42CrMo4 (equivalent to the national standard 42CrMo) is a commonly used medium-carbon alloy structural steel, featuring high strength, good toughness and hardenability. It is widely used in the manufacture of important components such as shafts, gears and connecting rods. Its typical heat treatment process is as follows:

1. Pretreatment (normalizing or annealing)

- ** Objective ** : Eliminate internal stress after forging or rolling, homogenize the structure, and improve machinability.

- ** Process ** :

Normalizing: Heat to 850-880℃, hold for a while and then air cool.

- ** Full annealing ** : Heat to **820-850℃**, hold at that temperature, and then cool slowly in the furnace (≤30℃/h) to below 500℃ before removing from the furnace.

2. Quenching

- ** Objective ** : To obtain a high-hardness martensitic structure in preparation for subsequent tempering.

- ** Process Parameters **

- ** Heating Temperature ** : **830-860℃** (30-50℃ above Ac3).

- ** Holding time ** : Depending on the size of the workpiece (usually 1.5 to 2 minutes /mm).

- ** Cooling medium ** : Oil cooling (Small parts can be quenched with water for oil cooling, but the risk of cracking should be noted).

- ** Hardness after quenching ** : approximately 50 to 55 HRC**.

3. Tempering

- ** Objective ** : Eliminate quenching stress and adjust hardness and toughness.

- "Process Selection" :

- Low-temperature tempering (150-250℃) :

It is used for parts with high hardness requirements (such as wear-resistant parts), with a hardness of 48 to 52 HRC.

- Medium-temperature tempering (350-500℃) :

Used for springs or parts that require elasticity, with a hardness of 40 to 48 HRC.

- High-temperature tempering (500-650℃) :

The quenched and tempered structure (tempered sorbite) was obtained, with the best comprehensive mechanical properties and a hardness of 28 to 36 HRC (typical quenched and tempered hardness).

- ** Holding time ** : Generally 2 to 4 hours, followed by air cooling after tempering.

- ** Note ** : Avoid tempering in the range of **250~350℃** (as tempering brittleness is prone to occur).

4. Additional processing (as required)

- Induction hardening:

For local surface hardening (such as the tooth surface of gears), the hardness can reach 55 to 60 HRC, and the depth is 0.5 to 3mm.

- ** Nitriding/nitriding ** :

Nitriding at 500 to 550℃ can enhance surface hardness and wear resistance (up to over 1000 HV).

"Critical Control Points"

1. ** Heating protection ** : Prevent decarburization (protective atmosphere or salt bath furnace can be used).

2. ** Cooling rate ** : Stirring is required during oil cooling to prevent quenching deformation or cracking.

3. ** Tempering Brittleness ** : Rapid cooling (water cooling or oil cooling) is recommended after high-temperature tempering.

Typical application process examples (quenching and tempering treatment)

Normalizing (860℃×2h, air cooling) → rough machining → quenching (850℃×1h, oil cooling) → high-temperature tempering (580℃×3h, air cooling) → finish machining.

2. Final performance: Tensile strength ≥900 MPa, yield strength ≥650 MPa, elongation ≥14%.

According to the specific dimensions of the workpiece, performance requirements and equipment conditions, the process parameters need to be appropriately adjusted. It is suggested to verify and optimize the process through experiments.