Microstructure and functionality of a uniquely graded super duplex stainless steel designed by a novel arc heat treatment method

摘要

A novel arc heat treatment technique was applied to design a uniquely graded super duplex stainless steel (SDSS), by subjecting a single sample to a steady state temperature gradient for 10 h. A new experimental approach was used to map precipitation in microstructure, covering aging temperatures of up to 1430 degrees C. The microstructure was characterized and functionality was evaluated via hardness mapping. Nitrogen depletion adjacent to the fusion boundary depressed the upper temperature limit for austenite formation and influenced the phase balance above 980 degrees C. Austenite/ferrite boundaries deviating from Kurdjumov-Sachs orientation relationship (OR) were preferred locations for precipitation of a at 630-1000 degrees C, chi at 560-1000 degrees C, Cr2N at 600-900 degrees C and R between 550 degrees C and 700 degrees C. Precipitate morphology changed with decreasing temperature; from blocky to coral-shaped for a, from discrete blocky to elongated particles for chi, and from polygonal to disc shaped for R. Thermodynamic calculations of phase equilibria largely agreed with observations above 750 degrees C when considering nitrogen loss. Formation of intermetallic phases and 475 degrees C-embrittlement resulted in increased hardness. A schematic diagram, correlating information about phase contents, morphologies and hardness, as a function of exposure temperature, is introduced for evaluation of functionality of microstructures.