Knowledge

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 manufacturing important components such as shafts, gears and connecting rods. Its typical heat treatment process is as follows:

 

1. Pre-treatment (normalizing or annealing)

Purpose: To eliminate internal stress after forging or rolling, homogenize the structure, and improve machinability.

Process:

Normalizing: Heat to 850~880℃, hold the temperature and then air cool.

Full annealing: Heat to 820~850℃, hold the 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 and prepare for subsequent tempering.

Process parameters:

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

Holding time: Depending on the size of the workpiece (typically 1.5 - 2 min/mm).

Cooling medium: Oil quenching (small parts can be water quenched followed by oil quenching, but the risk of cracking should be noted).

Hardness after quenching: Approximately 50 - 55 HRC.

 

3. Tempering

Purpose: To eliminate quenching stress, adjust hardness and toughness.

Process selection:

Low-temperature tempering (150~250℃):

Used for parts with high hardness requirements (such as wear-resistant parts), hardness 48~52 HRC.

Medium-temperature tempering (350 - 500℃): It is used for springs or parts that require elasticity, with a hardness of 40 - 48 HRC.

High-temperature tempering (500 - 650℃):

Obtain tempered microstructure (tempered sorbite), with the best comprehensive mechanical properties, hardness 28 - 36 HRC (typical tempered hardness).

Holding time: Generally 2 - 4 hours, air-cooled after tempering.

Note: Avoid tempering in the 250 - 350℃ range (easily causes temper brittleness).

 

4. Additional treatments (as required)

Induction hardening:

For local surface hardening (such as gear tooth surfaces), hardness can reach 55 to 60 HRC, with a depth of 0.5 to 3 mm.

Nitriding / Nitriding treatment:

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

 

Key control points:

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

2. Cooling rate: Stir when oil cooling to avoid quenching deformation or cracking.

3. Tempering brittleness: It is recommended to cool quickly (water cooling or oil cooling) after high-temperature tempering.

 

Typical application process example (tempering treatment)

1. 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 workpieces, performance requirements and equipment conditions, the process parameters need to be adjusted appropriately. It is recommended to optimize the process through experimental verification.

 

Vigor has more than 20years experience and the professional team in producing and supplying 42CrMo4 castings, forgings and the raw bar material machined parts. If you have any question or any parts need to be developed, please feel free to contact us at info@castings-forging.com