The Most Consequential Decision in Any ASTM A668 Forging Order
When engineers and procurement teams specify ASTM A668 / A668M forged steel components, the single most consequential decision is not the supplier — it is the class. ASTM A668 defines six discrete performance tiers, each representing a different combination of base chemistry, heat treatment route, tensile strength, yield strength, ductility, and toughness.
Choosing the wrong class in either direction has direct, measurable consequences: under-specifying leads to premature fatigue failure or plastic deformation in service; over-specifying adds material cost, processing complexity, and weldability constraints without delivering any engineering benefit. This guide gives you the framework to land on the right class the first time.
ASTM A668 class selection must be driven by the mechanical properties actually required at the critical cross-section of the finished component — not by specifying the highest class available. Every step up in class increases unit cost by 15–30%, processing time, and weldability risk.
What ASTM A668 Covers — and What It Doesn't
ASTM A668 is published by ASTM International (Committee A01) and covers carbon and alloy steel forgings for general industrial use. It applies to open die forgings, seamless rolled rings, and custom forged profiles produced by hot mechanical working from steel ingots. It does not cover hot-rolled or cold-finished bar stock, pressure vessel forgings (see ASTM A266), high-temperature alloy forgings (see ASTM A182), or railroad-specific components.
ASTM A668M is the metrically-equivalent companion specification. Engineering requirements — chemical composition limits, mechanical property minimums, heat treatment routes, inspection methods, documentation standards — are completely identical. Only the unit system differs: A668M uses SI (MPa, mm, kg) while A668 uses inch-pound (ksi, in, lb). Both designations are equally valid; specify whichever matches your engineering drawing units.
ASTM A668 Classes A Through F — Engineering Purpose of Each Grade
The six ASTM A668 classes are not simply a strength ladder from weakest to strongest. Each class was designed with a specific engineering purpose, and the heat treatment route is just as defining as the mechanical property targets. Understanding why each class exists prevents misapplication.
Complete Mechanical Properties — ASTM A668 Class A Through Class F
The table below presents the mandatory minimum mechanical property requirements for all six ASTM A668 / A668M classes. These are standard minimums. Actual certified production values typically exceed these, particularly for elongation and Charpy impact toughness. Client-specified enhanced targets — such as sub-zero Charpy at −46°C — can be accommodated through alloy selection and heat treatment parameter adjustment.
| Class | Min Tensile (MPa / ksi) | Min Yield 0.2% (MPa / ksi) | Elongation in 2 in. (%) | Reduction of Area (%) | Heat Treatment Route |
|---|---|---|---|---|---|
| Class A | ≥ 415 / ≥ 60 | ≥ 205 / ≥ 30 | ≥ 25 | ≥ 50 | Annealed or Normalized |
| Class B | ≥ 485 / ≥ 70 | ≥ 250 / ≥ 36 | ≥ 22 | ≥ 45 | Normalized or N+T |
| Class C | ≥ 550 / ≥ 80 | ≥ 310 / ≥ 45 | ≥ 20 | ≥ 40 | Normalized + Tempered |
| Class D | ≥ 620 / ≥ 90 | ≥ 415 / ≥ 60 | ≥ 18 | ≥ 38 | Quenched + Tempered |
| Class E | ≥ 690 / ≥ 100 | ≥ 515 / ≥ 75 | ≥ 16 | ≥ 35 | Quenched + Tempered |
| Class F | ≥ 795 / ≥ 115 | ≥ 655 / ≥ 95 | ≥ 14 | ≥ 30 | Quenched + Tempered |
Tensile Strength Visual Ladder — Class A Through Class F
As class increases from A to F, tensile and yield strength rise — but elongation drops from ≥ 25% (Class A) to ≥ 14% (Class F), and reduction of area drops from ≥ 50% to ≥ 30%. For applications combining high strength with sub-zero or impact-critical service, Class D or E is almost always preferable to Class F. Always add a supplementary Charpy V-notch impact requirement at your service temperature when ordering any Q+T class forging.
How to Select the Right ASTM A668 Class — 4-Step Engineering Framework
Answer the four questions below in sequence to identify the correct ASTM A668 class for your application. This framework covers the vast majority of industrial forging specifications across all global industries where ASTM A668 forgings are used.
The Hidden Cost of Over-Specifying
A common and costly procurement error is defaulting to Class D or Class E for applications where Class C fully meets all stress analysis requirements. Over-specifying creates three concrete problems:
- Material cost increase: Q+T grades require higher alloy content and more tightly controlled ingot chemistry. Class D material cost is typically 15–25% higher than Class C for the same forging geometry.
- Processing complexity: Quench-and-temper heat treatment requires a controlled quench tank, polymer quench medium management, post-quench dimensional check, and tighter furnace temperature control than normalizing. This adds lead time and cost to every order.
- Weldability restriction: As carbon equivalent (CE) rises with class, pre-heat and post-weld heat treatment (PWHT) requirements become more stringent. Class E and F forgings require careful welding procedure qualification. Class B and C can typically be welded without pre-heat at modest section thicknesses.
Using this framework is about engineering precision — selecting the class that meets requirements with adequate margin, not compromising safety. Over-specifying wastes budget that could be reinvested in better inspection, extended warranties, or spare part stock. Under-specifying is never acceptable.
Heat Treatment Explained — How Each Route Defines the Final Microstructure
The heat treatment route is not a secondary specification detail — it is definitional to the class. You cannot achieve ASTM A668 Class F mechanical properties through normalizing. You should not apply quench and temper to a Class A forging intended for carburizing. The following explains the metallurgical rationale for each route.
Annealing and Normalizing — Classes A and B
Class A forgings are supplied in the annealed or normalized condition specifically to optimize the material for subsequent surface hardening by the end-user. The primary goal is machinability and a uniform, predictable response to carburizing or carbonitriding — not maximum bulk strength. Annealing (furnace cooling from above Ac₃) or normalizing (air cooling from above Ac₃) produces a ferritic-pearlitic microstructure with consistent hardness, typically 150–200 HB, across the full cross-section. This uniformity gives predictable carburizing depth (typically 2–8 mm) and surface hardness response (58–63 HRC) after the customer's own heat treatment step.
Class B uses normalizing or normalizing-and-tempering to produce a fine-grained, homogeneous ferritic-pearlitic structure with moderate strength and excellent toughness — suited to structural and general load-bearing applications that do not require through-hardening.
Normalizing + Tempering — Class C
Class C adds a tempering step (typically at 550–650°C) after normalizing. This relieves residual stresses from the normalizing step, reduces hardness variation across thick cross-sections, and improves toughness. Class C is the most cost-effective route to a minimum 550 MPa tensile strength with good weldability — making it the preferred specification for large components that will be field-welded during installation, such as hydroelectric turbine rings and main shafts. The post-temper dimensional stability also reduces corrective machining during assembly.
Quenching + Tempering — Classes D, E, and F
Q+T is the most mechanically demanding and process-critical heat treatment route in the ASTM A668 system. The process involves: heating to the austenitizing temperature (typically 850–930°C), holding at temperature until full cross-section soak is achieved, then rapidly quenching in water or polymer solution to suppress diffusional transformation and form martensite or lower bainite. The subsequent temper (typically 510–680°C depending on target class) converts brittle as-quenched martensite into tempered martensite — delivering the high strength-toughness combination that defines Classes D, E, and F.
Three process variables are especially critical:
- Austenitizing temperature: Too low leaves undissolved carbides, reducing hardenability. Too high coarsens the austenite grain size, which degrades final impact toughness.
- Quench rate and hardenability: The center of a 600 mm diameter shaft cools substantially slower than its surface during quenching. Without adequate hardenability from alloying elements (Cr, Mo, Ni), martensite only forms near the surface, leaving the core under-strength. Always request hardenability calculations for large sections.
- Tempering temperature: Class D is typically tempered at 600–660°C to achieve 620 MPa tensile minimum. Class F requires a lower tempering temperature (~510–570°C) to achieve the 795 MPa minimum — but this limits toughness. This is precisely why Class F has the lowest minimum elongation (14%) and reduction of area (30%) in the standard.
For forgings with diameter greater than 400 mm, achieving Class E or F properties through the full wall thickness requires alloy chemistry designed for that section size. Carbon equivalent alone is not sufficient — hardenability must be assessed against the actual forging diameter. Always request a hardenability calculation or documented prior production experience at your section size and class when ordering from any supplier.
Application Mapping — Which ASTM A668 Class for Which Industry and Component
| Industry | Typical Component | Recommended Class | Key Engineering Rationale |
|---|---|---|---|
| Hydroelectric power | Turbine thrust rings, main shafts, runner crowns | Class C | Medium strength, excellent weldability for field assembly, stable under cyclic hydraulic loads |
| Cement manufacturing | Rotary kiln pinion shafts, girth gear blanks | Class A | Case hardening required; core must remain tough under 3,500 kW continuous torsional drive load |
| Mining & mineral processing | Ball mill ring gears, gearbox shafts, crusher components | Class D | High fatigue life requirement under sustained high-cycle torque loading; Q+T gives uniform microstructure |
| Oil & gas upstream | API 6D valve bodies, gate slabs, seat ring blanks | Class D (NACE) | Q+T to ≤ 22 HRC; specify S ≤ 0.005% for sulfide stress cracking resistance per NACE MR0175 / ISO 15156 |
| Thermal power generation | Boiler feed pump barrel casings, impellers | Class E | High internal pressure (up to 250 bar) requires maximum pressure-containment strength with adequate ductility |
| Marine & shipbuilding | Propeller shafts, rudder stocks, stern tube components | Class D | Cyclic bending fatigue in cold seawater; specify supplementary Charpy V-notch at −20°C minimum |
| Sugar & ethanol processing | Mill roller shafts, drive gear shafts, crushing spindles | Class C / Class D | Class C for roller shafts (combined bending + torsion); Class D for gear shafts requiring higher fatigue life |
| Oil & gas wellhead | Connector hubs, valve seats, bearing rings (API 6D CL 2500) | Class F | Maximum bulk strength for extreme-pressure service; seamless ring rolling geometry matches component shape |
| General heavy engineering | Press frames, structural cross-heads, machine bases | Class B | Normalized grade delivers adequate strength at minimum cost; fully weldable without pre-heat at normal sections |
Class A vs. Class F: The Extreme Ends Compared Directly
These two classes anchor opposite ends of the ASTM A668 spectrum and are engineered for fundamentally different purposes:
Class A is not a "weak" grade — it is a purpose-engineered case-hardening substrate. Its low bulk hardness (typically 160–200 HB as-supplied) is not a limitation; it is a requirement. A high bulk hardness in a pinion shaft blank would block uniform carburizing depth and impair the fatigue performance of the final case-hardened gear teeth. The minimum 25% elongation and 50% reduction of area requirements serve a specific purpose: they guarantee a tough, ductile core beneath the hardened case — preventing brittle fracture during impact loading.
Class F achieves the highest bulk strength in the ASTM A668 system (≥ 795 MPa tensile / ≥ 655 MPa yield) through a Q+T heat treatment with a lower-than-normal tempering temperature. This delivers maximum pressure-containment capability for seamless rings, valve seats, and wellhead hubs — but at the cost of ductility (minimum 14% elongation, 30% reduction of area). Specifying Class F for an application that Class D or E could fully serve wastes material cost, processing cost, and toughness margin.
Class A = maximum ductility, optimized for subsequent surface hardening, not for bulk strength. Class F = maximum bulk strength in the ASTM A668 system, minimum ductility, used only for extreme-pressure containment and high-contact-stress rings. These two grades serve entirely different engineering functions and are never interchangeable.
Where ASTM A668 Forgings Are Specified Globally
ASTM A668 / A668M is referenced in engineering specifications across all major industrial regions. While it originates from ASTM International (USA), it is widely adopted alongside European EN standards, API specifications, and regional procurement requirements. Jiangsu Liangyi supplies certified ASTM A668 forgings to procurement teams and engineering contractors in the following regions and industries:
All custom ASTM A668 open die forgings and seamless rolled rings are exported globally under FOB, CIF, CPT, DAP, or DDP Incoterms. EN 10204 3.1 material test certificates (signed by our own Quality Manager) are issued as standard on every order. EN 10204 3.2 third-party inspection co-signature by a client-nominated accredited inspection body is accommodated on request. Lead time: 4–16 weeks depending on class, weight, and inspection requirements.
Frequently Asked Questions — ASTM A668 Grade Selection
Class A and Class F represent opposite ends of the ASTM A668 spectrum. Class A (415 MPa minimum tensile, annealed or normalized) is engineered as a case-hardening substrate — its low bulk hardness (150–200 HB) optimizes machinability and uniform response to carburizing or induction hardening by the end-user. Class F (795 MPa minimum tensile, quenched and tempered) is the highest-strength grade, used for seamless rolled rings, wellhead connector hubs, and high-pressure valve seats where maximum bulk strength and pressure-containment capability are required. These two grades serve entirely different engineering purposes and should never be substituted for one another.
Technically possible but not good engineering practice. Over-specifying a class adds 15–25% material cost without adding meaningful safety if your stress analysis already shows adequate margin with Class C properties. It also introduces weldability constraints — Class D has a higher carbon equivalent than Class C, requiring more stringent pre-heat and post-weld heat treatment. Specify the class that meets your engineering requirements with appropriate safety factors applied, not the highest class available.
Yes, with appropriate chemistry and heat treatment controls. NACE MR0175 / ISO 15156 requires a maximum hardness of 22 HRC (approximately 237 HV10 or 248 HBW) for carbon and low-alloy steel in H₂S-containing environments. Class D Q+T, when the tempering temperature is adjusted to achieve ≤ 22 HRC, typically satisfies this requirement. Specify sulfur content ≤ 0.005% through ladle refining and vacuum degassing to minimize hydrogen-induced cracking (HIC) susceptibility. Class F, with its higher minimum yield strength, is generally not suitable for sour service because achieving the required 795 MPa+ strength at ≤ 22 HRC hardness is metallurgically incompatible.
ASTM A668 specifies maximum limits for phosphorus and sulfur (and carbon in some classes), but does not mandate a specific alloy chemistry for most classes. This gives manufacturers the flexibility to select appropriate steel grades for the section size and hardenability requirements of each specific component. For Classes D, E, and F, the chemistry must be sufficient to achieve Q+T mechanical properties through the full cross-section at your actual forging size. For large cross-sections, always request a hardenability calculation or evidence of prior production experience at your specific section size and weight.
Charpy V-notch impact testing is not a standard requirement in ASTM A668 — it is a supplementary requirement that must be explicitly stated in the purchase order. Specify: (1) the test temperature (e.g., −20°C for North Atlantic marine service, −46°C for Arctic or Middle East offshore applications); (2) minimum average impact energy in joules; (3) the number of specimens per test set (typically 3 per heat). Classes C, D, and E are better suited to sub-zero impact requirements than Class F, because their higher tempering temperatures produce greater ductility margins in the final microstructure.
ASTM A668 uses tensile testing as the primary mechanical acceptance criterion, not hardness. However, typical Brinell hardness correlations are: Class A: 140–200 HB (annealed/normalized for machinability); Class B: 170–220 HB; Class C: 190–240 HB; Class D: 220–280 HB; Class E: 250–310 HB; Class F: 280–360 HB. Actual values vary with section size and specific alloy chemistry. Hardness survey testing from surface to center is available as a supplementary requirement for all classes.
Every ASTM A668 forging order includes a complete documentation package:
Standard (all orders): EN 10204 3.1 Material Test Certificate covering chemical composition, full mechanical test results (tensile, yield, elongation, reduction of area), heat treatment records with temperature-time charts, ultrasonic testing (UT) report per ASTM A388, magnetic particle testing (MT) report per ASTM E709, dimensional inspection report, and packing list.
Available on request: EN 10204 3.2 co-signature by a client-nominated accredited third-party inspection body, supplementary Charpy impact test reports, hardness survey reports, radiographic testing (RT), positive material identification (PMI), and material traceability documentation referencing NACE MR0175 / API 6D purchase order requirements. Learn more by contacting our engineering team directly.
Work With an ASTM A668 Specialist — Free Engineering Review on Every Inquiry
Jiangsu Liangyi is a dedicated ASTM A668 Class A through Class F forging manufacturer with over 25 years of production experience, supplying engineers and procurement teams in 50+ countries. Our production and quality system operates under ISO 9001:2015. Our in-house engineering team reviews every inquiry at no charge — checking class selection, forgeability, heat treatment feasibility, and inspection requirements before quoting. No minimum order quantity. EN 10204 3.1 material test certificate issued as standard on every order.