What are the special requirements for commonly used precision machining processes when manufacturing stainless steel bearing raceways

Stainless steel bearings, particularly martensitic stainless steel bearings such as AISI 440C, are valued for their combined corrosion resistance and hardness. However, the manufacturing of these bearing raceways requires high-standard precision machining processes, such as grinding and superfinishing. Due to the inherent metallurgical properties of stainless steel, these machining steps place even more stringent requirements on process parameters and equipment accuracy than those for ordinary bearing steel.

1. Special Requirements for Grinding: Preventing Thermal Damage and Work Hardening

Grinding is a critical process for forming raceway geometry and achieving dimensional and tolerance accuracy. Given the high hardness and low thermal conductivity of 440C stainless steel, the grinding process must be strictly controlled to avoid introducing surface defects that could impact bearing life.

1. Strict Grinding Heat Control

Heat accumulation: Due to the low thermal conductivity of 440C stainless steel, the grinding heat generated during grinding is difficult to dissipate quickly, leading to transient high temperatures on the workpiece surface. This high temperature can cause tempering softening of the raceway surface, reducing raceway hardness and significantly shortening contact fatigue life.

Coolant Strategy: A high-flow, high-pressure coolant must be used for robust cooling. Selecting a grinding fluid with excellent lubrication and cooling properties is crucial to ensure effective temperature control in the grinding zone and prevent the formation of a thermal damage layer.

Low Feed, High Speed: To reduce grinding forces and heat generation, a low radial feed rate and appropriate grinding wheel speed are generally required. The self-sharpening properties of the grinding wheel should also be optimized to maintain a sharp cutting edge and avoid blunting, which can cause a sudden increase in heat.

2. Suppressing Work Hardening and Residual Stress

440C's Work Hardening Tendency: During cutting or grinding, martensitic stainless steel's surface grains are prone to plastic deformation, resulting in work hardening. This hardened layer increases the difficulty of subsequent cutting and can worsen surface roughness.

Residual Stress Control: Incorrect grinding parameters can easily introduce residual tensile stress on the raceway surface. Tensile stress significantly reduces a material's fatigue strength and stress corrosion resistance. Qualified precision grinding should generate favorable residual compressive stress on the surface, which is crucial for improving the rolling contact fatigue life of bearings.

II. Special Requirements for Superfinishing: Achieving High Surface Integrity

Superfinishing, also known as honing, is the final step in bearing raceway machining. Its goal is to eliminate surface waviness and roundness error left by grinding, achieving extremely low surface roughness.

1. Controlling Waviness and Topographic Accuracy

Eliminating Grinding Marks: The core of superfinishing lies in using an abrasive stone to oscillate the raceway at low pressure, high frequency, and short strokes, effectively removing microscopic peaks left by grinding. For high-hardness materials like 440C, the stone selection (e.g., particle size, binder) and pressure control require even more precise control to ensure that only the peaks are removed without disrupting the raceway's macroscopic geometry.

Roundness and Groove Profile Accuracy: Ultra-finishing significantly improves raceway profile shape errors. High-precision stainless steel bearings require raceway roundness and groove curvature to micron or even submicron levels, which demands the ultimate rigidity and motion control accuracy of the ultra-finishing equipment.

2. Surface Roughness and Lubricant Film

Extremely Low Ra: Ultra-finishing can reduce the surface roughness (Ra) of stainless steel raceways to Ra 0.02 microns or even lower. This ultra-smooth surface helps form a stable hydrodynamic lubricant film, reducing friction and heat generation, which is crucial for high-speed performance.

Surface Integrity Optimization: During the ultra-finishing process, by controlling the oilstone pressure and cutting fluid, a uniform cold plastic deformation layer with residual compressive stress is formed on the raceway surface. This layer improves the raceway's wear resistance and spalling resistance, and is crucial to the reliability of stainless steel bearings.

3. Cleanliness and Anti-pollution Requirements

Stainless steel bearings require particular attention to cleanliness, as their application environments are often contamination-sensitive.

Deburring and Cleaning: Thorough deburring and washing are essential after grinding and superfinishing. Even tiny metal particles or abrasive residue, even at the micron level, can cause secondary damage to the raceways at high speeds, leading to noise and premature failure.

Rust Prevention and Passivation: Unlike ordinary bearing steels, stainless steel bearings typically require passivation after machining to remove residual free iron and contaminants on the surface, promote the formation of a protective chromium oxide film, and ensure corrosion resistance meets design requirements.