4037 Alloy Steel Wire

4037 is a medium-carbon molybdenum alloy steel widely used for high-strength heat-treated fasteners. The molybdenum addition provides deep hardenability, high tempering resistance, and consistent mechanical properties in heavy sections — characteristics that boron steels like 10B21 cannot match in larger diameters or at elevated tempering temperatures.

Nevers and Company stocks 4037 in coil form for cold-heading applications, sourced from major US CHQ mills with A2LA-accredited labs. Every coil is USA melted and processed, with full chemistry and mechanical property verification on the mill cert.

Chemistry (AISI / SAE 4037)

Element	Min	Max
Carbon (C)	0.35	0.40
Manganese (Mn)	0.70	0.90
Phosphorus (P)	—	0.035
Sulfur (S)	—	0.040
Silicon (Si)	0.15	0.35
Molybdenum (Mo)	0.20	0.30

Note: 4037 carries molybdenum but not chromium. (4137 and 4140 are the chromium-molybdenum grades — sometimes confused with 4037.) For chromium-bearing alloy CHQ wire, contact us about 4140.

Typical Mechanical Properties

As-drawn (cold-headed condition):

Property	Typical Value
Tensile strength	95,000 – 115,000 PSI
Yield strength	80,000 – 100,000 PSI
Reduction of area	45 – 55%

Quenched & tempered (typical for Grade 8):

Property	Typical Value
Tensile strength	150,000 – 180,000 PSI
Yield strength	130,000 – 160,000 PSI
Core hardness	HRC 33 – 39

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

Common Applications

SAE J429 Grade 8 hex cap screws, bolts, and studs
High-strength socket products (SHCS, BHCS, FHCS in Class 12.9)
Aerospace and defense fasteners requiring documented alloy traceability
Heavy-equipment hardware in sections too large for 10B21 to through-harden
Suspension and powertrain components requiring fatigue resistance

When to Specify 4037 Over 10B21

Choose 4037 when:

Section diameter exceeds approximately 1.0” and through-hardness is required
Tempering temperatures above 1000°F are needed (boron steels lose strength at elevated tempering temperatures)
The application spec explicitly requires alloy steel (common in OEM specs)
Fatigue performance is critical (alloy steels generally outperform boron steels in cyclic loading)

For lighter sections and standard Grade 5 applications, 10B21 is usually the more cost-effective choice.

Available Sizes

Nevers stocks 4037 in diameters from 0.250” to 0.750” (6.35 mm – 19.05 mm). Common stocked sizes:

0.312” – 0.437” for standard Grade 8 hex caps
0.500” – 0.625” for heavy structural bolts
0.687” – 0.750” for large-diameter applications

Available Conditions

Nevers stocks 4037 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

4037 production requires tight chemistry control on molybdenum and manganese, plus consistent inclusion levels for cold heading. Nevers sources only from major US mills with proven alloy CHQ practice, and verifies chemistry against the AISI spec on every cert. We maintain full chemistry records for traceability — important for aerospace, defense, and OEM audits.

Frequently Asked Questions

What is 4037 used for? 4037 is a medium-carbon molybdenum alloy steel used primarily for heat-treated high-strength fasteners, including SAE J429 Grade 8 hex cap screws, socket head cap screws (Class 12.9), and heavy-equipment hardware.

Is 4037 the same as 4140? No. 4037 contains molybdenum but no chromium. 4140 contains both chromium and molybdenum and reaches higher strength levels. Choose 4140 for applications requiring tensile strength above 180,000 PSI or where wear resistance is critical.

What’s the difference between 4037 and 10B21? 4037 is an alloy steel with deeper hardenability, better tempering resistance, and superior performance in heavy sections. 10B21 is a boron-treated low-carbon steel that’s more cost-effective for smaller Grade 5 applications. Both can meet Grade 5 specs; only 4037 (or 4140) reliably meets Grade 8 in larger sections.

Can 4037 be cold headed? Yes, but it requires spheroidize-annealed condition and careful tooling selection. 4037 work-hardens faster than low-carbon grades, so heading operations often use multi-blow processes with intermediate annealing for complex parts.

What heat treatment is typical for Grade 8 from 4037? Austenitize at 1525–1575°F, oil quench, then temper at 800–1100°F depending on required strength. Grade 8 typically targets HRC 33–39 core hardness after tempering.

Does Nevers stock 4037 in annealed condition? Yes. We stock 4037 in spheroidize-annealed condition for cold heading. As-rolled or normalized conditions are available on mill order.