What is steel? Properties, grades and manufacturing uses

Key summary 

Steel is an alloy of iron and carbon, often with other elements added to tune its properties. It offers high tensile strength, hardness and ductility at a low material cost. There are four main groups of steel: 

• Carbon steel: structural and construction components 

• Alloy steel: automotive gears and heavy-duty frames 

• Stainless steel: corrosion-resistant medical and food-processing equipment 

• Tool steel: cutting tools and injection molds 

Steel is used across industrial machinery, automotive, aerospace and consumer products. Its mechanical performance is predictable, low-carbon grades weld well, and heat treatments such as quenching and tempering let you tailor its properties. Grades commonly CNC machined from stock include: 

• 1018 carbon steel: easy to machine and weld 

• 4140 alloy steel: high tensile strength and fatigue resistance 

• 304 stainless steel: strong corrosion resistance 

This guide covers steel's composition and properties, compares its main types and grades, outlines typical applications and explains what to consider when CNC machining steel parts. 

What is steel? 

Steel is an alloy of iron, carbon and other elements. High strength and low cost make it a core structural material in engineering. It is made in a blast furnace/basic oxygen furnace or an electric arc furnace, then goes through secondary refining. 

Changing the carbon and alloying content controls steel’s hardness, ductility and response to heat treatment. Heat treatment then tunes its tensile strength, hardness and ductility, and low-carbon grades weld well. There are four groups: carbon, alloy, stainless and tool steel. Specialized grades add wear resistance, high-temperature stability or corrosion resistance. Commonly machined grades include 1018, 4140 and 304 stainless steel. 

How is steel made? 

The production of steel requires a multi-stage process that removes impurities from raw iron ore and introduces precise quantities of carbon and alloying agents. Industrial steel manufacturing follows two primary methods: 

1. Blast furnace / Basic oxygen furnace (BF-BOF): Raw iron ore, coke and limestone are reduced in a blast furnace to produce liquid iron. This molten iron contains high carbon levels, which are subsequently reduced by blowing high-purity oxygen through the metal in a basic oxygen furnace. 

2. Electric arc furnace (EAF): This process uses recycled steel scrap as the primary feedstock. High-power electric arcs melt the scrap metal. This approach enables efficient chemical control and typically requires less energy and produces lower emissions than primary iron-ore-based steelmaking. 

3. Secondary metallurgy: After the initial melting, the liquid steel undergoes precise chemical adjustments and deoxidation. Elements such as chromium, nickel or manganese are added to meet exact grade specifications before the metal is cast into slabs, billets or blooms. 

Properties of steel 

Steel is the default manufacturing material across industrial sectors due to its highly predictable mechanical performance and unmatched economic efficiency. A primary driver for its widespread selection is its exceptional strength-to-cost ratio. This allows engineers to design highly loaded structural components without escalating production budgets.  

The metal possesses high tensile strength, which prevents catastrophic failure under massive structural loads. These are combined with structural hardness that provides robust wear resistance under continuous friction. 

Beyond the structural load capacity, the alloy retains excellent ductility, meaning that it can undergo significant plastic deformation during forming processes without cracking or fracturing. This property is paired with superior weldability in low-carbon variations, allowing complex machined or cast components to be securely joined into permanent industrial assemblies. The precise balance of these characteristics can be further optimized through thermal treatments such as quenching and tempering to meet specific operational requirements. 

The table below outlines the core material characteristics of standard unalloyed medium carbon steel: 

Types and grades of steel 

Industrial steel is categorized into four primary groups based on its composition: 

• Carbon steel: Variants classified by carbon content, typically used for structural panels, high-strength wires and construction materials. 

• Alloy steel: Infused with elements such as manganese or nickel, commonly used in automotive gears and heavy-duty frames. 

• Stainless steel: Formulated with chromium, it provides excellent corrosion resistance, ideal for medical instruments and food processing equipment. 

• Tool steel: Hardened alloys containing tungsten or molybdenum, primarily used to manufacture cutting tools and injection molds. 

Discover the different options using our full overview of machinable metals.

How is steel used in manufacturing? 

Engineers and manufacturers select specific steel grades when components demand reliable long-term performance under high mechanical loads or harsh operating conditions. Its high structural rigidity makes it a foundational material across the automotive, aerospace, industrial machinery and consumer goods sectors. 

Typical applications include: 

• Industrial machinery: Heavy-duty machine frames, structural gears, hydraulic cylinders and power transmission shafts. 

• Automotive engineering: Engine blocks, suspension brackets, structural chassis components and crash-reinforcement beams. 

• Aerospace manufacturing: High-strength fasteners, landing gear components and specialized actuators requiring extreme fatigue resistance. 

Which steels can be CNC machined? 

CNC machining is highly effective for fabricating precise, low-volume steel components and functional prototypes. While steel is generally harder to cut than aluminum or brass, specific alloys are optimized to enhance chip formation, reduce tool wear and improve surface finish during subtractive processing. 

Common machinable steel alloys available through our platform include: 

• Carbon steel 1018: A low carbon steel known for excellent weldability, a tough core and smooth surface finish. It is highly machinable and frequently selected for shafts, spindles and simple structural components. 

• Alloy steel 4140: A chromium-molybdenum alloy that offers exceptional tensile strength, toughness and high fatigue resistance. It is widely used for heavy-duty gears, industrial bolts and demanding structural applications. 

• Stainless steel 304: The most common stainless steel grade, providing excellent corrosion resistance and good formability. To learn more about how this alloy compares to other corrosion-resistant materials, read our dedicated comparison guide on stainless steel versus aluminum. 

Our global manufacturing network supports your project requirements from functional one-off prototypes up to thousands of end-use production components. Beyond standard machining, we offer high-volume fabrication services including injection molding and production-grade tooling. 

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