Metallurgy classifies metals into two broad categories: ferrous and nonferrous. This distinction is fundamental for manufacturing, material selection, and recycling. These categories are defined by a single element, which determines a material’s inherent characteristics, including strength, weight, and magnetic properties. Understanding this distinction clarifies the nature of common alloys like steel.
Why Steel is Classified as Ferrous
Steel is classified as a ferrous metal because its primary and most abundant component is iron (Fe). As an alloy, steel is iron combined with a small amount of carbon, which dramatically increases the material’s hardness and tensile strength. Standard carbon steel typically contains iron concentrations of 98% or more, making it an iron-based material.
The defining characteristic of ferrous metals is their high iron content, which steel inherits directly. This presence of iron atoms gives most steel the property of ferromagnetism, meaning it is strongly attracted to a magnet. The magnetic behavior is a direct chemical consequence of iron being the base element. Even when other elements are introduced, the material remains ferrous because iron is the foundation of its chemical structure.
Understanding Ferrous and Nonferrous Metal Categories
The classification of metals is based on the presence or absence of iron in their chemical makeup. Ferrous metals are defined as those that contain iron, often exceeding 50% of the total weight. This iron content is responsible for common properties, including high tensile strength, heavy density, and susceptibility to rust when exposed to moisture and oxygen.
Nonferrous metals do not contain iron as their primary constituent. Common examples include aluminum, copper, zinc, and brass. Because they lack iron, these materials offer superior resistance to corrosion, are often lighter, and generally do not exhibit magnetic properties. The difference in magnetism provides the simplest real-world test: ferrous metals attract a magnet while nonferrous metals do not.
How Different Steel Compositions are Classified
All steel is technically ferrous because it is an iron-based alloy, but certain compositions challenge the simple magnetic test. The most common example is stainless steel, created by adding a minimum of 10.5% chromium to the iron-carbon base. This chromium forms a protective, passive layer on the surface that prevents the steel from rusting, a property ferrous metals typically lack.
Some stainless steel grades, such as austenitic types like 304 and 316, contain nickel and have a crystal structure that makes them non-magnetic in their annealed state. Despite lacking magnetic attraction, they are still classified as ferrous because iron remains the principal element. Other stainless steel types, specifically ferritic and martensitic grades, maintain a crystal structure that makes them strongly magnetic. This demonstrates that magnetism is a characteristic, but not the ultimate determinant, of the ferrous classification.