01 Core Heat Treatment Mechanisms Affecting Gear Life

• Tooth surface contact fatigue strength: Resistance to pitting (pockmarks) and spalling under cyclic contact stress.
• Tooth root bending fatigue strength: Resistance to fatigue cracking and fracture at the root under cyclic bending stress.
• Surface hardness and wear resistance: Ability to resist material loss on the tooth surface under sliding and rolling friction.
• Core toughness and strength: Providing tough support for the hard surface layer to prevent tooth crushing or fracture under heavy loads.

1. Quenching and Tempering (Modulation Treatment)

• Process: Quenching + high-temperature tempering.
• Mechanism: Primarily targets core properties of gears, producing a tempered sorbite structure that balances high strength and good toughness. A strong, tough core acts like a building's foundation, providing solid support for subsequent surface hardening layers. Insufficient core strength can lead to plastic deformation of the hard surface layer under heavy loads, causing surface cracking or indentation.
• Contribution to life: Improves overall gear strength and toughness, especially tooth root bending fatigue strength, enabling gears to withstand significant impact loads. It is typically a pre-heat treatment (applied to blanks) for carburized and nitrided gears.

2. Surface Hardening (e.g., Induction Hardening, Flame Hardening)

• Process: Rapidly heats the gear surface to austenitizing temperature, followed by quick cooling to harden the surface while retaining the original core structure and properties.
• Mechanism: Forms a high-hardness martensite layer on the gear surface, significantly improving surface hardness and wear resistance. Induction hardening is widely used due to its fast heating rate, minimal deformation, and high efficiency.
• Contribution to life:
  • Markedly enhances tooth surface contact fatigue strength and wear resistance, effectively resisting pitting and wear.
  • Maintains core toughness, allowing gears to withstand moderate impact.
   
• Limitation: Steep hardness gradient with stress concentration at the interface between the hardened layer and core, potentially causing spalling of the hardened layer under extreme heavy loads.

3. Carburizing and Quenching

• Process: Heats and holds low-carbon steel gears (e.g., 20CrMnTi) in a carbon-rich medium to diffuse carbon atoms into the surface, followed by quenching and low-temperature tempering.
• Mechanism: The most widely used and comprehensive gear strengthening process, achieving the perfect combination of "hard surface and tough core":
  • Surface layer: High-carbon martensite structure with exceptional hardness (HRC 58-62), wear resistance, and contact fatigue strength.
  • Core: Low-carbon martensite or sorbite structure with high strength and good toughness.
  • Significant residual compressive stress in the surface layer offsets tensile stress from external loads, greatly improving bending fatigue strength.
   
• Contribution to life: Comprehensively extends gear service life, especially suitable for high-load, high-impact, and severe friction applications (e.g., automotive gearboxes, rear axle gears). It is one of the most reliable processes for ensuring long gear life.

4. Nitriding

• Process: Heats gears (500-580℃) in a nitrogen-rich medium to diffuse nitrogen atoms into the surface, forming an extremely hard nitride layer.
• Mechanism:
  • Ultra-high surface hardness (HV 800-1200) with excellent wear resistance and anti-scuffing performance.
  • Minimal thermal deformation due to low processing temperature and no need for quenching.
  • Improves fatigue strength and corrosion resistance.
   
• Contribution to life: Particularly suitable for precision gears, gears operating under high-wear, moderate-impact conditions, and those in high-temperature or corrosive environments.
• Limitation: Thin nitrided layer (0.1-0.8mm) results in lower load-bearing capacity than carburized gears, with poor impact resistance.

5. Carbonitriding

• Process: Simultaneously diffuses carbon and nitrogen atoms, combining the advantages of carburizing and nitriding.
• Mechanism: Integrates benefits of both processes: higher wear resistance and fatigue strength than carburizing, lower processing temperature with minimal deformation; deeper diffusion layer and better load-bearing capacity than nitriding.
• Contribution to life: An excellent alternative to carburizing, especially for medium-low carbon steel gears under moderate loads requiring wear resistance, fatigue resistance, and controlled deformation.