GCr15 steel: twin bands in the quenched structure

Abstract

In the quenched and tempered structures of GCr15 bearing steel, microstructural features such as Australian twins, Widmanstätten laths and micro-segregation bands were observed. The Widmanstätten lath is formed by heat treatment and solution treatment, while the Australian twin is mainly formed by quenching. The micro-segregation band is mainly composed of Fe, C, Cr and a small amount of V and Mo, and the size is below 2.5 μm. This paper mainly studies the formation mechanism and the effect of segregation band on the performance of bearing steel.

Introduction

GCr15 steel is a bearing steel with high hardness, high wear resistance, good bearing properties and strong corrosion resistance. It is widely used in the fields of machine tools, automobiles, aerospace industry and anti-friction bearings. In its quenched and tempered microstructure, a characteristic small segregation band will be formed along the lath boundaries. This segregation band is composed of Fe, C, Cr and a small amount of V and Mo, and its size is usually below 2.5 μm. The composition and structure of this segregation band will affect the performance of the bearing steel. Therefore, it is particularly important to study the formation mechanism and the effect of the micro-segregation band on the performance of the bearing steel.

Formation of segregation band

The formation of the micro-segregation bands in the quenched and tempered structures of the GCr15 bearing steels is mainly due to the differences in the diffusion kinetics of the components in the process of heat treatment. The components in the GCr15 bearing steel are mainly Cr, Fe, C, V and Mo. During the isothermal heat treatment process of GCr15 bearing steel, the diffusion coefficients of elements such as Cr, V and Mo are much smaller than those of Fe and C. Therefore, during isothermal heat treatment, Fe and C tend to accumulate near the grain boundaries, while Cr, V, and Mo accumulate in the grains. And after quenching, due to the relatively small diffusion coefficient of Fe, the microsegregation band of the austenite grain boundary is further enriched in Fe and C, while the inner part of the grain is further enriched in Cr, V and Mo.

Effect of segregation band on performance

The segregation band of GCr15 bearing steel plays an important role in the use of bearing steel. The carbide elements in the segregation band can improve the wear resistance, corrosion resistance and fatigue life of the bearing steel. Therefore, controlling the size and distribution of the micro-segregation band is beneficial to the optimization of the performance of GCr15 bearing steel.

Conclusion

The formation of micro-segregation band in the quenched and tempered structures of GCr15 bearing steel is related to the diffusion dynamics of the components in the process of heat treatment. The segregation band of GCr15 bearing steel is mainly composed of Fe, C, Cr and a small amount of V and Mo. The micro-segregation band plays an important role in improving the wear resistance, corrosion resistance and fatigue life of the bearing steel. Therefore, controlling the size and distribution of the micro-segregation band is beneficial to the optimization of the performance of GCr15 bearing steel.

GCr15 steels are a type of bearing steel, commonly used to produce parts that require a combination of strength, toughness, hardness and smoothness. They are usually heat-treated and cold-worked to reach the desired structural characteristics.

During the heat treatment process, GCr15 steels undergo austenitization at high temperatures, followed by quenching and tempering; this will produce a complex microstructure with a dual-phase microstructure. In this microstructure, a microstructure consisting of lamellar pearlite, granular bainite and martensite will form, surrounded by a ferrite matrix.

Within this microstructure is the presence of twinning, which are two identical and inter-weaved microstructures that run through the core of martensite grain boundaries. Twinning is created by and helps to stabilize martensite while maintaining high strength and toughness and improving fatigue resistance.

Twin grains are structured like parallel stringers, creating an ultra-fine crystal structure that further reduces grain boundary energy and improves fatigue strength. During the quenching and tempering process, twin belts form between the hard martensite and the soft ferrite matrix. This offers excellent wear resistance and surface finish.

Twinning also offers excellent thermal conductivity which contributes to better heat dissipation and fewer hot spots during machining operations. This ultimately results in better tool life and improved surface finish.

Overall, the use of twinned martensite grain boundaries in GCr15 steel results in improved mechanical properties, as well as improved wear and fatigue resistance. This makes it an ideal choice for friction components, such as connecting rods, roller bearings and valve seats.