Abstract
This paper presents the results of microstructural examinations on gas carbonitrided layers using scanning electron microscopy (SEM), wavelength dispersive spectrometry X-ray microanalysis (WDS), magnetic force microscopy (MFM), electron backscattered diffraction (EBSD), and X-ray diffraction. The main objective of the work is to discuss the microstructural and morphological changes occurring in the layers depending on the process parameter such as temperature and atmosphere composition. The carbonitrided layers comprise two sublayers: an outer layer enriched in nitrogen and an inner layer enriched in carbon. It is found that both the microstructure and the thickness depend on the technological parameters of the thermochemical treatment. The treatment temperature is particularly important, increasing both the total and the nitrogen-rich sublayer thickness regardless of the atmosphere type. The nitrogen potential has a positive influence on the thickness of the nitrogen-stabilised S-phase sublayer. Layers treated for 5 h at temperatures of 400 and 450 °C are composed of an internal carbon-enriched S-phase sublayer and an outer nitrogen-enriched S-phase sublayer that can be divided into ferromagnetic and paramagnetic zones. In the outer sublayer, symmetry distortion of the fcc crystal system is observed and stacking faults may have occurred. With layers produced at 500 °C for 5 h, the outer nitrogen-enriched sublayer contains fine precipitates of nitrides and carbides.
Highlights
Austenitic stainless steel is a widely used structural material for applications where high corrosion resistance is required, in medical, chemical, and food processing industries
No software enhancement of the maps was performed on the results presented in this article
The total layer thickness responded a similar way to the treatment temperature for both
Summary
Austenitic stainless steel is a widely used structural material for applications where high corrosion resistance is required, in medical, chemical, and food processing industries. Increasing the mechanical properties of such steel is possible through the use of surface treatment such as nitriding or/and carburization. It is found that if the process of treatment is conducted below 500 ◦ C, the formation of a new phase takes place [4,5]. This phase, called “expanded austenite” or “S-phase” demonstrates good mechanical properties such as hardness or wear resistance and has the corrosion properties comparable with those of austenitic stainless steel [2,6,7,8]. The majority of scientific research in this area concerns the surface treatment of austenitic stainless steels due to their excellent
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