Properties of Steel, Stainless Steel, and Carbon Steel

Steel is the world’s most essential engineering and construction material, appearing in every aspect of our lives, including home appliances, vehicles, and various construction products. Although we consider steel an individual metal, it is neither a single product nor is it technically a metal.

In fact, there are more than 3,500 steel grades with different physical, chemical, and environmental properties. And to be accurate, steel is an alloy, meaning it is not a pure element, so it is not actually a metal. It is composed of iron, a metal, but the carbon that goes into producing steel eliminates it from being classified as a metal.

Most of today’s steel was developed in the past 20 years, making it stronger but up to 35% lighter than in the past. So, if an older structure like the Eiffel Tower were to be rebuilt today, it would require about one-third of the steel used initially!

How is Steel Produced?

To begin, iron is made by removing oxygen and other impurities from iron ore. Combined with carbon, recycled steel, and small amounts of different elements, it becomes steel. The carbon content of steel is around 2%, with small amounts of manganese, silicon, phosphorus, sulfur, and oxygen making up the rest.

Steel is produced using two main methods: the blast furnace-basic oxygen furnace (BF-BOF) and the electric arc furnace (EAF). The difference between the methods lies in the type of raw materials they consume. For BF-BOF, iron ore, coal, and recycled steel are the materials, while EAF uses recycled steel and electricity.

Approximately 70% of steel is produced using the BF-BOF method. Iron ore is first reduced to iron and converted to steel in the basic oxygen furnace. After casting and rolling, the steel is delivered as coils, plates, or bars.

Steel made in an electric arc furnace uses electricity to melt recycled steel, while alloys are added to adjust it to the desired chemical composition. Various downstream processes, including casting, reheating, and rolling, are similar to the BF-BOF method.

General Characteristics of Steel

Steel has the following common characteristics:

• Strength: Steel is a high-strength material, specifically in tension, ideal for structural loads.
• Durability: Steel is highly durable, remaining rigid for over 100 years or more.
• Versatility: Steel is an incredibly versatile material, with its various grades having thousands of applications. 
• Machinability: Most steel grades are machinable, while free-cutting steel grades are highly machinable.
• Weldability: Some steel grades need special welding procedures, but most are readily weldable.
• Corrosion resistance: If steel is alloyed with chromium, nickel, and molybdenum, it will better resist corrosion.
• Conductivity: Steel typically has lower thermal and electrical conductivity than other metals but is often used as a robust, heat-resistant shielding material. 
• Recycling: Steel is entirely recyclable.

What are the Different Steel Types?

Steel is one of the most versatile and valuable materials, consisting primarily of iron (Fe) and carbon (C), offering over 3,500 grades containing numerous combinations of iron and carbon.  Steel’s characteristics and strength are affected by those mixtures and the inclusion of other elements, giving steel infinite applications.

Here are the four main types of steel, how they are classified, and the methods of heat treatment that improve its mechanical properties:

1. Carbon Steel

In addition to carbon and iron, carbon steels contain small amounts of other components. They are the most common of the four steel grades, accounting for 90% of total steel production. Carbon steel is classified into three subgroups depending on the amount of carbon in the metal:

• Low carbon or mild steels (up to 0.3% carbon)
• Medium carbon steels (0.3–0.6% carbon)
• High carbon steels (more than 0.6% carbon)

Steel makers produce these carbon steels in large quantities since they are relatively inexpensive and versatile enough for large-scale construction projects.

2. Alloy Steel

Alloy steels combine steel with other alloying elements such as nickel, copper, chromium, or aluminum. Combining these elements increases strength, hardness, ductility, wear resistance, corrosion resistance, machinability, and toughness. The alloying elements added to the base iron and carbon structure typically do not total more than 5% of the alloy steel’s composition.

3. Stainless Steel

Stainless steel grades are alloyed with 10–20% chromium, nickel, silicon, manganese, and carbon. Because of their increased capacity to withstand adverse weather conditions, these steels have high corrosion resistance and are safe for outdoor construction. Stainless steel grades are also commonly used in electrical devices.

For example, 304 stainless steel is widely used for its ability to tolerate the environment while keeping electrical materials safe.

Although various stainless steel grades, including 304 stainless steel, are used in building construction, it is often chosen for its sanitary properties. Examples of these applications include pipes, pressure vessels, medical devices, food processing machinery, and cutting instruments.

4. Tool Steel

As the name suggests, tool steel is excellent for drilling equipment and cutting tools. Adding tungsten, cobalt, molybdenum, and vanadium improves the material’s heat resistance and general durability. Since it maintains its shape despite excessive use, tool steel is preferred for most hand tools.

Additional Steel Classifications

In addition to the four groups, steel is often classified using different variables, including:

• Finishing method: hot rolled, cold rolled, cold finished, etc.
• Composition: carbon range, alloy, stainless, etc.
• Production method: electric arc furnace, basic oxygen furnace, etc.
• Microstructure: austenitic, ferritic, pearlitic, martensitic, etc.
• Physical strength: using ASTM standards
• De-oxidation process: killed or semi-killed
• Heat treatment: annealed, tempered, normalized, etc.
• Quality nomenclature: commercial quality, pressure vessel quality, drawing quality, etc.

Methods of Heating and Hardening Steel

As mentioned, steel is an alloy manufactured from iron and other elements. Different types of steel depend on what other parts are used alongside iron.

For quality assurance purposes, all the following have to be present for an alloy to be called steel:

• Aluminum
• Carbon
• Manganese
• Nitrogen
• Oxygen
• Phosphorus
• Silicon
• Sulfur

Although other elements may be added to change the properties of steel, those eight must be present. These elements’ ratios affect the steel's hardness, durability, flexibility, etc. Steel is engineered for its application using these elements, but the metal alloy must undergo a heat-treating process to be shaped and cut into a final product.

Engineers create the correct shape and quality steel they need using one of several approaches to heat treat steel.  Those processes include but are not limited to the following:

• Annealing: Heating and slowly cooling steel refine it, making it softer.
• Carburizing: The carburization process transforms low-carbon steel into high-carbon steel by exposing it to a dense carbon atmosphere, usually in a furnace.
• Case hardening: Carburizing and quickly cooling steel keeps the center soft while the outside layer hardens.
• Cyanide hardening: Similar to case hardening, this method uses molten cyanide salt for the hard case instead of carbon.
• Decarburization: Removing carbon from the steel alloy with heat or oxidation.
• Nitriding: Adding nitrogen to the steel surface with heat and a nitrogen-rich liquid or gas.
• Drawing or tempering: Reheating steel to a specific temperature to remove hardness.

Taking unrefined steel alloy through various heat treatment processes is the only way to make finished steel parts. Even though not every steel product must go through all the heat-treating steps, all steel needs to be treated.

Most Common Steel Grades

Although no steel grade is right for every application, the following list represents the steel grades that are commonly used and regarded as the top series from each type:

• Carbon steels: A36, A529, A572, 1020, 1045, and 4130
• Alloy steels: 4140, 4150, 4340, 9310, and 52100
• Stainless steels: 304, 316, 410, and 420
• Tool steels: D2, H13, and M2

Physical Properties of Steel

The physical properties of steel include high strength, low weight, durability, ductility, and corrosive resistance. Although steel is a lightweight material, it has exceptional strength, with steel’s strength-to-weight ratio being the lowest of any building material. And “ductility” refers to steel’s capability to be molded to form various shapes.

Steel’s dimensional stability is another of its benefits since it maintains dimensions even after many years of extreme environmental conditions. It’s also an excellent electricity conductor and cools rapidly from scorching temperatures after being subjected to water or oil. Unlike its fundamental component, iron, steel does not corrode easily after exposure to moisture and water.

Steel grades are classified on the metal's composition and physical properties. The deciding factor for the steel grade is its chemical composition. Higher carbon content produces a harder and more robust material, while high-quality steel with less carbon is more ductile. Today’s steel manufacturing focuses on less carbon and a rapid cooling process by quenching to retain the metal’s desirable physical properties.

Other steel types, including galvanized and stainless steel, are corrosion-resistant. Galvanized steel is coated with zinc to protect it from corrosion, while stainless steel contains about 10 percent chromium in its composition.

Mechanical Properties of Carbon Steel vs. Alloy Steel vs. Stainless Steel vs. Tool Steel

The following table highlights the differences in mechanical properties of the four main steel types:

	Carbon Steel	Alloy Steel	Stainless Steel	Tool Steel
Density	7.85	7.85	7.75-8.1	7.72-8.0
Elasticity (GPa)	190-210	190-210	191-210	190-210
Poisson’s Ratio	0.27-0.3	0.27-0.3	0.27-0.3	0.27-0.3
Tensile Strength	276-1882	758-1882	515-827	640-2000
Yield Strength	186-758	366-1793	207-552	380-440
Elongation %	10-32	4-34	12-40	5-25
Brinell Hardness	86-388	149-627	137-595	210-620

 

Types of Steel and Their Uses

Carbon steel

Carbon steel contains few alloying elements (other than carbon) and is classified as low-, medium-, or high-carbon, depending on the carbon content. Low-carbon steel (up to 0.3% C) makes rivets, wire, and stampings in the lower carbon ranges and structural shapes, gears, cold-forged parts, and welded tubes in the middle and upper ranges.

Medium-carbon steel (0.3-0.5% C), sometimes called machinery steel, works well for producing gears, shafts, connecting rods, and seamless tubing. High-carbon steel (over 0.5% C) is used for springs, knives, hand tools, taps, dies, and milling cutters.

Alloy steel

Alloy steel is commonly used to manufacture pipes, particularly those for energy-related applications. It also manufactures heating elements in appliances like toasters, silverware, pots and pans, and corrosion-resistant containers.

Stainless steel

Stainless steel--which typically includes chromium, nickel, or molybdenum--is the most corrosion-resistant of the four groups. Unsurprisingly, it is often used in food handling, processing, medical instruments, hardware, and appliances.

Tool steel

Because of its hardness, resistance to wear, toughness, and resistance to softening at high temperatures, tool steel is well-suited for producing tools, including reamers, drills, machine dies, and hand tools.  

In summary

Steel is durable, strong, and versatile, providing cost-effective, sustainable, and robust design solutions for home and business owners, architects, engineers, and contractors. Steel is easy to mass produce to a uniform quality, ideal for lightweight designs, and can be transported and handled relatively quickly.

Steel is sustainable and the most recycled material on Earth. As a non-combustible material, it provides long-term safety benefits to occupants and holds up well during earthquakes, hurricanes, and other extreme events. Machinable, weldable, versatile, and recyclable, it’s no wonder steel is everywhere, from bridges, buildings, and tunnels to cars, ships, and planes!