Low-Temperature Plasma Nitriding for Austenitic Stainless Steel Layers with Various Nickel Contents Fabricated via Direct Laser Metal Deposition

Shinichiro Adachi,Motoo Egawa,Nobuhiro Ueda,Takuto Yamaguchi

doi:10.3390/coatings10040365

Shinichiro Adachi, Motoo Egawa + Show 2 more

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https://doi.org/10.3390/coatings10040365

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Abstract

In this study, low-temperature plasma nitriding is applied to austenitic stainless steels at temperatures below 450 °C. This enhances the wear resistance of the steels with maintaining corrosion resistance, by producing expanded austenite (known as the S-phase), which dissolves excessive nitrogen. Austenitic stainless steels contain nickel, which has the potential to play an important role in the formation and properties of the S-phase. In this experiment, austenitic stainless steel layers with different nickel contents were processed using direct laser metal deposition, and subsequently treated using low-temperature plasma nitriding. As a result, the stainless steel layers with high nickel contents formed the S-phase, similar to the AISI 316L stainless steel. The thickness and Vickers hardness of the S-phase layers varied with respect to the nickel contents. Due to lesser chromium atoms binding to nitrogen, the chromium content relatively decreased. Moreover, there was no evident change in the wear and corrosion resistances due to the nickel contents.

Highlights

Conventional nitriding for austenitic stainless steel is typically performed at treatment temperatures exceeding 500 ◦ C; this improves the surface hardness by synthesizing chromium or iron nitride
Blended AISI 316L stainless steel and nickel powder were supplied as the feeding powder for direct laser metal deposition
The AISI 316L stainless steel powder has a diameter of −212/+63 μm, and the nickel powder has a diameter of −150/+45 μm

Summary

Introduction

Conventional nitriding for austenitic stainless steel is typically performed at treatment temperatures exceeding 500 ◦ C; this improves the surface hardness by synthesizing chromium or iron nitride. The formation of chromium nitride causes a reduction of the passive film on the stainless steel, thereby significantly deteriorating corrosion resistance. It is difficult to apply conventional nitriding to austenitic stainless steels under corrosive environments. Low-temperature nitriding at treatment temperatures of less than 450 ◦ C can form a supersaturated solid solution of nitrogen in the face-centered cubic lattice of austenitic stainless steel, which is known as the S-phase. The S-phase possesses a high hardness exceeding 1000 HV; it significantly enhances wear resistance. The S-phase does not disturb the formation of the passive film, because the S-phase is not a nitride. The corrosion resistance is maintained after nitriding [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]

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