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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.
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 difference between carbon steel and alloy steel?
Carbon steel contains only iron and carbon as its primary components, while alloy steel includes additional elements such as chromium, molybdenum, nickel, or vanadium. These alloying elements significantly improve properties like hardenability, toughness, heat resistance, and fatigue strength, making alloy steel 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 enhances its resistance to pitting corrosion and chloride attack compared to 304. This makes 316 the preferred choice for marine fittings, chemical processing equipment, and any application involving exposure to saltwater or aggressive chemical media.
What does "heat treatment" mean for steel, and how does it affect properties?
Heat treatment refers to controlled heating and cooling processes applied to steel, such as quenching, tempering, annealing, or precipitation hardening. These processes alter the steel's microstructure, allowing significant improvements in hardness, tensile strength, toughness, or ductility depending on the specific treatment applied and the steel grade used.
Which steel grade is best suited for CNC machining of high-strength structural parts?
AISI 4140 (Chromium-Molybdenum Steel) is one of the most widely used grades for high-strength CNC machined structural parts. It offers excellent hardenability, good machinability in the annealed condition, and achieves outstanding strength and toughness after quenching and tempering. For extreme load applications, AISI 4340 is preferred due to its ultra-high-strength characteristics.
What makes tool steels like D2, H13, and S7 different from structural steels?
Tool steels are specifically engineered to withstand the extreme conditions involved in cutting, forming, or shaping other materials. They prioritize hardness, wear resistance, dimensional stability, and in some cases red hardness (retention of hardness at elevated temperatures) or impact toughness — properties that go far beyond what standard structural steels provide.
Is 17-4 PH stainless steel suitable for aerospace applications?
Yes. 17-4 PH (Precipitation Hardening) stainless steel is widely used in aerospace due to its combination of high mechanical strength — comparable to high-strength alloy steels — and good corrosion resistance. Its low work-hardening tendency also makes it more predictable during CNC machining, and it performs reliably in demanding structural and environmental conditions found in aerospace assemblies.
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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