AISI 4340 Steel Round Bar – High Strength Alloy Steel for Critical Applications
Introduction
If you’re searching for a **high-strength alloy steel round bar** that delivers exceptional **toughness, fatigue resistance, and hardenability**, **AISI 4340 steel round bar** is one of the best options available.
As a **nickel-chromium-molybdenum alloy steel**, 4340 is widely used across **aerospace, automotive, defense, and heavy machinery** industries. It is known for maintaining its mechanical properties even under **high-stress and high-temperature conditions**.
Whether you are an **engineer, fabricator, or 4340 steel supplier**, understanding the properties and applications of this steel grade can help you make the right material choice.
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**Chemical Composition of AISI 4340 Steel Round Bar**
The unique chemical composition of AISI 4340 provides a balance of **strength, toughness, and hardenability**.
| **Element** | **Content (%)** |
| ————— | ————— |
| Carbon (C) | 0.38 – 0.43 |
| Manganese (Mn) | 0.60 – 0.80 |
| Silicon (Si) | 0.15 – 0.35 |
| Nickel (Ni) | 1.65 – 2.00 |
| Chromium (Cr) | 0.70 – 0.90 |
| Molybdenum (Mo) | 0.20 – 0.30 |
| Sulfur (S) | ≤ 0.04 |
| Phosphorus (P) | ≤ 0.035 |
The combination of **nickel, chromium, and molybdenum** enhances strength, **toughness**, and **wear resistance**, making 4340 ideal for demanding engineering applications.
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### **Mechanical Properties of 4340 Steel Round Bar**
AISI 4340 round bar exhibits outstanding mechanical properties, which can be further improved through **heat treatment**.
| **Property** | **Typical Range** |
| ————————- | —————————————– |
| Tensile Strength | 860 – 1980 MPa |
| Yield Strength | 740 – 1860 MPa |
| Elongation | 11 – 23% |
| Hardness | 24 – 53 HRC (depending on heat treatment) |
| Impact Resistance | Excellent |
| Fracture Toughness (K_IC) | 53 – 110 MPa√m |
These properties make it one of the most **versatile and reliable alloy steels** available for **high-load and fatigue-prone components**.
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### **Equivalent Grades of 4340 Steel**
| **Standard** | **Equivalent Grade** |
| ———— | ——————– |
| AISI / SAE | 4340 |
| UNS | G43400 |
| EN | 36CrNiMo4 / 1.6511 |
| BS | 817M40 |
| DIN | 1.6565 |
| JIS | SNCM439 |
These equivalents allow 4340 round bars to be sourced globally under different specifications without compromising performance.
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### **Heat Treatment and Hardening**
AISI 4340 responds exceptionally well to **heat treatment**, allowing for customizable hardness and strength depending on the intended use.
* **Annealing:** 830–850°C followed by slow cooling in the furnace
* **Normalizing:** 870–900°C followed by air cooling
* **Hardening:** 830–870°C followed by oil quenching
* **Tempering:** 450–650°C depending on the required balance between hardness and toughness
This flexibility in heat treatment makes **4340 steel round bar** a favorite for engineers seeking **specific performance characteristics**.
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### **Machinability and Fabrication**
In the **annealed condition**, 4340 has a **machinability rating of approximately 57%**, compared to AISI 1212 at 100%. It can be **machined, welded, and formed** effectively with proper techniques. Post-weld heat treatment is recommended to restore its mechanical properties.
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### **Key Features of AISI 4340 Steel Round Bar**
* Exceptional **strength-to-weight ratio**
* Excellent **fatigue and impact resistance**
* High **ductility and toughness**
* Suitable for **through-hardening** and **surface hardening**
* Consistent **mechanical properties across sections**
* Reliable **performance in elevated temperature environments**
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### **Applications of 4340 Steel Round Bar**
Due to its unique combination of strength, ductility, and fatigue resistance, **AISI 4340 steel round bar** is widely used in:
* **Aircraft landing gear and engine components**
* **Automotive crankshafts, connecting rods, and drive shafts**
* **Heavy-duty gears, axles, and shafts**
* **High-strength fasteners and bolts**
* **Hydraulic systems and power transmission components**
* **Oil & gas drilling tools and machinery parts**
These applications demand materials capable of withstanding **high mechanical stress and fatigue**, making 4340 an ideal choice.
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### **Equivalent International Standards**
AISI 4340 steel is also known as **EN24 / 817M40 / 36CrNiMo4**, depending on regional standards. Most **4340 steel suppliers and stockists** carry it in **round bar, square bar, and forged block** forms.
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### **Conclusion**
**AISI 4340 round bar** is a premium-grade **nickel-chromium-molybdenum alloy steel** that delivers outstanding **strength, toughness, and durability**. Its adaptability through **heat treatment**, combined with excellent **fatigue and wear resistance**, makes it an indispensable material for **critical engineering components**.
If you’re looking for a **reliable 4340 steel round bar supplier**, ensure your source provides **mill test certificates (MTCs)**, **ultrasonic testing**, and **precise dimensional tolerances** to guarantee performance and quality.
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Contact us today to get a quotation or learn more about our AISI 4340 steel round bar price, stock availability, and technical specifications.
