ASTM A29 AISI 4340 Alloy Steel Bar

Hot forged, hot rolled and cold drawn round bar supplied in QT (quenched & tempered) condition, 20 mm to 600 mm diameter, with heat treatment and rough machining done in our own workshop.

Equivalent to: AISI / SAE 4340 · DIN 36CrNiMo4 (1.6511) · JIS SNCM439 · BS 817M40 · GB 40CrNiMoA

4340 Steel Stock in Warehouse

Specification at a Glance

• Supply form — Round bar (flats and blocks on request)

• Process — Hot forged / hot rolled / cold drawn

• Condition — QT (quenched & tempered), typically 28–34 HRC, or annealed

• Diameter range — 20–600 mm

• Standard — ASTM A29

• Certificate — EN 10204 3.1 standard, 3.2 on request

• Lead time — 15–30 days depending on size and heat treatment

Equivalent Grades

Chemical Composition

4340 is a nickel-chromium-molybdenum alloy steel specified in ASTM A29 and SAE J404. The nickel addition is what separates it from 4140: it lifts toughness, especially through thick sections and at low temperature, which is why 4340 is the default choice for highly stressed shafts and landing-gear type components.

Mechanical Properties (Quenched & Tempered)

Values below are typical for the QT condition. Where your drawing calls for a specific strength level, we quench and temper to order and verify with test pieces taken from the actual heat.

Typical Applications

• Aerospace and power transmission — landing gear parts, crankshafts, connecting rods

• Heavy machinery — high-tensile shafts, heavy-duty gears and pinions

• Oil and gas — drill collars, tool joints, high-strength fasteners

• General engineering — spindles, rams, high-load bolts and studs

Heat Treatment Practice

4340 is normally quenched in oil from 830–860 °C and tempered between 400–600 °C depending on the strength/toughness balance you need. Tempering in the 200–450 °C band is avoided for highly stressed parts because of temper embrittlement. If your application needs a different combination, tell us the working section and required properties and we will propose the cycle.

What We Do In-House

Most suppliers in this business are traders who subcontract everything. We run our own processes, which means shorter cycles and one point of accountability when something needs fixing.

• Heat treatment shop — quenching and tempering to your target hardness, with furnace charts recorded per batch

• Machining shop — rough turning, peeling and cutting to length, so your bar arrives ready for finish machining

• Testing — ultrasonic per ASTM A388 / EN 10228-3, hardness mapping across the section, tensile and impact testing

• Certification — EN 10204 3.1 as standard, 3.2 available with third-party witness (ABS, BV, CCS, GL)

• Export packing — bundled, end-capped and rust-protected for sea freight

Frequently Asked Questions

Q1: What is the difference between 4340 and 4140?

A1: 4340 adds about 1.65–2.00 % nickel to a similar Cr-Mo base. That gives it higher toughness and better hardenability through large sections. For most general shaft work 4140 is more economical; for high-stress, impact-loaded or large-section parts, 4340 is the safer choice. See our AISI 4140 alloy steel round bar page for a full comparison.

Q2: Can you supply 4340 already quenched and tempered?

A2: Yes. We heat treat in our own shop and verify hardness on multiple points across the bar section. Tell us the required hardness range together with the working diameter, because achievable core hardness depends on section size.

Q3: What certificates come with the shipment?

A3: EN 10204 3.1 mill certificate is included as standard, showing full heat analysis and mechanical test results. If you need 3.2 certification we arrange third-party inspection with ABS, BV, CCS or GL.

Q4: What is your minimum order quantity?

A4: For standard sizes held in stock we accept trial quantities. For forged-to-order sizes the MOQ depends on the melting batch — contact us with your specification and we will confirm.

Request a Quote

Send us your diameter, length, quantity, required hardness and destination port. You will get a quotation with the full technical proposal within 24 hours.