Influência da martensita induzida por deformação e seu processo de reversão no desempenho mecânico do aço inoxidável lean duplex 2304

Abstract

Duplex stainless steels belong to a category whose predominant microstructure is formed by the ferrite (α) and austenite (γ) phases, which give the material good mechanical and corrosion performance and, therefore, are widely used in severe industrial applications. However, due to the thermomechanical treatment to which it is subjected, new phases can be formed, which directly interferes in the materials properties. The α’-martensite, for example, is formed when the steel is subjected to a high degree of deformation, where the austenite is partially transformed into martensite and, during the annealing step, this process is reversed, generating grain refining in the material. This α’-martensite phase increases the mechanical strength of the material but decreases its toughness. The study material is a type 2304 lean duplex stainless steel, supplied by the company APERAM, cold-rolled at UFMG laboratories with a 74% thickness reduction. After annealing at temperatures of 400°C, 500°C, 550°C and 600°C for 1800 s, the influence of thermomechanical treatments on the formation of the α'-martensite phase, its reversion and their effects on material texture and mechanical performance were studied. After cold rolling, approximately 53% of α’-martensite was identified through X-ray diffraction (DRX), which began its reversion at 550°C, reaching 45% of α’-martensite. An increase in hardness was noticed during annealing at 400°C, justified by an increase in the percentage of α' martensite. The chromium-rich α' phase, known as Embrittlement at 475°C, may have occurred in a small amount, due to the short annealing time. In the tensile tests, all specimens showed brittle fracture aspects, which indicates the presence of deleterious phases and that annealing up to 600°C for 30 min was not sufficient to reverse all the α'-martensite.