlation model is established between the SPT ductile-to-brittle transition temperature (DBTT) and the standard Charpy impact transition temperature. This study presents a viable methodology for safety assessment of in-service cryogenic pressure vessels through minimally invasive testing and performance prediction.