Role of Alloying Elements in Bearing Steel for Precision Rolling Bearings

1. Role of Silicon (Si) in GCr15 Bearing Steel

Facilitates the formation of body-centered cubic ferrite structures. It does not form carbides in steel and, located to the left of iron in the periodic table, primarily dissolves interstitially in iron. It has little effect on the diffusion coefficient of carbon in austenite or the rate of austenite formation, but it raises the A1 point, relatively slowing down the formation of austenite. It slightly hinders or has no effect on the size of austenite grains during heating. It delays the pearlite transformation, shifting the C-curve to the right and moving the "nose" of the C-curve to higher temperature regions. It lowers the Ms point and increases the stability of supercooled austenite, thereby reducing the critical cooling rate for quenching and improving the hardenability of the steel. It significantly slows down the decomposition of martensite at lower temperatures but does not slow it down during tempering at 400～500°C. It significantly hinders the aggregation of carbides and the elimination of various distortions during tempering. Furthermore, it generally delays the recovery, recrystallization, and carbide aggregation processes of the α-phase in quenched steel, thereby inhibiting the reduction of hardness and strength and enhancing the tempering stability of the steel. It can increase the recrystallization temperature of the α-phase, significantly enhance temper brittleness, alter the phase structure of the steel, and increase the amount of pearlite. The main purpose is to increase the hardenability of the steel; fully hardened parts can achieve high and uniform comprehensive mechanical properties after high-temperature tempering, especially a high yield ratio, significantly strengthening the ferrite, and improving the toughness of the steel within a certain range.

2. Role of Chromium (Cr) in GCr15 Bearing Steel

A γ-phase region closing element; when the content reaches a certain level, the γ-phase region is closed, shrinking the γ region on the phase diagram to a very small range. Exceeding this content causes an alloy γ to α phase transformation, facilitating the formation of body-centered cubic ferrite structures. It can form carbides in steel. As a transition metal element located to the left of iron in the periodic table, it reduces the eutectoid point carbon content and the maximum solubility of carbon in γ-iron. Adding a large amount can make the γ-phase region disappear, resulting in a full ferrite structure. It is a carbide-forming element that reduces the diffusion coefficient of carbon in austenite, thus greatly delaying the transformation process of pearlite to austenite. In steel, the formation of special carbides that are not easily dissolved slows down the formation speed of austenite. It raises the A1 point, relatively slowing down the formation of austenite. It significantly pushes the recrystallization temperature of the α-phase to high temperatures, causing obvious temper brittleness in the steel, and strongly prevents the further development of martensite decomposition. It can alter the phase structure of the steel and increase the amount of pearlite. It increases the hardenability of the steel; fully hardened parts can achieve high and uniform comprehensive mechanical properties after high-temperature tempering, especially a high yield ratio, significantly strengthening the ferrite, and improving the toughness of the steel within a certain range. If insoluble special carbides are formed, insufficient holding time during heating will result in austenite with extremely uneven composition. It has a moderate hindering effect on the size of austenite grains during heating, delays the pearlite transformation, lowers the Ms point, increases the stability of supercooled austenite, thereby reducing the critical cooling rate for quenching and improving the hardenability of the steel. It significantly hinders the aggregation of carbides and the elimination of various distortions during tempering, and generally delays the recovery, recrystallization, and carbide aggregation processes of the α-phase in quenched steel, thereby inhibiting the reduction of hardness and strength.

5.3 Role of Manganese (Mn) in GCr15 Bearing Steel

An austenite stabilizing (γ-region opening) element; if it reaches a certain quantity, it can completely suppress the appearance of the α-phase region and replace it with the γ-phase. Therefore, if the γ-region is quenched to room temperature, austenite is easily obtained. It can increase the recrystallization temperature of the α-phase, cause obvious temper brittleness in the steel, alter the phase structure of the steel, and increase the amount of pearlite. It can form carbides in steel. As a transition metal element located to the left of iron in the periodic table, it lowers A3 and A1; adding a large amount can even lower A3 below room temperature, resulting in the steel retaining an austenitic structure at room temperature. It can change the eutectoid transformation temperature; lowering the A1 point relatively increases the degree of superheat, which increases the formation speed of austenite. It can refine pearlite and is beneficial to austenite formation, while aiding the growth of austenite grains during heating. It delays the pearlite transformation, lowers the Ms point, increases the stability of supercooled austenite, thereby reducing the critical cooling rate for quenching and improving the hardenability of the steel. To increase the hardenability of the steel, fully hardened parts can achieve high and uniform comprehensive mechanical properties after high-temperature tempering, especially a high yield ratio, significantly strengthening the ferrite, and improving the toughness of the steel within a certain range.