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Major Steel Categories and Typical Grades
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
Grade Low Carbon Steel (e.g., AISI/SAE 1018, 1045)
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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.
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Typical Applications: Mechanical structural parts, shafts, bolts, gears, frames, welded structures.
Grade Medium Carbon Steel (e.g., AISI/SAE 1045, 4140)
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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.
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Typical Applications: Crankshafts, connecting rods, high-strength bolts, mold bases, gears, hydraulic cylinder tubes.
Grade High Carbon Steel (e.g., AISI/SAE 1095, C45)
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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.
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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.
Grade AISI/SAE 4140 — Chromium-Molybdenum Steel
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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.
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Typical Applications: High-strength shafts, gears, connecting rods, oil drilling equipment, molds.
Grade AISI/SAE 4340 — Nickel-Chromium-Molybdenum Steel
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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.
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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.
Grade Austenitic Stainless Steel (e.g., 304, 316)
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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.
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Typical Applications: Food and medical equipment, chemical containers, marine fittings, architectural decoration, consumer goods.
Grade Martensitic Stainless Steel (e.g., 410, 440C)
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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.
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Typical Applications: Cutlery, surgical instruments, bearings, valve parts, pump shafts.
Grade Precipitation Hardening Stainless Steel (e.g., 17-4 PH)
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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.
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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.
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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.
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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.
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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)
Q1 What is the main difference between carbon steel and alloy steel?
Carbon steel relies primarily on carbon content to control its mechanical properties, while alloy steel incorporates additional elements such as chromium, molybdenum, nickel, or vanadium. These alloying elements significantly enhance properties like hardenability, toughness, heat resistance, and fatigue strength — making alloy steel suitable for more demanding structural and engineering applications.
Q2 Why is stainless steel more corrosion-resistant than regular steel?
Stainless steel contains at least 10.5% chromium, which reacts with oxygen in the environment to form a thin, stable, self-repairing passive oxide film on the surface. This film acts as a barrier that prevents further oxidation and corrosion, giving stainless steel its characteristic resistance to rust and chemical attack.
Q3 What is the difference between 304 and 316 stainless steel?
Both 304 and 316 are austenitic stainless steels with excellent corrosion resistance and formability. The key difference is that 316 contains 2–3% molybdenum, which significantly improves its resistance to pitting and crevice corrosion — especially in chloride-rich environments such as marine or chemical processing applications. 316 is generally preferred when exposure to saltwater or harsh chemicals is expected.
Q4 When should tool steel be chosen over regular alloy steel?
Tool steel should be selected when the application demands extreme hardness, superior wear resistance, dimensional stability under repeated stress, or the ability to retain hardness at elevated temperatures (red hardness). These characteristics make tool steel the ideal choice for manufacturing molds, cutting tools, dies, punches, and other high-wear industrial components.
Q5 How does heat treatment affect the properties of carbon steel?
Heat treatment processes such as quenching and tempering can dramatically improve the mechanical properties of carbon steel. For example, AISI 1045 medium carbon steel in its normalized condition has a tensile strength of 570–700 MPa, but after quenching and tempering, both tensile strength and hardness can be significantly increased. The specific outcome depends on the carbon content, the heat treatment parameters, and the desired balance between strength and toughness.
Q6 Is AISI 4340 steel suitable for CNC machining?
Yes, AISI 4340 is machinable, but it is typically machined in its annealed or normalized condition before final heat treatment to achieve the best results. In its hardened state, it becomes significantly more difficult to machine. Modern CNC equipment with appropriate tooling and cutting parameters can handle 4340 effectively, making it a practical choice for high-strength aerospace, defense, and heavy industrial components.
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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