SAE 8620

What is SAE 8620?

SAE 8620 is a low-carbon nickel-chromium-molybdenum case-hardening steel containing 0.18 to 0.23 percent carbon. Carburising and hardening produce a wear-resistant surface over a tough core. It is the default carburising grade in the American system. The alloy contains roughly half a percent each of nickel and chromium and one-fifth of a percent molybdenum. This composition provides core strength and hardenability without making the case brittle or substantially increasing the cost of the bar.

SAE 8620 is designated 20NiCrMo2-2 with material number 1.6523 in the European system. Other designations include EN 362 in the older British series, SNCM220 in the Japanese system and G86200 in the UNS index. The first two digits of the SAE designation identify the nickel-chromium-molybdenum series. The final two digits indicate the mean carbon content multiplied by one hundred.

SAE 8620 bar is supplied in a soft condition. Areas that are not carburised remain soft. Component drawings therefore specify the case depth, surface hardness and core hardness together with the grade designation.

Laxcon Steels lists SAE 8620 in its grades reference as an alloy steel grade. The same steel is written 8620, 20NiCrMo2-2, EN 362, 1.6523 and G86200.

Chemical ranges per SAE J404 as printed in ASTM A322 Table 1. Phosphorus and sulphur are maximums.

What is the chemical composition of SAE 8620?

Composition limits in weight percent are carbon 0.18 to 0.23, manganese 0.7 to 0.9, sulphur 0.04 maximum, phosphorus 0.035 maximum, silicon 0.15 to 0.35, chromium 0.4 to 0.6, nickel 0.4 to 0.7 and molybdenum 0.15 to 0.25.

What is SAE 8620 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

Mechanical properties

SAE 8620 is generally supplied in the as-rolled condition at approximately 280 Brinell maximum, or roughly 30 HRC. This condition permits turning, milling, hobbing and broaching. The hardness value applies to the supplied bar rather than the finished component.

Carburising and hardening produce a surface hardness of about 60 HRC. Published working ranges extend from roughly 58 to 63 HRC, depending on the carburising cycle and the resulting surface carbon content. The core retains its initial low carbon content. It therefore develops strength and toughness rather than high hardness. Its strength varies with section size and quenching conditions. The principal specified properties are effective case depth, surface hardness and core hardness. Components made from the same bar can have different properties when these values differ.

Case-hardening process

Carburising takes place at 900 to 925 degrees Celsius. The component remains at a controlled carbon potential until carbon has diffused to the required depth. Case depth has a diffusion-based relationship with time. Doubling the case depth requires roughly four times as long at temperature. Deep cases are therefore more expensive, and 8620 is normally assigned the shallowest case that can carry the contact stress.

Two hardening routes are available after carburising. Direct quenching from the carburising temperature is faster and cheaper, but it produces the greatest distortion and the coarsest structure. Alternatively, the component can be cooled and reheated to a lower hardening temperature before quenching. This additional cycle refines the grain and improves toughness and dimensional control. Low-temperature tempering then relieves quenching stress while retaining case hardness.

Distortion is a principal processing constraint. A component heated to 900 degrees Celsius, held for several hours and quenched will undergo dimensional change. The grinding allowance on running surfaces must therefore be considered in relation to the effective case depth. Grinding beyond the case removes the load-carrying layer.

Composition and hardenability of SAE 8620 and SAE 8620 H

SAE 8620 H is the hardenability-controlled form of SAE 8620.

Weight percent limits for the plain grade and its hardenability-controlled version, from the Laxcon Steels grade reference. Sulphur and phosphorus figures are ceilings.

The H grade has wider alloying ranges and the same maximum sulphur and phosphorus contents. Acceptance is based on a hardenability band rather than analysis alone. The Jominy end-quench curve records hardness at fixed distances from the water-quenched end of a bar. The analysis may vary within the permitted ranges if the steel reaches the guaranteed hardening depth. This guarantee indicates whether the core beneath a gear tooth can support the case.

Machining and welding

SAE 8620 has good machinability. Cutting normally occurs in the as-supplied condition before carburising. Gear teeth, splines and internal features can therefore be produced while the material is soft and hardened afterwards. Copper plating or a stop-off compound protects threads, tapped holes and other surfaces that must remain soft. These areas may instead be machined after treatment to remove the carburised layer.

SAE 8620 has the best welding behaviour among the grades in this group. Its International Institute of Welding carbon equivalent is about 0.52 when calculated from the midpoints of the SAE composition ranges. This value is slightly above 0.45. Welding still requires preheating and low-hydrogen consumables. Welding normally precedes carburising so that the complete assembly receives the same case-hardening cycle.

Applications

SAE 8620 is used for carburised gears, including automotive ring and pinion sets and industrial reduction gearing. Other applications include pinions, gear shafts, camshafts, splined shafts, sprockets, chain parts, hydraulic and pneumatic valve components, guide pins, bushings and rollers. It is also used for tooling that requires a hard surface over a tough body, including press tooling and mould bases with local carburising.

Where SAE 8620 is not the right choice

SAE 8620 is unsuitable for components that will not be carburised. Its carbon content of approximately 0.2 percent does not permit useful through hardening. Untreated bar remains a mild alloy steel. SAE 4140 or SAE 4340 is more appropriate when a section requires uniform strength throughout.

SAE 8620 is also unsuitable when a drawing omits the case specification. The grade designation alone does not define effective case depth, surface hardness or core hardness. Different values can produce components with substantially different service lives.

Heavy sections and components with severe core loading can exceed the hardenability of SAE 8620. Its modest alloy content limits the depth of core hardening. Large gears and heavily shock-loaded components may instead require three percent nickel grades such as EN 36C. A carburised case provides wear resistance but not corrosion resistance. SAE 8620 has no corrosion resistance, and its hardened surface rusts as readily as its soft surface.

Other alloy steel grades