Effect of Heat Treatment Process on the Microstructure and Hardness of 20CrMoH Steel Gear Forgings
20CrMoH steel is a high-quality alloy structural steel. Due to its composition of alloying elements such as chromium (Cr) and molybdenum (Mo), it exhibits excellent hardenability, strength-toughness balance, and machinability. It is a commonly used material for high-load gear forgings in automotive, construction machinery, and other fields. Its final properties, especially microstructure and hardness, largely depend on heat treatment processes. Different processes lead to significant differences by altering phase transformations, carbon distribution, and grain states in the steel. The following is a detailed analysis from three aspects: preliminary heat treatment processes, final heat treatment processes, and influence of key process parameters.
After forging, gear forgings form an inhomogeneous microstructure (such as overheated grains, Widmanstätten structure, banded pearlite, etc.) and retain forging stress. Preliminary heat treatment (normalizing or annealing) is required to eliminate defects and lay the foundation for subsequent processing and final heat treatment.
- Process Characteristics: The forging is heated to 30-50°C above Ac₃ (austenitization critical temperature, approximately 880-920°C), held for a sufficient time to fully austenitize the microstructure, and then air-cooled to room temperature.
- Influence on Microstructure:
The rapid cooling (air cooling) in normalizing can inhibit the reticular precipitation of ferrite along grain boundaries, refine grains, and transform the microstructure into uniform fine pearlite + a small amount of ferrite (the pearlite lamellae are finer), eliminating the Widmanstätten structure and coarse grains after forging.
- Influence on Hardness:
The mixed structure of fine pearlite and ferrite has moderate hardness, usually 180-220HBW, which not only meets the requirements of subsequent cutting processing (machinability is good when hardness is below 250HBW) but also provides a uniform original microstructure for final heat treatment such as carburizing.
- Process Characteristics: Full annealing (heating to 20-30°C above Ac₃, followed by slow cooling with the furnace after holding) or isothermal annealing (holding at the pearlite transformation temperature range after heating) is commonly used.
- Influence on Microstructure:
Slow cooling allows sufficient carbon diffusion, resulting in more uniform pearlite + ferrite (the pearlite lamellae are thicker and more 弥散分布), completely eliminating forging stress and composition segregation. In the case of spheroidizing annealing (for high-carbon regions), carbides can be spheroidized to further improve machinability.
- Influence on Hardness:
The microstructure after annealing is softer, with a hardness usually of 160-190HBW, which is lower than that after normalizing. It is suitable for forgings with complex shapes and high cutting difficulty, but the production cycle is longer.
Gears need to meet the performance requirements of "high surface hardness for wear resistance and high core toughness for impact resistance". Therefore, the final heat treatment is mainly carburizing-quenching + low-temperature tempering; some low-load gears may adopt quenching and tempering.
This is the core process for 20CrMoH steel gears, achieving performance matching through "carburizing to enrich surface carbon content → quenching to obtain martensite → low-temperature tempering to eliminate stress".
Some low-load gears (such as auxiliary gears with small torque transmission) may adopt quenching and tempering as the final heat treatment to pursue a balance between strength and toughness.
- Process Characteristics: Quenching at 860-880°C (water-cooled or oil-cooled) followed by high-temperature tempering at 600-650°C.
- Influence on Microstructure: Forming tempered sorbite (fine carbides uniformly distributed in the ferrite matrix), with refined and uniform grains.
- Influence on Hardness: Moderate hardness (220-280HBW), balancing strength (σb ≥ 800MPa) and toughness (impact energy ≥ 60J), but the surface has no high-hardness layer and poor wear resistance.
Heat treatment process parameters (temperature, holding time, cooling rate) directly determine the stability of microstructure and hardness. The common influences are as follows:
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