Disclaimer: This is the policy I follow for material selection. Your situation may call for a different decision. You should not adopt this policy without doing your own analysis of your specific situation.
Through hardened parts:
AISI 1045 — only for parts less than 240 HB
AISI 4140 — critical sections below 4 inches and/or teeth smaller than 3 NDP
AISI 4340 — required for critical sections over 6 inches and/or teeth bigger than 3 NDP
Avoid AISI 4150 and AISI 4350 due to concern over quench cracks.
AISI 8620 — only for critical sections under 3 inches and teeth smaller than 4 NDP
AISI 4320 — critical sections over 3 inches and/or teeth larger than 4 NDP
AISI 4820 — critical sections over 5 inches and/or teeth larger than 2.5 NDP
AISI 9310 — use whenever core hardness over 32 HRC is required
Avoid AISI 4620 in favor of AISI 8620
Never use AISI 1018 to 1020 for parts with teeth bigger than 32 NDP.
For flame or induction hardened parts, follow the through hardened policy.
For nitride parts, AISI 4140 and AISI 4340 are pre-hardened to 285-325 HB if specialty nitralloy grades are not used.
Warning about pre-hardened materials: Strain hardened “stress-proof” and “fatigue-proof” are great materials for shafts, but should be avoided for toothed parts. Their hardness is achieved by “work hardening” that is undone by other thermal processing. Do not use them if you intend to induction harden seal diameters.
Warning about “free machining” grades: the addition of lead, sulphur, calcium or aluminum to aid in chip break-up reduces tooth bending strength by 30% or more. Do not use these materials for gears without a thorough analysis and load testing.