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4340 Alloy Steel Round Bar

4340 steel round bar (UNS G43400) is a nickel-chromium-molybdenum alloy steel specified for high-strength, heavy-duty parts and demanding shafting applications after appropriate heat treatment. Actual strength, toughness and hardness depend on section size, delivery condition and processing history.

Download 4340 Alloy Steel Round Bar — Mozaic technical reference (PDF)

4340 Alloy Steel Round Bar — Technical Specifications

Reference data; confirm the exact order specification and material condition
Property or designation Reference information
Steel / UNS SAE/AISI 4340 / G43400
Reference chemistry specification ASTM A29/A29M or the applicable ordered product standard
Nickel (Ni) 1.65–2.00 wt%
Chromium (Cr) 0.70–0.90 wt%
Molybdenum (Mo) 0.20–0.30 wt%
Carbon (C) 0.37–0.43 wt%
Manganese (Mn) 0.60–0.80 wt%
Condition note Mechanical properties depend strongly on heat treatment and bar cross-section

Technical values shown are general reference information, not a mill test report. Chemical limits and mechanical properties are governed by the applicable material specification, heat/lot and supply condition.

Applications and Grade Comparison

4340 is frequently chosen for loaded shafts, aircraft and industrial components where high hardenability and toughness are important. Compare 4140 alloy steel for other chromium-molybdenum uses, and 8620 steel for carburized-case applications. An alloy name alone does not define strength or hardness.

Precision Centerless Grinding and Bar Preparation

Mozaic can grind, straighten, cut and prepare 4340 bars to purchase-order requirements. Include the exact grade and heat-treatment condition, target mechanical properties if applicable, required finished diameter, bar length, tolerance and quantity. See Mozaic’s centerless grinding services.

Common Questions

How does 4340 differ from 4140?

4340 contains a significant nickel addition, typically improving hardenability and toughness in appropriately processed components. The best choice depends on geometry and required properties.

Is 4340 bar always supplied hardened?

No. Supply condition must be specified explicitly and confirmed in the quote and certifications.

Request a Quote

For order-specific sourcing and grinding availability, request a quote, email sales@mozaicprecision.com, or call 847-637-8379.

Reference source: technical grade information. This page is not a substitute for the relevant purchase specification or mill certification.

Machining and Heat Treatment of 4340 Steel

4340 steel round bar is a high-strength, nickel-chromium-molybdenum alloy widely used in aerospace, automotive, oil & gas, and heavy machinery industries. Its performance depends heavily on proper machining and heat treatment processes, which enhance strength, toughness, and wear resistance.

Machining 4340 Steel Round Bar

4340 steel is known for its good machinability, allowing it to be shaped into precision components such as shafts, gears, and fasteners. To achieve the best results, proper tooling and cutting practices are critical:

  • Cutting Tools: Carbide or high-speed steel tools are recommended for efficient machining.
  • Lubrication: Adequate lubrication and cooling fluids are essential to minimize heat buildup and extend tool life.
  • Controlled Speeds & Feeds: Using appropriate speeds prevents premature tool wear and ensures a smooth surface finish.

Heat Treatment of 4340 Steel Round Bar

Heat treatment is one of the most important processes in fabricating 4340 steel, as it enhances mechanical properties and tailors performance for specific applications. Common heat treatment methods include:

  • Annealing: Heats the steel to a set temperature, then cools it slowly to improve machinability and relieve stresses.
  • Normalizing: Refines the microstructure and increases toughness by heating and cooling in air.
  • Tempering: Reduces hardness while improving ductility and toughness after hardening or quenching.
  • Quenching & Tempering: Rapidly quenches the heated steel in water or oil, followed by tempering to achieve a strong balance of hardness, strength, and toughness.

Optimizing Performance of 4340 Steel

The choice of heat treatment depends on the end-use application—whether the part requires maximum wear resistance, impact toughness, or a balance of both. Following proper machining practices and heat treatment procedures ensures 4340 steel bars perform reliably under the most demanding conditions.

4340 Steel from Mozaic Precision Metals

At Mozaic Precision Metals, we supply high-quality 4340 alloy steel round bars manufactured to precise tolerances with superior surface finishes. Our leadership team, with over 60 years of combined precision grinding expertise, ensures consistent quality for critical applications in aerospace, automotive, construction, and beyond.

Contact us today to request a quote for 4340 steel round bar and learn how Mozaic Precision Metals can provide the strength, toughness, and precision your project demands.

Applications of 4340 Alloy Steel

4340 alloy steel is a high-strength, low-alloy steel known for its toughness, fatigue resistance, and excellent hardenability. Its ability to maintain strength under high stress makes it a preferred material across industries that demand durability and reliability. Below are some of the most common applications of 4340 steel bar stock.

Aerospace Industry

4340 alloy steel is widely used in aerospace applications requiring superior strength-to-weight performance. Typical uses include:

  • Landing gear components
  • Aircraft engine parts
  • Structural assemblies subjected to heavy loads

Automotive Applications

In the automotive sector, 4340 steel delivers durability and wear resistance for high-performance parts, including:

  • Crankshafts and camshafts
  • Transmission gears and axles
  • High-stress fasteners and connecting rods

Oil & Gas Industry

Thanks to its toughness and resistance to fatigue, 4340 alloy steel is trusted for oilfield equipment such as:

  • Drill collars
  • Drill bits
  • Downhole tools and high-pressure components

Tooling & Die Applications

4340’s combination of machinability, strength, and wear resistance makes it suitable for:

  • Forging dies
  • Tooling components
  • High-strength molds

Heavy Machinery

4340 alloy steel is essential in the construction and mining industries for equipment parts requiring resilience under constant wear. Applications include:

  • Excavator and crane components
  • Bulldozer parts
  • Industrial shafts and couplings

Military & Defense

4340 steel is trusted in defense applications due to its ability to withstand extreme stress and impact. Typical uses include:

  • Vehicle armor components
  • Weapons and firearm parts
  • Military vehicle shafts and gears

4340 Alloy Steel from Mozaic Precision Metals

At Mozaic Precision Metals, we supply precision-ground 4340 alloy steel bars engineered for superior performance across industries. With over 60 years of combined expertise in precision grinding, our leadership team ensures consistent quality and tight tolerances for every order.

Whether you need 4340 bar stock for aerospace, automotive, oil and gas, or defense applications, Mozaic Precision Metals delivers materials designed to meet the highest standards of strength and reliability.

Contact us today to request a custom quote and secure high-quality 4340 steel for your next project.