Study on Step-Annealing Heat Treatment on Properties of Grade 6A Duplex Stainless Steels of ASTM A890: Fid-Bau Portal

Study on Step-Annealing Heat Treatment on Properties of Grade 6A Duplex Stainless Steels of ASTM A890

Nithin Raj, P. / Raha, B.

Improvement in properties of duplex steel grade 6A is always a thrust area of interest from application point of view and in that heat treatment plays an important role. The precipitations and phase ratio are key factor in determining the properties of super duplex stainless steels. The furnace cooling or step-annealing at lower temperature than normal annealing temperature and for a short time was not included in ASTM A890-18 version has now been added in 18a version for grade 6A like other duplex grades. For studying effect of step annealing on the phase ratio and properties of SDSS, the specimens were soaked at temperatures in the range of 1130–1190 °C and next furnace cooled to 1100 °C and holding for 15 min followed by water quenching. The micrograph of specimen directly quenched from 1100 °C revealed high ferrite content whereas the micrograph of the specimens subjected to step-annealing process revealed higher austenite content with more equiaxed structure. The step-annealing increased the austenite percentage and improved the impact toughness by 24%. However, the hardness of the specimen reduced as the step-annealing temperature increased. The polarization curves revealed a cathodic shift in the curve of the specimen directly quenched from 1100 °C compared to the step-annealed specimens. The reason for this reduction was investigated by special etching techniques, electron microscopy and energy-dispersive spectroscopy. Undissolved Cr₂N precipitations were observed in the specimen directly quenched from 1100 °C, leading to the reduction in corrosion resistance.