What HRC Hardness Can GCr15 Bearing Steel Achieve?

1. Introduction to GCr15 Bearing Steel

GCr15 is the most commonly used high-carbon chromium bearing steel in China, conforming to the GB/T 18254-2016 standard. Its international approximate grades are AISI 52100 (USA), SUJ2 (Japan), and 100Cr6 (Europe). This is a through-hardening bearing steel that can achieve a hardness of HRC 58~64 after proper heat treatment. It is widely used for manufacturing rings and rolling elements of rolling bearings.

2. Chemical Composition (GB/T 18254)

Note: The above data is based on GB/T 18254-2016. The high carbon content ensures high hardness after quenching, and the chromium element improves hardenability and wear resistance.

3. Mechanical Properties and Hardness

The mechanical properties of GCr15 are highly dependent on the heat treatment state:

Note: GCr15 is not a quenched and tempered steel. In practical use, it is almost always in the quenched and low-temperature tempered state to achieve high hardness. The tensile strength after quenching and tempering is approximately 2000~2400 MPa (reference value, usually not specified due to brittleness).

4. GCr15 Heat Treatment Process

Standard heat treatment sequence:

• Spheroidizing Annealing: Heat and hold at 780~810°C, then slow cool to below 650°C followed by air cooling. Obtains a spheroidized pearlite structure with a hardness of 179~207 HB, preparing for subsequent machining and quenching.
• Quenching: Heat at 830~860°C, cool in oil or a salt bath. Key point: Heating temperature must be precisely controlled; overheating can produce coarse martensite leading to brittleness and cracking.
• Cold Treatment: Immediately after quenching, perform deep cryogenic treatment at -70~-80°C for 1~2 hours to promote the transformation of retained austenite and stabilize dimensions.
• Low-Temperature Tempering: Temper at 150~170°C for 2~4 hours to relieve quenching stresses, maintaining hardness at HRC 58~64.

5. Typical Applications

Main uses of GCr15:

• Bearing Components: Inner rings, outer rings, balls, rollers, and needles for rolling bearings
• Measuring Tools: Gauge blocks, ring gauges, plug gauges, micrometer anvils
• Precision Spindles: Grinding machine spindles, precision machine tool mandrels
• Wear-Resistant Parts: Fuel injection pump components (plungers, delivery valves)
• Cold Work Dies: Stamping dies, cold heading dies (for light-load applications as a substitute for Cr12MoV)

6. CNC Machining Guidelines for GCr15

Important Notes:

• GCr15 must be machined by CNC in the spheroidized annealed state; after quenching, only grinding is possible.
• Machinability in the spheroidized annealed state is acceptable, but carbide particles accelerate tool wear; coated cemented carbide tools are recommended.
• Machining allowances must account for quenching distortion (typically leave 0.15~0.30 mm grinding stock).
• After quenching, distortion is significant, and grinding is prone to burning and cracks; strictly control grinding parameters and coolant.

7. Hardness Comparison of GCr15 with Common Steels

The core advantage of GCr15 lies in its high purity (low P, S) and extremely high contact fatigue strength, which cannot be replaced by ordinary structural steels. However, note that its service temperature must not exceed 150°C, as hardness will significantly decrease above this temperature.

8. Summary

GCr15 bearing steel can achieve a hardness of HRC 58~64, provided it undergoes correct quenching (830~860°C oil cooling) and low-temperature tempering (150~170°C). Its hardness in the spheroidized annealed state is 179~207 HB, making it suitable for CNC machining. When selecting GCr15, note that it is not a quenched and tempered steel and is not suitable for high-temperature environments. For precision machining of GCr15, our company is equipped with CNC lathes and CNC machining centers. Please feel free to send your drawings for consultation.