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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
⚙️ 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.
⚙️ 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.
⚙️ 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.
⚙️ 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.
⚙️ 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.
⚙️ 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.
⚙️ 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.
⚙️ 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
What is the difference between carbon steel and alloy steel?
Carbon steel primarily contains iron and carbon, with minimal other alloying elements. Alloy steel intentionally adds elements such as chromium, molybdenum, nickel, or vanadium to improve specific properties like hardenability, toughness, heat resistance, or corrosion resistance beyond what carbon alone can achieve.
When should I choose 316 stainless steel over 304?
Choose 316 stainless steel when the application involves exposure to chlorides, saltwater, or aggressive chemicals. The molybdenum content in 316 provides significantly better pitting and crevice corrosion resistance compared to 304, making it the preferred choice for marine, medical, and chemical processing environments.
What makes 4340 alloy steel suitable for aerospace applications?
AISI 4340 is a nickel-chromium-molybdenum steel that offers an exceptional combination of ultra-high strength and excellent toughness and fatigue resistance. Even at very high strength levels, it resists crack propagation under cyclic loading — a critical requirement for aerospace components such as landing gear and drive shafts.
Can tool steels like D2 and H13 be CNC machined?
Yes, tool steels can be CNC machined, but they require careful selection of cutting tools, speeds, and feeds due to their high hardness and wear resistance. D2 and H13 are typically machined in an annealed (softened) state and then heat-treated to final hardness after machining to achieve optimal dimensional accuracy and tool life.
How does heat treatment affect the properties of medium carbon steel like AISI 1045?
Heat treatment significantly enhances the mechanical properties of AISI 1045. In its normalized condition, it has a tensile strength of around 570–700 MPa. After quenching and tempering, tensile strength can increase to over 900 MPa, with substantially improved hardness and wear resistance, while maintaining adequate toughness for demanding structural applications.
What is precipitation hardening stainless steel and why is 17-4 PH popular?
Precipitation hardening (PH) stainless steel achieves high strength through a specialized aging heat treatment that causes fine precipitates to form within the steel matrix, dramatically increasing hardness and strength. 17-4 PH is popular because it combines near-alloy-steel mechanical strength with good corrosion resistance, excellent machinability, and low distortion during heat treatment — making it ideal for aerospace, nuclear, and high-performance 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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