Mild steel, often used in construction and general manufacturing, is characterized by its high ductility and low cost. It is a low-carbon steel, with content typically ranging from \(0.05\text{%}\) up to about \(0.25\text{%}\) by weight. While highly formable and easy to weld, mild steel cannot be hardened through the simple heating and quenching process used for higher-carbon alloys. However, it is possible to significantly increase the surface hardness using specialized thermochemical treatments. This allows the component to benefit from the metal’s inherent toughness while gaining necessary exterior wear resistance.
The Critical Role of Carbon Content
The inability of mild steel to harden deeply stems from its low concentration of carbon atoms. Traditional hardening involves a rapid microstructural change that depends entirely on sufficient carbon. Heating steel above \(900^{\circ}\text{C}\) (\(1650^{\circ}\text{F}\)) transforms its structure into austenite, a solid solution capable of dissolving a large amount of carbon.
Once in the austenitic phase, carbon atoms are dissolved and distributed uniformly throughout the crystal lattice. Hardening is achieved by rapidly cooling this austenite (quenching), which prevents the carbon atoms from diffusing out. This rapid cooling locks the carbon in place, forcing the crystal structure to contort into a highly strained, hard, and brittle phase called martensite. This martensitic structure is the source of the steel’s increased hardness.
Mild steel, however, lacks the necessary carbon to form a stable, high-carbon martensitic structure throughout its entire volume. The low carbon content means that even if the steel is quenched, the resulting structure will only be marginally harder than the original material. With carbon percentages well below the \(0.4\text{%}\) to \(0.5\text{%}\) generally required for effective through-hardening, the base material resists the deep microstructural change. The core remains soft because the carbon atoms are too few to create the dense, distorted lattice required for high hardness.
Case Hardening: Achieving Surface Strength
Since the core material cannot be hardened, the practical solution for mild steel is a process called case hardening, which is a thermochemical treatment that modifies only the surface layer. Case hardening works by artificially increasing the carbon or nitrogen content of the steel’s exterior, creating a hard “case” over the soft core. The most common method of case hardening is carburization, where the steel is heated in a carbon-rich environment.
During carburization, the steel is typically heated to temperatures around \(930^{\circ}\text{C}\) (\(1700^{\circ}\text{F}\)) while being exposed to a carbon source, such as a carbon-rich gas, liquid, or solid compound. At this elevated temperature, carbon atoms diffuse into the surface of the steel, effectively transforming the low-carbon skin into a high-carbon alloy. The depth of this high-carbon layer, known as the case depth, is precisely controlled by varying the temperature and the duration of the treatment.
After the carbon has sufficiently diffused into the surface, the component is quenched rapidly to transform the newly created high-carbon case into hard martensite. This results in a dual-property material: a surface layer with a carbon content high enough to harden effectively, bonded to a low-carbon core that remains soft and ductile. Other related surface treatments, such as nitriding, achieve a similar effect by diffusing nitrogen instead of carbon into the surface layer, often at lower temperatures.
Characteristics and Applications of Hardened Mild Steel
The defining characteristic of case-hardened mild steel is the combination of a hard exterior and a tough interior. The outer case, which has been successfully converted into martensite, exhibits high resistance to wear, abrasion, and surface indentation. This hard layer can achieve a carbon content of up to approximately \(0.8\text{%}\), giving it surface durability.
In contrast, the inner core of the component retains the original properties of mild steel, meaning it remains soft, tough, and highly ductile. This duality is beneficial because while the hard surface resists wear, the soft core acts as a shock absorber. It prevents the hard, brittle case from fracturing or cracking under heavy impact or sudden shock loads. The material effectively resists surface damage while maintaining resistance to catastrophic failure.
This specific combination of properties makes case-hardened mild steel suitable for components that experience constant friction and stress. Common applications include various automotive and machinery parts.
Common Applications
- Gears, where the tooth surfaces need to be wear-resistant but the underlying body must withstand high bending forces.
- Shafts, which require a durable surface to maintain dimensional accuracy.
- Pins, where a durable surface is required to extend service life.
- Bearings, where the core must absorb operational stress while the surface resists wear.