Development of High Fracture Toughness in Maraging Steels.

If 300 grade maraging steel is heat-treated to contain a distribution of tough austenite in an aged martensitic matrix, the fracture toughness can be improved significantly with only minimal decreases in the ultimate tensile strength. Studies have been carried out to determine the reasons for this increase. In particular the structure has been characterized by scanning SEM and transmission electron microscopy TEM, plastic zone sizes and shapes have been determined by microhardness and etching techniques, plane-strain crack tip opening displacements have been measured under loading, and fracture surfaces have been examined both by scanning and replica electron fractography. In addition, a survey of the fatigue crack propagation FCP characteristics have been carried out. Some testing was done at -196 C. The fracture toughness of the specially treated steel was equivalent to what was obtained for conventionally treated steel at room temperature. This process was extended to the ultra high strength 350 grade steel and again a significant improvement in toughness was obtained although the magnitude of the increase was not as large as was obtained for the 300 grade steel. A model based on the energy absorbed during plastic deformation of the austenite in the plastic enclave ahead of the crack tip has been proposed and is in good agreement with the experimental results. In some cases, heat treatments were used that result in formation of metastable austenite which transforms to martensite at crack tip on loading. This is a mode of plastic deformation that is responsible for very high toughness.