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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.
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 (FAQ)
What is the main difference between carbon steel and alloy steel?
Carbon steel is primarily composed of iron and carbon, with minimal other alloying elements. Alloy steel intentionally incorporates elements such as chromium, nickel, or molybdenum to enhance specific properties like hardenability, toughness, or heat resistance — making it suitable for more demanding structural and engineering applications.
Why is AISI 316 stainless steel preferred over 304 in marine or chemical environments?
AISI 316 contains molybdenum, which significantly improves its resistance to pitting corrosion and chemical attack — especially from chlorides found in seawater and harsh chemical environments. AISI 304, while highly corrosion resistant in general use, lacks this additional protection, making 316 the preferred choice for marine fittings, chemical containers, and similar applications.
When should I choose 4340 alloy steel over 4140?
AISI 4340 (Nickel-Chromium-Molybdenum steel) is chosen when parts must withstand extreme loads and require ultra-high strength combined with excellent toughness and fatigue resistance — such as aircraft landing gear or heavy-duty drive shafts. AISI 4140 is more cost-effective and sufficient for general high-strength applications like shafts, gears, and molds where the extreme demands of 4340 are not required.
What makes tool steels like D2, H13, and S7 different from structural steels?
Tool steels are specifically engineered to achieve extreme hardness, wear resistance, red hardness (ability to retain hardness at elevated temperatures), and dimensional stability under demanding service conditions. Unlike structural steels that prioritize strength and toughness for load-bearing, tool steels are optimized for cutting, forming, and impact applications such as dies, molds, punches, and blades.
Can high carbon steel be welded?
High carbon steel (carbon content above 0.60%) has poor weldability due to its increased brittleness and susceptibility to cracking in the heat-affected zone. Welding is generally not recommended without special precautions such as preheating, controlled interpass temperatures, and post-weld heat treatment. For applications requiring welding, low or medium carbon steels are typically preferred.
What is precipitation hardening stainless steel and why is it used in aerospace?
Precipitation hardening (PH) stainless steel, such as 17-4 PH, undergoes a special aging heat treatment that causes fine precipitates to form within the steel matrix, dramatically increasing its strength and hardness. This allows it to achieve mechanical properties comparable to high-strength alloy steels while retaining good corrosion resistance. Its combination of high strength, corrosion resistance, and low work-hardening tendency makes it ideal for aerospace structural components, high-speed centrifuge 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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