Austenitic stainless steels have been widely utilized in industries, infrastructures, housing structures, kitchen components, and medical tools. Higher hardness and strength as well as more improvement of wear and corrosion toughness are often required in the industrial and medical applications. Fine-grained stainless steel (FGSS) provides a solution to increase the strength without loss of ductility and toughness. Deeper research and development in manufacturing of FGSS is required to make full use of its properties toward its applications in industries and medicals. First, its mechanical properties and microstructure is introduced as a basic knowledge of FGSS with comparison to the normal stainless steels. Mechanical and laser machinability of FGSS is stated and discussed to finish the products in seconds. Its performance in metal forming and diffusion bonding is explained to explore its applications in third. Its surface treatment and tooling is discussed to describe the grain-size effect on the low temperature plasma nitriding and to demonstrate its effectiveness in die-making in forth. Finally, every aspect in manufacturing of FGSS sheets and solids is summarized as a conclusion.
Part of the book: Engineering Steels and High Entropy-Alloys
Austenitic stainless steel type AISI304 sheets and plates as well as fine-grained type AISI316 (FGSS316) substrates and wires were employed as a work material in the intense rolling, the piercing and the plasma nitriding. AISI304 sheet after intense rolling had textured microstructure in the rolling direction. Crystallographic state changed itself to have distorted polycrystalline state along the shearing plane by piercing, with the strain induced phase transformation. FGSS316 substrates were plasma nitrided at 623 K for 14.4 ks to have two-phase fine nanostructure with the average grain size of 100 nm as a surface layer with the thickness of 30 μm. FGSS316 wires were also plasma nitrided at the same conditions to form the nitrided surface down to the depth of 30 μm. This nitrided wire was further uniaxially loaded in tensile to attain more homogeneously nitrided surface nano-structure and to form the austenitic and martensitic fiber structure aligned in the tensile direction. Each crystallographic structure intrinsic to metals and metallic alloys was tailored to have preferable micro−/nano-structured cells by metal forming and nitrogen supersaturation. The crystallographic change by metal forming in a priori and posterior to nitriding was discussed to find out a new way for materials design.
Part of the book: Electron Crystallography
The high-density plasma nitriding at 673 K and 623 K was employed to make 10% of nitrogen supersaturation on AISI316 base austenitic stainless steels. The processing parameters and nitrogen-hydrogen gas flow ratio were optimized to increase the yield of N2+ ion and NH-radical for efficient nitriding. The nitrided AISI316 specimens were prepared for multidimensional analysis to describe the fundamental features of low-temperature plasma nitriding. First, macroscopic evaluation revealed that nitrogen supersaturation induced the γ-lattice expansion and the higher nitrogen content than 4% of mass in depth. The mesoscopic analysis describes the holding temperature and initial grain-size effects on the microstructure changes. Plastic straining, grain-size refinement, and nitrogen zone-boundary diffusion processes advance with nitrogen supersaturation to drive the inner nitriding behavior. The microscopic analysis explains the microstructure refinement, the two-phase structuring, and the microstructure modification. Through this multi-dimensional analysis, the essential characteristics of the low-temperature plasma nitriding of 316 austenitic stainless steels were precisely understood to extend the engineering treatise on the bulk nitrogen stainless steels for surface modification and treatment of stainless steels by nitriding. This plasma nitriding was applied to strengthen and harden the AISI316 wire surfaces toward its application on surgery wires.
Part of the book: Stainless Steels