Estudo da presença de ferrita delta com a variação de níquel e tratamento térmico em um aço AISI 304L com ênfase nas propriedades mecânicas

Abstract

The properties of austenitic stainless steels can be degraded in service when containing delta ferrite in the microstructure. In small quantities, delta ferrite reduces susceptibility to solidification cracks and increases tensile strength, but reduces ductility. The distribution and quantity of the formed delta ferrite depend on the chemical composition of the steel and the speed of solidification. In these steels the addition of nickel stabilizes the austenite phase at room temperature, but control is necessary, because the change in the residual volumetric percentage of delta ferrite influences the mechanical properties. Thus, in this work, the ferrite delta phase in 304L steel after thermal treatments was analyzed. The influence of delta ferrite and heat treatments on mechanical properties was analyzed using optical microscopy (OM), thermodynamic calculations by calphad method, energy dispersion X-ray spectroscopy (EDS), delta ferrite quantification by magnetic permeability (ferritoscopy) method and hardness and tensile assays. The results obtained showed the decrease of this phase with the increase of annealing time, reaching practically its elimination with the treatment. The tensile behavior of the solubilized material decreases with respect to mechanical resistance, but ductility increases in the treated condition. The material without solubilization treatment presented properties opposite to the material with treatment, with maximum resistance and superior yield. It was also possible to observe that the solubilized samples presented a lower amount of ferrite and the ferrite aspect also differs from the non-solubilized samples, due to their dissolution and spartoidization after solubilization treatment. It was concluded that it is necessary to control the volumetric fraction of delta ferrite formed in austenitic stainless steels, being less than 3% in fully solubilized steels and up to 20% in fused steels, since it is mainly from this microconstituent that the sigma phase precipitates.