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Steel is an alloy primarily composed of iron and carbon. Its properties are precisely controlled by adjusting the carbon content, adding other alloying elements (such as chromium, nickel, molybdenum, vanadium), and through heat treatment processes.
The steel family is vast, mainly categorized into carbon steel, alloy steel, stainless steel, and tool steel. This covers a full spectrum of choices ranging from high ductility to ultra-high strength, and from general weather resistance to extreme corrosion resistance. Modern CNC technology can efficiently machine various steels, achieving a perfect combination of complex geometries and high dimensional accuracy.
1. Carbon Steel
● Low Carbon Steel (e.g., AISI/SAE 1018, 1045)
⚙️ Characteristics: Carbon content is typically below 0.25%. It has lower strength but excellent toughness, ductility, and weldability. It is easy to perform cold forming and machining.
High Ductility
Excellent Weldability
Easy Machining
🔩 Typical Applications: Mechanical structural parts, shafts, bolts, gears, frames, welded structures.
Shafts
Bolts
Gears
Frames
Welded Structures
● Medium Carbon Steel (e.g., AISI/SAE 1045, 4140)
⚙️ Characteristics: Carbon content is approximately 0.25%–0.60%. After heat treatment (quenching and tempering), it achieves a good balance of strength, hardness, and wear resistance. It is the most commonly used quenched and tempered structural steel.
High Strength
Good Hardness
Wear Resistance
🔩 Typical Applications:
Crankshafts
Connecting Rods
High-Strength Bolts
Mold Bases
Gears
Hydraulic Cylinder Tubes
● High Carbon Steel (e.g., AISI/SAE 1095, C45)
⚙️ Characteristics: Carbon content is higher than 0.60%. It can achieve very high hardness and wear resistance, but brittleness increases and weldability is poor. It is typically used for making tools and parts requiring extremely high surface hardness.
Very High Hardness
High Wear Resistance
Increased Brittleness
🔩 Typical Applications:
Cutlery
Springs
Saw Blades
Bearing Parts
High-Strength Wire
📊 AISI 1045 (Medium Carbon Steel) — Typical Properties (Normalized Condition)
| Property | Value | Notes |
|---|---|---|
| Tensile Strength (MPa) | 570 – 700 | |
| Yield Strength (MPa) | ≥ 310 | |
| Elongation at Break (%) | ≥ 16 | Good overall mechanical properties |
| Hardness (Brinell) | ~ 170 | |
| Density (g/cm³) | 7.85 | |
| Note: Properties can be significantly improved after heat treatment. | ||
2. Alloy Steel
Alloy steel contains elements such as molybdenum, chromium, and nickel added to carbon steel to enhance strength, toughness, hardenability, or heat resistance.
● AISI/SAE 4140 (Chromium-Molybdenum Steel)
⚙️ Characteristics: A versatile medium-carbon alloy steel. The addition of molybdenum and chromium gives it excellent hardenability, strength, and toughness, allowing uniform properties to be obtained across larger cross-sections.
Excellent Hardenability
High Strength
Superior Toughness
🔩 Typical Applications:
High-Strength Shafts
Gears
Connecting Rods
Oil Drilling Equipment
Molds
● AISI/SAE 4340 (Nickel-Chromium-Molybdenum Steel)
⚙️ Characteristics: The representative of ultra-high-strength alloy steel. The addition of nickel allows it to maintain excellent toughness and fatigue resistance even at very high strength levels, making it suitable for parts subjected to extreme loads.
Ultra-High Strength
Excellent Fatigue Resistance
Superior Toughness
🔩 Typical Applications:
Aircraft Landing Gear
Heavy Vehicle Drive Shafts
High-Strength Fasteners
Critical Military Equipment Parts
3. Stainless Steel
Stainless steel uses chromium (≥10.5%) as the main alloying element to form a passive film, thereby providing superior corrosion resistance.
● Austenitic Stainless Steel (e.g., 304, 316)
⚙️ Characteristics: Non-magnetic, excellent corrosion resistance, and superb formability and weldability. Due to the addition of molybdenum, 316 has better pitting corrosion resistance and chemical resistance (especially to chlorides) than 304.
Non-Magnetic
Excellent Corrosion Resistance
Superb Weldability
🔩 Typical Applications:
Food & Medical Equipment
Chemical Containers
Marine Fittings
Architectural Decoration
Consumer Goods
● Martensitic Stainless Steel (e.g., 410, 440C)
⚙️ Characteristics: Can be heat-treated to achieve high hardness and strength. It has certain corrosion resistance and wear resistance, but its toughness and weldability are relatively poor.
High Hardness
Wear Resistance
Heat Treatable
🔩 Typical Applications:
Cutlery
Surgical Instruments
Bearings
Valve Parts
Pump Shafts
● Precipitation Hardening Stainless Steel (e.g., 17-4 PH)
⚙️ Characteristics: Through special heat treatment (precipitation hardening), it achieves mechanical properties close to high-strength alloy steel while maintaining good corrosion resistance. It has a low tendency for work hardening.
Ultra-High Strength
Good Corrosion Resistance
Low Work Hardening
🔩 Typical Applications:
Aerospace Structural Parts
High-Speed Centrifuge Components
Nuclear Industry Parts
High-End Molds
📊 AISI 304 Stainless Steel — Typical Properties (Solution Annealed Condition)
| Property | Value | Notes |
|---|---|---|
| Tensile Strength (MPa) | ≥ 515 | |
| Yield Strength (0.2% Offset, MPa) | ≥ 205 | |
| Elongation at Break (%) | ≥ 40 | Excellent plasticity and toughness |
| Hardness (Brinell) | ≤ 201 | |
| Density (g/cm³) | 8 |
4. Tool Steel
Tool steel is specifically developed for manufacturing tools (such as molds and cutlery), pursuing extreme hardness, wear resistance, red hardness, and dimensional stability.
🛠️ D2
High Carbon, High Chromium Cold Work Steel
High wear resistance with moderate toughness, used for high-life cold stamping dies and fine blanking dies.
🔥 H13
Hot Work Die Steel
Maintains high strength and wear resistance at high temperatures with good resistance to thermal fatigue. Used for die-casting molds and hot forging dies.
💥 S7
Impact Resisting Tool Steel
Renowned for its excellent toughness. Used for chisels, punches, and shear blades subjected to high impact loads.
❓ Frequently Asked Questions (FAQ)
Q What is the main difference between carbon steel and alloy steel?
Carbon steel is primarily composed of iron and carbon, with minimal other elements. Alloy steel intentionally adds elements such as chromium, molybdenum, nickel, or vanadium to enhance specific properties like hardenability, toughness, strength, or heat resistance beyond what carbon alone can achieve.
Q Why is 316 stainless steel preferred over 304 in marine or chemical environments?
316 stainless steel contains molybdenum, which significantly improves its resistance to pitting corrosion and chemical attack — especially from chlorides found in seawater and industrial chemicals. While 304 offers good general corrosion resistance, 316 is the superior choice for harsh or chloride-rich environments.
Q What makes AISI 4340 suitable for aerospace and military applications?
AISI 4340 is a nickel-chromium-molybdenum ultra-high-strength alloy steel that combines exceptional tensile strength with excellent toughness and fatigue resistance. This rare combination makes it ideal for critical components such as aircraft landing gear and military equipment parts that must endure extreme mechanical loads without fracturing.
Q When should tool steel be chosen over standard alloy steel?
Tool steel should be chosen when the application demands extreme hardness, wear resistance, red hardness (retention of hardness at elevated temperatures), and high dimensional stability — such as in cutting tools, stamping dies, molds, and punches. Standard alloy steels prioritize a balance of strength and toughness rather than the extreme surface hardness required by tooling applications.
Q How does heat treatment affect the properties of medium carbon steel like AISI 1045?
Heat treatment — particularly quenching and tempering — can significantly increase the tensile strength, yield strength, and hardness of AISI 1045 beyond its normalized condition values. The process allows engineers to tailor the mechanical properties for demanding structural applications such as crankshafts, gears, and high-strength bolts.
Q What is precipitation hardening stainless steel and where is it used?
Precipitation hardening (PH) stainless steel, such as 17-4 PH, undergoes a specialized aging heat treatment that causes fine precipitates to form within the microstructure, dramatically increasing strength. It achieves mechanical properties comparable to high-strength alloy steels while retaining good corrosion resistance — making it ideal for aerospace structural parts, high-speed centrifuge components, nuclear industry parts, and high-end molds.
Finishes and Post-Processing Options
The zinc alloy substrate is highly suitable for various surface treatments to enhance decoration, protection, or functionality:
Electroplating: Such as chrome, nickel, gold, or silver plating, providing a highly decorative appearance, excellent wear and corrosion resistance. This is the most common choice.
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Passivation or Chromate Conversion Coating:
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Powder Coating:
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Painting:
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Physical Vapor Deposition (PVD):
Design and Machining Considerations
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Process Selection:
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Design Considerations:
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Material Selection:
Zamak 3 and 5 are suitable for the vast majority of general-purpose parts requiring good appearance and strength; ZA-8 is better for wear and bearing applications; for making rapid tooling, choose specialized alloys like Kirksite.
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