AISI 4340 — Premium Through-Hardening Low Alloy Steel

Product Description

SHUNFU METAL offers premium AISI 4340 through-hardening low alloy steel, a versatile 1.8% Nickel-Chromium-Molybdenum steel renowned for its exceptional combination of high strength, outstanding toughness, and excellent fatigue resistance. This high-performance material delivers superior mechanical properties in relatively large sections through optimized quench and temper heat treatment processes. With its balanced alloy composition providing excellent hardenability, AISI 4340 maintains consistent properties throughout the cross-section, making it the preferred choice for critical components subjected to high permanent and fluctuating stresses.

As an ISO 9001 certified manufacturer with advanced heat treatment facilities, SHUNFU METAL ensures consistent quality through rigorous process controls. Each batch of our AISI 4340 steel undergoes comprehensive testing by CNSA (China National Standards Association) to verify compliance with international standards, including ASTM A434/A29 and AS1444 specifications. Available in both black (hot rolled/forged) and bright (cold drawn/peeled) conditions with various hardness levels, our AISI 4340 steel delivers the perfect combination of properties for demanding applications in aerospace, automotive, oil and gas, defense, and heavy machinery industries, where reliability and performance are paramount.

Key Advantages

• Exceptional Hardenability: Superior through-section hardening capability ensures consistent properties in large sections
• Outstanding Toughness: Excellent impact resistance even at high strength levels with Charpy values up to 50J
• Superior Fatigue Resistance: Exceptional performance under cyclic loading conditions
• High Strength-to-Weight Ratio: Achieves tensile strengths up to 1150 MPa while maintaining good ductility
• Excellent Dimensional Stability: Minimal distortion during heat treatment due to balanced alloy composition
• Good Machinability: Can be machined in both the annealed and pre-hardened conditions
• Surface Hardening Capability: Suitable for nitriding and induction hardening for enhanced surface properties
• Versatile Heat Treatment Options: Can be tempered across a wide range of temperatures to achieve desired property combinations
• Extended Low-Temperature Toughness: Maintains good impact values at reduced temperatures
• CNSA Verification: Each batch independently tested by China National Standards Association

Chemical Composition

SHUNFU METAL’s AISI 4340 steel features a carefully controlled chemistry to achieve optimal mechanical properties:

Element	Minimum %	Maximum %
Carbon (C)	0.37	0.44
Silicon (Si)	0.10	0.35
Manganese (Mn)	0.55	0.90
Phosphorus (P)	–	0.040
Sulfur (S)	–	0.040
Chromium (Cr)	0.65	0.95
Molybdenum (Mo)	0.20	0.35
Nickel (Ni)	1.55	2.00

Role of Key Alloying Elements

• Nickel (1.55-2.00%): Primary alloy element that significantly enhances hardenability, toughness, and impact resistance, especially at low temperatures
• Chromium (0.65-0.95%): Improves hardenability, wear resistance, and high-temperature strength
• Molybdenum (0.20-0.35%): Enhances hardenability, prevents temper embrittlement, and improves high-temperature strength and creep resistance
• Carbon (0.37-0.44%): Carefully balanced to provide optimal combination of strength and toughness
• Manganese (0.55-0.90%): Contributes to hardenability and overall strength
• Silicon (0.10-0.35%): Acts as a deoxidizer and improves strength

Mechanical Properties

SHUNFU METAL’s AISI 4340 steel is supplied in the hardened and tempered condition with mechanical properties that vary according to section size:

Diameter (mm)	Tensile Strength (MPa)	Approximate Mechanical Condition	0.2% Proof Stress (MPa)	Elongation (A%)	Impact Value Charpy (J)
≤ 60	1000 – 1150	V	≥ 835	≥ 12	≥ 42
> 60 ≤ 100	930 – 1080	U	≥ 740	≥ 12	≥ 42
> 100 ≤ 178	930 – 1080	U	≥ 720	≥ 14	≥ 50
> 178 ≤ 240	900 – 1000	T	≥ 690	≥ 14	≥ 35

Surface Finish and Machining Allowance

SHUNFU METAL offers AISI 4340 in both black (hot rolled/forged) and bright (cold drawn/peeled) conditions with the following specifications:

Surface Finish

• Black Condition: Supplied with +/- tolerance according to DIN 1013 or better
• Bright Condition:
  • Up to 25mm diameter: Cold drawn with h10 tolerance
  • 25 to 75mm diameter: Cold drawn to h10 or peeled to k12
  • 75 to 210mm diameter: Peeled to k12
  • Over 210mm diameter: Peeled to -0/+2mm tolerance

Machining Allowances

Recommended machining allowances for AISI 4340 round bar (mm on diameter):

Bar Diameter (mm)	Black (hot rolled or forged)		Bright (drawn or peeled bar)
	Part Length <120mm	Part Length >120mm	Part Length <120mm	Part Length >120mm
0-50	1.5mm	1.5 + 6mm/m	1.0mm	1.0 + 4mm/m
50-100	2.3mm	2.3 + 6mm/m	1.0mm	1.0 + 4mm/m
100-150	4.5mm	4.5 + 6mm/m	1.0mm	1.0 + 4mm/m
150-210	6.5mm	6.5 + 6mm/m	1.5mm	1.5 + 4mm/m
210-450	–	–	1.5mm	1.5 + 6mm/m

Important Note: Hot-rolling surface defects are retained in cold drawing. For bright bar in the range of cold drawing (up to 50mm), it is essential to take machining allowance into account. Peeled bar is generally free of surface defects. A certain allowance for surface defects is recommended however, as minor defects are permitted by various national standards.

Heat Treatment Guidelines

SHUNFU METAL provides the following heat treatment guidelines for optimal processing of AISI 4340 steel:

Process	Temperature	Cooling Method
Annealing	850°C	Slow furnace cool
Normalizing	850 – 920°C	Air cool
Hardening	850 – 860°C	Oil quench
Tempering	450 – 650°C	Air cool

Detailed Heat Treatment Procedures

Hardening

Heat components slowly to 850°C, hold until uniform, then quench vigorously in oil (or polymer with an equivalent quench rate) to between 80 and 110°C.

Tempering

While still warm, re-heat to 450 to 650°C, hold one hour per 25mm of section (2 hours minimum) and cool in air. Select tempering temperatures according to the required mechanical properties – refer to tempering curve.

Stress Relieving

For pre-hardened steel, stress relieving is achieved by heating to between 500 to 550°C. Re-treated bars or forgings should be heated to 25°C below tempering temperature. Annealed components should be heated to 600 to 650°C. Hold in this temperature range for 1 to 2 hours, furnace cool to 450°C, then air cool.

Surface Hardening Options

AISI 4340 can be surface hardened by various methods to enhance surface properties:

Nitriding

Nitride at approximately 510°C for 10 to 60 hours, depending upon required case depth. Surface hardness achievable is 600 to 650HV.

Induction or Flame Hardening

AISI 4340 can be surface hardened to 58HRC (water quench) or surface hardened to between 53 to 55HRC (oil quench). Tempering at 200°C is recommended after surface hardening.

Welding Considerations

Welding of AISI 4340 is not generally recommended because of the danger of quench cracking. If welding must be carried out, the following precautions should be taken:

• Pre-heat to 200 to 300°C and maintain this temperature while welding
• Use low hydrogen electrodes
• Immediately after welding, stress relieve at 550 to 650°C
• Avoid rapid cooling after welding

Applications

AISI 4340 from SHUNFU METAL is specifically designed for components exposed to high permanent and fluctuating stresses where excellent fatigue and toughness properties are required:

Aerospace Industry

• Landing gear components
• Aircraft structural parts
• Propeller and rotor hubs
• Shaft and torque tube assemblies
• Fastening devices for high-stress applications

Automotive and Transportation

• Heavy-duty gears and planetary gear systems
• Axles for heavy vehicles
• Pinions for power transmission
• Crankshafts and connecting rods for high-performance engines
• Transmission shafts

Industrial Equipment

• Bushes and sleeves for heavy machinery
• Heavily loaded shafts
• Power transmission components
• Heavy equipment components subject to impact loading
• Industrial fasteners for critical applications

Oil and Gas Industry

• Downhole tools
• Mud motor components
• Pump shafts
• Valve components
• Pressure vessels (non-sour service)

Defense Industry

• Ordnance components
• Weapon system parts
• Armor components
• Military vehicle parts

Specifications and Compliance

SHUNFU METAL’s AISI 4340 steel meets or exceeds the following specifications:

• AS1444: Grade 4340
• ASTM A434/A29: Grade 4340
• DIN 1013: Dimensional tolerances for black condition
• JIS SCNM447: Similar Japanese specification

Certifications

• ISO 9001: Quality Management System
• CNSA: China National Standards Association verification
• AS9100: Quality Management System for aerospace industry (on request)

Alternative Grades Comparison

SHUNFU METAL offers several alternative grades that may be considered instead of AISI 4340 for specific applications:

Grade	Why it might be chosen instead of AISI 4340
AISI 4140	Lower cost and better availability in a range of sizes. However, in general slightly lower impact properties achieved when heat-treated to similar strength levels as 4340.
Atlas 6582	Superior impact properties and through-hardening when heat-treated to similar strength levels as 4340.
Atlas 6580	Substantially higher yield strength achievable, especially in larger sections, when heat treated to similar impact properties as 4340. Higher toughness is also achieved in Atlas 6580.

Product Forms and Sizes

SHUNFU METAL offers AISI 4340 steel in various forms to meet diverse application requirements:

Round Bars

• Diameter Range: 10mm to 450mm (0.4″ to 18″)
• Length Range: Standard lengths up to 6000mm (236″), custom lengths available on request
• Surface Condition: Black (hot rolled/forged) or Bright (cold drawn/peeled)
• Mechanical Condition: Hardened and tempered to specified hardness levels

Custom Forms (Available Upon Request)

• Rectangular Bars
• Forged Blocks
• Custom Shapes
• Near-net Shape Forgings

Technical Support and Services

SHUNFU METAL provides comprehensive technical support to help customers maximize the performance of our AISI 4340 steel:

• Material Selection: Expert assistance in selecting the optimal grade for specific applications
• Heat Treatment Consultation: Recommendations for optimal heat treatment parameters
• Metallurgical Support: Expert advice on microstructure development and properties
• Failure Analysis: Investigation and root cause analysis of material-related issues
• Custom Testing: Additional mechanical, chemical, or metallurgical testing as required
• Machining Recommendations: Guidelines for optimal machining parameters
• Custom Certifications: Special certification packages available on request
• Just-in-Time Delivery: Inventory management and delivery scheduling to support lean manufacturing

Why Choose SHUNFU METAL for AISI 4340

Selecting SHUNFU METAL as your supplier for AISI 4340 steel provides numerous advantages:

• Material Expertise: Specialized knowledge in high-performance alloy steels for critical applications
• Consistent Quality: Rigorous quality control procedures ensure batch-to-batch consistency
• Advanced Testing: Comprehensive mechanical testing for each production batch
• CNSA Verification: Each production batch undergoes independent testing by the China National Standards Association
• Precise Heat Treatment: Computer-controlled heat treatment processes ensure optimal mechanical properties
• Global Supply Capability: Ability to serve customers worldwide with consistent quality
• Custom Solutions: Ability to tailor products to meet specific customer requirements
• Complete Traceability: Full documentation from melt to final inspection
• Technical Support: Comprehensive application and processing guidance
• Competitive Pricing: Excellent value proposition for premium quality material

Request Information

Contact SHUNFU METAL today to discuss your AISI 4340 steel requirements. Our technical team is ready to assist you with:

• Detailed technical specifications
• Material selection guidance
• Custom size and property requirements
• Price quotations and delivery schedules
• Quality certification and documentation
• Technical support for your application