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China Steel CNC Machining Services - Reliable Suppliers & Factory for Precision Steel Parts
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 — covering a full spectrum 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.
Major Steel Categories and Typical Grades
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. 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 approximately 0.25%–0.60%. After heat treatment, achieves a good balance of strength, hardness, and wear resistance. 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 higher than 0.60%. Achieves very high hardness and wear resistance, but brittleness increases and weldability is poor. Used for 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 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, suitable for parts under 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. Has certain corrosion resistance and wear resistance, but 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), achieves mechanical properties close to high-strength alloy steel while maintaining good corrosion resistance. 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)
Q What is the difference between carbon steel and alloy steel?
Carbon steel contains iron and carbon as its primary elements, with minimal other alloying additions. Alloy steel incorporates additional elements such as chromium, molybdenum, nickel, or vanadium to enhance specific properties like hardenability, toughness, heat resistance, or corrosion resistance beyond what carbon steel can achieve.
Q Why is AISI 4140 so widely used in CNC machining?
AISI 4140 (Chromium-Molybdenum steel) offers an excellent combination of machinability, hardenability, strength, and toughness. It responds well to heat treatment, allowing engineers to tailor its mechanical properties for demanding applications such as shafts, gears, and mold bases, making it one of the most versatile alloy steels for CNC machined components.
Q What is the key difference between 304 and 316 stainless steel?
Both 304 and 316 are austenitic stainless steels with excellent corrosion resistance. The key difference is that 316 contains molybdenum (approximately 2–3%), which significantly improves its resistance to pitting corrosion and chemical attack, especially in chloride-rich environments such as marine or chemical processing applications. 316 is therefore preferred where higher corrosion resistance is critical.
Q When should I choose tool steel over other steel types?
Tool steel should be chosen when the application demands extreme hardness, superior wear resistance, dimensional stability under stress, or the ability to maintain hardness at elevated temperatures (red hardness). Typical use cases include cutting tools, stamping dies, molds, punches, and blades where standard carbon or alloy steels would wear out too quickly.
Q Can stainless steel be CNC machined as easily as carbon steel?
Stainless steel is generally more challenging to CNC machine than carbon steel due to its work-hardening tendency, lower thermal conductivity, and tendency to build up on cutting tool edges. Austenitic grades like 304 and 316 are particularly prone to work hardening. Proper tooling selection, cutting speeds, and coolant strategies are essential to achieve good results and extend tool life when machining stainless steel.
Q How does heat treatment affect the properties of carbon and alloy steels?
Heat treatment processes such as quenching, tempering, annealing, and normalizing can dramatically alter the mechanical properties of carbon and alloy steels. For example, AISI 1045 in the normalized condition has a tensile strength of 570–700 MPa, but after quenching and tempering, its strength, hardness, and wear resistance can be significantly increased. Alloy steels like 4140 and 4340 are specifically designed to respond favorably to heat treatment across larger cross-sections.
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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