Standard National Material Gb 40cr Steel Round Bar Quenching Process - China

GB 40Cr steel round bar is an alloy structural steel with excellent hardenability, strength, and toughness.

It is widely used in the manufacture of parts subject to moderate loads and impact (such as shafts and gears).

The quenching process for round bar steel should be determined based on part size and performance requirements.

Hardenability: Better than 45 steel.

Round bar diameters ≤50mm can be fully hardened by water quenching.

Larger diameters require oil quenching or a two-liquid quenching process.

Typical Quenching Process Steps

1 - Pretreatment

Purpose: Relieve forging/rolling stresses, refine grain size, and improve the machinability of GB 40Cr steel round bar steel.

Process: Normalizing (850-870°C for 1-2 hours, air cooling) or annealing (800-820°C for 2-4 hours, slow cooling to below 500°C, air cooling).

2 - Quenching Heating

Heating Temperature: 830-860°C (adjusted based on part size; the upper limit for larger diameters to ensure homogenization of the austenite).

Holding Time: Calculated based on the diameter of the GB 40Cr steel round bar, generally 1.5-2.5 minutes/mm (e.g., 75-125 minutes for a φ50mm bar).

Consider also the furnace load and heating equipment (box-type furnaces require an appropriate holding time).

Heating Medium: Air (box-type furnaces), protective atmosphere (to prevent oxidation and decarburization, recommended for critical parts).

3 - Cooling (Quenching)

Cooling Medium Selection:

- Diameter ≤ 30mm: Water quenching (20-40°C fresh water).

This provides a fast cooling rate and ensures hardenability, but is prone to deformation and cracking.

- Diameter 30-50mm: Water-oil dual-liquid quenching (water quenching for 3-5 seconds, then switching to oil cooling) to reduce stress.

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