Estudo experimental e por meio de simulação computacional do tratamento térmico de têmpera para o aço inoxidável X22CrMoV12-1

Abstract

The development of stainless steels was one of the main technological advances within the metallurgical sector, once those materials can be used under the most diverse applications with different combinations of mechanical properties and high oxidation and electrochemical corrosion resistance. Within this context, martensitic stainless steels can be highlighted due to its capability of going through martensitic hardening during quenching, enabling the attainment of high levels of strength and performance under elevated temperatures, especially when its chemical composition is engineered. Therefore, martensitic stainless steels with additions of Mo and V, such as X22CrMoV12-1, show prominence in virtue of its high hot resistance and satisfactory corrosion resistance for application under aggressive conditions, as is the case of components for steam turbines. Its properties, however, are directly connected to the quenching and tempering process. Hence, the objective of this study is to evaluate through experimental analysis and computational simulation the effects of the geometry, dimension, time and temperature of the austenitization stage upon the microstructure, grain size and hardness of the material, in order to provide understanding of how those factors can be controlled to guarantee processing improvement and suitability. To fulfill this goal, analyses were conducted under different austenitization temperatures (980 ºC to 1020 ºC) and times (0,5 h, 1 h, 2 h and 3 h), and both cases demonstrated directly proportional tendencies between temperature, time and grain size, while the hardness showed a peak value followed by a decrease (which happened for austenitization at 1020 ºC on the first case, and for times superior to 2h, on the second analysis). Additionally, aiming possibility of grain refinement, a thermal treatment of normalizing was added preceding the process of quenching, and the combination resulted in the highest value of hardness (299 HBW) and significant grain refinement (ASTM 4, when quenched and tempered, and ASTM 5 with the addition of the normalizing stage). Lastly, computational simulations were carried out considering bars of different transversal sections (rectangular and round) and dimension, enabling the study of the minimum time needed for the austenitization process. Therefore, the held discussion over processing parameters allowed basis to the refinement of the thermal treatment of the studied martensitic stainless steel, aiming the achievement of desired properties for different applications and requirements.