High-Strength AISI 8640 Alloy Steel

Product Overview

AISI 8640 is a nickel-chromium-molybdenum alloy steel of the oil-hardening type, recognized for its excellent combination of hardenability, toughness, and strength properties. Developed as a versatile low-alloy steel, 8640 offers a balanced performance profile that makes it suitable for demanding engineering applications requiring high fatigue resistance and wear durability.

Different from 8620 steel and 8630 steel, 8640 steel features a higher 0.38–0.43% carbon content, which delivers superior surface hardness after quenching & tempering, carburizing or nitriding treatments, while nickel additives maintain stable ductility under low-temperature shock loads.

Product Features

Chemical Composition

Mechanical Properties

1. As-Drawn Condition (25 mm Round, Cold Drawn)

2. Oil-Quenched and Tempered Condition (845°C quench, 540°C temper, 100 mm round)

3. High-Strength Heat-Treated Condition (Oil Quenched, Fine Grained, Tempered at 205°C)

Forging Process

• Preheating: 650°C – 750°C, slow heating rate to eliminate temperature gradient inside billets

• Initial forging temperature: 1150°C – 1200°C

• Final forging temperature: 850°C Min

• If forming stops below 850°C, the material enters the brittle zone; cold forging deformation will create micro-cracks on surface and core.

Heat Treatment

Normalizing

• Heating temperature: 860°C – 900°C

• Soaking time: 30–90 mins

• Cooling mode: Still air cooling to room temperature

• Function: Refine coarse forged grains, homogenize alloy element distribution, reduce internal stress, improve uniformity for follow-up quenching. Normalized hardness: 240–280 HB.

Annealing

• Heating temperature: 760°C – 790°C

• Long-term holding: 2–4 hours

• Slow furnace cooling (≤30°C/h) down to 500°C, then air cool

• Function: Convert flaky pearlite to spherical cementite, cut cutting resistance drastically. Annealed hardness ≤220 HB for high-precision turning, milling and drilling.

Quenching & Tempering

• Austenitizing temperature: 830°C – 860°C

• Soaking duration: 45–120 mins

• Quenching medium: Warm oil (60–80°C oil bath recommended; water quenching forbidden to avoid cracking)

• Temper temperature: 560°C – 620°C, hold 120–180 mins, air cool

• Key reminder: Complete tempering must be carried out within 4 hours after oil quenching to prevent delayed quenching cracks.

Supply Sizes, Tolerance & Stock

1. Product Types & Size Range

2. Surface Finish, Dimensional Tolerance & Straightness

We maintain over 10,000 tons of in-stock 8640 steel monthly for fast delivery. Stock changes daily; contact our sales team for real-time inventory.

Applications

AISI 8640's exceptional combination of strength, toughness, and hardenability makes it suitable for a diverse range of demanding applications across multiple industries:

Aerospace

• Landing gear components

• Airframe structural parts

• High-strength fasteners

Automotive & Heavy Equipment

• Gears and gear shafts

• Crankshafts

• Axles and connecting rods

• Drive train components

Industrial Machinery

• Forged hand tools and wrenches

• Screws and bolts

• Pistons and pins

• Wear-resistant components

Military & Defense

• Track shoes and undercarriage components

• Ordnance parts

• High-strength structural elements

Spring Applications

High-performance springs requiring elastic limit and fatigue resistance

AISI 8640 Comparison with Other Alloy Steels

8640 steel vs 8620 steel

Both adopt Ni-Cr-Mo alloy systems but differ in carbon content and core strength.

• 8620 steel is low-carbon (0.18–0.23% C) designed only for carburizing. It forms a thin hardened surface while keeping a soft core, fit for light-load small gears and miniature shafts with low cyclic impact.

• 8640 steel features medium carbon (0.38–0.43% C) and supports full quenching & tempering. It delivers uniform high strength across thick cross-sections, perfect for heavy-duty transmission gears and large mining shafts enduring continuous shock loads. 8620 offers better raw machinability and lower cost, while 8640 provides superior fatigue lifespan for heavy equipment.

8640 steel vs 8630 steel

8630 steel serves as a mid-transition grade between 8620 and 8640 with limited deep hardening capacity.

• 8630 steel maintains stable hardness only on workpieces thinner than 50 mm; thick forgings easily develop soft inner cores under repeated loads, suited for static medium-load small-size parts with low fatigue demand.

• 8640 steel eliminates core softening on blanks up to 100 mm thick, with 15–20% higher tensile strength and excellent low-temperature toughness. It acts as a direct high-performance upgrade for components requiring long-term variable load resistance.

8640 steel vs 4140 steel

• 4140 steel is standard Cr-Mo alloy steel free of nickel, widely used for general medium-strength mechanical parts at lower material cost. Without nickel additives, AISI 4140 suffers weak low-temperature crack resistance and uneven hardness on thick stock, prone to brittle fracture in cold environments. It works for regular brackets and room-temperature mild-load shafts.

• AISI 8640’s triple Ni-Cr-Mo formula greatly boosts hardenability and impact toughness. It performs reliably on large forgings, oilfield downhole parts and off-road vehicle components that demand high safety against sudden overloads.

8640 steel vs 4340 steel

4340 steel is premium high-toughness Ni-Cr-Mo steel for aerospace and military applications, yet carries high raw and processing expenses.

• AISI 4340 owns extreme overload resistance but needs ultra-precise heat treatment; minor temperature deviation causes mass rejects. It applies to ultra-high-reliability aviation and military structural parts.

• AISI 8640 cuts redundant top-tier performance while fully meeting civilian heavy machinery standards. It has a wider heat treatment window with lower production failure rates, delivering optimal cost-performance for mining, truck and energy equipment without extreme service pressure.

Quick Material Selection Reference

• Pick 8620: Light precision parts only needing surface wear resistance

• Pick 8630: Small-section static medium-load components

• Pick 4140: Ordinary room-temperature machinery with moderate strength needs

• Pick 8640: Heavy shafts, gear blanks, mining/oil gear, low-temperature heavy-load parts

• Pick 4340: Aerospace and military components with extreme safety requirements

FAQ

Q1: What is the difference between AISI 8640 and AISI 4140?

A1: Both are medium-carbon low-alloy steels, but the key distinction lies in nickel content. 8640 contains 0.40–0.70% nickel, which significantly improves impact toughness and low-temperature ductility compared to 4140. While 4140 offers similar hardenability, 8640 is preferred for components subjected to shock loading or colder service environments. For applications requiring extreme hardenability in very large cross-sections, 4340 remains a step above both.

Q2: What international standards cover AISI 8640?

A2: AISI 8640 is referenced across multiple standards. In the U.S., it is covered under ASTM A322, A331, A519, and A829, as well as SAE J404 and J412. The UNS designation is G86400. Globally, equivalent grades appear in DIN (1.6546), ISO 683, and BS specifications.

Q3: What are the most common applications for 8640 steel?

A3: The material is widely used in automotive drivetrain components (gears, axle shafts, crankshafts), aerospace structural fasteners, heavy machinery parts, and forged hand tools. Its combination of high fatigue strength and impact resistance also makes it a standard choice for military track shoes and ordnance components where reliability under cyclic loading is critical.

Q4: How does the machinability of 8640 compare to other alloy steels?

A4: AISI 8640 has a machinability rating of approximately 65% relative to AISI 1212 free-machining steel (which is rated at 100%). This puts it on par with 4140 and slightly better than 4340 (rated around 55%). In the annealed condition, machinability improves, making it suitable for standard turning, drilling, and milling operations using carbide or coated tools.

Q5: Does 8640 steel require post-quench tempering?

A5: Absolutely. Tempering is not optional—it is a mandatory step after oil quenching. As-quenched 8640 is extremely hard and brittle, with high residual internal stresses. Without tempering, the material is prone to sudden fracture under service loads. The tempering process relieves these stresses, adjusts hardness to the required level, and develops the desired toughness for the intended application.

For technical specifications, pricing, and availability of AISI 8640 alloy steel products, please contact us. We offer customized cutting, machining, and heat treatment services to meet your specific component requirements.