Steel Composition Explained: Properties, Grades & Applications
From skyscrapers to surgical tools, steel shapes our world. Find out what goes into making it, and how different grades unlock different properties.
Steel is one of the world’s most important engineering metals, used in construction, automotive, aerospace, medical devices and countless everyday products. Its appeal comes from the way its composition can be adjusted to deliver the right balance of strength, ductility, toughness and corrosion resistance.
This article will explain what steel is made of and how different compositions translate into performance on the shop floor and in the field.
What Is Steel Made of?
Steel is an iron-carbon alloy, usually containing 0.05% to 2% carbon by weight. Carbon strengthens the iron while keeping it workable, and additional elements like manganese, nickel, chromium and molybdenum are added to enhance properties such as hardness, weldability and corrosion resistance.
While iron provides the base structure, it's the carbon and these other alloying elements that transform it into a material strong enough for jet engines, precise enough for surgical instruments, and versatile enough for everyday manufacturing.
Understanding steel composition helps you select the right grade for your project, whether you need corrosion resistance or high strength.
Steel Performance and Manufacturability
Steel can be CNC machined, CNC turned, fabricated, or even 3D printed through DMLS. Its manufacturability in each of these processes depends largely on its carbon content, which influences hardness, ductility, and ease of machining.
- Low-carbon steels (under 0.3% carbon) are easier to machine, weld, and form, making them ideal for sheet metal work and general fabrication.
- Medium-carbon steels (0.3% to 0.6%) offer a balance between strength and workability, commonly used in machinery parts.
- High-carbon steels (0.6% to 1.4%) provide excellent hardness and wear resistance but become more brittle and harder to machine.
Think of it as a sliding scale. More carbon results in steel that’s harder and stronger, but less forgiving. Your application dictates where on that scale you need to land.
Types of Steel
Steel is not one material but a family of alloys. Common categories include:
- Carbon steel: Primarily iron and carbon. Cost-effective, strong, but prone to corrosion.
- Alloy steel: Includes elements like nickel, chromium or vanadium for improved strength, toughness or heat resistance.
- Mild steel: A low-carbon subtype of carbon steel (<0.3% carbon) that is easy to weld and form, commonly used in general fabrication.
- Stainless steel: Contains at least 10.5% chromium for corrosion resistance.
- Maraging steel: High-alloy, low-carbon steel that gains strength through aging rather than carbon content. Used for high-performance tooling and precision parts where toughness and low distortion matter.
- Tool steel: High-carbon steel engineered for edge retention, wear resistance and durability in cutting and forming tools. Ideal when hardness and abrasion resistance are the priority. Learn more in our tool steel manufacturing blog.
Properties Linked to Composition
Different elements in the mix give steel different behaviours, and each one plays a role:
- Strength and hardness: Carbon does most of the heavy lifting here, while alloying elements like chromium and molybdenum can boost strength even further. Heat treatment can then dial those properties up or down.
- Toughness: This is all about absorbing impact without cracking. Steel with more nickel is better at enduring colder conditions.
- Corrosion resistance: Chromium is the key player. Once you hit about 10.5%, it forms a protective layer on the surface. Molybdenum adds extra protection in harsher environments like salt or chemicals.
- Machinability: Small additions of sulphur or phosphorus help chips break cleanly during machining. Grade 303 stainless, available through Protolabs Network, is particularly good at this.
- Weldability: The more carbon a steel has, the trickier it is to weld. Low-carbon steels weld easily, while higher-carbon grades may need preheating or specific techniques to avoid cracking.
- Magnetism: Most carbon and alloy steels are magnetic. Austenitic stainless steels like 304 and 316 are generally non-magnetic, while martensitic grades such as 17‑4 PH are magnetic.
Heat Treatment Options
Heat treatment adjusts steel's microstructure to achieve specific properties. The process you choose lets you fine-tune performance:
- Annealing: Softens the metal and makes it easier to work with. It’s useful when you want to relieve stress or get the material ready for forming.
- Normalising: Brings the grain structure back into balance so the steel behaves consistently. It’s often used after machining or forging.
- Quenching and tempering: First you harden the steel by cooling it fast, then you temper it to bring back some flexibility. Choose this when you need strength without the brittleness.
- Case hardening: Hardens just the outside while keeping the inside tougher and more flexible. It’s a good fit for gears, shafts and fasteners.
Finishes and Corrosion Protection
Even steels with good corrosion resistance can benefit from protective finishes. Protolabs offers a range of finishing options to enhance both performance and appearance:
- Passivation: Cleans the surface and boosts its natural resistance to rust. It doesn’t change the look or dimensions and is common in medical and food‑grade parts.
- Black oxide: Adds a thin, non‑reflective layer with light corrosion protection. It’s mainly used for appearance or to cut glare on tools.
- Zinc plating and galvanising: Puts a protective zinc layer on the surface so the zinc corrodes before the steel. Galvanising is thicker than plating and better for outdoor use.
- Electroless nickel plating: Adds a hard, even coating that improves wear and corrosion resistance. It works well on complex shapes and functional parts.
- Powder coating: A durable painted finish in a range of colours. It protects against impact and corrosion but can add thickness to tight‑tolerance features.
Selecting the Right Steel
Start with your functional requirements, then work backward to your metal material selection. Here's a decision framework:
For Corrosion Resistance
- Marine or chemical environments → 316L stainless steel or other CNC stainless steels
- Food contact or medical devices → 304L stainless steel (with passivation) or other CNC stainless steels
- General corrosion protection → 17-4 PH stainless steel or carbon steel with protective coating
For High Temperature Applications
For Strength and Hardness
- Maximum hardness needed → Tool steels (D2, O1, A2) or hardened 420/440C stainless
- High strength with some ductility → 17-4 PH stainless steel orc 3D printing stainless steels (can harden to 44 HRC)
- Moderate strength, good toughness → Alloy steel 4340
For Machinability and Cost
- Easiest machining, lowest cost → Mild steel 1018 or S275JR
- Good machining with better properties → Carbon steel EN8 or alloy steel 1215
- Tight tolerances after heat treatment → 17-4 PH stainless or pre-hardened tool steels
For Lightweight or Complex Geometries
- Parts with internal channels or lattice structures → 3D printing stainless steels (316L)
Protolabs Capabilities With Steel
Protolabs provides multiple manufacturing processes for steel parts, from rapid prototyping to production runs.
- CNC machining: Stainless grades 304L, 316L, 17-4 PH available directly, with an extended range through the Protolabs Network.
- Direct metal laser sintering: Stainless steel 316L is available for metal 3D printing. See our DMLS alloys list.
- Sheet metal fabrication: Available via Protolabs Network for stainless steel 304, stainless steel 316L, Steel 1018.
Protolabs Steel Materials
| Steel Category | Grades | Strengths | Typical Usage |
| Stainless Steel | 304L, 316L, 17-4 PH | Corrosion-resistant, versatile | Medical, aerospace, general parts |
| Stainless Steel (DMLS) | 316L | Strong, complex geometries possible | Aerospace, automotive, medical |
| Mild Steel | S275JR | Low cost, easy to weld | Construction, fabrication |
| Carbon Steel | EN8 | Balanced strength and machinability | Shafts, automotive parts |
|
Maraging Steel (DMLS) |
Maraging Steel 1.2709 | Very high strength | Tooling, high-performance parts |
| Steel Category | Grades | Strengths | Typical Usage |
| Stainless Steel | 304/304L, 316/316L, 303, 17-4 PH, 416, 2205 Duplex, 420, 440C, 430, 301, 15-5 | Range from easy machining to wear resistance | Precision, automotive, marine |
| Mild Steel | 1018, C45, S355J2, 1045, S235JR, A36, S275JR | Affordable, easy to machine | Construction, frames |
| Alloy Steel | 1.7131, 4140, 4340, 1215 | High strength and wear resistance | Automotive, industrial equipment |
| Tool Steel | D2, A2, O1, A3, S7, H13 | Hard, wear- and heat-resistant | Cutting tools, dies |
|
|
Various grades | Lightweight, formable | Consumer electronics, robotics, industrial equipment |
Frequently Asked Questions
What is steel made of?
expand_less expand_moreIron and carbon, with alloying elements added to adjust performance.
What’s the difference between carbon and stainless steel?
expand_less expand_moreCarbon steels are strong but prone to corrosion. Stainless steels contain chromium for corrosion resistance.
Can steel be 3D printed?
expand_less expand_moreYes, direct metal laser sintering can build strong, functional parts from stainless steel powder.
Which steel grade is easiest to machine?
expand_less expand_moreMild steels like 1018 and S275JR are good choices. For stainless steels, 303 is designed for machinability.
Does steel need surface finishing?
expand_less expand_moreStainless steels often don’t, but carbon and alloy steels usually need coatings like zinc plating or powder coat to protect against corrosion and improve durability.
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