4340 carries more nickel (1.65–2.00% vs 0.85–1.15%) and hardens deeper; 9840 is the cost-optimized grade common in Mexican and South American specs.

Grades · Alloy & Engineering Steel · 9840

9840

Data verified

9840 is a Ni-Cr-Mo through-hardening steel popular in Latin American specifications for shafts and high-strength parts — between 4140 and 4340 in alloy cost and hardenability.

Hardness30–42 HRC typical

AISI9840

GB40CrNiMo (near)

CategoryAlloy & Engineering Steel

Chemical composition

Element	Content (%)
C	0.38–0.43
Si	0.15–0.35
Mn	0.70–0.90
Cr	0.70–0.90
Ni	0.85–1.15
Mo	0.20–0.30

Weight percent, per the governing standard ranges. Actual values per heat are on the mill certificate.

Properties

Hardness (Q&T)	30–42 HRC typical
Tensile strength	1000–1250 MPa (Q&T)
Density	7.85 g/cm³

Heat treatment

Quench 845 °C oil; temper 500–650 °C to specification.

Equivalent grades

Standard	Designation
aisi	9840
gb	40CrNiMo (near)

Compare side by side: 9840 vs 4340

Equivalents overlap in chemistry and use but are not always identical — see our equivalents guide before substituting in certified designs.

Applications & Processing

Applications: Heavy shafts, gears, mining and agricultural machinery, high-strength bolts.

Processing: As 4340 with slightly easier practice; weld with preheat and qualified procedure.

FAQ

9840 vs 4340?: 4340 carries more nickel (1.65–2.00% vs 0.85–1.15%) and hardens deeper; 9840 is the cost-optimized grade common in Mexican and South American specs.

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Source: SAE J404 published ranges · Last updated 2026-11-03. Data is indicative — verify critical values against the governing standard and mill certificate before use.