# ASTM A668/A668M steel forgings: Class A to N

ASTM A668/A668M is the general-industrial specification for untreated and heat-treated carbon and alloy steel forgings. It defines six carbon steel classes (A to F) and seven alloy steel classes (G, H, and J to N). Every class, size range and property limit is tabulated below in ksi and MPa.

Maintained by Jiangyin Jiangnan Metal Co., Ltd. Current ASTM edition A668/A668M-21. Last reviewed 14 September 2026.

ASTM A668/A668M is a standard specification published by ASTM International covering untreated and heat-treated carbon and alloy steel forgings for general industrial use. There are 13 classes. Six are carbon steel (A, B, C, D, E, F) and seven are alloy steel (G, H, J, K, L, M, N). Minimum tensile strength runs from 47 ksi (324 MPa) for Class A up to 170 ksi (1172 MPa) for Class N in sections up to 4 in (102 mm). Required properties drop as section size grows, so a class means little without the section size beside it.

- **Designation:** ASTM A668 / A668M (inch-pound and metric companion)
- **Full title:** Standard Specification for Steel Forgings, Carbon and Alloy, for General Industrial Use
- **Current edition:** A668/A668M-21, ASTM International, West Conshohocken, PA
- **Committee:** A01 on Steel, Stainless Steel and Related Alloys, Subcommittee A01.06
- **Classes:** 6 carbon (A to F) plus 7 alloy (G, H, J, K, L, M, N), 13 in total
- **Excluded:** Hot-rolled and cold-finished bars fall outside the scope
- **Common requirements:** Specification A788/A788M applies to forgings ordered to A668
- **Supplied by:** Jiangyin Jiangnan Metal Co., Ltd., Jiangyin, Jiangsu, China. Open-die forgings, seamless rolled rings and forged bars to A668 Classes A to N

## What ASTM A668 covers

A668 is the catch-all forging specification. Where a forging has no dedicated standard of its own, it is normally ordered to A668 with a class and a size. ASTM publishes separate specifications for forgings used in pressure vessels, railroad service, turbine generators, gearing, and service with special temperature requirements. Those take precedence over A668 wherever they apply.

Hot-rolled and cold-finished bars fall outside the scope, since A668 material has to be forged by hot mechanical work. The specification also stops short of fixing the full chemistry. Apart from the limits in the table below, the analysis is agreed between purchaser and manufacturer under the rules of A788/A788M. Nor does A668 name a grade. Class K can be reached with 4140, 4340 or several other alloy steels, depending on section size and hardenability.

Appendix X1 of the standard maps the current classes onto the classes of the superseded specifications A235, A237 and A243. That mapping is worth having when you re-order against an old drawing.

## Class index: how to pick a class

A class is a strength level rather than a grade name. Use the table below to narrow the choice down, then check the size-dependent limits in the next section.

ASTM A668 class index, strength level and typical use

| Class | Steel type | Min tensile at smallest section | Typical use |
|---|---|---|---|
| A | Carbon | 47 ksi (324 MPa) | As-forged blanks, clevises, non-critical weldments |
| B | Carbon | 60 ksi (414 MPa) | General machined parts, flanges, spacers |
| C | Carbon | 66 ksi (455 MPa) | Rings, hubs, low-stress shafting |
| D | Carbon | 75 ksi (517 MPa) | Heavy shafts, rolls, press components. The most commonly ordered carbon class |
| E | Carbon | 85 ksi (586 MPa) | Normalized and tempered shafts needing higher yield without alloying |
| F | Carbon | 90 ksi (621 MPa) | Quenched and tempered carbon forgings, top of the carbon range |
| G | Alloy | 80 ksi (552 MPa) | Large alloy sections where through-hardening matters more than peak strength |
| H | Alloy | 90 ksi (621 MPa) | Normalized and tempered alloy shafts and rings |
| J | Alloy | 95 ksi (655 MPa) | Turbine and generator shafting, wicket gate shafts |
| K | Alloy | 105 ksi (724 MPa) | Quenched and tempered drive shafts, pinions, heavy couplings |
| L | Alloy | 125 ksi (862 MPa) | High-load gear blanks, crankshafts, hydraulic cylinder components |
| M | Alloy | 145 ksi (1000 MPa) | High-strength small-section forgings, tool holders, press rams |
| N | Alloy | 170 ksi (1172 MPa) | Highest strength class. Peak values apply only to sections up to 4 in (102 mm) |

There is no Class I. The letter is skipped to avoid confusion with the numeral 1, so the alloy sequence runs G, H, J, K, L, M, N.

## Chemical requirements

A668 fixes only the residual and manganese limits. Carbon, silicon and the alloying elements are set by the grade chosen to meet the mechanical properties, subject to the heat analysis and product analysis rules of A788/A788M.

ASTM A668 chemical composition limits, heat analysis, mass %

| Element | Classes A to F and AH to FH (carbon) | Classes G to N and GH to NH (alloy) |
|---|---|---|
| Manganese, max | 1.35 | Not specified, set by grade |
| Phosphorus, max | 0.050 | 0.040 |
| Sulfur, max | 0.050 | 0.040 |

Alloy classes carry tighter phosphorus and sulfur limits than carbon classes. Put a lower sulfur ceiling on the purchase order if the part needs transverse toughness or ultrasonic inspection to a fine acceptance level.

## Mechanical requirements by class and section size

Each row below is a complete requirement. Minimum tensile, yield, elongation and reduction of area all step down as the ruling section grows, so the class on its own does not define the properties. Elongation is measured over a 2 in (50 mm) gauge length. Hardness values are Brinell.

ASTM A668 mechanical property requirements, Classes A to N, in ksi and MPa

| Class | Steel type | Section size | Tensile strength, min | Yield strength, min | Elongation in 2 in, min | Reduction of area, min | Hardness, HBW |
|---|---|---|---|---|---|---|---|
| A | Carbon | ≤ 20 in (≤ 508 mm) | 47 ksi (324 MPa) | — | — | — | 183 max |
| B | Carbon | ≤ 20 in (≤ 508 mm) | 60 ksi (414 MPa) | 30 ksi (207 MPa) | 24% | 36% | 120-174 |
| C | Carbon | ≤ 12 in (≤ 305 mm) | 66 ksi (455 MPa) | 33 ksi (228 MPa) | 23% | 36% | 137-183 |
| C | Carbon | > 12 to 20 in (> 305 to 508 mm) | 66 ksi (455 MPa) | 33 ksi (228 MPa) | 22% | 34% | 137-183 |
| D | Carbon | ≤ 8 in (≤ 203 mm) | 75 ksi (517 MPa) | 37.5 ksi (259 MPa) | 24% | 40% | 149-207 |
| D | Carbon | > 8 to 12 in (> 203 to 305 mm) | 75 ksi (517 MPa) | 37.5 ksi (259 MPa) | 22% | 35% | 149-207 |
| D | Carbon | > 12 to 20 in (> 305 to 508 mm) | 75 ksi (517 MPa) | 37.5 ksi (259 MPa) | 20% | 19% † | 149-207 |
| D | Carbon | > 20 in (> 508 mm) | 75 ksi (517 MPa) | 37.5 ksi (259 MPa) | 19% | 30% | 149-207 |
| E | Carbon | ≤ 8 in (≤ 203 mm) | 85 ksi (586 MPa) | 44 ksi (303 MPa) | 25% | 40% | 174-217 |
| E | Carbon | > 8 to 12 in (> 203 to 305 mm) | 85 ksi (586 MPa) | 44 ksi (303 MPa) | 23% | 37% | 174-217 |
| E | Carbon | > 12 to 20 in (> 305 to 508 mm) | 83 ksi (572 MPa) | 43 ksi (296 MPa) | 22% | 35% | 174-217 |
| F | Carbon | ≤ 4 in (≤ 102 mm) | 90 ksi (621 MPa) | 55 ksi (379 MPa) | 20% | 39% | 187-235 |
| F | Carbon | > 4 to 7 in (> 102 to 178 mm) | 85 ksi (586 MPa) | 50 ksi (345 MPa) | 20% | 39% | 174-217 |
| F | Carbon | > 7 to 10 in (> 178 to 254 mm) | 85 ksi (586 MPa) | 50 ksi (345 MPa) | 19% | 37% | 174-217 |
| F | Carbon | > 10 to 20 in (> 254 to 508 mm) | 82 ksi (565 MPa) | 48 ksi (331 MPa) | 19% | 36% | 174-217 |
| G | Alloy | ≤ 12 in (≤ 305 mm) | 80 ksi (552 MPa) | 50 ksi (345 MPa) | 24% | 40% | 163-207 |
| G | Alloy | > 12 to 20 in (> 305 to 508 mm) | 80 ksi (552 MPa) | 50 ksi (345 MPa) | 22% | 38% | 163-207 |
| H | Alloy | ≤ 7 in (≤ 178 mm) | 90 ksi (621 MPa) | 60 ksi (414 MPa) | 22% | 44% | 187-235 |
| H | Alloy | > 7 to 10 in (> 178 to 254 mm) | 90 ksi (621 MPa) | 58 ksi (400 MPa) | 21% | 42% | 187-235 |
| H | Alloy | > 10 to 20 in (> 254 to 508 mm) | 90 ksi (621 MPa) | 58 ksi (400 MPa) | 18% | 40% | 187-235 |
| J | Alloy | ≤ 7 in (≤ 178 mm) | 95 ksi (655 MPa) | 70 ksi (483 MPa) | 20% | 50% | 197-255 |
| J | Alloy | > 7 to 10 in (> 178 to 254 mm) | 90 ksi (621 MPa) | 65 ksi (448 MPa) | 20% | 50% | 187-235 |
| J | Alloy | > 10 to 20 in (> 254 to 508 mm) | 90 ksi (621 MPa) | 65 ksi (448 MPa) | 18% | 48% | 207-255 |
| K | Alloy | ≤ 7 in (≤ 178 mm) | 105 ksi (724 MPa) | 80 ksi (552 MPa) | 20% | 50% | 212-269 |
| K | Alloy | > 7 to 10 in (> 178 to 254 mm) | 100 ksi (689 MPa) | 75 ksi (517 MPa) | 19% | 50% | 207-269 |
| K | Alloy | > 10 to 20 in (> 254 to 508 mm) | 100 ksi (689 MPa) | 75 ksi (517 MPa) | 18% | 48% | 207-269 |
| L | Alloy | ≤ 4 in (≤ 102 mm) | 125 ksi (862 MPa) | 105 ksi (724 MPa) | 16% | 50% | 255-321 |
| L | Alloy | > 4 to 7 in (> 102 to 178 mm) | 115 ksi (793 MPa) | 95 ksi (655 MPa) | 16% | 45% | 235-302 |
| L | Alloy | > 7 to 10 in (> 178 to 254 mm) | 110 ksi (758 MPa) | 85 ksi (586 MPa) | 16% | 45% | 223-293 |
| L | Alloy | > 10 to 20 in (> 254 to 508 mm) | 110 ksi (758 MPa) | 85 ksi (586 MPa) | 14% | 40% | 223-293 |
| M | Alloy | ≤ 4 in (≤ 102 mm) | 145 ksi (1000 MPa) | 120 ksi (827 MPa) | 15% | 45% | 293-352 |
| M | Alloy | > 4 to 7 in (> 102 to 178 mm) | 140 ksi (965 MPa) | 115 ksi (793 MPa) | 14% | 40% | 285-341 |
| M | Alloy | > 7 to 10 in (> 178 to 254 mm) | 135 ksi (931 MPa) | 110 ksi (758 MPa) | 13% | 40% | 269-331 |
| M | Alloy | > 10 to 20 in (> 254 to 508 mm) | 135 ksi (931 MPa) | 110 ksi (758 MPa) | 12% | 38% | 269-341 |
| N | Alloy | ≤ 4 in (≤ 102 mm) | 170 ksi (1172 MPa) | 140 ksi (965 MPa) | 13% | 40% | 331-401 |
| N | Alloy | > 4 to 7 in (> 102 to 178 mm) | 165 ksi (1138 MPa) | 135 ksi (931 MPa) | 12% | 35% | 331-401 |
| N | Alloy | > 7 to 10 in (> 178 to 254 mm) | 160 ksi (1103 MPa) | 130 ksi (896 MPa) | 11% | 35% | 321-388 |
| N | Alloy | > 10 to 20 in (> 254 to 508 mm) | 160 ksi (1103 MPa) | 130 ksi (896 MPa) | 11% | 35% | 321-402 |

† This reduction-of-area value does not fit the adjacent size ranges for the same class and looks like a transcription error in the source table. Check it against ASTM A668/A668M-21 Table 2 before using it on a purchase order.

MPa and mm values are converted (1 ksi = 6.894757 MPa, 1 in = 25.4 mm) and rounded. Where the metric specification A668M is invoked on the order, use the SI values published in A668M rather than these conversions.

## Required heat treatment by class

The class determines which heat treatment routes are permitted. Where more than one route is listed, the manufacturer may choose, unless the purchase order restricts it. If the application needs a specific condition, say quenched and tempered rather than normalized and tempered, put it on the order.

ASTM A668 heat treatment condition by class

| Class | Steel type | Permitted heat treatment | Hardness-only designation |
|---|---|---|---|
| A | Carbon steel | Untreated (as-forged) | AH |
| B | Carbon steel | Annealed, or normalized, or normalized and tempered | BH |
| C | Carbon steel | Annealed, or normalized, or normalized and tempered | CH |
| D | Carbon steel | Annealed, or normalized, or normalized and tempered | DH |
| E | Carbon steel | Normalized and tempered, or double normalized and tempered | EH |
| F | Carbon steel | Quenched and tempered, or normalized, quenched and tempered | FH |
| G | Alloy steel | Annealed, or normalized, or normalized and tempered | GH |
| H | Alloy steel | Normalized and tempered | HH |
| J | Alloy steel | Normalized and tempered, or normalized, quenched and tempered | JH |
| K | Alloy steel | Normalized, quenched and tempered | KH |
| L | Alloy steel | Normalized, quenched and tempered | LH |
| M | Alloy steel | Normalized, quenched and tempered | MH |
| N | Alloy steel | Normalized, quenched and tempered | NH |

## What the “H” suffix means

A668 allows a forging to be certified and marked on hardness testing alone. When that option is taken the class carries an H suffix: AH, BH, CH, DH, EH, FH, GH, HH, JH, KH, LH, MH and NH. Heat treatment and hardness range stay the same as the base class. No tension test is carried out, so the certificate shows no tensile, yield, elongation or reduction-of-area values.

The H variant is cheaper and quicker, and it suits volume parts running on an already qualified process. It will not do where a design code, a customer specification or a third-party inspector expects reported tensile properties. Order the base class when the mill certificate has to carry tensile data.

## Product forms we forge to A668

A668 governs the material condition rather than the shape, so any open-die or ring-rolled geometry can be ordered to it. These are the forms we run most often.

- **Forged shafts.** Solid and hollow, stepped or plain, including hydro turbine wicket gate shafts, main shafts, generator rotor shafts and steam turbine shafting.
- **Seamless rolled rings.** Bearing races, slew rings, flange rings, gear blanks and tyre rings.
- **Forged bars.** Round, square, rectangular, flat and hexagonal sections.
- **Hollow bars and sleeves.** Bushings, cylinders, barrels, casings, shells, hubs and housings, trepanned or bored.
- **Blocks, discs and plates.** Die blocks, tube sheets, end caps and cover plates.
- **Near-net shapes.** Upset flanges, stepped billets and pre-machined blanks that cut customer machining time.

## Where A668 forgings are used

Because A668 is the general-industrial specification, it turns up wherever a forging is needed but no application-specific standard governs. In our order book the recurring sectors are hydroelectric and steam turbine equipment, heavy machinery and presses, mining and cement plant, marine propulsion and deck machinery, oil and gas surface equipment, and wind turbine structural components.

Classes D and E cover most carbon-steel shafting. Classes J and K cover most alloy shafting and are the usual choice for wicket gate shafts and hydro turbine hollow shafts. Classes L to N appear mainly on smaller high-load parts, where the section size stays inside the range that can still meet the tensile minimums.

## Manufacturing capability

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory in Jiangyin, Jiangsu Province, China, supplying forged bars, shafts, rings, blocks and hollow sections to ASTM, EN, DIN, JIS and GB specifications. We forge, heat treat, test and rough-machine in house, and ship with a mill test certificate traceable to the heat number.

Jiangyin Jiangnan Metal Co., Ltd. open-die forging capability

| Item | Capability |
|---|---|
| Process | Open-die forging, radial ring rolling, upsetting, trepanning |
| Single-piece weight | 50 kg to 30,000 kg |
| Forged bar diameter | 80 mm to 1,200 mm |
| Rolled ring outside diameter | 300 mm to 5,000 mm |
| Maximum shaft length | Up to 12,000 mm |
| Heat treatment | Annealing, normalizing, quenching and tempering. Trolley-hearth and pit furnaces with recorded charts |
| Testing | Spectrometer chemical analysis, tensile, impact, Brinell hardness, UT to ASTM A388, MT to ASTM E709 |
| Machining | Rough turning, boring, drilling and finish machining to drawing |
| Documentation | EN 10204 3.1 mill test certificate. 3.2 with third-party inspection on request |
| Standards supplied | ASTM, ASME, EN, DIN, JIS, GB, AISI/SAE, UNS |

## What to specify when ordering A668 forgings

A668 leaves several choices to the purchase order. Send these seven items with the enquiry and we can quote without going back and forth.

1. **Class, and whether the H option applies.** For example “A668 Class D” or “A668 Class DH”.
2. **Ruling section size** in inches or millimetres, since the property limits depend on it.
3. **Finished or rough dimensions**, with machining allowance if the part ships in the rough.
4. **Grade, if you need a particular one.** 1045, 4140, 4340 and so on. Leave it open and we will propose a grade that meets the class at your section size.
5. **Heat treatment condition**, where the class permits more than one route.
6. **Test and inspection requirements.** Test location and orientation, impact testing and temperature, ultrasonic acceptance class, surface NDT.
7. **Certification.** EN 10204 3.1 or 3.2, marking requirements, and any supplementary requirements from A668 or A788/A788M.

Send drawings in PDF, STEP or DWG to sales@steelforgepieces.com and we will return a quotation with weight, price and lead time.

## Frequently asked questions

### What is ASTM A668?

ASTM A668/A668M is the Standard Specification for Steel Forgings, Carbon and Alloy, for General Industrial Use, published by ASTM International. It covers untreated and heat-treated forgings made by hot mechanical work, and sets out their required heat treatment and mechanical properties. The current edition is A668/A668M-21.

### How many classes does ASTM A668 have?

Thirteen. Six carbon steel classes (A, B, C, D, E and F) and seven alloy steel classes (G, H, J, K, L, M and N). The letter I is skipped to avoid confusion with the numeral 1.

### What does the H suffix mean in classes such as AH, DH or KH?

The H suffix means the forging is certified and marked on hardness testing alone. Heat treatment and hardness range match the base class, but no tension test is carried out, so the certificate reports no tensile, yield, elongation or reduction-of-area values.

### What is the difference between A668 Class D and Class E?

Class D has a minimum tensile strength of 75 ksi (517 MPa) and can be supplied annealed, normalized, or normalized and tempered. Class E requires 85 ksi (586 MPa) minimum tensile in sections up to 12 in (305 mm) and must be normalized and tempered, or double normalized and tempered. Class E also carries a higher hardness range, 174 to 217 HBW against 149 to 207 HBW for Class D.

### Which A668 class is the strongest?

Class N. It requires a minimum tensile strength of 170 ksi (1172 MPa) and a minimum yield of 140 ksi (965 MPa) in sections up to 4 in (102 mm), with a hardness range of 331 to 401 HBW. Above 4 in the minimums drop, reaching 160 ksi (1103 MPa) tensile in the 10 to 20 in range.

### Does ASTM A668 apply to hot-rolled or cold-finished bar?

No. Hot-rolled and cold-finished bars sit outside the scope. A668 material has to be forged by hot mechanical work. Forged bar in round, square, rectangular, flat or hexagonal section is within scope. Rolled bar is not.

### What chemical composition does ASTM A668 require?

A668 fixes only a few limits. Carbon classes A to F allow a maximum of 1.35% manganese, 0.050% phosphorus and 0.050% sulfur. Alloy classes G to N are held to 0.040% phosphorus and 0.040% sulfur, with no manganese ceiling in the specification. The rest of the analysis follows from the grade chosen to reach the required properties, under the rules of A788/A788M.

### Is there a metric version of ASTM A668?

Yes. A668M is the metric companion. Unless the order specifies the M designation, forgings are supplied to inch-pound units. The two are separate documents and their values are not exact conversions of one another, so state which one applies on the purchase order.

### Which steel grade should I use for A668 Class K?

The specification does not name grades, so the choice comes down to section size and hardenability. In sections up to about 7 in (178 mm), 4140 or 42CrMo4 normally reaches Class K in the quenched and tempered condition. In heavier sections a nickel-bearing grade such as 4340 or 34CrNiMo6 is usually needed to hold 105 ksi (724 MPa) minimum tensile through the ruling section.

### Who manufactures ASTM A668 forgings?

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, supplying ASTM A668 Class A to N forged shafts, seamless rolled rings, forged bars, hollow bars, blocks and discs. Enquiries: +86 189 2135 9659, sales@steelforgepieces.com.

### What is the lead time for ASTM A668 forgings from China?

For stock sections in common carbon and alloy grades, rough-forged and heat-treated parts typically ship in 25 to 40 days, depending on size, heat treatment route and testing scope. Large one-off shafts and rings that need a dedicated ingot take longer. Send the drawing and required class for a firm lead time.

## Sources and scope of this page

The property tables on this page are a working summary prepared by Jiangyin Jiangnan Metal Co., Ltd. for buyers and design engineers. ASTM International is the only authoritative source for the requirements of A668/A668M, and the standard is available from [astm.org](https://www.astm.org/a0668_a0668m-21.html). Where this page and the standard disagree, the standard governs. Do not treat the values here as the sole basis for a design or a purchase order without checking the current edition.

Page prepared 14 September 2026 and reviewed by the technical sales team at Jiangyin Jiangnan Metal Co., Ltd.

## Request a quotation for A668 forgings

Send a drawing, the class and the section size. We reply with weight, price and lead time, usually within one working day.
Email sales@steelforgepieces.com
Call +86 189 2135 9659
Jiangyin Jiangnan Metal Co., Ltd., No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China

## Related grades and specifications

The alloy grades below are the ones most often used to meet A668 alloy classes. Each has its own data sheet.

- [AISI 4140](https://www.steelforgepieces.com/Alloy-Steel/AISI-4140.html)
- [42CrMo4](https://www.steelforgepieces.com/Alloy-Steel/42CrMo4.html)
- [1.7225](https://www.steelforgepieces.com/Alloy-Steel/1.7225.html)
- [SCM440](https://www.steelforgepieces.com/Alloy-Steel/SCM440.html)
- [AISI 4340](https://www.steelforgepieces.com/Alloy-Steel/AISI-4340.html)
- [34CrNiMo6](https://www.steelforgepieces.com/Alloy-Steel/34CrNiMo6.html)
- [1.6582](https://www.steelforgepieces.com/Alloy-Steel/1.6582.html)
- [AISI 4130](https://www.steelforgepieces.com/Alloy-Steel/AISI-4130.html)
- [34CrMo4](https://www.steelforgepieces.com/Alloy-Steel/34CrMo4.html)
- [AISI 4145](https://www.steelforgepieces.com/Alloy-Steel/AISI-4145.html)
- [18CrNiMo7-6](https://www.steelforgepieces.com/Alloy-Steel/18CrNiMo7-6.html)
- [30NiCrMo8](https://www.steelforgepieces.com/Alloy-Steel/30NiCrMo8.html)
- [817M40](https://www.steelforgepieces.com/Alloy-Steel/817M40.html)
- [Carbon steel forgings](https://www.steelforgepieces.com/Carbon-Steel/)

## Contact

Jiangyin Jiangnan Metal Co., Ltd.  
Open-die forging factory  
No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China  
Telephone: +86 189 2135 9659  
Email: sales@steelforgepieces.com  
Web: https://www.steelforgepieces.com/Alloy-Steel/ASTM-A668.html