Abstract
The purpose of this study is to improve the surface properties of austenitic stainless steel using the double-folded electrode screen plasma nitriding (SPN) process. In general, the S-phase is well-known for its excellent properties such as improved hardness and wear resistance along with sustained corrosion resistance. The concentrated nitrogen via SPN process was injected to form S-phase with time at 713 K. This study was carried out under the conditions of 44 at% of nitrogen injection, which was higher than 25 at% known as the condition of no precipitation of S-phase formed by the SPN process, and 20 K higher than the maximum temperature without precipitation phase. The hardness analysis of stainless steel sample treated by the SPN process at 713 K showed a much higher value than the typical nitriding hardness at a depth of lower nitrogen than the maximum nitrogen concentration. The SPN 20 hr treated specimen showed the average value of 2339 HV while 40 hr showed the average value of 2215 HV. The result is attributed to the concentrated nitrogen formed in the SPN process reacting with the alloying elements contained in the base material to form fine precipitates, thus producing a synergy effect of the extreme hardening effect; that is, the movement of precipitates and dislocations due to the GP-zone (Guinier-Preston zone).
Highlights
This study was carried out under the conditions of 44 at% of nitrogen injection, which was higher than 25 at% known as the condition of no precipitation of S-phase formed by the Screen Plasma Nitriding (SPN) process, and 20 K higher than the maximum temperature without precipitation phase
This study reports the result showing the high hardness even when considering that the error range is high since the micro hardness is very large during the process of controlling the phase formation through aging by slow cooling after injecting concentrated nitrogen in the precipitation zone of S-phase for the study of the formation of more hardened surface despite the loss of nitride layer by precipitate
The SPN process enables the diffusion of a high concentration of nitrogen atoms into al.stainless steel (ASS) at relatively low temperatures compared to other nitriding processes such as gas nitriding
Summary
Stainless steel (ASS) at low temperatures has been extensively studied to understand the factors influencing the formation of nitrogen expanded austenite (S-phase) [1] [2] [3] [4] [5]. The S-phase on the surface of ASS is thermodynamically unstable when it is oversaturated with interstitial atoms such as nitrogen and carbon. This is because the state of oversaturation induces internal stress that distorts lattice and increases the volume of ASS [6]. The formation of cracks can initiate from the locations with high composition-induced stress in ASS
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