Formation and Properties of Nitrocarburizing S-Phase on AISI 316L Stainless Steel-Based WC Composite Layers by Low-Temperature Plasma Nitriding

Abstract

Stainless steel-based WC composite layers fabricated by a laser cladding technique, have strong mechanical strength. However, the wear resistance of WC composite layers is not sufficient for use in severe friction and wear environments, and the corrosion resistance is significantly reduced by the formation of secondary carbides. Low-temperature plasma nitriding and carburizing of austenitic stainless steels, treated at temperatures of less than 450 °C, can produce a supersaturated solid solution of nitrogen or carbon, known as the S-phase. The combined treatment of nitriding and carburizing can form a nitrocarburizing S-phase, which is characterized by a thick layer and superior cross-sectional hardness distribution. During the laser cladding process, free carbon was produced by the decomposition of WC particles. To achieve excellent wear and corrosion resistance, we attempted to use this free carbon to form a nitrocarburizing S-phase on AISI 316 L stainless steel-based WC composite layers by plasma nitriding alone. As a result, the thick nitrocarburizing S-phase was formed. The Vickers hardness of the S-phase ranged from 1200 to 1400 HV, and the hardness depth distribution became smoother. The corrosion resistance was also improved through increasing the pitting resistance equivalent numbers due to the nitrogen that dissolved in the AISI 316 L steel matrix.

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

“…Some studies have shown that surface laser treatment can significantly increase the thickness and hardness of the nitrided layer and form a more uniform nitrided layer structure [196]. Surface laser treatment can also promote the formation of precipitates in the nitrided layer, which further increases hardness and wear resistance [197]. When the nitriding temperature is increased from 500 • C to 560 • C, the surface hardness increases by about 55%.…”

The “Expanded” Phases in the Low-Temperature Treated Stainless Steels: A Review

“…Expanded austenite was observed to form not only in austenitic stainless steels, both in massive specimens [8,11,46,47,146] and coatings obtained with various techniques [147][148][149][150][151][152][153][154], but also in stainless steels, in which the high N solubilization might induce the transformation of ferrite or martensite into austenite, as duplex [104,116,132,[134][135][136], martensitic [65,91,95], ferritic [121,122], and precipitation hardening [65,71,155,156] stainless steels.…”

Effects of Solid-Solution Carbon and Eutectic Carbides in AISI 316L Steel-Based Tungsten Carbide Composites on Plasma Carburizing and Nitriding

“…Thus, high corrosion resistance is maintained, and pitting corrosion in chlorinated environments is inhibited [19][20][21][22][23][24][25][26][27][28]. In a previous study, AISI 316L stainless-steel-based tungsten carbide composite layers fabricated via laser metal deposition were subjected to low-temperature plasma nitriding, and the formation of the S phase was confirmed at temperatures of 400-450 • C and a processing time of 4 h [29]. The hardness of the stainless-steel parts in the composite layers increased from 500-700 HV to 1200-1400 HV.…”

“…A few papers have reported low-temperature nitriding and carburizing for austenitic stainless-steel fabrication via directed energy deposition and powder bed fusion [22,24,32,46,47]. However, to the best of our knowledge, none of the studies published to date investigate tungsten carbide composites except the previous study [29]. The effect of original solidsolution carbon present in stainless-steel plates on low-temperature nitriding has been well-researched in previous studies, establishing the formation of thin carburizing S phase layers at the interface between the nitriding layer and substrate [21,26,48,49].…”