Aisi 8620Edit
AISI 8620 is a nickel-chromium-molybdenum alloy steel that is widely used in applications where a tough core and a hard, wear-resistant surface are both required. It is typically supplied in a machinable condition and is most renowned for its suitability to surface hardening processes, particularly carburizing, which yields a hard outer case while preserving a ductile interior. This combination makes 8620 a staple material in automotive, power transmission, and heavy-machinery components where reliability and load-bearing capability matter.
In the broad category of alloy steels, 8620 sits among steels designed to balance formability, strength, and hardenability. The alloy’s composition, which includes modest carbon content along with nickel, chromium, and molybdenum, gives it better hardenability and toughness than plain carbon steels, while remaining more forgiving to machine than higher-alloy alloys. The standard composition ranges are roughly carbon 0.18–0.23%, manganese about 0.60–0.90%, silicon 0.15–0.35%, chromium 0.40–0.60%, nickel 0.60–0.90%, and molybdenum 0.15–0.30%. This combination allows the material to respond well to carburizing heat treatments, where carbon is diffused into the surface to produce a high-hardness layer.
Composition and properties - Core ideas: AISI 8620 belongs to the family of Alloy steels designed for improved toughness and surface hardness. It is often discussed in the same breath as other carburizable steels used for gears and shafts. - Carburized case and core: After a carburizing cycle and oil or neutral-quench tempering, the surface hardness typically reaches around the upper 50s to low 60s on the Rockwell C scale, while the core remains substantially softer and tougher. This distinct gradient supports wear resistance at the surface without sacrificing the ability to absorb shock in the interior. - Machinability and formability: In its annealed or normalized condition, 8620 machines readily compared with other low-to-medium carbon steels, making it convenient to produce complex shapes before heat treatment.
Production, heat treatment, and processing - Carburizing and hardening: The primary value of 8620 lies in its response to carburizing. Carbon is introduced into the surface, producing a hard outer case that resists wear, while the core retains toughness for impact loading and bending stresses. The carburized layer is typically followed by a quench and temper step to stabilize hardness and reduce residual stresses. - Heat treatment options: Beyond carburizing, 8620 can be heat treated by normalization, quenching and tempering, or annealing, depending on the required balance of strength, hardness, and machinability. Normalizing or annealing improves machinability and reduces internal stresses, whereas quench-and-temper routes tune hardness and strength for specific service conditions. - Welding considerations: Welding 8620 can be challenging due to its alloying elements and carbon content. Preheating and post-weld heat treatment are often recommended to minimize cracking and distortion, and many designs avoid sewing carburized or hardened components together without careful metallurgical planning. - Machining and fabrication: The material’s machinability is generally favorable in the pre-heat-treated state, which is why many components are produced as blanks or forgings and then heat-treated to final properties. Post-heat-treatment machining is often more difficult due to increased hardness in the surface layer.
Applications - Automotive and power transmission: The characteristic combination of a hard surface and tough core makes 8620 a popular choice for gears, shafts, and other load-bearing components in transmissions and drive trains. It is common in intermediate-strength gears and pinions where wear resistance on the surface matters but ductility cannot be sacrificed. - Industrial and heavy machinery: Bearings, coupling gears, and certain crankshaft or connecting-rod elements are among the uses where the material’s durability under cyclic loading is valuable. The carburized-case property is especially useful in wear-prone surfaces. - Aerospace and general engineering: In some cases, 8620 or its heat-treated derivatives find roles in aerospace-related components where a good balance of toughness and surface hardness is beneficial, though modern manufacturers may choose alternative alloys based on specific performance or certification requirements.
Standards and specifications - Classification: AISI/SAE 8620 identifies the alloy and its intended carburizing-leaning applications. The same material is discussed in various regional standards under the nickel-chromium-molybdenum alloy steel family. - References and equivalences: In practice, engineers consult material specifications and supplier datasheets that summarize composition ranges, heat-treatment recipes, and mechanical properties to ensure compatibility with designed service conditions. When designing or sourcing, engineers may also compare with other carburizing steels in the same family to optimize cost and performance.
Other considerations - Cost and availability: 8620 is widely available and generally cost-effective for carburized components given its robust performance and well-understood heat-treatment behavior. - Alternatives: Depending on geometry, performance requirements, and production volume, other carburizable steels or differently treated alloys may be chosen. For example, steels in the same general class may be selected for higher surface hardness, better corrosion resistance, or greater weldability, depending on the application.
See also - Alloy steel - Carburizing - Heat treatment - Quenching (metallurgy) - Tempering (metallurgy) - Gear - Shaft (mechanical significance) - Welding - Machining
Given its characteristic blend of toughness and surface hardness after carburizing, AISI 8620 remains a pragmatic choice for engineered parts where reliable performance under repetitive loading is required.