AISI 8620 is a low-alloy case-hardening steel widely used for mechanical components that require a combination of a tough core, good machinability, and a hard wear-resistant surface. It contains carbon together with nickel, chromium, and molybdenum, giving the alloy better strength, toughness, and hardenability than many ordinary low-carbon steels. Because of these characteristics, AISI 8620 is commonly selected for gears, shafts, bushings, pins, sprockets, cam components, bearing-related parts, and other precision components exposed to repeated loads or surface wear. It is particularly useful when a component must remain relatively tough internally while the outer surface needs increased hardness.

The carbon content of AISI 8620 is relatively low, typically around 0.18–0.23%. This low carbon level helps maintain ductility and toughness in the core of a finished component. Nickel improves toughness, chromium increases hardenability and wear resistance, and molybdenum contributes to strength and helps reduce temper brittleness. This alloying combination makes AISI 8620 suitable for carburizing, where carbon is introduced into the surface before hardening. After carburizing and heat treatment, the surface can achieve high hardness while the core remains considerably tougher and more resistant to impact.

AISI 8620 is often supplied in annealed, normalized, or otherwise machinable conditions before final heat treatment. Its machinability is generally considered good for an alloy steel, especially when machining is performed before carburizing and hardening. CNC turning and CNC milling are both commonly used to manufacture AISI 8620 components. Turning is suitable for cylindrical parts such as shafts, pins, sleeves, gear blanks, and bushings, while milling is commonly used for flats, slots, pockets, keyways, mounting surfaces, and more complex geometries.

During CNC machining of AISI 8620, cutting conditions should be selected according to the material condition, part geometry, and required tolerance. Carbide cutting tools are commonly used for production machining because they provide good tool life and allow higher cutting speeds. Sharp cutting edges and stable workholding are important when producing close-tolerance features. Excessive cutting forces can affect thin walls, slender shafts, or other relatively flexible structures, so toolpath strategy, depth of cut, and fixture design should be considered carefully.

Coolant can improve chip removal, control cutting temperature, and reduce tool wear during machining. Consistent cooling is especially useful when machining deep pockets, internal features, or long turning operations where heat accumulation may influence dimensional stability. Proper chip control is also important because long or poorly controlled chips can damage finished surfaces or interfere with automated machining operations. For production parts, optimized feeds, speeds, insert geometry, and coolant delivery can significantly improve machining efficiency.

AISI 8620 can be machined to produce relatively precise features before heat treatment, including threads, grooves, bores, shoulders, keyways, splines, and mating surfaces. However, manufacturers must consider dimensional changes that may occur during carburizing and heat treatment. Components requiring very tight final tolerances may therefore be machined slightly oversized before heat treatment and then finished afterward by grinding, hard turning, honing, or another precision process. This approach helps compensate for distortion and allows critical dimensions to meet final drawing requirements.

Carburizing is one of the most important treatments associated with AISI 8620. During carburizing, the component is heated in a carbon-rich environment so that carbon diffuses into the surface layer. The part is subsequently quenched and tempered to produce a hard case surrounding a comparatively tough core. Depending on the application and process parameters, carburized AISI 8620 can achieve surface hardness levels around the high 50s to low 60s HRC. The required case depth should be determined by component size, loading conditions, wear requirements, and engineering specifications.

This combination of surface hardness and internal toughness explains why AISI 8620 is frequently used for gears and other transmission components. Gear teeth experience repeated contact stress and wear, making surface hardness important. At the same time, the component must resist impact and fatigue without becoming excessively brittle. A properly carburized AISI 8620 gear can provide the hard working surface needed for wear resistance while maintaining sufficient toughness beneath the surface.

Heat treatment must be considered early in the manufacturing process rather than treated as a separate final operation. Carburizing, quenching, and tempering can cause distortion, especially in thin sections, asymmetric parts, long shafts, or components with uneven wall thickness. CNC machining plans may therefore include additional material on critical surfaces for final finishing after heat treatment. Manufacturers may also use controlled heat-treatment procedures, suitable fixturing, and appropriate machining sequences to reduce deformation.

In addition to carburizing, AISI 8620 parts may receive other finishing and surface treatment processes depending on the application. Black oxide can provide a dark appearance and mild corrosion protection when used together with oil or another protective coating. Zinc plating may be selected when stronger corrosion protection is required, particularly for components operating in relatively moderate environments. Nickel plating can provide additional corrosion resistance and surface protection and may also be chosen for appearance or functional requirements.

Phosphate coatings can also be applied to some AISI 8620 components to improve corrosion resistance, assist lubricant retention, or provide a suitable surface for subsequent coatings. Painting and powder coating are possible for parts where exposed surfaces require additional environmental protection, although coating thickness must be considered around threaded holes, precision fits, and other dimensionally sensitive features. For high-precision components, surface treatment specifications should identify which surfaces can be coated and which surfaces must remain masked.

Grinding and polishing are technically finishing operations rather than chemical surface treatments, but they can be important after AISI 8620 has been hardened. Precision grinding is frequently used to restore dimensional accuracy, improve roundness, control bearing fits, or achieve low surface roughness on hardened surfaces. Gear grinding may be used on demanding transmission components where tooth profile accuracy and surface quality are critical. Hard turning can also be an option for certain geometries and production requirements.

Designers should specify both the material condition and heat-treatment requirements clearly on engineering drawings. Simply specifying “AISI 8620” may not provide enough information for manufacturing because the final properties depend strongly on carburizing depth, surface hardness, core hardness, quenching conditions, and tempering. If only certain surfaces require case hardening, this should also be clearly identified. Critical dimensions that must be maintained after heat treatment should be distinguished from dimensions that can tolerate normal process variation.

When selecting AISI 8620 for CNC-machined parts, engineers should also consider whether its case-hardening capability is actually required. If a component only needs moderate strength and does not require a hard wear-resistant surface, another steel may provide a simpler manufacturing route. However, for gears, shafts, pins, bushings, and other mechanical parts that combine contact wear with impact or cyclic loading, AISI 8620 offers an effective balance of machinability, toughness, and surface hardenability.

For custom manufacturing projects, CNC machining and heat treatment should be planned as an integrated process. Machining allowances, carburized case depth, distortion control, finishing operations, surface treatment, and inspection requirements all influence final part performance. By coordinating these factors from the beginning, manufacturers can produce AISI 8620 components with reliable dimensions, durable surfaces, and mechanical properties suited to demanding industrial applications.