Microstructure and Mechanical Properties of Low-Carbon Steels Quenched and Tempered after Gas Carburizing, Carbonitriding, and Nitriding

Abstract

In this study, low-carbon steel specimens were subjected to gas carburizing, carbonitriding, or nitriding at 800 °C, followed by oil quenching and tempering at 300 °C. The investigation focused on the effects of carbon and nitrogen on the resulting microstructure and mechanical properties. The carburized specimens consisted of martensite, fine pearlite, and a small amount of retained austenite. In contrast, the carbonitrided specimens were composed of martensite and retained austenite, while the nitrided specimens exhibited a microstructure of martensite, retained austenite, and pores. The amount of retained austenite in the specimens after quenching was observed to be higher in specimens with a larger amount of nitrogen. X-ray diffraction (XRD) analysis revealed that the carbonitrided and nitrided specimens after tempering had higher amounts of γ’-Fe ₄ N phase as the nitrogen content increased. Furthermore, transmission electron microscopy (TEM) and selected area electron diffraction (SAED) analysis confirmed the precipitation of fine γ’-Fe ₄ N and ε-Fe ₃ (N, C) phases in the subsurface region of the tempered carbonitrided samples with a high nitrogen-to-carbon ratio. Except for the nitrided specimens where pore formation occurred, surface hardness after tempering increased in correlation with nitrogen content. Carbonitriding with a higher nitrogen-to-carbon ratio markedly improved the wear resistance of low-carbon steel.