Acceleration of Plasma Nitriding at 550 °C with Rare Earth on the Surface of 38CrMoAl Steel

Dongjing Liu,Lin Hong,Mufu Yan,Yuan You,Tingjie Han,Hongtao Chen,Rui Li

doi:10.3390/coatings11091122

Dongjing Liu, Lin Hong + Show 5 more

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https://doi.org/10.3390/coatings11091122

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Abstract

In order to explore the effect of the addition of rare earth (RE) to a steel microstructure and the consequent performance of a nitrided layer, plasma nitriding was carried out on 38CrMoAl steel in an atmosphere of NH3 at 550 °C for 4, 8, and 12 h. The modified layers were characterized using an optical microscope (OM), a microhardness tester, X-ray diffraction (XRD), a scanning electron microscope (SEM), a transmission electron microscope (TEM), and an electrochemical workstation. After 12 h of nitriding without RE, the modified layer thickness was 355.90 μm, the weight gain was 3.75 mg/cm2, and the surface hardness was 882.5 HV0.05. After 12 h of RE nitriding, the thickness of the modified layer was 390.8 μm, the weight gain was 3.87 mg/cm2, and the surface hardness was 1027 HV0.05. Compared with nitriding without RE, the ε-Fe2-3N diffraction peak was enhanced in the RE nitriding layer. After 12 h of RE nitriding, La, LaFeO3, and a trace amount of Fe2O3 appeared. The corrosion rate of the modified layer was at its lowest (15.089 × 10−2 mm/a), as was the current density (1.282 × 10−5 A/cm2); therefore, the corrosion resistance improved.

Highlights

Plasma nitriding is a widely used chemical heat treatment technology. This method uses the active nitrogen atoms generated during the cathode sputtering process at a relatively low temperature (350–570 ◦C) to accumulate on the surface of a workpiece, diffuse into the substrate, and form a nitriding-modified layer with excellent performance
The 1 × 1 × 1 cm3 rare earth (RE) lanthanum was cut into 1/8 cube blocks, and the RE was hung on the cathode platform with the iron wire, in order to maximize the sputtering of RE
The nitrided layer without RE was clearly divided from the matrix, and the microstructure of the nitrided layer was uniform

Summary

Introduction

Plasma nitriding is a widely used chemical heat treatment technology This method uses the active nitrogen atoms generated during the cathode sputtering process at a relatively low temperature (350–570 ◦C) to accumulate on the surface of a workpiece, diffuse into the substrate, and form a nitriding-modified layer with excellent performance. The methods most commonly used to increase the thickness of the nitride layer are to increase the nitriding temperature or to extend the process time [8]. These methods have some disadvantages, such as the coarsening of the structure and reduced hardness [9,10]. Rare earth (RE) elements have been proven to be effective catalysts for chemical heat treatment [11]

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