Microstructure/Mechanical Characterization of Plasma Nitrided Fine-Grain Austenitic Stainless Steels in Low Temperature

Abstract

Fine-grained austenitic stainless steels (FGSS) were plasma nitrided below 700 K to describe their microstructure evolution during the nitrogen supersaturation process and to investigate the post-stressing effect on the microstructure and mechanical properties of nitrided FGSS. Normal- and fine-grained AISI304 plates were nitrided at 623 K and 673 K to investigate the grain size effect on the nitrogen supersaturation process as well as the microstructure evolution during the nitriding process. Fine-grained AISI316 (FGSS316) wires were nitrided at 623 K to demonstrate that their outer surfaces were uniformly nitrided to have the same two-phase, refined microstructure with high nitrogen solute content. This nitrided FGSS316 wire had a core structure where the original FGSS316 core matrix was bound by the nitrided FGSS316 layer. The nitrided wire had higher stiffness, ultimate strength, and elongation in the uniaxial tensile testing than its un-nitrided wires. The core microstructure was refined and homogenized by this applied loading together with an increase of nitrided layer hardness.

Nitrogen Supersaturation into AISI420 Mold for Precise Machining

“…That is, this XRD analysis in Figure 3 proves that nitrogen supersaturation advances into AISI420 specimen without significant precipitation of iron and chromium nitrides. As studied in [7,11], the αand γ-phase formation is commonly seen in the nitrided martensitic and austenitic stainless steels. This two phase formation in Figure 3, is induced by the local cluster-fluctuation in the nitrogen supersaturation under the different affinity of nitrogen atoms to iron, nickel and chromium atoms in the cells [19].…”

Massive Nitrogen Supersaturation to CoCrMo alloys for Surface Microstructure Control

“…Among several candidate surface treatments, the lowtemperature plasma nitriding process is highlighted as a nitrogen-supersaturation hardening treatment of austenitic stainless steels [9][10][11]. Since no iron and chromium nitrides were synthesized, the high chromium solute content was preserved even in the nitrided layer without any loss of intrinsic corrosion toughness to AISI300 series stainless steels [10].…”

“…Since no iron and chromium nitrides were synthesized, the high chromium solute content was preserved even in the nitrided layer without any loss of intrinsic corrosion toughness to AISI300 series stainless steels [10]. The nitrided layer thickness reached 50 m after plasma nitriding AISI304 and AISI316 substrates for 14.4 ks at 673 K. This nitrided layer had a fine-grained, two-phase microstructure so that the average and surface hardness reached to 1,200 HV1N and 1,400 HV1N, respectively, in [11][12][13]. In a similar manner to the lowtemperature plasma nitriding of austenitic stainless steels, CoCr and CoCrMo alloys were also nitrogen-supersaturated to have high nitrogen solubility at the vicinity of their surfaces [14][15][16].…”

“…Since the original CoCrMo specimen has no specific crystallographic structure, this crystallographic change and orientation are induced by the nitrogen supersaturation. As stated in the studies by Farghali et al[11], Aizawa[12], and Aizawa et al[26], the normal polycrystalline austenitic stainless steel matrix was modified to have a two-phase, nano-grained microstructure by nitrogen supersaturation via the plasma nitriding at 673 K. Each nanostructured grain had a crystallographic orientation to (111) by the plastic straining. When nitriding the austenitic stainless steels, the nitrogen solute enriched to the chromium atoms, while the nitrogen solute starved to the iron and nickel atoms.…”