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. Offering moderate cost, good hardenability, and excellent surface hardness after carburizing, it is the preferred material for transmission components such as gears, shafts, and pins. Based on national standards and technical literature, this article systematically analyzes the chemical composition, mechanical properties, heat treatment processes, international grade equivalents, and key CNC machining considerations for 20Cr.
1. 20Cr Basic Information and Standard Overview
20Cr is a low-carbon chromium alloy structural steel with a carbon content of 0.18%–0.24%, classifying it as a carburizing steel. “20” indicates an average carbon content of 0.20% (20 hundredths of a percent), and “Cr” denotes chromium as the primary alloying element, present at approximately 0.70%–1.10%. The addition of chromium significantly enhances hardenability, allowing 20Cr to achieve a high surface hardness of 58–62 HRC after carburizing and quenching, while maintaining good toughness and plasticity in the core.
With over 50 years of application history in China’s mechanical industry, 20Cr is widely used in transmission parts subjected to moderate loads, such as automotive transmission gears, camshafts, piston pins, worms, and spline shafts. Due to its excellent combination of properties and moderate price, it remains one of the most frequently specified materials in CNC machining orders for manufacturing enterprises.
2. Chemical Composition and Alloy Design Principles
According to GB/T 3077-2015, the chemical composition requirements for 20Cr are as follows:
| Element | Content Range (wt%) | Function Description |
|---|---|---|
| Carbon (C) | 0.18–0.24 | Low-carbon design ensures core toughness; provides carbon concentration gradient for carburizing |
| Silicon (Si) | 0.17–0.37 | Deoxidizer; increases strength but reduces toughness, controlled at a low level |
| Manganese (Mn) | 0.50–0.80 | Improves hardenability, reduces hot brittleness; purifies sulfur |
| Chromium (Cr) | 0.70–1.10 | Core alloying element; improves hardenability, wear resistance, and tempering stability |
| Phosphorus (P) | ≤0.035 | Impurity element; increases cold brittleness, strictly controlled to upper limit |
| Sulfur (S) | ≤0.035 | Impurity element; causes hot brittleness, strictly controlled to upper limit |
| Nickel (Ni) | ≤0.30 | Residual element; slightly beneficial but not required by standard |
| Copper (Cu) | ≤0.25 | Residual element from steelmaking raw materials |
The alloy design of 20Cr embodies the principle of “small amounts, multiple elements”: the carbon content is kept low to ensure core toughness; a chromium content of only about 1% can more than double the hardenability compared to 20 carbon steel. This design achieves an excellent balance between cost and performance.
3. Mechanical Properties and Heat Treatment Condition
3.1 Mechanical Properties in the Quenched and Tempered Condition (GB/T 3077-2015)
| Property | Value | Notes |
|---|---|---|
| Tensile Strength (Rm) | ≥835 MPa | After quenching + low-temperature tempering |
| Yield Strength (ReL) | ≥540 MPa | After quenching + low-temperature tempering |
| Elongation (A) | ≥10% | — |
| Reduction of Area (Z) | ≥40% | — |
| Impact Energy (AKU2) | ≥47 J | — |
| Annealed Hardness | ≤179 HB | As-delivered condition |
3.2 Properties After Carburizing and Quenching
After a standard carburizing and quenching process (carburizing temperature 920–940°C, quenching temperature 780–820°C, low-temperature tempering 180–200°C), 20Cr can achieve:
- Surface Hardness: 58–62 HRC (case depth 0.8–1.2 mm)
- Core Hardness: 30–45 HRC (ensuring adequate toughness)
- Case Microstructure: Fine martensite + uniformly distributed granular carbides
- Core Microstructure: Low-carbon martensite + small amount of ferrite
Reference critical temperatures for 20Cr: Ac1 (temperature at which austenite begins to form on heating) is approximately 766°C, Ac3 (temperature at which ferrite completely transforms to austenite on heating) is approximately 838°C, and Ms (martensite start temperature) is approximately 355°C. These parameters are crucial for developing heat treatment processes.
4. Detailed Heat Treatment Process
1. Normalizing: Temperature 870–900°C, hold then air cool. Purpose: Refine grain size, eliminate forging stresses, and prepare the microstructure for subsequent machining. Hardness after normalizing is approximately 143–179 HB, providing good machinability.
2. Carburizing: For pack carburizing, temperature 900–930°C; for gas carburizing, temperature 920–940°C. Carbon potential is controlled at 0.8%–1.0% C. Carburizing time depends on the required case depth (approximately 0.5–1 hour per 0.1 mm).
3. Quenching After Carburizing: A single quench method (direct air cool to 780–820°C followed by oil quench) or a double quench method (two quenches, first at high temperature then low temperature, for improved precision) is recommended. The cooling medium is quenching oil.
4. Low-Temperature Tempering: 180–200°C × 2 hours, water cool or air cool. Purpose: Relieve quenching stresses, stabilize microstructure and dimensions, and maintain high hardness.
5. Stress Relief Annealing: If grinding is required after carburizing and quenching, a stress relief temper at 150–160°C × 4–6 hours is recommended to prevent grinding cracks.
5. International Grade Equivalents
20Cr has equivalent grades in major international industrial standard systems:
| Standard System | Grade | Notes |
|---|---|---|
| China GB | 20Cr | GB/T 3077-2015 |
| USA ASTM/AISI | 5120 | Chemically similar, slightly higher chromium content |
| Japan JIS | SCr420(H) | JIS G4053, most common equivalent grade |
| Germany DIN | 20Cr4 / 1.7027 | DIN EN 10084 carburizing steel standard |
| UK BS | 527A20 / 1.7027 | BS 970 standard |
| France AFNOR | 20MC5 / 18C3 | — |
| Russia GOST | 20Х | ГОСТ 4543-71 |
It is important to note that while grades are equivalent, differences exist in trace element limits (e.g., phosphorus, sulfur) and mechanical property requirements between standards. For export orders, it is recommended to require suppliers to provide material test certificates (MTC/EN 10204 3.1) conforming to the target country’s standard.
6. CNC Machinability and Process Parameters
The machinability of 20Cr depends on its heat treatment condition:
- Annealed/Normalized Condition: Machinability rating is 65% (relative to a 100% baseline for 20 carbon steel), considered upper-mid range. Recommended cutting speed for carbide tools is 120–180 m/min.
- After Carburizing and Quenching: Surface hardness of 58–62 HRC necessitates grinding. CBN (Cubic Boron Nitride) or vitrified bond grinding wheels are recommended.
Recommended CNC Machining Process Parameters:
- Roughing: Depth of cut 1.5–3.0 mm, feed rate 0.2–0.4 mm/rev, cutting speed 150–200 m/min (coated carbide tools)
- Finishing: Depth of cut 0.2–0.5 mm, feed rate 0.08–0.15 mm/rev, cutting speed 180–250 m/min
- Drilling: HSS drill cutting speed 15–25 m/min, carbide drill cutting speed 60–80 m/min
- Coolant: Emulsion (5%–8% concentration) is recommended; ensure adequate cooling to prevent built-up edge.
Machining Challenges: Although 20Cr has low hardness in the annealed condition, the addition of chromium makes the chips tougher, which can easily lead to built-up edge formation. It is recommended to use tools with a large rake angle and small nose radius, combined with ample cutting fluid to improve chip breaking.
7. Typical Applications and Material Selection Advice
Based on long-term practice in the domestic machinery industry, the core applications for 20Cr include:
- Automotive Components: Transmission gears, synchronizer hubs, steering knuckle kingpins, clutch release forks
- Construction Machinery: Hydraulic pump rotors, worms, spline shafts, sprockets
- General Machinery: Machine tool spindles, camshafts, mandrels, piston pins
- Fasteners: High-strength pins, double-ended studs (requiring carburizing + quenching)
Alternative Material Suggestions:
- If higher core strength is required → Upgrade to 20CrMnTi (core strength increases by approximately 20%)
- If only moderate strength is needed without carburizing → 45 carbon steel or 40Cr (quenched and tempered) can be selected
- If parts are large and hardenability is insufficient → Upgrade to Ni-Cr-Mo carburizing steels like 20CrNiMo or 18CrNiMo7-6
- If cost-sensitive and loads are light → 20 carbon steel (carbon steel carburizing) can be selected
8. Material Procurement and Quality Verification
The procurement price for 20Cr bar and plate stock varies by specification and heat treatment condition:
- Hot-rolled round bar (annealed): Approximately 6–9 RMB/kg (Φ16–Φ100)
- Hot-rolled plate (annealed): Approximately 7–10 RMB/kg
- Forged blank: Approximately 10–15 RMB/kg (including forging cost)
Procurement Considerations:
- Specify Material Standard: The contract must reference GB/T 3077-2015
- Request MTC (Material Test Certificate): Must include heat number, chemical composition, mechanical properties, and heat treatment condition
- Recommended Incoming Inspection: PMI (Positive Material Identification) to verify Cr content 0.70%–1.10%; hardness testing (annealed condition ≤179 HB)
- Visual Inspection: Bar surfaces must be free from cracks, folds, scars, inclusions, and other defects
9. Summary
As a classic chromium-series low-carbon alloy structural steel, 20Cr holds an irreplaceable position in the mechanical manufacturing industry due to its excellent carburizing performance, moderate cost, and broad applicability. From a material selection perspective, 20Cr is best suited for components requiring “high surface hardness for wear resistance and good core toughness,” making it an ideal base material for processes like CVD (Chemical Vapor Deposition) composite heat treatment. Correct heat treatment process control and incoming material quality acceptance are critical links to ensure the performance of 20Cr parts.
Turn this machining question into a manufacturable part
Need this material or process for your next CNC project?
Send your STEP, STP, IGES, DXF, PDF, material, quantity, surface finish, and tolerance requirements. We will review manufacturability and reply with practical quotation guidance.
