Two‐Step Nitriding Behavior of Pure Iron with a Nanostructured Surface Layer

Abstract

A two-step gas nitriding treatment (280 C for 2 h plus 550 C for 6 h) is designed for pure iron with a nanostructured surface layer induced by surface mechanical attrition treatment (SMAT). The microstructure and surface properties of the SMAT sample after two-step nitriding treatment are investigated in comparison with those of the SMAT sample after single-step nitriding treatment (550 C for 6 h). Experimental results show that a much thicker compound layer (up to 25 μm in thickness) is generated on the surface of the SMAT sample after twostep nitriding treatment, in which the mean grain size of the Fe 2-3 N phase is below 25 nm. The enhanced nitriding kinetic may be attributed to the formation of a large number of Fe(N) solid solutions and a small amount of dispersed Fe 2-3 N particles on the SMAT sample after first low-temperature nitriding, which enhances the thermal stability of nanostructured iron grains. Due to the enhanced nitriding kinetic and refined nitride structure, the SMAT sample after two-step nitriding treatment displays higher surface hardness and improved wear resistance relative to its single-step nitrided counterpart.

Effect of ultrasonic surface deep rolling combined with oxygen boost diffusion treatment on fatigue properties of pure titanium

“…Tong et al 23 proved that nitriding at low temperature can effectively produce a high hardness surface layer by means of surface nanocrystallization treatment. Other studies also proved that it is feasible to improve surface properties by the combination of surface mechanical treatment and low-temperature diffusion treatment 24,25 . Thus, the effect of USDR plus OBD at lower diffusion temperature on fatigue properties should be further investigated.…”

A Review—Effect of Accelerating Methods on Gas Nitriding: Accelerating Mechanism, Nitriding Behavior, and Techno-Economic Analysis

“…(1) Nitriding time: The pretreatment of surface mechanical nano-crystallization can shorten the nitriding time of gas nitriding to 70%. Sun et al [127] performed mechanical grinding on the surface of pure iron and then carried out nitriding treatment in an ammonia atmosphere. It was found that the nitriding depth of the SMAT sample was greater than that of the untreated sample, and the nitriding time was also reduced by about 30%.…”

The Effect of the Transformation of ε-Fe2-3N into γ′-Fe4N Phase on the Fatigue Strength of Gas-Nitrided Pure Iron

“…In this study, ε phase nitride, which had not been transformed and left a small amount, was not a significant problem to confirm whether the γ' phase nitride of thick layer, which was formed by 2stage gas nitriding, was able to improve fatigue strength compared to γ' phase nitride formed at once. According to the literature [12,19,20], however, it was recorded that the ε phase formed in the first stage was mostly transformed into γ' phase in the second stage for a short time. Nevertheless, this study showed different results than expected that ε phase formed in the first stage would be completely transformed into the γ' phase in the second stage during 2-stage gas nitriding.…”

“…Therefore, this study is intended to investigate the effect on fatigue strength by forming thick iron nitride consisted of γ phase through 2-stage gas nitriding. 2-stage gas nitriding is a technique in which an iron nitride layer consisting of ε phase, which is grown to be relatively thick compared to that consisting of γ phase at the same time, is first formed on the surface, and then iron nitride consisting of ε phase is phase-transformed to γ phase by controlling K N [12,19,20]; i.e., it was intended to investigate how the fatigue strength of iron nitride consisting of γ phase, which was phase-transformed in ε phase through 2-stage gas nitriding, and that of iron nitride consisting of γ phase, grown at one time through phase-controlled gas nitriding, are compared.…”