20Cr is one of the most widely used low-alloy carburizing steels in China, conforming to the national standard GB/T 3077-2015 “Alloy Structure Steels”. It is a low-carbon chromium series alloy structure steel, where “20” indicates an average carbon content of 0.20%, and “Cr” denotes chromium as the primary alloying element. After carburizing and quenching, 20Cr achieves high surface hardness (58~62 HRC) while maintaining good toughness and plasticity in the core. It is widely used in transmission components subjected to medium loads, such as automotive transmission gears, camshafts, and piston pins.
1. Basic Information of 20Cr
20Cr is a low-carbon chromium series alloy structure steel with a carbon content of 0.18%~0.24% and a chromium content of approximately 0.70%~1.10%. With over 50 years of application history in China, it offers excellent comprehensive performance and moderate cost, making it one of the most frequently specified carburizing steel materials in CNC machining orders.
2. Chemical Composition
According to the GB/T 3077-2015 standard, the chemical composition requirements for 20Cr are as follows:
| Element | Content (wt%) |
|---|---|
| Carbon (C) | 0.18~0.24 |
| Silicon (Si) | 0.17~0.37 |
| Manganese (Mn) | 0.50~0.80 |
| Chromium (Cr) | 0.70~1.10 |
| Phosphorus (P) | ≤0.035 |
| Sulfur (S) | ≤0.035 |
| Nickel (Ni) | ≤0.30 |
Chromium is the core alloying element. Although its content is only about 1%, it increases the hardenability to more than twice that of 20 steel. The low-carbon design ensures core toughness, while chromium enhances hardenability and wear resistance.
3. Mechanical Properties
According to GB/T 3077-2015, the mechanical properties of 20Cr specimens after quenching + low-temperature tempering are:
| Property | Value |
|---|---|
| Tensile Strength (Rm) | ≥835 MPa |
| Yield Strength (ReL) | ≥540 MPa |
| Elongation (A) | ≥10% |
| Reduction of Area (Z) | ≥40% |
| Impact Energy (AKU2) | ≥47 J |
| Annealed Hardness | ≤179 HB |
Properties after Carburizing and Quenching: After standard carburizing and quenching (carburizing at 920~940°C, quenching at 780~820°C, low-temperature tempering at 180~200°C), the surface hardness is 58~62 HRC, core hardness is 30~45 HRC, and the carburized case depth is 0.8~1.2mm.
Reference critical temperatures: Ac1 approx. 766°C, Ac3 approx. 838°C, Ms approx. 355°C.
4. Heat Treatment Process
Normalizing: Hold at 870~900°C and air cool to refine grain size and eliminate forging stress. Hardness is approximately 143~179 HB.
Carburizing: Gas carburizing at 920~940°C with a carbon potential of 0.8%~1.0%C. Each 0.1mm case depth requires approximately 0.5~1 hour.
Quenching: Oil quench from 780~820°C to obtain a high-hardness martensitic structure.
Low-Temperature Tempering: Temper at 180~200°C for 2 hours to relieve quenching stress while maintaining high hardness.
5. Typical Applications
- Automotive components: Transmission gears, synchronizer hubs, steering knuckle kingpins
- Construction machinery: Hydraulic pump rotors, worms, splined shafts, sprockets
- General machinery: Machine tool spindles, camshafts, mandrels, piston pins
- Fasteners: High-strength pin shafts (requiring carburizing + quenching)
6. CNC Machinability
The machinability rating of 20Cr in the annealed/normalized state is 65% (relative to 20 steel), which is considered above average.
Recommended Cutting Parameters:
- Roughing: Cutting speed 150~200 m/min, depth of cut 1.5~3.0mm
- Finishing: Cutting speed 180~250 m/min, depth of cut 0.2~0.5mm
- Drilling: Carbide drill 60~80 m/min
- Coolant: Emulsion concentration 5%~8%
Machining Notes: Although the hardness is low in the annealed state, the chromium content makes the chips tougher and prone to built-up edge formation. It is recommended to use tools with a large rake angle and ample cutting fluid.
7. International Grade Comparison
| Standard System | Grade |
|---|---|
| China GB | 20Cr |
| USA AISI | 5120 |
| Japan JIS | SCr420(H) |
| Germany DIN | 20Cr4 / 1.7027 |
| Russia GOST | 20Х |
8. Procurement Guide
Market price reference: Hot-rolled round bars 6~9 RMB/kg, hot-rolled plates 7~10 RMB/kg, forged blanks 10~15 RMB/kg.
Incoming inspection recommendations: Spectroscopic analysis to confirm Cr content 0.70%~1.10%, annealed hardness ≤179 HB, and surface free from cracks, folds, and other defects.
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