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Abstract
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Grain coarsening due to the high temperature exposure deteriorates mechanical properties of
the high nitrogen austenitic stainless steels (HNASSs) produced by solution nitriding. To
improve mechanical properties, the grains of nickel and manganese-free Fe-23Cr-2.4Mo-1.2N
HNASS plates fabricated by pressurized solution nitriding were refined using a two-stage heat
treatment process. Structural and mechanical properties were investigated using X-ray diffraction,
optical microscopy, scanning and transmission electron microscopy, hardness and
tensile testing and compared with that of the conventional AISI 316L steel. The results show
that the as-produced HNASS exhibits uniform deformation up to failure without necking and
brittle inter-granular fracture. By grain refinement, the yield and tensile strengths as well as the
elongation to failure are increased by 17.8, 21.2, and 108.3 pct, respectively, as compared to the
as-produced HNASS. However, despite more than a double increase in tensile toughness and
elongation to failure, the brittle inter-granular fracture is not suppressed. The HNASSs plastically
deform through formation of straight slip bands. TEM observations indicate development
of planar arrays of dislocations in tensile-deformed HNASSs. The enhancement in tensile
strength and toughness by grain refinement is discussed on the basis of straight slip bands
formation, number of dislocations in pile-ups, and incompatibility strain developed between
adjacent grains.
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