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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. 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 & 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
What is the main difference between carbon steel and alloy steel?
Carbon steel is primarily composed of iron and carbon, with minimal other elements. Alloy steel intentionally adds elements such as chromium, nickel, or molybdenum to enhance specific properties like hardenability, toughness, corrosion resistance, or heat resistance — making it suitable for more demanding engineering applications.
Why is AISI 4140 one of the most widely used alloy steels in CNC machining?
AISI 4140 offers an excellent combination of strength, toughness, and hardenability. Its chromium-molybdenum composition allows it to maintain uniform mechanical properties across large cross-sections after heat treatment, making it ideal for high-stress components such as shafts, gears, and mold bases in CNC machining operations.
What is the difference between 304 and 316 stainless steel?
Both are austenitic stainless steels with excellent corrosion resistance and weldability. The key difference is that 316 contains molybdenum, which significantly improves its resistance to pitting corrosion and chemical attack — especially from chlorides. This makes 316 the preferred choice for marine environments, chemical processing, and medical applications.
When should I choose tool steel over standard alloy steel for a part?
Tool steel should be chosen when the part requires extreme hardness, superior wear resistance, or the ability to maintain performance at elevated temperatures — such as in molds, dies, cutting tools, and punches. Standard alloy steel is more appropriate for structural components where toughness and machinability are prioritized over extreme hardness.
Can high carbon steel be welded?
High carbon steel (carbon content above 0.60%) has poor weldability due to its tendency to form hard, brittle microstructures in the heat-affected zone, which can lead to cracking. If welding is necessary, special pre-heating and post-weld heat treatment procedures are required. For applications requiring weldability, low or medium carbon steel is strongly recommended.
What makes 17-4 PH stainless steel suitable for aerospace applications?
17-4 PH (precipitation hardening) stainless steel achieves mechanical properties comparable to high-strength alloy steels while retaining good corrosion resistance. Its low work-hardening tendency and ability to be precisely heat-treated to different strength levels make it highly suitable for aerospace structural parts, high-speed rotating components, and other critical applications where both strength and corrosion resistance are essential.
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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