Thermal Fatigue Failure Mechanism of High Temperature Bearing Steel And Alloy Optimization Direction

In the power systems of modern industrial equipment and high-speed trains, bearings must operate stably under conditions of high speed, high load, and continuous thermal cycling. This is particularly true for traction motors in rail vehicles, high-speed main shafts, and high-temperature motors in the metallurgical industry. These bearings are subjected to prolonged exposure to frictional heat and external thermal sources, making them highly susceptible to thermal fatigue, which can lead to issues such as peeling, hardness degradation, and structural instability. Therefore, understanding the thermal fatigue failure mechanisms of high-temperature bearing steels and optimizing these mechanisms through alloy design is crucial for enhancing the reliability of high-temperature bearings.

The main influence mechanism of high temperature heat cycle on bearing steel

In the high temperature environment, the repeated heating and cooling process of the raceway surface of the bearing will produce thermal stress concentration. These thermal stresses coupled with rolling contact stress are the important inducers that promote the early fatigue crack of the bearing. The specific mechanism is as follows:

Thermal stress alternating loading: the temperature gradient produces repeated tensile and compressive cyclic stress between the surface and the core;

Material structure change: martensite tempering, carbide coarsening and redistribution in steel lead to the decrease of hardness;

Lubricating film instability: high temperature destroys the stability of oil film, contact surface friction increases, micro-welding intensifies;

Oxidation induced spalling: the rupture of high temperature oxidation layer forms oxide debris, which accelerates the propagation of fatigue crack.

Typical failure mode analysis: thermal fatigue spalling and hardness regression

In practical applications, the most common failure modes of high temperature bearings include:

Rolling surface patch peeling (Spalling): mainly caused by thermal fatigue, showing dark gray shallow pit-like peeling, the depth is 1~3μm;

Surface hardness degradation: with the passage of time, the surface hardness gradually decreases from more than 60HRC to 55HRC or even below, losing contact fatigue strength;

Chain development of crack source: microcracks are gradually connected under the action of thermal cycle to form through peeling;

Network carbide precipitation: carbides in steel are re-extracted and grow, forming a toughening zone at the grain boundary, which becomes the preferred path of crack.

The evolution characteristics of the structure of bearing steel at high temperature

At present, the mainstream high temperature bearing steels include AISI M50, M50NiL, JIS SUJ2 (modified), Cr4Mo4V, etc., which show the following microstructure evolution under thermal fatigue conditions:

The tempered martensite is transformed into tempered sorbite or re-austenite, and the hardness decreases;

The carbide coarsening and aggregation reduce the uniformity of the structure, and fatigue cracks are easy to occur;

The grain is coarse, and the fine crystal strengthening effect is lost, resulting in the shortening of contact fatigue life;

The residual austenite disappears or the unstable transformation occurs, resulting in volume change and cracking is easy to occur.

Material performance standards in thermal fatigue environments

High temperature bearing steel usually needs to meet the following standards or test parameters:

GB/T 18254 "High carbon chromium bearing steel": basic steel performance standard;

AMS 6491 (M50) and AMS 6278 (M50NiL): heat treatment and performance requirements for aviation bearing steel;

ISO 683-17: General standard of heat treatment alloy steel for rolling bearings;

Hardness retention performance: at 150°C~300°C temperature, hardness is maintained at least 58HRC;

Thermal crack resistance: Crack propagation threshold ΔK is greater than or equal to 15 MPa√m;

Direction of alloy fine tuning: optimization design for thermal fatigue

For the failure mechanism caused by thermal fatigue, the alloy composition and heat treatment can be fine-tuned from the following aspects:

Add molybdenum (Mo) and vanadium (V): refine carbide, improve high temperature hardness and heat crack resistance;Add nickel (Ni): stabilize residual austenite, improve impact toughness and thermal treatment dimensional stability;

Optimize the content of C in 0.25%~0.35%: control the number and morphology of carbides, reduce grain boundary embrittlement;

Temperature control tempering treatment: secondary tempering (540~560°C) to enhance tempering stability and inhibit hardness degradation;

Developing rare earth steel: improving the morphology of inclusions, improving the adhesion of scale, and reducing the source of spalling.

New material trend and engineering case reference

Some high-end applications have adopted the following new high temperature bearing steel:

M62 (Cr-Mo-V-Ni series): used for the main bearing of aero engine, with excellent thermal crack resistance;

Cronidur 30 (nitrogen alloy martensitic steel): corrosion resistant, heat tempering resistant, can be used in high speed motors;

Hybrid ceramic hybrid bearing steel: combined with Si₃N₄ rollers to reduce friction heating and improve the ability of limiting temperature.

Actual cases show that the gear box bearing with M50NiL steel and oil mist lubrication system can still maintain its complete structure without signs of spalling or cracking after running on the rail train for more than 2 million kilometers.

The bearing reliability is extended from the material nature

Thermal fatigue has become a critical bottleneck limiting the lifespan of high-temperature bearings, primarily due to the unstable response of material microstructures to thermal cycling. By deeply understanding the mechanisms of thermal fatigue failure and precisely optimizing the alloy elements, heat treatment parameters, and microstructure control, it is possible to achieve longer bearing life and higher reliability in high-temperature environments. In high-heat applications such as high-speed trains, metallurgical equipment, and wind turbine spindles, only by integrating thermal, mechanical, and material factors in a comprehensive design can a robust 'protective wall' for high-temperature bearing performance be truly established.