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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)
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.
● 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.
● 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.
● 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.
● 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.
● 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.
● 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.
● 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
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, strength, or heat resistance. Alloy steels are generally better suited for demanding structural and high-performance applications.
Q 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 passive oxide layer on the surface. This self-repairing film acts as a barrier that prevents further oxidation and corrosion, giving stainless steel its characteristic resistance to rust and chemical attack.
Q What is the advantage of 316 stainless steel over 304?
316 stainless steel contains molybdenum (approximately 2–3%), which significantly improves its resistance to pitting corrosion and chemical attack, particularly from chloride environments such as seawater or salt spray. This makes 316 the preferred choice for marine fittings, chemical processing equipment, and medical implants where 304 may be susceptible to localized corrosion.
Q How does heat treatment affect the properties of steel?
Heat treatment processes such as quenching, tempering, annealing, and precipitation hardening can dramatically alter a steel's mechanical properties. For example, quenching and tempering AISI 1045 or 4140 can greatly increase hardness and tensile strength beyond their normalized values. Precipitation hardening of 17-4 PH stainless steel can bring its strength close to that of high-strength alloy steels while retaining good corrosion resistance.
Q Which steel grade is best suited for CNC machining?
The best steel grade for CNC machining depends on the application requirements. Low carbon steels like AISI 1018 are easy to machine and weld. Medium carbon steels like AISI 1045 and 4140 offer a good balance of machinability and mechanical performance. Stainless steels like 304 and 316 are machinable but require appropriate tooling due to work hardening tendencies. Modern CNC equipment can efficiently handle all major steel categories to achieve complex geometries and tight tolerances.
Q What makes tool steel different from other steel types?
Tool steels are specially formulated to withstand the extreme conditions encountered during cutting, forming, and shaping operations. They are characterized by very high hardness, superior wear resistance, red hardness (the ability to retain hardness at elevated temperatures), and excellent dimensional stability. Grades such as D2, H13, and S7 are engineered for specific tooling applications — from cold work dies to hot forging molds and high-impact chisels.
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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