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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
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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.
Typical Applications Mechanical structural parts, shafts, bolts, gears, frames, welded structures.
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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.
Typical Applications Crankshafts, connecting rods, high-strength bolts, mold bases, gears, hydraulic cylinder tubes.
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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.
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.
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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.
Typical Applications High-strength shafts, gears, connecting rods, oil drilling equipment, molds.
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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.
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.
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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.
Typical Applications Food and medical equipment, chemical containers, marine fittings, architectural decoration, consumer goods.
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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.
Typical Applications Cutlery, surgical instruments, bearings, valve parts, pump shafts.
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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.
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 difference between carbon steel and alloy steel?
Carbon steel is primarily composed of iron and carbon, with minimal other elements. Alloy steel intentionally incorporates additional elements such as chromium, nickel, molybdenum, or vanadium to enhance specific properties like hardenability, toughness, corrosion resistance, or high-temperature strength beyond what carbon alone can provide.
Q Why is 316 stainless steel preferred over 304 in marine or chemical environments?
316 stainless steel contains molybdenum (typically 2–3%), which significantly improves its resistance to pitting and crevice corrosion caused by chlorides. This makes it far more suitable than 304 for marine fittings, chemical processing equipment, and any environment with high chloride exposure.
Q Can high carbon steel be welded?
High carbon steel (carbon content above 0.60%) is generally considered difficult to weld. The high carbon content increases the risk of hardening and cracking in the heat-affected zone. When welding is necessary, preheating and post-weld heat treatment are typically required to minimize these risks.
Q What makes 17-4 PH stainless steel suitable for aerospace applications?
17-4 PH stainless steel combines the corrosion resistance of stainless steel with mechanical properties approaching those of high-strength alloy steels, achieved through a precipitation hardening heat treatment process. Its low work-hardening tendency and dimensional stability during heat treatment make it ideal for precision aerospace structural components.
Q What is the key advantage of AISI 4340 over 4140 steel?
AISI 4340 contains nickel in addition to chromium and molybdenum, which gives it superior toughness and fatigue resistance at ultra-high strength levels. While 4140 is an excellent general-purpose alloy steel, 4340 is specifically chosen for the most demanding applications — such as aircraft landing gear and heavy-duty drive shafts — where both extreme strength and impact toughness are critical.
Q How do I choose between D2, H13, and S7 tool steels for my application?
The choice depends on the primary operating condition: D2 is best for cold work applications requiring maximum wear resistance (e.g., stamping and blanking dies). H13 is designed for elevated-temperature applications, offering thermal fatigue resistance ideal for die-casting and hot forging. S7 prioritizes impact toughness over wear resistance, making it the right choice for tools subjected to repeated shock loads such as chisels and punches.
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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