Effect of Nitrocarburizing and Tempering on Microstructure and Wear Performance of SKH51 High-Speed Tool Steel

Traditional heat treatment of high-speed tool steels requires multiple thermal cycles, increasing processing
time and energy costs. This study investigates an integrated approach combining tempering and nitrocarburizing processes to optimize both processing efficiency and surface properties of SKH51 high-speed tool steel. Various heat treatment sequences were examined: quenching–double tempering (Q2T), austenitizing at a temperature of 1220 C for a duration of 15 min. Subsequently, the specimen underwent
tempering at 570 ^oC for 3 hours and was cooled to room. In addition, it is a combination of quenching
followed by single (QNC) or double (Q2NC) nitrocarburizing at 570 ^oC for 3 hours, and quenching–double
tempering followed by single (Q2TNC) or double (Q2T2NC) nitrocarburizing. Microstructural analysis
revealed that carbon and nitrogen atoms can diffuse into the martensite matrix when the steel is heated at
570 C for 3 hours in a nitrocarburizing atmosphere. However, the thickness of the nitrocarburized layer
was slightly greater for pre-tempered steel compared to direct tempering in the nitrocarburizing atmosphere.
XRD analysis confirmed the formation of both e-nitride (Fe2-3(N,C)) and c¢-nitride (Fe4(N,C))
phases in the compound layer, with the substrate microstructure influencing phase development. While
double nitrocarburizing increased compound layer thickness from 4 to 8 lm, it also generated continuous
porosity, particularly in pre-tempered specimens, which compromised mechanical performance. All heat
treatment sequences effectively reduced the retained austenite content from 24.0 to 1.5-2.1%. Tribological
properties were assessed using microhardness depth profiling, scratch test, and ball-on-disk wear test. The
wear volume of the QNC specimen was close to that of the Q2TNC specimen. Additionally, the connected
porosity in the compound layer developed after two nitrocarburizing cycles led to a decrease in scratch
resistance and wear resistance, despite the high surface hardness. These findings establish that a single cycle of direct tempering in the nitrocarburizing atmosphere (QNC) offers a practical pathway for reducing
processing steps while maintaining desired surface properties in tool steel manufacturing.