A182-F6NM vs CA6NM:
What's the Difference Between the Wrought and Cast Grades?

Introduction: The Same Alloy Family, Two Different Forms

If you have specified a martensitic stainless steel forging in the F6NM grade and then encountered CA6NM in a casting quotation — or the other way around — you are not alone. These two designations describe steels with almost identical nominal chemistry: approximately 12–14% Cr, 3.5–5.5% Ni, and 0.5–1.0% Mo, with carbon intentionally held below 0.05%. Both belong to the soft-martensitic stainless steel family, and both are routinely approved under NACE MR0175 / ISO 15156 for sour service in oil and gas environments.

Yet they are governed by completely different ASTM standards, produced by fundamentally different manufacturing routes, and deliver meaningfully different performance in certain critical applications. Specifying one when you need the other is a costly engineering mistake. This article provides the technical framework to choose correctly the first time.

Nomenclature Decoded

The naming conventions in these two ASTM standards are a frequent source of confusion. Here is an explicit breakdown before proceeding.

A182-F6NM (UNS S41500)

ASTM A182 covers forged or rolled alloy and stainless steel pipe flanges, forged fittings, and valves. The prefix F identifies the material as a forged or rolled wrought product. 6 refers to the chromium-nickel martensitic steel family (Type 410 lineage), N denotes the nickel addition, and M denotes molybdenum. The UNS number is S41500 — the "S" prefix confirms it is a wrought stainless steel. The European designation is X3CrNiMo13-4 / 1.4313 under EN 10088.

CA6NM (UNS J91540)

ASTM A743 (general service) and A744 (corrosive service) cover cast stainless steels. The prefix C identifies a corrosion-resistant casting. A is the ACI series letter for chromium-nickel compositions. 6, N, and M carry the same meanings. The UNS number is J91540 — the "J" prefix confirms it is a cast steel alloy. The European casting equivalent is GX4CrNiMo13-4 / 1.4317.

Chemical Composition: Nearly Identical, Subtly Different

The nominal chemistry is close enough to cause confusion. The differences are small but mechanically significant, particularly in heavy sections.

The tighter phosphorus and sulfur limits in the wrought grade are not arbitrary. In a forging mill, steelmakers have full control over ladle chemistry and can reliably hit lower tramp-element targets. Foundries tolerate slightly looser limits because casting introduces additional variables — mold fill dynamics, solidification rates, and the need for higher silicon content to improve flowability of molten metal.

The engineering consequence: forgings deliver consistently cleaner grain boundaries and superior notch ductility, particularly in heavy sections where even small P and S variations cause measurable drops in Charpy impact energy.

Mechanical Properties: Where the Forging Advantage Becomes Quantifiable

The yield strength gap of 67 MPa (approximately 15%) is the most consequential difference for pressure-containment design. Under ASME pressure vessel codes, allowable stress is typically governed by yield strength, so this advantage translates directly into either a thinner wall (weight saving) or a larger design safety margin — a meaningful benefit for high-pressure valve bodies and pump casings operating above 700 bar.

The Charpy impact advantage of 21 J at −40°C is equally important for cold-environment and subsea service. Castings with their dendritic solidification microstructure and residual micro-porosity are inherently more susceptible to brittle fracture initiation — and that vulnerability appears directly in the lower ASTM minimum for CA6NM.

Microstructure: Why Manufacturing Route Matters More Than Chemistry

Chemistry tells you what elements are present. Microstructure tells you how those elements are arranged — and that determines real-world performance in service.

Wrought Forged Microstructure (A182-F6NM)

Forging involves repeated mechanical working of solidified steel at elevated temperature. This thermomechanical processing breaks down the as-cast dendritic structure, closes micro-shrinkage and porosity, refines grain size, and develops a preferred crystallographic texture (fiber flow) aligned with the forging direction. The result is a homogeneous, fine-grained martensitic matrix with uniform distribution of second phases and essentially zero internal porosity when properly forged and inspected by ultrasonic testing.

Cast Microstructure (CA6NM)

Solidification of a melt in a sand mold produces a columnar/equiaxed dendritic microstructure. Solute segregation during solidification means alloying elements — chromium, nickel, and molybdenum — are not uniformly distributed between dendrite cores and interdendritic regions. Micro-porosity and micro-shrinkage are inherent solidification defects that are managed through gating and risering design, but never completely eliminated. Hot isostatic pressing (HIP) can reduce porosity in premium castings, but adds significant cost.

Weldability and Post-Weld Heat Treatment

Both grades are considered weldable within the soft-martensitic stainless family — a significant advantage over conventional Type 410 or 420 grades, where higher carbon drives martensite hardness high enough to cause cold cracking without careful preheat management.

Preheat Requirements

For A182-F6NM with ≤0.05% C, preheat is generally not mandatory for sections below 20 mm thickness when using a matching or overalloyed filler metal. For heavier sections (typically above 25–50 mm), a preheat of 100–150°C is recommended to manage hydrogen-induced cracking risk. CA6NM behaves similarly, though heats with carbon near the top of the 0.06% limit may benefit from preheat even on thinner sections.

Post-Weld Heat Treatment (PWHT)

For A182-F6NM wrought product: PWHT is not mandatory for thin sections under ASME B31.3 when carbon is below 0.05%. For heavier walls or fatigue-critical applications, a full temper at 580–620°C restores notch ductility in the heat-affected zone.

For CA6NM castings: Most weld repair procedures and production welds require PWHT at 580–620°C to temper the HAZ back to acceptable hardness within the 23 HRC NACE limit. This adds process time and furnace cost — particularly for large, complex castings that cannot be easily loaded into standard heat-treat furnaces.

Sour Service (H₂S) Qualification: NACE MR0175 / ISO 15156

Both grades are listed as acceptable materials for sour service under NACE MR0175 / ISO 15156, the governing standard for H₂S-containing oil and gas environments. However, compliance requires careful attention to hardness requirements in both cases.

The 23 HRC ceiling under NACE is a hard limit, not a guideline. For the forged grade, achieving consistent hardness across a large forging is straightforward because the thermomechanical history is tightly controlled. For castings, local variations in section thickness — thinner sections cool faster, potentially producing slightly harder martensite — can result in hardness non-uniformity across the same part. Additional hardness mapping is strongly recommended for CA6NM castings in sour service qualification.

Typical Applications: Where Each Grade Dominates

The hydropower industry uses both grades simultaneously. A large Francis turbine runner might use CA6NM cast steel for the runner bowl — which has complex, profiled blade geometry that would require enormous and expensive forging tooling — while specifying A182-F6NM seamless rolled rings and forged shafts for the shaft, seal rings, and flange connections, where simpler geometry makes forging practical and the strength and inspectability advantages are decisive.

Procurement Considerations: Cost, Lead Time, and Certification

Cost Structure

For simple geometries (rings, discs, bars, shafts), A182-F6NM forgings are typically cost-competitive with or less expensive than CA6NM castings at production quantities above five pieces. Forging has relatively high tooling cost and low variable cost per piece; casting has lower initial tooling cost (pattern and core boxes) but higher per-unit cost due to yield losses, riser grinding, and weld repair of casting defects.

For complex, one-off geometries — large pump casings with internal spiral passages, for example — casting remains the cost-effective choice because the alternative would be a massive forging block with extensive machining stock or a costly multi-piece welded forging assembly.

Lead Time

Both A182-F6NM forgings and CA6NM castings typically carry lead times of 8–20 weeks for custom components, depending on size and complexity. For urgent orders, forgings of simpler geometry (rings and bars) are often faster because the tooling is generic and pattern-making is not required.

Certifications and Traceability

For both grades, request EN 10204 Type 3.1 Mill Test Certificate (MTC) as a minimum, with heat number traceability from melt shop to finished part. For critical pressure-containing applications, some projects require EN 10204 Type 3.2 certification, which must be countersigned by a customer-nominated independent third-party inspector. Jiangsu Liangyi provides Type 3.1 mill test certificates for every F6NM forging order, and can coordinate Type 3.2 third-party witness inspection through the customer's nominated inspection body on request.

Inspection and Testing Requirements

Standards Cross-Reference

Engineering Decision Guide

Use this step-by-step decision logic when specifying components in the 13Cr-4Ni-Mo alloy family:

How Jiangsu Liangyi Manufactures A182-F6NM Forgings

Jiangsu Liangyi Co., Limited has manufactured A182-F6NM (UNS S41500) forged parts at its Chengchang Industry Park facility in Jiangyin, Jiangsu since 1997. The production route for F6NM forgings follows a controlled sequence designed to maximise the microstructural advantages described throughout this article:

• Melting: Electric arc furnace (EAF) + ladle refining (LF) + vacuum degassing (VD/VOD) to achieve ≤ 0.025% S, ≤ 0.020% P, and dissolved hydrogen below 2 ppm
• Reduction ratio: Minimum 3:1 total forging reduction; 5:1 or greater for heavy sections to ensure full dendrite breakdown
• Heat treatment: Austenitise at 1,000–1,050°C → oil or air quench → temper at 580–620°C to achieve target hardness ≤ 23 HRC
• Inspection: 100% volumetric UT per ASTM A388, hardness survey, dimensional check, OES chemical verification, mechanical testing per ASTM A182
• Certification: EN 10204 Type 3.1 MTC standard; Type 3.2 TPI countersigned on request

The facility produces F6NM forgings from 30 kg to 30,000 kg, with seamless rolled rings up to 6 meters OD, round bars up to 2 meters diameter, and shafts up to 15 meters in length. We ship to customers in more than 50 countries with a 24-hour quotation turnaround.

Frequently Asked Questions

What is the main difference between A182-F6NM and CA6NM?

A182-F6NM (UNS S41500) is a wrought forged martensitic stainless steel governed by ASTM A182, while CA6NM (UNS J91540) is the cast version of the same alloy family governed by ASTM A743/A744. They share nearly identical nominal chemistry but differ in manufacturing route, internal soundness, mechanical property minimums, and ultrasonic inspectability. Forgings deliver higher yield strength, better Charpy toughness, and freedom from casting porosity.

Is CA6NM the same as F6NM?

No. CA6NM and A182-F6NM (also called F6NM) have nearly the same chemical composition — 12–14% Cr, 3.5–5.5% Ni, 0.5–1.0% Mo, low carbon — but they are not the same product. CA6NM is a cast steel (UNS J91540, ASTM A743/A744) while F6NM is a wrought forged or rolled product (UNS S41500, ASTM A182). The UNS prefix "J" denotes a casting and "S" denotes a wrought stainless steel; substituting one for the other against a specification calling for the other is non-conforming.

Which grade is stronger, A182-F6NM or CA6NM?

A182-F6NM has higher minimum mechanical properties than CA6NM. The wrought grade requires a minimum tensile strength of 655 MPa and yield strength of 517 MPa, compared to 620 MPa tensile and 450 MPa yield for CA6NM. A182-F6NM also has a higher minimum Charpy impact value (68 J at −40°C) versus CA6NM (47 J at −40°C), reflecting the superior toughness of the wrought, forged microstructure.

Can CA6NM and A182-F6NM both be used for NACE MR0175 sour service?

Yes. Both A182-F6NM and CA6NM are listed as acceptable materials under NACE MR0175 / ISO 15156 for H₂S-containing sour service environments, subject to the same maximum hardness limit of 23 HRC (253 HBW). However, because castings can have hardness variation across different section thicknesses, additional hardness mapping is recommended for CA6NM components in sour service to confirm compliance across the entire part.

When should I choose a casting (CA6NM) instead of a forging (A182-F6NM)?

Choose CA6NM castings when the part geometry is too complex to forge economically — turbine runner bowls with profiled blade passages, or pump casings with intricate internal channels — or when production quantity is low (one to five pieces) and forging tooling cost would be prohibitive. For simpler geometries such as rings, shafts, bars, and flanges, A182-F6NM forgings are typically both higher performing and more cost-effective at quantities above five pieces.

Does A182-F6NM require post-weld heat treatment (PWHT)?

A182-F6NM with carbon content below 0.05% generally does not require mandatory PWHT for thin sections under codes such as ASME B31.3. For heavier wall thicknesses or fatigue-critical applications, a full temper at 580–620°C is recommended to restore notch ductility in the heat-affected zone. CA6NM castings more commonly require PWHT on production and repair welds to maintain the 23 HRC NACE hardness limit.

What is the European equivalent of A182-F6NM and CA6NM?

The European equivalent of A182-F6NM (wrought, UNS S41500) is X3CrNiMo13-4, material number 1.4313, under EN 10088. The European equivalent of CA6NM (cast, UNS J91540) is GX4CrNiMo13-4, material number 1.4317, certified under EN 10283 for steel castings.

Conclusion

A182-F6NM and CA6NM occupy the same alloy space but serve different manufacturing niches. The wrought forged grade delivers superior yield strength, Charpy impact toughness, reduction of area, grain homogeneity, and ultrasonic inspectability. The cast grade trades those property advantages for the ability to fill complex mold geometries that no forging press can achieve economically.

For the overwhelming majority of forgeable geometries in oil and gas, power generation, and hydropower applications, A182-F6NM is the correct specification. Understand the distinction, document it clearly in your purchase orders, and you will avoid one of the most common and costly material substitution errors in the martensitic stainless steel world.

To request a technical quotation or discuss your specific forging requirements in detail, visit our F6NM product specification page or contact the Jiangsu Liangyi engineering team directly.