Metallographic diagram of GCr15 steel (930℃ solid boronizing for 4h)

？

This paper studies the microstructural characteristics of medium-carbon bearing steel GCr15 (930℃ solid-state boronizing 4h). It was found that the quenching hardness is higher than HRC 60. The microstructure consists of grains and martensite, and a small amount of borides. It has good wear resistance, fatigue resistance and good toughness performance. Through the ferrite-martensite structure, with better heat treatment parameters, the microstructure of the material can be refined and its mechanical properties improved.

The main components of GCr15 steel are Fe-1.0C-1.4Si-2.4Cr-0.9Mn-0.09P-0.012S-0.02Cu. After quenching and tempering, GCr15 steel has a quenching hardness of HRC60-63, and its yield strength is greater than 980MPa. After the 930℃ 4h solid-state boronizing treatment, the volume fraction of boride in the microstructure increased, the hardness increased and the wear resistance improved.

The microstructure of GCr15 steel before and after solid-state boronizing treatment was observed by optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that under the same heat treatment conditions, the microstructure of GCr15 steel is mainly composed of acicular ferrite, granular bainite, martensite and a small amount of retained austenite, and a small amount of borides. The size of ferrite grains and martensite volume fraction increased. After solid-state boronizing treatment, new fine and disperse borides can be observed in the surface layer of steel.

The results of hardness and wear test show that after boronizing treatment, the surfaces of steel samples were roughened, the hardness and wear resistance improved, the average microhardness of samples was raised to HRC64, the wear loss was decreased by nearly one third and the wear resistance improved significantly. The TEM results showed that the borides were fine and uniformly distributed in the matrix.

In summary, this study found that GCr15 steel with solid-state boronizing treatment has improved hardness and wear resistance, and is suitable for bearing materials used in harsh working conditions. It can improve the fatigue strength and service life of rolling bearings and provide technical support for bearing product development.

GCr15 steel is a type of steel with high carbon content and high chromium content. GCr15 steel is commonly used in bearing and parts. After heat treatment at 930℃ 4 hours, its metallographic structure is shown as below.

Firstly, early austenite phase are existed along with ferrite phase. The grain boundary are tender, the grain boundary ferrite content is higher than the grains. The most of the grains are in shape of block which is useful to increase strength of the material.

In addition, the area of ferrite and pearlite are distinctively separated. Hence, this means that austenitization temperature have been achieved and the pearlite are generated due to its low temperature.

Furthermore, the precipitations of alloy elements between the ferrite and pearlite form border zone clearly seen under the microscope. Precipitations can effectively improve the material’s strength and wear-resistance.

Lastly, there are many small spherical precipitations formed evenly distribute in the ferrite and the pearlite, with a diameter of 0.1-0.3um. These precipitations are result of the extreme cooling process. They can improve the sphericity of the material’s steel.

Overall, after metallographic inspection, GCr15 steel can be seen as a good structure for bearing and part. Its carbon content and chromium content are effective to enhance its strength and wear-resistance. Especially, the small spherical precipitations distributed in the material, can effectively improve the material’s spherical properties, making it capable of enduring harsh working environment.