Effect of Warm Forging on the Microstructure and Corrosion Behavior of Austenitic Stainless Steel 316LN

Abstract

The as-received as well as deformed microstructure and electrochemical corrosion behavior of warm-forged and annealed 316LN austenitic stainless steel have been investigated by using electron backscatter diffraction (EBSD) and electrochemical corrosion test methods. An attempt has been made to understand the effect of warm forging and annealing on the microstructure and electrochemical corrosion behavior of 316LN stainless steel. As-received and deformed microstructures of a sample were observed by EBSD, and from micrographs, it was observed that the refinement of grain occurs after deformation. Microstructural refinement occurs after appropriate annealing of the forged sample because of the revision of alpha ferrite and strain-induced martensite into austenite, which is formed after forging. Annealing treatment after warm forging gives better mechanical strength and optimum corrosion resistance because of the formation of homogeneous and equiaxed strain-free austenite grains. After warm forging, around 85 % of grains show a low misorientation angle of 2.5°, which shows that a lot of dislocation generation occurred during warm forging deformation and that these dislocations aligned themselves to form subgrain boundaries also known as recovery during annealing dislocation. The corrosion rate drastically increases after deformation but decreases after the annealing of deformed samples, which indicates that after appropriate annealing the corrosion resistance and repassivation capacity of materials improved because of the formation of stable oxide of chromium. The value of corrosion current density (Ecorr) shifted toward the passive direction in the Tafel plot after annealing of deformed samples, which indicates that a more protective passive film of Cr2O3 formed on the surface of the annealed sample after warm forging. The surfaces of the samples after an immersion test in the FeCl3 solution have been observed by scanning electron microscopy, and from micrographs, it has been observed that the number of pits is reduced after the annealing of a warm-deformed sample as compared with undeformed samples. The pitting potential of the samples was calculated in a simulated pressurized water reactor environment using the Tafel potentiodynamic polarization test.