Case hardening is a thermochemical process used to treat metal components, primarily low-carbon steel, to enhance their surface properties. This technique involves infusing elements like carbon or nitrogen into the metal’s outer layer at high temperatures to create a hard, wear-resistant skin, known as the case. The purpose is to achieve a component with a durable surface while retaining a softer, more ductile inner core. This balance is desirable for parts that must withstand friction and abrasion without risking catastrophic failure under impact.
The Fundamental Goal of Case Hardening
Case hardening is performed to engineer a dual-property metal component that combines the features of two different material states. The objective is to create a surface with high hardness, typically reaching up to 64 on the Rockwell C scale, which provides excellent resistance to wear, erosion, and indentation. This hard outer layer extends the component’s service life, especially in applications involving metal-to-metal contact and sliding friction.
Beneath this hardened surface lies the core, which remains in its original, tough state. This tough core is characterized by high ductility and superior impact resistance. The inner material can absorb sudden shock loads and bending stress without becoming brittle, which is a risk with materials that are hardened throughout.
The resulting structure resists fatigue failure because the hard case develops beneficial compressive residual stresses on the surface. These internal stresses counteract the tensile stresses that typically lead to the initiation of surface cracks under cyclic loading. This engineered contrast makes the component suitable for demanding mechanical environments where both surface longevity and structural integrity are required.
Core Methods of Case Hardening
The desired dual-property structure is achieved through various thermochemical diffusion processes, with the most common being carburizing and nitriding. Carburizing involves the introduction and diffusion of carbon atoms into the surface of a low-carbon steel part. The workpiece is heated to a high temperature, typically between 850°C and 950°C, in a carbon-rich environment (gas, liquid salt, or solid pack).
During this high-temperature exposure, carbon atoms diffuse into the steel’s surface, increasing the carbon content in the outer layer. After sufficient case depth is achieved, the part is rapidly quenched, transforming the carbon-enriched surface into a hard microstructure called martensite. This process creates a relatively deep hardened layer, often up to several millimeters, which is ideal for parts requiring significant strength.
Nitriding introduces nitrogen atoms into the steel surface, often performed on steels containing elements like chromium, aluminum, or molybdenum. This process occurs at much lower temperatures, usually between 500°C and 600°C, by exposing the component to a nitrogen-rich atmosphere, such as ammonia gas or plasma. The nitrogen reacts with the alloying elements in the steel to form extremely hard, stable nitride compounds.
A primary advantage of nitriding is that it does not require a final quench to achieve hardness, minimizing the risk of part distortion and internal stress. While nitriding generally produces a shallower case depth than carburizing, its resulting surface hardness is often higher. This makes it effective for precision components with tight dimensional tolerances.
Common Applications and Material Requirements
Case-hardened components are indispensable in mechanical systems where parts are subjected to high contact pressures and continuous relative motion. Common applications include transmission gears, which require hard tooth surfaces to resist wear while the gear body maintains toughness to absorb torque and impact loads. Other essential parts include camshafts, crankshafts, pins, and various bearings, all of which benefit from a wear-resistant surface.
For example, in a lock shackle, case hardening ensures the outer material is hard enough to resist filing, while the soft core prevents the shackle from fracturing under a sharp blow. The successful application of this process is linked to the material selection, as case hardening is typically applied to low-carbon steels. These steels generally have a carbon content below 0.3% and cannot be fully hardened throughout.
The low carbon content of the core material allows it to remain tough and ductile after the surface has been chemically altered and heat-treated. Using low-carbon or low-alloy steel also offers a cost advantage over high-carbon tool steels. The specific grade of steel and the hardening method chosen are matched to the component’s function to optimize both performance and manufacturing feasibility.