Avaliação das propriedades mecânicas e magnéticas de aços maraging graus 300 e 350 de ultra-alta resistência: relação entre propriedades e tamanho de grão da austenita prévia

Abstract

Maraging steels have low carbon content (<0.02%) and high nickel content. They consist of Ni-Co-Mo-Ti quaternary alloys that have high mechanical strength and excellent toughness, and the name of this alloy is attributed to the hardening mechanism that occurs in a martensitic transformation (Mar) + aging (aging). The high mechanical strength of maraging steels makes it an ideal material for applications in a wide range of fields from high pressure vessels operating in critical processes, aeronautical components, underwater hulls to sports equipment. This research aimed to evaluate the relationships between microstructure and mechanical properties of the 18% Ni grade 300 and 350 maraging steel, and their alterations due to the series of heat treatments. Thus, the objective of this research was to establish relationships between prior austenitic grain size, which is altered as a function of solution annealing temperature. For this purpose, solution treatments were carried out at four different temperatures (840oC / 1h, 950oC / 1h, 1050oC / 1h and 1150oC / 1h), followed by aging at two temperatures (480oC / 3h and 560oC / 1h), with microstructural characterization, measurement of grain size and hardness test, after each step. X-ray measurements were also carried out to evaluate the presence of reverse austenite. In order to evaluate the magnetic properties, Vibrant Sample Magnetometer (VSM) and Barkhausen noise tests were performed. The effects of hydrogen embrittlement on the mechanical properties of the maraging 300 steel were evaluated by Slow Strain Rate Test (SSRT) both in air and in hydrogenated aqueous solution of 3.5% NaCl applying a cathodic potential. It was observed that both diffused hydrogen in the samples and the grain size influence the mechanical properties of the maraging 300 steel. The samples of this steel with smaller prior austenitic grain size presented higher mechanical resistance. The samples tested in air presented better performance than the samples tested in hydrogenated aqueous solution. Hydrogen induced cracking was observed in the grain boundaries of the hydrogenated samples. The grain size of the prior austenite also influenced the magnetic properties of maraging 300 and 350 steels.