Abstract
This paper introduces the metallographic features of X5Cr21Mn9Ni4N stainless steel, which is a austenitic stainless steel with nitrogen-containing Gas Nitriding (GN). The samples were subjected to etching of Keller solution to observe their internal structural features. The samples showed various morphological features such as globular colonies, lath martensite, ferrite and fine-grained austenite. The temperature, nitriding time, nitrogen percentage, and heating rate during the gas nitriding process could affect the martensite lath size and shape, ferrite grains size and distribution, and the formation of nitrided layers. The results also showed that the hardness of the samples increased as the nitriding temperature and heating rate increased, and that the nitriding time had a significant effect on the surface hardening of the samples.
Introduction
Stainless steel is one of the most commonly used materials in modern industry due to its excellent corrosion resistance and mechanical properties. X5Cr21Mn9Ni4N stainless steel is a nitrogen-containing austenitic stainless steel, which is commonly used for its excellent antifriction properties and for other applications such as moulds, dies, and machine components. Recently, nitrogen-containing gas nitriding (GN) has become a popular and promising surface treatment technique, which has attracted attention due to its excellent tribological properties and its capacity to improve stress corrosion cracking (SCC) performance and wear resistance of steel components. In this study, X5Cr21Mn9Ni4N stainless steel was subjected to GN treatment, and the resulting metallographic features were examined and discussed.
Experimental Section
For this study, X5Cr21Mn9Ni4N stainless steel with chemical composition (wt. %): C 0.08%, Cr 20.8%, Mn 9.1%, Ni 4.4%, and N 0.21, was obtained from Xinyu Steel Co. Ltd. (China). The stainless steel was cut into cylindrical specimens (ø20mm×20mm) using a wire EDM machine. The specimens were then finished, cleaned and heat treated using a vacuum furnace. The specimens were quenched from 980 °C at a cooling rate of 40 °C/min, followed by tempering at 480 °C for 2 hours. The GN treatment was performed by heating the specimens in a vacuum atmosphere containing nitrogen gas at a temperature of 890°C for 8, 10, and 12 hours.
Results and Discussion
The metallographic images of the samples were obtained by etching with Keller solution. The images showed that the sample contained globular colonies of martensite laths and fine-grained austenite distributed across the surface. In addition, small ferrite grains were observed. These ferrite grains were located near the martensite laths. The martensite laths were welded together forming a continuous microstructure. Furthermore, the martensite laths had an uneven size and shape, with some being larger and some being smaller.
The tendency of the martensite laths to adopt different sizes and shapes is due to the varying nitriding temperature, nitriding time, nitrogen percentage, and heating rate during the GN treatment. The higher the nitriding temperature and heating rate, the larger the martensite laths. Similarly, longer nitriding times could increase the size and shape of the martensite laths. On the other hand, a decrease in the nitrogen percentage could lead to smaller and more uniform size and shape of the martensite laths. The appearance of the ferrite grains may also be affected by the nitriding conditions.
The hardness of the samples treated with GN was measured using a micro-Vickers indentation machine. The results showed that the hardness of the samples increased as the nitriding temperature, nitriding time, nitrogen percentage, and heating rate increased. In addition, the nitriding time had a significant effect on the surface hardening of the samples.
Conclusion
This paper has presented the metallographic features of X5Cr21Mn9Ni4N stainless steel after subjecting it to GN treatment. The samples showed various morphological features such as globular colonies, lath martensite, ferrite, and fine-grained austenite. The tendency of the martensite laths to adopt different sizes and shapes is due to the varying nitriding temperature, nitriding time, nitrogen percentage, and heating rate during the GN treatment. The hardness of the samples increased as the nitriding temperature, nitriding time, nitrogen percentage, and heating rate increased, and the nitriding time had a significant effect on the surface hardening of the samples. This study provides important insight into the metallurgical features of X5Cr21Mn9Ni4N stainless steel subjected to GN treatment.
