Why add boron instead of more alloy?

About 0.002% boron gives the hardenability boost of a much larger chromium or molybdenum addition, at a fraction of the cost — which is why boron steels dominate cold-headed fastener production.

Grades · Carbon Steel · 10B21

10B21

Data under review

10B21 is a boron-treated cold-heading steel: a trace of boron multiplies the hardenability of a 0.20% carbon steel, letting fasteners reach property classes 8.8–10.9 after quenching and tempering. It is supplied as wire rod and wire for high-volume bolt production.

AISI10B21

GBML10B21 family (near)

CategoryCarbon Steel

Chemical composition

Element	Content (%)
C	0.18–0.23
Mn	0.70–1.00
P	≤0.035
S	≤0.035
B	0.0005–0.003

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

Properties

Tensile strength	800–1050 MPa (Q&T fastener duty, property class 8.8–10.9 range)
Density	7.85 g/cm³

Heat treatment

Cold head in the annealed/drawn condition, then quench 850–880 °C oil and temper 400–550 °C to the fastener property class.

Equivalent grades

Standard	Designation
aisi	10B21
gb	ML10B21 family (near)

Compare side by side: 10B21 vs SAE 1020 · 10B21 vs 1022

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

Applications & Processing

Applications: High-strength bolts and screws (classes 8.8/10.9), automotive fasteners, cold-headed parts.

Processing: Boron effect depends on soluble boron — nitrogen must be tied up (Ti/Al practice at the mill); verify hardenability on the certificate for critical fasteners.

FAQ

Why add boron instead of more alloy?: About 0.002% boron gives the hardenability boost of a much larger chromium or molybdenum addition, at a fraction of the cost — which is why boron steels dominate cold-headed fastener production.

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Source: SAE J403 / ASTM A29 boron grade ranges · Last updated 2026-11-03. Data is indicative — verify critical values against the governing standard and mill certificate before use.