Specification Overview
ASTM A322 is a standard specification from ASTM International that governs hot-wrought alloy steel bar production, including 4140 steel—one of the most widely used chromium-molybdenum (Cr-Mo) alloy steels for forging, heat treatment, and machining, among others. The specification defines chemistry requirements, permissible manufacturing processes, and quality standards to ensure consistent, high-quality alloy steel bar products.
4140 alloy steel under ASTM A322 is a chromium-molybdenum alloy steel that offers a strong balance of strength, toughness, fatigue resistance, and hardenability, making it suitable for a wide range of high-stress mechanical and structural applications. The nominal average alloying elements of 0.95% Chromium and 0.20% Molybdenum work together to significantly improve hardenability compared to plain carbon steels, allowing 4140 to achieve high strength through heat treatment even in larger cross-sections.
Per ASTM International, ASTM A322 covers hot-wrought alloy steel bars intended for applications including forging, heat treating, cold drawing, machining, and many structural components. The steel shall be made by open-hearth, basic-oxygen, or electric-furnace primary processes, with optional secondary melting via electro-slag remelting or vacuum arc remelting. Bars must be free of pipe, cracks, flakes, and injurious seams, laps, or segregation that would interfere with machining or fabrication of suitable parts.
Available Product Forms
All bar forms are supplied in the Hot-Wrought condition per ASTM A322 requirements. Bar applications include forging, heat treating, cold drawing, machining, and structural components. Custom-cut lengths, discs, and blocks are available upon request. Contact our materials specialists for non-standard dimensions or specific heat treat conditions.
Typical Chemical Composition – 4140 Alloy Steel
| Element |
Symbol |
Range (%) |
Notes |
| Carbon |
C |
0.38–0.43% |
Provides base strength and hardness |
| Chromium |
Cr |
0.80–1.10% |
Improves hardenability and wear resistance |
| Molybdenum |
Mo |
0.15–0.25% |
Enhances strength, toughness, and hardenability |
| Manganese |
Mn |
0.75–1.00% |
Deoxidizer; improves hardness and strength |
| Silicon |
Si |
0.15–0.35% |
Deoxidizer; minor strength contribution |
| Iron |
Fe |
Balance |
Base metal |
Chemical composition ranges are per ASTM A322. Consult the current issue of ASTM A322 for precise minimum/maximum requirements. Aero-Vac provides full mill test reports (MTRs) with certified chemistry for every order.
Key Properties & Specification Details
| Property / Attribute |
Details |
| Specification Number |
ASTM A322 (ASTMA322) |
| Issuing Organization |
ASTM International |
| Material Type |
Alloy Steel – Chromium-Molybdenum (Cr-Mo) |
| Steel Grade |
4140 (UNS G41400) |
| Nominal Composition |
0.95Cr – 0.20Mo (Average Alloying Elements) |
| Condition |
Hot-Wrought (Various heat treatment conditions available) |
| Available Forms |
Bars (Round, Flat, Hex, Square) |
| Hardenability |
Excellent – responds well to through-hardening heat treatment |
| Machinability |
Good – approximately 66% of AISI 1212 in annealed condition |
| Weldability |
Good – pre-heat and post-weld heat treatment recommended |
| Magnetic Properties |
Magnetic (ferritic/martensitic structure) |
| Certification Available |
AS9100D / ISO 9001, Certificate of Conformance, MTR |
Typical Applications
Aerospace & Defense: Structural components, actuator rods, hydraulic system shafts, landing gear components, and high-stress brackets requiring excellent strength-to-weight ratio and fatigue resistance.
Oil & Gas: Drill collars, pump shafts, tool joints, downhole tooling, and wellhead components where high strength and toughness are critical in demanding subsurface environments.
Automotive: Crankshafts, connecting rods, axle shafts, spindles, and transmission components requiring high fatigue strength and excellent hardenability throughout the cross-section.
Industrial & Tooling: Gears, dies, jigs and fixtures, molds, punch tooling, and machine spindles where wear resistance and toughness under cyclic loading are required.
General Engineering & Structural: High-stress structural parts, heavy-duty bolts and fasteners, pressure vessel components, and general engineering applications requiring superior mechanical properties over plain carbon steels.
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