How to achieve 4140+QT 80 ksi and 110 ksi levels?
In order to make 4140 steel reach the yield strength level of 80 ksi (about 550 MPa) and 110 ksi (about 758 MPa), the following heat treatment process can be used:
1. Material Basis & Pretreatment
- 4140 steel composition:
– Carbon (C): 0.38-0.43%
– Chromium (Cr): 0.8-1.1%
– Molybdenum (Mo): 0.15-0.25%
– Manganese (Mn): 0.75-1.0%
– Silicon (Si): 0.15-0.35% - Features: Medium carbon alloy steel with good hardenability, strength and toughness, suitable for high-strength structural parts.
- Pretreatment: Normalizing or annealing is required after forging or rolling to eliminate residual stress and uniform organization, in preparation for subsequent heat treatment.
2. Heat Treatment Process Parameters
| Target yield strength | Quenching temperature (°C) | Tempering temperature (°C) | Hardness (HRC) | Tensile strength (MPa) | Elongation (%) | Impact energy (J) |
| 110 ksi (758 MPa) | 845-855 | 480-540 | 28-32 | 860-1030 | 12-16 | 25-40 |
| 80 ksi (550 MPa) | 845-855 | 590-650 | 18-22 | 620-750 | 18-22 | 50-80 |
3. Performance Comparison Analysis
3.1 Mechanical properties
| Index | 110 ksi level | 80 ksi level | Difference explanation |
| Yield strength | 750-860 MPa | 550-620 MPa | 110 ksi strength is 34%-55% higher |
| Tensile strength | 860-1030 MPa | 620-750 MPa | 38%-66% higher |
| Hardness (HRC) | 28-32 | 18-22 | Hardness difference is about 10 HRC |
| Elongation (%) | 12-16 | 18-22 | 80 ksi toughness is increased by 50%-83% |
3.2 Toughness Performance
| Parameters | 110 ksi level | 80 ksi level | Key Mechanisms |
| Impact Energy (J) | 25-40 (Charpy V-Notch) | 50-80 (Charpy V-Notch) | 80 ksi impact toughness 100%-120% higher |
| Fracture Toughness | Lower (KIC 60-80 MPa√m) | Higher (KIC 90-110 MPa√m) | High Temperature Tempering Reduces Carbide Brittleness |
3.3 Microstructure
| Level | Metallographic Structure | Characteristic Description |
| 110 ksi level | Tempered Troostite | Fine carbides are distributed in the martensite matrix, with high strength but limited dislocation movement and low toughness. |
| 80 ksi level | Tempered sorbite | Spheroidal carbides are evenly distributed, the ferrite matrix is softened, dislocations are easy to slip, and plasticity and toughness are significantly improved. |
3.4 Application scenario comparison
| Level | Typical application scenario | Key points for controlling failure risks |
| 110 ksi level | Gears, aircraft landing gear, high-load shafts | Prioritize the prevention of fatigue fracture and overload brittle fracture, and require surface strengthening (such as nitriding). |
| 80 ksi level | Hydraulic connecting rods, building bolts, molds | Focus on avoiding plastic deformation and impact fracture, suitable for welding and cold processing. |
4. Key Points of Process Implementation
4.1 110 ksi level
- Quenching: heating at 845-855°C, holding time 1 hour/25mm, oil quenching or water quenching (thin section).
- Tempering: tempering at 480-540°C, holding time 1-2 hours, air cooling.
- Surface treatment: It is recommended to perform shot peening or carburizing to improve fatigue life (fatigue limit can be increased by 20%-30%).
4.2 80 ksi level
- Quenching: heating at 845-855°C, holding time 1 hour/25mm, oil quenching.
- Tempering: tempering at 590-650°C, holding time 1-2 hours, air cooling or furnace cooling (≤30°C/h).
- Weldability: welding performance is improved after high temperature tempering, preheating to 150°C before welding, stress relief annealing after welding (620°C×2 h).
5. Common Misunderstandings & Solutions
| Misunderstanding phenomenon | Cause analysis | Solution |
| 110 ksi workpiece impact energy is insufficient | empering temperature is too low (<480°C) | Increase to 480-540°C, extend the holding time to 3 h. |
| 80 ksi strength exceeds the standard (>600 MPa) | Insufficient tempering temperature (<590°C) | Use step tempering: 590°C×2 h + 620°C×1 h. |
6. Summary of Selection Recommendations
- Choose 110 ksi if:
– The component is subjected to high-frequency alternating loads (such as engine crankshafts).
– It is allowed to sacrifice some toughness in exchange for lightweight design (in the aviation field). - Choose 80 ksi if:
– The component needs to withstand impact or dynamic loads (excavator buckets).
– Subsequent welding or cold forming is required (building steel structure connectors).
By precisely controlling the tempering temperature and combining sufficient quenching, 4140 steel can be flexibly adjusted to different strength grades to meet diverse engineering needs. In practical applications, it is recommended to optimize parameters through small batch tests.
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