Talk given by

Prof. S. Sankaran

Department of Metallurgical and Materials Engineering

IIT Madras

Chennai, India

Abstract:

Nicrofer 6025HT or alloy 602CA is a high temperature corrosion resistant nickel base super alloy with high creep strength in carburizing and oxidizing atmosphere. Alloy 602CA contains 5% (by volume) of Cr₂₃C₆ precipitates which act as reservoirs for the formation of a protective Cr₂O₃ coating in the event of exposure to oxidation or corrosion at high temperatures. For this reason it is extensively used in thermal, petrochemical and power plant equipments. Corrosion due to Na₂SO₄ salt deposition is a major challenge and also involves high cost in such applications.

In the present study, an attempt is made to improve the high temperature corrosion resistance and mechanical properties of the Alloy 602CA sheets through two different surface modification techniques namely, High Reduction Cold Rolling (HRCR) and Ultrasonic Nanocrystal Surface Modification (UNSM). The UNSM is an energy intensive plasticity induced grain refinement process which essentially modifies the surface resulting in graded microstructure along the cross-section. Cold rolling on the other hand is a much simpler process and if the highest possible reduction per single pass is achieved, it leads to formation of graded microstructure across the cross-section. This results in significant increase in surface hardness and strength while preserving ductility and toughness. This study is to understand the extent of gradation in grain size obtained from surface to core of sheets by various High reduction cold rolling (HRCR) regimes in comparison to UNSM process. The study also addresses microstructural characterization in terms of grains size, dislocation density, residual stresses, salt-corrosion and grain boundary diffusion characteristics.