Surface Hardening and Nitride Precipitation in the Nitriding of Fe-M1-M2 Ternary Alloys Containing Al, V, or Cr
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I.
INTRODUCTION
NITRIDING is one of the most popular surface hardening treatments for steels, which improves fatigue-, wear- and corrosion-resistance properties. Surface hardening by nitriding is caused by the formation of a hard compound layer on the alloy surface, solid-solution hardening of a ferrite (a) matrix by nitrogen (N), and precipitation hardening by fine alloy nitrides or alloying element (M)-N clusters. Precipitation hardening has the most significant effects on surface hardening in the nitriding of alloyed steels. Therefore, the influence of the alloying elements on the nitriding behavior has been extensively investigated using Fe-M binary alloys. In the nitriding of Fe-low Cr alloys, disk-shaped CrN particles with the B1 structure form on the {001}a planes and hold a Baker–Nutting orientation relationship with the a matrix: (001)a//(001)CrN, [100]a//[110]CrN.[1–5] Although the equilibrium alloy nitride on Ti is also TiN with the B1 structure, it has been reported that metastable Ti-N clusters are formed prior to the precipitation of TiN during the nitriding of Fe-Ti alloys on a basis of rather indirect experiments such as X-ray diffraction, internal friction, or N absorption/desorption measurements.[6–8] Very fine mono-layered Ti-N clusters were later observed in a nitrided Fe-Ti alloy using high-resolution transmission electron microscopy.[9] In addition, M-N clusters were reported to form in the nitriding of Fe-V,[9] Fe-Mn,[10] and Fe-Mo alloys.[11] GORO MIYAMOTO, Associate Professor, KUNIO SHINBO, Assistant Technical Staff, and TADASHI FURUHARA, Professor, are with the Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi, 980-8577, Japan. Contact e-mail: [email protected] SHOTARO SUETSUGU, Graduate Student, formerly with the Tohoku University, is now with the KD Business Department, Toyota Tsusyo, Nagoya, Japan. Manuscript submitted January 30, 2015. Article published online September 3, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
Two types of Al nitrides were observed in the nitriding of Fe-Al alloys: metastable AlN with the B1 structure and stable AlN with the wurtzite structure.[9,12,13] In the nitriding of a recrystallized Fe-Al alloy, AlN (B1) is precipitated with sluggish kinetics and is subsequently transformed to stable AlN (wurtzite).[9,12–14]. Biglari et al.[12,14,15] reported that deformation prior to nitriding accelerates the precipitation of AlN (B1) due to the promotion of AlN (B1) nucleation at dislocations. A high density of dislocations in the lath martensite structure results in similar acceleration effects in the nitriding of an Fe-Al-C alloy.[16] Many practical nitriding steels contain a certain amount of Al, so that the enhancement of Al nitride precipitation is of particular interest to achieve increased surface hardness by nitriding. The addition of another alloying element that has a stronger affinity for N than Al, such as Cr, Ti, or V, would cause precipitation of those alloy nitrides at the early stage of nitriding and provide
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