8620 steel is a low-carbon alloy steel known for its exceptional ability to be case hardened, making it ideal for components requiring a tough core and a hard, wear-resistant surface. The four-digit designation, standardized by organizations like the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI), classifies the steel based on its primary alloying elements. This system allows manufacturers to quickly identify the material’s general nature. Due to its low initial carbon content and specific alloy additions, 8620 steel is the most widely used material for the carburizing process.
Defining the Chemical Blueprint
The composition of 8620 steel is designed to achieve specific performance after heat treatment. The first two digits, “86,” indicate that the steel belongs to the nickel-chromium-molybdenum alloy family. It contains a balanced mix of these elements, typically ranging from 0.40% to 0.70% Nickel, 0.40% to 0.60% Chromium, and 0.15% to 0.25% Molybdenum.
These alloying elements improve the steel’s hardenability, which is the depth to which the material can be hardened during quenching. Nickel enhances core toughness, while chromium contributes to wear resistance and core strength. Molybdenum aids in deep hardening and reduces the material’s susceptibility to becoming brittle during tempering. The final two digits, “20,” indicate the approximate average carbon content (0.20%). This low carbon level is fundamental, allowing the core to remain soft and ductile for subsequent processing.
Essential Mechanical Characteristics
Before specialized heat treatment, 8620 steel is typically in an annealed or normalized state, making it highly workable. In this softer condition, the steel exhibits good machinability, rated around 65% of the free-machining steel B1112. This ease of forming allows manufacturers to shape intricate parts economically before final hardening.
In its as-rolled or annealed state, the steel has moderate tensile strength (530 to 630 megapascals, or MPa) and a yield strength of approximately 385 MPa. This condition ensures high ductility and toughness, allowing the material to absorb significant energy and deform plastically before fracturing. Furthermore, 8620 steel shows good weldability using conventional methods, though preheating is often beneficial to prevent cracking.
The Role of Case Hardening
The true value of 8620 steel is unlocked through case hardening, or carburizing. This process involves placing the low-carbon steel part into a high-temperature, carbon-rich atmosphere (usually 900°C to 925°C). Carbon atoms diffuse into the surface layer, increasing the carbon content in the outer “case” while the inner “core” retains its original low carbon level.
Following carbon enrichment, the part is rapidly quenched, often in oil, causing the high-carbon case to transform into a very hard martensitic microstructure. The resulting surface hardness can reach 60 to 62 on the Rockwell C scale (HRC), providing exceptional resistance to wear and abrasion. Simultaneously, the low-carbon core remains tough and soft (32 to 36 HRC). This dual-property structure—a hard outer skin and a shock-absorbing inner core—is the defining characteristic of 8620 steel. A subsequent low-temperature tempering step relieves internal stresses and enhances case toughness, reducing the potential for grinding cracks.
Typical Industrial Uses
The combination of a hard, wear-resistant surface and a tough, shock-absorbing core makes 8620 steel highly desirable for specific mechanical components. Its primary use is in applications subjected to high surface friction and impact loading.
The automotive industry relies heavily on 8620 steel for manufacturing transmission gears, differential pinions, and camshafts. These components require a hardened surface to withstand constant meshing and friction, coupled with a tough core that absorbs torsional forces and shocks without failing. Beyond the automotive sector, 8620 is used to produce arbors, bushings, heavy-duty pins, and splined shafts in general machinery and agricultural equipment. The material’s ability to achieve high fatigue resistance after case hardening also makes it a suitable choice for various fasteners and aircraft components.