Duplex Stainless Steel Composition: Cr-Ni-Mo-N Alloy Elements, Casting Grades and RFQ Guide

Duplex stainless steel composition is a controlled balance rather than a recipe based on chromium alone. Chromium, nickel, molybdenum and nitrogen affect the ferrite-austenite structure, localized-corrosion screening, strength, heat-treatment response and susceptibility to detrimental intermetallic phases. Carbon, silicon, manganese, copper, tungsten, phosphorus and sulfur also matter. The casting route and section thickness then decide how that chemistry solidifies and responds to solution heat treatment.

This guide is for pump, valve, chemical-processing and general industrial buyers who need to interpret a “2205,” “2507,” “super duplex,” ASTM A890 or ASTM A995 request. It explains the difference between wrought and cast designations, provides practical chemistry and PREN tables, compares sand and investment casting, and builds an RFQ checklist. It does not select an alloy for a particular fluid, approve a pressure design or guarantee corrosion life.

Clarify 2205, 2507, 2207 and cast-grade language

“2205” and “2507” are widely used names for wrought duplex and super-duplex alloy families. A casting may have related corrosion and phase-balance objectives, but it is normally ordered using a casting designation such as an ASTM A890/A995 grade and corresponding cast UNS number. A request for “2207” is not complete enough to manufacture from: the purchaser should supply the governing standard, grade, UNS, composition table or approved material specification.

Incoming term	What it may mean	Required clarification	Risk if accepted unchanged
2205	Wrought S31803/S32205 family or an informal chemistry target	Product form, UNS, standard edition, condition and whether a cast substitution is permitted	Wrought properties and heat treatment may be applied incorrectly to a casting.
2507	Wrought S32750 super-duplex family	Exact cast grade, corrosion basis, NDT and purchaser approval	“Super duplex” becomes a marketing label without controlled limits.
2207	Possible typo, internal label, supplier term or another intended alloy	Ask for the original standard, UNS or complete chemistry/property table	The wrong alloy may be quoted while both parties believe the designation is understood.
ASTM A890 grade	General corrosion-resistant duplex casting	Edition, numbered grade/UNS, heat treatment and supplementary requirements	A general casting route may be applied to a pressure part without the correct requirements.
ASTM A995 grade	Pressure-containing duplex casting for valves, flanges, fittings or similar parts	Edition, grade, common requirements, design code and acceptance plan	The material standard may be mistaken for complete pressure-system certification.

How Cr, Ni, Mo and N build the duplex structure

Chromium and molybdenum promote ferrite and contribute to passivation and localized-corrosion resistance in suitable environments. Nickel promotes austenite and helps balance a ferrite-rich Cr-Mo chemistry. Nitrogen is a strong austenite former, adds strength in solid solution and contributes strongly to common pitting-resistance calculations. The correct result depends on all four elements plus the actual thermal history.

During casting, a duplex alloy normally solidifies predominantly through ferrite and forms austenite during cooling and solution treatment. Section thickness, segregation and cooling rate can create local differences. Too little austenite may reduce toughness and change corrosion behavior; unsuitable thermal exposure can precipitate sigma, chi or other harmful phases. This is why a bulk spectrometer report cannot replace heat-treatment and phase-related evidence.

Element	Primary selection role	Interaction to remember	RFQ evidence
Cr	Passivation, oxidation response and ferrite formation	High Cr also raises the need to control intermetallic precipitation during harmful thermal exposure	Complete heat/product analysis and approved solution treatment.
Ni	Austenite formation and phase balance	Required amount changes with Cr, Mo, N, Cu, W and cooling history	Grade chemistry plus ferrite/phase verification when ordered.
Mo	Localized-corrosion resistance in many chloride duties and ferrite promotion	Mo-bearing grades can form detrimental phases if time-temperature history is unsuitable	Grade compliance and applicable ASTM A923 test plan.
N	Austenite formation, strength and pitting-resistance contribution	Must be measured by a suitable laboratory method; ordinary handheld XRF does not determine N	Reported nitrogen value linked to heat and lot.
C	Carbide response and weldability control	A low maximum still requires a valid carbon-analysis method	Combustion or another approved laboratory analysis.
Cu / W	Intentional grade-specific corrosion or alloy-balance additions	They change grade identity and may change the PREN expression used	Current specification limits, not a generic “super duplex” table.
Mn / Si / P / S	Melting, deoxidation, castability and residual control	They remain part of compliance even when Cr-Ni-Mo-N drives the keyword	Full certificate and product-analysis rules.

Cast versus wrought composition and property data

Nickel Institute guidance on duplex stainless steels explains the interactions of chromium, molybdenum, nitrogen and nickel, and provides nominal wrought-alloy tables. Those tables are useful for understanding alloy families, but they are not casting acceptance limits. A casting develops a dendritic structure, section-dependent solidification and a different test-bar relationship. A wrought plate certificate cannot certify a valve body poured to an unidentified “equivalent” chemistry.

Evidence type	Wrought product	Cast product	Substitution question
Designation	Wrought UNS/product specification such as plate, pipe or forging	Cast UNS and ASTM A890/A995 grade	Has the design authority approved the different product form?
Structure	Thermomechanically processed and solution annealed	Dendritic cast structure followed by grade-specific solution treatment	Are phase and intermetallic requirements appropriate to the cast section?
Mechanical tests	Samples from the wrought product under its product standard	Separately cast, attached or product samples under the casting specification	Do the samples represent the critical component section?
Defect model	Laminations, surface and fabrication/weld concerns	Shrinkage, inclusions, gas, hot tears, core shift and repair	Are NDT and repair rules written for a casting?
Dimensions	Mill-product tolerances plus fabrication/machining	Pattern/tooling, mould/shell, solidification, heat treatment and machining allowances	Has the drawing separated as-cast from machined tolerances?

PREN calculation with worked chemistry examples

A commonly used screening formula is PREN = %Cr + 3.3 × %Mo + 16 × %N. Some tungsten-bearing specifications use a modified expression such as %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N. Always use the formula required by the governing specification. PREN compares selected chemistry terms; it does not measure phase balance, segregation, surface condition, crevice design, velocity, deposits or actual corrosion rate.

Illustrative input only	Cr %	Mo %	N %	Calculated PREN	What it does not prove
Example A	22.0	3.0	0.18	22 + 9.9 + 2.88 = 34.78	That the material is a specific 2205 cast grade or safe in seawater.
Example B	25.0	4.0	0.27	25 + 13.2 + 4.32 = 42.52	That the casting is super-duplex compliant, free of sigma phase or correctly heat treated.
Example C with W	25.0	3.5	0.25	Incomplete until W and the specified formula are supplied	That an assumed W value can be inserted from another grade.

The numbers are calculation demonstrations, not chemical targets or EB Castworld acceptance ranges. Use the current purchased casting standard for actual limits. A higher calculated number may improve screening for some localized-corrosion environments, but can coexist with harmful intermetallic phases, casting discontinuities or unsuitable mechanical behavior.

When PREN belongs in the purchase order

Include a PREN requirement only when the governing standard, customer specification or responsible corrosion engineer defines the formula, analysis basis and minimum. State whether the calculation uses heat analysis or product analysis and how rounding is handled. If tungsten is present, do not assume a formula. PREN should sit beside, not replace, the cast grade, heat treatment, phase testing and service-compatibility review.

ASTM A890 versus ASTM A995 selection

ASTM A890/A890M-26 covers corrosion-resistant duplex castings for general application. ASTM A995/A995M-20 covers duplex castings for valves, flanges, fittings and other pressure-containing parts. A995 states that grade selection is the purchaser’s responsibility based on design and service conditions, mechanical properties and corrosion characteristics. It also notes that listed grades, when properly balanced and heat treated, commonly develop approximately 30-60% ferrite with austenite making up the balance, although the specification does not itself set a ferrite limit.

For an ordinary corrosion-resistant guide or support, A890 may be the relevant product specification. For a pressure-containing valve body, A995 and its common requirements may be relevant, together with the applicable design code, pressure tests, NDT and documentation. Choosing A995 does not make a casting code compliant by itself; design, manufacturing and inspection obligations still have to be satisfied.

Sand casting or investment casting?

Sand casting is often evaluated for larger pump casings, valve bodies, heavy sections and lower-volume parts. Investment casting is often evaluated for smaller complex internals, impellers, discs, brackets or fittings where ceramic-shell tooling and reduced machining are justified. Both routes can produce duplex stainless steel when the alloy, geometry and process are qualified. The best route depends on mass, wall variation, cores, surface, tolerances, quantity, NDT access and pressure risk.

Factor	Sand-casting question	Investment-casting question	Buyer input
Size and section	Can pattern, cores, risers and chills control heavy regions?	Can wax, shell and gating support the mass and fill path?	Drawing, section map and quantity.
Internal passage	Can cores be supported, vented, cleaned and measured?	Is a ceramic core feasible and inspectable?	Flow-path tolerance and cleaning standard.
Tolerance/surface	Which faces need machining stock?	Which features can remain as cast without false net-shape assumptions?	Datums, CTQs, finish and minimum wall.
Quality evidence	Are RT/UT/PT methods suitable for section and geometry?	Can thin sections and complex overlaps be interpreted?	NDT method, zones and acceptance.
Repeat volume	Pattern/core-box investment and change control	Wax tooling, assembly and shell repeatability	Prototype, batch and annual demand.

See the general lost-wax investment-casting process guide for process steps. The older SAF2205 centrifugal-casting note describes a tubular process context; centrifugal pipe, sand-cast pump bodies and investment-cast internals are different product forms.

Evidence needed before changing the casting route

A route change can alter tooling, solidification, test-bar representation, surface, tolerance and NDT interpretation. Before moving a part from sand to investment casting or the reverse, compare the approved drawing, minimum wall, hot spots, core feasibility, machining stock and acceptance standards. Treat the change as an engineering deviation requiring written purchaser approval, not as a supplier-only production shortcut.

Solution heat treatment and rapid cooling

Duplex castings require grade-appropriate solution heat treatment and rapid cooling. The treatment must develop the required austenite-ferrite structure and minimize detrimental phases. The controlling variables include furnace uniformity, load support, section thickness, minimum metal temperature, soak, transfer and quench effectiveness. A furnace set point copied into a certificate is not enough.

ASTM A923-25 explains that certain duplex steels are susceptible to intermetallic formation during exposure at approximately 320-955 °C. It also states that chemical and mechanical compliance does not necessarily demonstrate the absence of detrimental phases. The standard’s sodium-hydroxide etch, Charpy impact and ferric-chloride corrosion methods have different purposes and defined grade tables. The applicable method, specimen location and acceptance must be ordered.

The heat-treatment process and verification guide provides a broader furnace, quench and record framework. Final cycles remain controlled by the exact grade and approved procedure.

Ferrite measurement and phase-balance limitations

ASTM A800/A800M-20 describes composition-based, magnetic and metallographic methods for estimating ferrite in cast stainless steels. Each has uncertainty. Composition calculations depend on analytical accuracy. Magnetic readings can change with surface roughness, curvature and phases other than ferrite/austenite. Metallographic point counting depends on sampling, preparation, etching and point count.

Method	Useful for	Main limitation	How to specify
Chemistry-based estimate	Melt control and initial phase-balance prediction	Analytical variation and segregation can shift local phase content	State formula, analysis source and whether result is informational.
Magnetic instrument	Rapid local screening on accessible surfaces	Curvature, roughness, calibration and other phases affect response	Instrument, calibration, surface, locations and number of readings.
Metallographic point count	More direct quantitative phase estimate on a prepared sample	One section may not represent the slowest-cooled casting region	Sampling location, preparation, etch, field count and acceptance.
ASTM A923 methods	Detecting harmful intermetallic effects in covered grades	Not a universal ferrite-percentage test or a guarantee against every corrosion mechanism	Applicable method, grade, specimen and pass/fail criterion.

Failure modes that chemistry alone cannot solve

Observation	Possible contributors	Evidence before changing alloy
Pitting or crevice attack	Fluid chemistry, temperature, deposits, crevice geometry, surface, phase condition or wrong grade	Full medium analysis, location map, chemistry, heat treatment, A923/corrosion evidence.
Brittle cracking	Intermetallic phase, casting discontinuity, impact, restraint, repair or low-temperature duty	Fractography, sample location, impact/microstructure, NDT and repair record.
Leakage	Shrinkage, inclusion, crack, machining breakthrough, joint or design issue	Pressure/leak test, RT/CT/PT, wall map and interface inspection.
Distortion	Mould/shell movement, residual stress, support, heat treatment or machining sequence	Datum measurements before/after treatment and machining.
Unexpected ferrite reading	Wrong chemistry, thermal history, local segregation, surface or measurement error	Repeat calibrated test, laboratory chemistry and representative metallography.

Do not respond to a failed part by requesting “more nickel” or “higher PREN” without identifying the mechanism. Preserve the as-found condition, photograph location and direction, record service variables and retain samples. Changing both chemistry and geometry in one field trial makes the result difficult to interpret.

How to write the chemical-composition section of an RFQ

List the exact cast grade and current standard edition, then reproduce the controlled limits or attach the purchaser specification. Include every required element and footnote. Define heat analysis, product analysis, sample frequency and permitted product-analysis variation. If positive material identification is required, specify which elements the method can measure and do not use ordinary XRF as a substitute for carbon or nitrogen analysis.

Element group	RFQ field	Verification note
C, P, S	Maximum values from the selected cast grade	Require appropriate laboratory methods; portable screening may be insufficient.
Cr, Ni, Mo	Minimum/maximum ranges	Report complete values and link them to heat/product analysis.
N	Grade range and analysis method	Do not omit the element that strongly affects phase balance and PREN.
Cu, W	Grade-specific range or maximum	Use only when the selected cast grade requires it.
Mn, Si and other residuals/additions	Complete standard/customer limits	Do not reduce certification to four headline elements.
PREN	Formula and required value only when contractually applicable	Treat as chemistry screening, not corrosion or phase acceptance by itself.

RFQ checklist for duplex stainless steel castings

1. Component, equipment, part number, drawing/model and revision.
2. Cast product standard, current edition, grade/UNS and permitted alternatives.
3. General or pressure-containing scope and applicable design code.
4. Complete fluid/gas chemistry, concentration, pressure, temperature, velocity, solids and upset cases.
5. Preferred casting route, quantity, tooling status and mandated melt/refining process.
6. Heat treatment, ferrite requirement and ASTM A923 method where applicable.
7. Mechanical tests, coupon/product sampling and low-temperature impact requirement.
8. Dimensions, datums, CTQs, surface, machining allowance and minimum finished wall.
9. Visual, PT, RT, UT, CT, pressure/leak or corrosion tests with coverage and acceptance.
10. Repair permission, WPS/PQR, repair map, reheat and reinspection.
11. Part-to-heat traceability, document package, witness points, marking and retention.
12. Failure evidence, trial quantity, destination and commercial schedule.

Related product and factory pages

For a drawing-based inquiry, use the duplex stainless steel pump and valve casting product page. Compare it with the different product forms represented by SAF2205 centrifugal cast pipe and general stainless steel investment castings.

Review Quality Assurance and Factory Capability, but treat those as general context rather than automatic qualification for a particular ASTM grade, pressure code or inspection level. Use the contact page to send drawings, service chemistry and the required inspection dossier.

Engineering and safety boundary

This article explains metallurgy and purchasing controls. It does not select a material, approve equivalence, prescribe a foundry heat-treatment cycle, certify pressure design, establish a welding procedure or guarantee corrosion resistance and service life. Molten metal, furnaces, quench systems, NDT chemicals, machining and pressure tests require qualified personnel and site-specific safety procedures. The purchaser and responsible engineer approve the final material, process and acceptance plan.

Authoritative references

• ASTM A890/A890M-26, Duplex Corrosion-Resistant Castings for General Application.
• ASTM A995/A995M-20, Duplex Stainless Steel Castings for Pressure-Containing Parts.
• ASTM A800/A800M-20, Estimating Ferrite Content of Stainless Steel Castings.
• ASTM A923-25, Detecting Detrimental Intermetallic Phase in Duplex Stainless Steels.
• Nickel Institute, Practical Guidelines for the Fabrication of Duplex Stainless Steels.