Steel is an alloy composed primarily of iron and a small percentage of carbon, which dramatically improves its utility over pure iron. The addition of carbon and other elements creates a material with an exceptional combination of characteristics, making it the most used engineering and construction material worldwide. Understanding how steel reacts to force, heat, and its environment is fundamental to its application, from massive bridges to specialized tools.
Mechanical Properties
Mechanical performance dictates steel’s suitability for structural applications and manufacturing, describing its behavior under applied loads. A fundamental measure is Yield Strength, which defines the maximum stress a steel can endure before it begins to deform permanently. For mild steel, this point can be as low as 200 megapascals (MPa), while high-strength alloys can resist stresses exceeding 2000 MPa before yielding.
Beyond the yield point is the Ultimate Tensile Strength (UTS), representing the maximum stress the material can withstand before it begins to neck and eventually fracture. The UTS for common structural steel typically ranges from 400 to 700 MPa, though specialized grades can reach up to 2400 MPa. Increasing the carbon content in the alloy generally elevates both the yield strength and the UTS, improving the material’s overall strength.
Hardness is a measure of steel’s resistance to localized permanent deformation, such as indentation or scratching. This property is closely linked to carbon content; higher carbon concentrations result in a harder steel, often at the expense of other traits. Harder steels are better suited for applications requiring wear resistance, like cutting tools and bearings.
The ability of steel to undergo plastic deformation without fracturing is known as Ductility, often quantified by the percentage of elongation before failure. Mild, low-carbon steels exhibit high ductility, allowing them to be stretched or bent significantly. Toughness describes the ability to absorb energy and deform plastically before fracturing, making tough steels resistant to sudden, catastrophic failure under impact. Steels that are very hard and strong often have low ductility and toughness, making them more susceptible to brittle failure.
Physical Properties
Physical characteristics define the intrinsic nature of steel. Steel is a dense material, with carbon and low-alloy steels typically ranging between 7.8 and 8.0 grams per cubic centimeter. This high density is a factor in applications where weight is either a benefit, such as ballast, or a constraint, like in aerospace design.
The Thermal Conductivity of steel is notably low compared to other common metals like copper or aluminum, with carbon steel averaging around 45 Watts per meter-Kelvin. This poor heat transfer capability is advantageous in high-temperature environments, helping steel components maintain structural integrity. However, steel also undergoes Thermal Expansion, meaning its size changes predictably with temperature fluctuations, a factor that must be accounted for in large structures like bridges.
Regarding its Electrical Properties, steel is a good conductor of electricity, though not as efficient as pure copper. Specialized alloys, known as electrical steels, are engineered to reduce electrical conductivity by incorporating elements like silicon. This increases the steel’s electrical resistance, which is desirable in transformer cores to minimize energy loss from eddy currents.
Most common steel types, including carbon and low-alloy variants, are strongly Ferromagnetic due to their high iron content. This property allows them to be magnetized and strongly attracted to magnetic fields, which is fundamental to the operation of motors, generators, and sorting processes. However, certain high-chromium stainless steel alloys, specifically austenitic types, are non-magnetic.
Chemical and Fabrication Characteristics
The interaction of steel with its surrounding environment is defined by its chemical properties, particularly its susceptibility to Corrosion. Standard carbon steel readily undergoes oxidation, commonly known as rust, when exposed to moisture and oxygen. This reaction forms iron oxide, which is structurally weak and progressive, requiring protective coatings to prevent material degradation.
To combat this, Stainless Steel alloys are created by adding a minimum of 11% chromium. The chromium reacts with oxygen to form a thin, stable, and self-repairing layer of chromium oxide on the surface, which prevents further oxidation. This chemical passivity makes stainless steel suitable for food processing, medical, and marine environments.
The ease with which steel can be joined by welding is its Weldability, a fabrication characteristic significantly affected by carbon content. Low-carbon steels are easily welded because they are less prone to cracking and embrittlement. Conversely, high-carbon steel requires special preheating and post-weld treatments to create a sound joint.
Machinability
Machinability refers to the ease of cutting and shaping steel with machine tools, which impacts manufacturing cost and speed. Harder, higher-strength steels generally have poorer machinability, causing more tool wear. Elements like sulfur and lead are sometimes intentionally added to create “free-machining” steels, which promote chip breakage and improve surface finish.
Malleability
Malleability is the ability of the steel to be flattened or rolled into thin sheets without fracturing. This property is maximized in low-carbon, ductile grades.