10B21 Boron-Treated Cold Heading Steel Wire

10B21 is a low-carbon manganese steel with a controlled boron addition (0.0005%–0.003%) that dramatically improves hardenability. It is widely used for SAE J429 Grade 5 fasteners and similar heat-treated parts, replacing 4037 and other alloy grades in many applications at a significantly lower cost.

Nevers and Company stocks 10B21 in coil form, sourced from major US CHQ mills with A2LA-accredited labs. All material is USA melted and USA processed, with full mill certs and chemistry verification on every coil.

Chemistry (AISI / SAE 10B21)

Element	Min	Max
Carbon (C)	0.18	0.23
Manganese (Mn)	0.80	1.10
Phosphorus (P)	—	0.040
Sulfur (S)	—	0.050
Boron (B)	0.0005	0.003
Aluminum (Al)	0.020 min (total)	—
Titanium (Ti)	as added to fix nitrogen	—

The boron must be in solid solution to develop hardenability. Aluminum and titanium are added to tie up free nitrogen and protect the boron addition.

Typical Mechanical Properties

As-drawn (cold-headed condition):

Property	Typical Value
Tensile strength	75,000 – 95,000 PSI
Yield strength	65,000 – 85,000 PSI
Reduction of area	55 – 65%

Quenched & tempered (post-heat-treat, typical for Grade 5):

Property	Typical Value
Tensile strength	120,000 – 140,000 PSI
Yield strength	92,000 – 120,000 PSI
Core hardness	HRC 25 – 34

Final properties depend on heat-treat parameters and section size.

Common Applications

• SAE J429 Grade 5 hex cap screws and bolts
• Heat-treated socket products (low-head and button-head SHCS where 4037 is overspec)
• Self-drilling and self-tapping screws (with appropriate case-hardening)
• Wheel studs, suspension bolts, and chassis hardware
• Track bolts, plow bolts, and OEM fasteners requiring through-hardness

Why 10B21 Instead of 4037?

For many Grade 5 applications, 10B21 delivers comparable post-heat-treat properties at a meaningfully lower cost per pound. 4037 (a molybdenum alloy steel) is preferred when:

• Section sizes exceed ~1” diameter (boron’s hardenability advantage diminishes in heavy sections)
• Tempering temperatures above 1000°F are required
• Specific OEM specs call for alloy steel

For high-volume Grade 5 production in smaller diameters, 10B21 is commonly chosen. We can help you evaluate whether 10B21 meets your spec or whether 4037 is required.

Available Sizes

Nevers and Company stocks 10B21 in diameters from 0.135” to 0.625” (3.43 mm – 15.88 mm). Common stocked sizes:

• 0.171” – 0.281” for small Grade 5 hardware
• 0.312” – 0.437” for standard hex cap screws
• 0.500” – 0.625” for larger structural bolts

Available Conditions

Nevers stocks 10B21 in two spheroidize-annealed conditions, both with phosphate-and-lubricant (P&L) coating applied as standard for cold heading:

• SAFS (Spheroidized Annealed at Finish Size) — our default. Annealing is the final processing step before coating, producing the softest, most ductile condition with the lowest internal stress. Choose SAFS for severe upsetting, deep extrusion, and demanding cold-heading where maximum formability matters.

• SAIP (Spheroidized Annealed In Process) — adds $2/cwt over SAFS. Annealing is followed by a light final cold draw (6–9% reduction) that sets tighter dimensional tolerances and produces a bright drawn finish. Choose SAIP for high-speed automated headers where consistent diameter and smooth feed behavior matter more than maximum formability.

See Cold Heading Wire Finishes: SAFS vs SAIP for a detailed comparison.

Quality Assurance

Boron steel quality depends on proper nitrogen control during melting. Nevers sources only from major US mills with proven boron-grade practice, and verifies aluminum and titanium levels on every mill cert. Coils with marginal chemistry are rejected at intake.

Frequently Asked Questions

What is 10B21 used for? 10B21 is a boron-treated low-carbon steel used primarily to manufacture Grade 5 hex cap screws, bolts, and other heat-treated fasteners. The boron addition provides excellent hardenability at lower cost than alloy steels.

Is 10B21 the same as 4037? No. 10B21 is a low-carbon boron steel; 4037 is a molybdenum alloy steel. Both can achieve similar Grade 5 properties after heat treatment, but 10B21 is more cost-effective for smaller sections, while 4037 performs better in heavy sections or at higher tempering temperatures.

Why does boron content have to be so precise? Free boron in solid solution provides hardenability, but boron readily combines with nitrogen, forming boron nitride that has no hardenability benefit. Mills add aluminum and titanium to fix the nitrogen and protect the boron. Too little boron and the steel won’t through-harden; too much provides no additional benefit and can cause brittleness.

Can 10B21 be welded? 10B21 can be welded but requires careful procedure controls due to the hardenability of the heat-affected zone. Welding is uncommon in fastener applications; consult a welding engineer for specific procedures.

What heat treatment does 10B21 require for Grade 5? Typical heat treatment is austenitize at 1550–1600°F, oil quench, then temper at 800–900°F to achieve HRC 25–34 core hardness and meet the J429 Grade 5 mechanical requirements.

Does Nevers verify boron and aluminum content on incoming material? Yes. Every coil arrives with a mill cert showing full chemistry, and we maintain chemistry records for traceability. Coils with chemistry outside the 10B21 specification — particularly low aluminum or marginal boron — are rejected before stocking.