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

To develop a micro-damage evaluation method applicable to in-service equipment under low-temperature conditions, this study systematically investigates into 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 micro-mechanism analysis. Results indicate that cryogenic conditions promote grain refinement while simultaneously suppressing dislocation slip. This leads to a reduction in dislocation density, thereby facilitating more uniform plastic deformation. 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 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 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.