AISI 8620 and AISI 9310 are two case-hardening (carburising) steels that do the same job. Both are produced with a low carbon content, the surface is saturated with carbon in a carburising furnace, and the part is then hardened and tempered. The result is a hard surface layer with a tough core — exactly what is wanted for gears, pinions and shafts. The difference between them is how strong that core is.
The element that makes the difference: nickel
In AISI 8620 the nickel content is in the 0.40–0.70% range. In AISI 9310 it rises to 3.25%, roughly five times as much. The same happens with chromium: 0.40–0.60% in 8620 against 1.20% in 9310.
The return on that increase is seen directly in the core. According to the three different carburising + heat treatment programmes on our datasheet, the core of AISI 9310 reaches a tensile strength of 1069–1289 MPa and delivers it together with 15–16% elongation. In other words the part is strong without being brittle. Since surface hardness is achieved by carburising in both steels, the real distinction is not at the surface but in the load-carrying core.
Comparison in numbers
| PROPERTY | AISI 8620 | AISI 9310 |
|---|---|---|
| Carbon (C) | 0.18 – 0.23% | 0.10% |
| Nickel (Ni) | 0.40 – 0.70% | 3.25% |
| Chromium (Cr) | 0.40 – 0.60% | 1.20% |
| Molybdenum (Mo) | 0.15 – 0.25% | 0.12% |
| Manganese (Mn) | 0.70 – 0.90% | 0.50% |
| Core after carburising | — | Rm 1069 – 1289 MPa · Elongation 15 – 16% · 331 – 375 HV |
| Carburising temperature | 900 – 925 °C | — |
| UNS | G86200 | G93106 |
| W.Nr (DIN/EN) | 1.6523 | — |
| AMS | 6274 | 6260 · 6265 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
AISI 8620 is sufficient — general machine gears, gearbox pinions, sprockets, medium-duty shafts, bushings and pins. It is easy to source, it machines well, and its carburising behaviour is predictable.
AISI 9310 is required — aerospace and helicopter power transmission gears, high-speed gearboxes, pinions working under impact loading. If the specification calls out AMS 6260 or AMS 6265, 8620 is not an acceptable substitute; those two AMS numbers describe 9310 directly.
Beyond case hardening
If the whole part is to be hardened, quenched and tempered steels are used instead of case-hardening steels: AISI 4140 and AISI 4340. We covered the difference between them in this article. Where higher strength is needed, 300M and AerMet 100 come into play.
Technical datasheets
AISI 8620 — chemical composition, mechanical values, standard equivalents
AISI 9310 — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.