Study of the relationships between microstructure and local mechanical properties in the ductile-to-brittle transition of a bainitic steel containing local segregations

Abstract

Although considered a 1st-class option to meet the drastic requirements of Reactor Pressure Vessels (RPV) environments, Mn-Mo-Ni bainitic alloys also have segregated microstructures that can lead to heterogeneous mechanical properties. The latter should explain the scatter and the brittle fracture sometimes observed along the ductile-to-brittle transition during qualification tests. The present study focuses on a highly segregated SA 508 Gr.3 (aka 20MND5) steel and investigates the evolutions of local mechanical properties with temperature. Microstructure analysis revealed positive and negative segregations mainly composed of lower and upper bainite, respectively. An experimental device was developed and combined with a systematic procedure to enable local microhardness measurements from room temperature to − 40 °C. Analysis of hardness maps revealed two critical parameters assumed to be involved in the macroscopic brittle fracture as the temperature decreases. The first one is a significant increase in the hardness of both phases, particularly a continuous increase in the lower bainite’s hardness, suggesting that this phase could preferentially lead to brittle fracture. A wide range of hardness values in selected positive segregation reinforced the idea of the possible existence of very local weak points that could explain the dispersion of the results of the macroscopic Charpy test. The second parameter is an extension in the brittle surfaces, leading to a greater probability of testing a brittle area due to an extension surface area of the weak points.