Ferro nodular temperado e particionado após recozimentos intercríticos: caracterização microestrutural e mecânica

Abstract

Over the past ten years, Quenching and Partitioning (Q&P) heat treatment has emerged as an alternative route for producing ductile irons with superior mechanical strength and ductility comparable to that of Austempered Ductile Iron (ADI) grades 1 to 5. However, the effects of different volume fractions of proeutectoid ferrite on the mechanical properties of these materials had not yet been systematically investigated. Accordingly, this study was conducted to correlate microstructural aspects with the mechanical behavior of a quenched and partitioned FUCO® FE 45012 ductile iron containing proeutectoid ferrite. To achieve this, the material was subjected to Q&P heat treatment after intercritical annealing, followed by microstructural and mechanical characterization. Initially, the as-cast microstructure was characterized, the critical temperatures of the stable three-phase field were determined, and the formation kinetics of proeutectoid ferrite and martensitic transformation were evaluated. Subsequently, to obtain microstructures with volume fractions of 10, 20, and 30% proeutectoid ferrite; 25, 35, 45, and 55% partitioned martensite; and 15, 25, 35, and 45% ausferrite, eight heat treatments were designed by varying the intercritical annealing time and quenching temperature. Following the heat treatments, the microstructures were characterized by optical microscopy (OM), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and phase mapping by electron backscatter diffraction (EBSD). The mechanical properties were evaluated by tensile, Brinell hardness, Vickers microhardness, and Charpy impact tests. The as-cast microstructure exhibited heterogeneities along the cross-section of the material bars, related to the graphite nodule distribution, size, and mean spacing. The designed heat treatments produced microstructures consisting of partitioned martensite, ausferrite, Widmanstätten ferrite side plates, and volume fractions of proeutectoid ferrite ranging from 2.3 to 33.2%. At intercritical annealing times longer than 30 min, polygonal proeutectoid ferrite grains enveloped the graphite nodules, resulting in a "bull's-eye" morphology. An increase in the volume fractions of proeutectoid ferrite led to lower yield strength, tensile strength, and Brinell hardness, and higher absorbed impact energy. The ultimate tensile strength ranged from 1077 to 1462 MPa; yield strength ranged from 833 to 1292 MPa; Brinell hardness ranged from 291 to 396 HBW; and impact energy ranged from 55.6 to 79.3 J, with elongation not exceeding 2%. Heterogeneities associated with the graphite nodules and proeutectoid ferrite grains preferentially distributed around them were found to contribute to the observed low percentage elongation. Microstructures containing proeutectoid ferrite exhibited a mixed-mode fracture mechanism, characterized by dimples, cleavage, and quasi-cleavage. The FUCO® FE 45012 quenched and partitioned after intercritical annealing exhibited tensile strengths and percentage elongations comparable to those of medium-strength, low-ductility ADIs, as well as higher tensile strengths and lower percentage elongations than those of dual-phase ADIs.