ASTM A182 F6NM Charpy Impact Test: Requirements, Temperatures and Low-Temperature Limits

A practical reference for buyers and QA engineers who specify forged 13Cr-4Ni martensitic stainless steel (UNS S41500 / AISI 415).

Quick answer: ASTM A182 lists the mechanical properties for Grade F6NM but does not assign a Charpy value, so impact testing is carried out only when you request it, either through the specification's supplementary clause or through the code that governs the part. When it is required, F6NM is tested with a standard 10 × 10 × 55 mm V-notch specimen to ASTM E23, and a common acceptance criterion is an average of 42 J across three specimens with no single value below 34 J. The test temperature is not fixed; it follows the part's minimum design metal temperature (MDMT), which is frequently -40 °C or lower for cold-service applications.

What Is ASTM A182 Grade F6NM?

ASTM A182/A182M is the specification that covers forged stainless and alloy-steel flanges, fittings and valve parts for pressure service. Grade F6NM is its soft martensitic stainless steel, the same material often listed as UNS S41500 or AISI 415. It contains roughly 13% chromium, about 4% nickel and just under 1% molybdenum, with the carbon content held low at 0.05% maximum.

This composition is what sets F6NM apart from ordinary 410. The nickel addition and the low carbon give it a level of toughness and weldability that a plain 13Cr martensitic steel cannot reach, which is why it is widely used in oil and gas, hydropower and pressure-vessel applications, and why impact testing appears on almost every F6NM order. It is the standard choice for forged A182-F6NM rings, flanges and valve bodies that have to stay tough at low temperatures.

Does ASTM A182 Require Impact Testing for F6NM?

By itself, no. The mechanical-property table for F6NM specifies tensile strength, yield strength, elongation, reduction of area and a maximum hardness, but it does not include a Charpy value. A forging can therefore be fully certified to A182 without any impact testing at all.

Notch toughness becomes a requirement in one of two ways. You can invoke the supplementary impact-test clause within A182 on your order, or the code that governs the part can impose its own rules, such as ASME B31.3, ASME Section VIII, NACE MR0175/ISO 15156 or a client specification. Whichever one applies will define the test temperature and the acceptance energy.

The important point is that you have to specify it. If your drawing calls for impact-tested F6NM, state the test temperature and acceptance values on the purchase order. If you leave them out, you may receive a fully compliant forging that carries no impact data.

F6NM Charpy Impact Test Temperature

There is no single test temperature for F6NM. It is set by the minimum design metal temperature, which is the coldest temperature the metal will experience in service, including upset and depressurization conditions. The Charpy specimens are tested at that temperature, or below it, to confirm that the steel remains ductile.

The test temperatures most commonly specified for F6NM are shown below.

Because F6NM retains good toughness well below zero, it covers most of this range comfortably. That low-temperature capability is the main reason engineers choose it over 410.

F6NM Low-Temperature Rating and Cryogenic Service: -40, -60 and -196 °C

This question comes up constantly: can F6NM be used at -196 °C, and what is its rating at -60 °C? It is worth setting out the engineering reality clearly, because selecting the wrong grade for low-temperature service is a common cause of brittle fracture.

F6NM is a martensitic stainless steel, and like any steel with a martensitic structure it has a ductile-to-brittle transition temperature. Below that point its Charpy energy falls sharply. The low carbon content, the nickel addition and a properly controlled double temper push that transition far lower than it sits in 410, which is why F6NM is the preferred low-temperature martensitic grade. It does, however, have a practical limit.

A drawing that calls for impact-tested F6NM at -40 or -60 °C is therefore something we can produce and certify. A specification that requires guaranteed Charpy values at -196 °C is a different matter. At that temperature a 13Cr-4Ni steel is well below its transition point and cannot provide dependable toughness, regardless of heat treatment. It is also worth noting that some data sheets list F6NM as suitable "to -196 °C". This refers to the temperature at which the part can remain in service with its strength intact, not to passing an impact test at that temperature. Genuine cryogenic impact applications belong to austenitic stainless steel or 9% nickel steel.

This is where our heat treatment makes the difference. With a controlled quench-and-double-temper cycle, we can supply forged F6NM (UNS S41500) parts qualified to the specified Charpy minimum at low temperatures, verified on every heat lot and supported by full test reports. At Jiangsu Liangyi Co., Limited, we have delivered impact-tested F6NM on demanding cold-climate export projects, including forged F6NM round bars for a Russian client tested at -60 °C (-75 °F) with results of 202 / 175 / 202 J and 100% shear, and F6NM seamless rolled rings for an Italian client. When a component genuinely requires -196 °C service, we recommend an austenitic or 9% nickel grade and supply it accordingly, so that a single source can cover your full temperature range.

F6NM Charpy Acceptance Criteria (Joules)

When impact testing is specified, acceptance is normally given as an average across three specimens together with a minimum allowable single value. The combination most often applied to F6NM is shown below.

Some codes and end users specify a different pair of values. An average of 27 J (20 ft-lbf), for example, is sometimes used for weld-zone qualification. The governing code or project specification always takes precedence, so the figures above should be treated as a typical mill default for the base material rather than a fixed ASTM requirement.

Charpy Specimen Size and Test Standard

Impact testing follows ASTM E23, or the equivalent ASME SA-370 and ISO 148-1. The reference specimen is the full-size Charpy V-notch bar, 10 × 10 × 55 mm, broken in a single pendulum blow with the absorbed energy reported in joules.

Where the section is too thin for a full-size bar, sub-size specimens of 7.5 mm or 5 mm are permitted, but the acceptance energy is reduced in proportion. This should be agreed before testing so that acceptable parts are not rejected on a technicality. Specimens are taken from prolongations or sacrificial test rings that have gone through the same quench-and-temper cycle as the production forging.

Why F6NM Performs Well at Low Temperatures

The toughness of F6NM comes from its heat treatment rather than from chance. After forging, the material is quenched and given a controlled double temper, which produces a microstructure of fine tempered martensite combined with a small fraction of stable reverted austenite. This austenite is the key to its performance: it blunts advancing cracks and lowers the ductile-to-brittle transition temperature well beyond what a 0.1% carbon martensitic steel could achieve.

The low carbon ceiling of 0.05% also helps, keeping the martensite soft and tough rather than hard and brittle, while the nickel depresses the transition temperature further. Together, these factors allow a well-treated F6NM forging to absorb well over 100 J at sub-zero temperatures, clearing typical acceptance limits with a comfortable margin, as the mill data below shows.

Real F6NM Charpy Results (Mill Certificate Data)

The table below is taken from an actual production mechanical-property report for forged A182 F6NM supplied to a power-plant project. It gives a realistic picture of what a properly heat-treated forging should deliver.

Charpy results four to five times the minimum are normal for F6NM that has been correctly quenched and double tempered. This is the same toughness that makes the grade a trusted choice for hydro turbine runners, valve bodies and subsea wellhead components.

How to Specify F6NM Impact Testing on Your Purchase Order

Including all four of the following on your order will help you avoid re-testing and rejected lots:

• Governing code or specification, for example ASME B31.3, NACE MR0175/ISO 15156 or a client specification.
• Test temperature, tied to the component MDMT (for example, -40 °C).
• Acceptance values, giving both the average and the minimum single value (for example, 42 J average and 34 J single).
• Specimen orientation and location, such as longitudinal or transverse, and the sampling depth (mid-wall, T/4, and so on).

Frequently Asked Questions

Need Impact-Tested F6NM Forgings?

Jiangsu Liangyi Co., Limited supplies forged A182-F6NM / UNS S41500 / AISI 415 components, including rings, flanges, valve bodies, turbine runners and wellhead parts, with full mechanical and impact test reports to your specified MDMT for low-temperature service. Contact us at forgepieces.com, +86-510-86107550 or sales@forgepieces.com.