Abstract
Duplex stainless steels were initially developed for the manufacture of thermomechanically formed products. However, the increasing demand for casting components of these materials has led to the application of a widely developed technology for this forming process. After undergoing a solution annealing heat treatment, these materials become thermodynamically metastable systems, since the concentration of solute atoms in solid solution is so high that they become saturated, causing them to seek a lower free energy state when exposed to different temperatures. These systems reach a more stable thermodynamic condition by the precipitation of various intermetallic phases, depending on the temperatures to which they are exposed. Thermal energy serves as a catalyst to “overcome” the energy barrier that separates the metastable and stable phases. The objective of this work was to determine the influence of several heat treatment temperatures on the microstructure and mechanical properties of an ASTM A 890/A 890M Gr6A super duplex stainless steel. The increase in hardness and the decrease in impact toughness of these materials in impact tests were found to be directly correlated with the increase in sigma phase concentration in their microstructure, which tended to precipitate into ferrite/austenite interfaces. When the sigma phase was completely dissolved by the heat treatment, the material's hardness was determined by the volumetric concentration of ferrite and austenite in the microstructure, and the energy absorbed in the impact test reached approximately 220 J at room temperature.
