Properties of 40Cr Steel

Formulation of 40Cr Heat Treatment Process

Quenching Process

• Heating temperature: 850℃
• Holding time: 80 minutes
• Cooling method: Oil quenching

Low-Temperature Tempering Process

• Temperature range for low-temperature tempering: 150-300℃
• Note: The first-type temper brittleness temperature of steel is 250-400℃. Due to alloying elements such as silicon, manganese and chromium in 40Cr, this temperature will increase.
• Selected temperature: 240℃
• Holding time: 60 minutes
• Cooling method: Air cooling

Medium-Temperature Tempering Process

• Temperature range for medium-temperature tempering: 350-500℃
• Selected temperature: 460℃
• Holding time: 50 minutes
• Cooling method: Air cooling

High-Temperature Tempering Process

• Temperature range for high-temperature tempering: 500-650℃
• Selected temperature: 620℃
• Holding time: 60 minutes
• Cooling method: Air cooling

Rockwell Hardness of 40Cr After Quenching and Tempering

1. 40Cr: 850℃ oil quenching (50.2 HRC) → 620℃ tempering (24.4 HRC)
2. 40Cr: 850℃ oil quenching (51.8 HRC) → 460℃ tempering (41.3 HRC)
3. 40Cr: 850℃ oil quenching (49.3 HRC) → 240℃ tempering (50.3 HRC)

Key Process Notes

• The original structure of 40Cr steel is spheroidal pearlite. Spheroidal structures have small contact areas, and chromium hinders carbon diffusion while having a slow diffusion rate itself. Thus, the upper limit of heating temperature and extended holding time are recommended. Otherwise, spheroidal cementite cannot be fully dissolved, leading to reduced hardness and strength after quenching.
• Higher heating temperatures increase the percentage of martensite (50%-99%) but may coarsen the structure. Theoretically, heating 40Cr to 850-870℃ with insulation allows full dissolution of alloying elements into austenite without excessive grain coarsening. Quenching at 850℃ achieves the highest hardness; temperatures above 850℃ reduce cooling rate during quenching, resulting in lower hardness.
• Excessively high quenching hardness significantly impairs strength and toughness, making it unsuitable for torque transmission and shock-resistant applications. The general quenching hardness of 40Cr is 48-55 HRC. For high torque requirements, HR42 is recommended. For high wear resistance, carburizing and quenching of low-carbon steel is preferred (carburizing depth: 0.5-2mm, related to carburizing time; hardness: 58-62 HRC).

Hardness of 40Cr After Quenching and Tempering

• 850-860℃ oil quenching: 55 HRC
• 150℃ tempering: 55 HRC
• 200℃ tempering: 53 HRC
• 300℃ tempering: 51 HRC
• 400℃ tempering: 43 HRC
• 500℃ tempering: 34 HRC
• 550℃ tempering: 32 HRC
• 600℃ tempering: 28 HRC
• 650℃ tempering: 24 HRC