Z6CNT18-10 Forgings: Titanium-Stabilised 18/10 Austenitic Stainless Steel (EN 1.4541, AISI 321)

Forged rings, seamless rolled rings, flanges, shafts, discs, sleeves, bushings, tube sheets and bars, produced to drawing by an open-die forging works in Jiangyin, China.

AFNOR

Z6CNT18-10

EN / Werkstoff

X6CrNiTi18-10, 1.4541

UNS / AISI

S32100, 321

Structure

Austenitic, non-magnetic

Rm, annealed

500 to 700 N/mm²

Rp0.2

min 190 N/mm²

Elongation A

min 40 %

PREN

17.0 to 19.0

1. Grade description

The AFNOR code reads directly: Z for high-alloy steel, 6 for about 0.06 % carbon, C-N-T for chromium, nickel and titanium, and 18-10 for the nominal chromium and nickel levels. The steel is fully austenitic and non-magnetic in the solution annealed condition. In chemistry it is the stabilised version of the 304 / 1.4301 family.

The grade appears in several French standards, which is worth knowing when an older European drawing lands on your desk. These include NF A 35-578 for stainless steel for high temperature application, NF A 35-573 for plates, wide strips and strips, NF A 35-574 for semi-finished products, bars and wire rod, NF A 36-607 for austenitic stainless steel parts obtained by flat die forging or drop forging for boilers and pressure vessels, NF A 36-209 for plates for boilers and pressure vessels, and NF A 35-559 and NF E 29-043 for bolting for pressure equipment. Variants in the same family are Z6CNT18-10B for high temperature application, Z6CNT18-10F for general purpose forgings, and TUZ6CNT18-10 and TSZ6CNT18-10 for seamless furnace tubes and longitudinally welded heat exchanger tubes.

For forging work the relevant one is Z6CNT18-10F, the general purpose forging designation. Most current enquiries, however, arrive against EN 10222-5, EN 10250-4, ASTM A182 F321 or ASTM A965 Gr F321, and dual certification is normal.

2. Equivalents and standards

Z6CNT18-10 maps onto two European grades, depending on whether the duty is corrosion driven or creep driven. X6CrNiTi18-10 (1.4541) is the corrosion grade. X8CrNiTi18-10 (1.4878) is the heat-resisting 321H type, with higher carbon and better creep strength.

Where the order is written to an American specification, the two usual routes are ASTM A182 / A182M Grade F321, the forging specification for 321 covering flanges, fittings, valves and parts for high temperature service, and ASTM A965 / A965M Grade F321, which covers austenitic stainless steel forgings for boilers, pressure vessels, high temperature parts and associated equipment. Either, or both at once, can be supplied.

3. Chemical composition

Two specifications are in play and they are not identical. The AFNOR high temperature specification is tighter on carbon, silicon, phosphorus and sulphur than the EN bar standard. A drawing calling Z6CNT18-10 to NF A 35-578 is melted to the left column; one calling 1.4541 to EN 10088-3 is melted to the right.

4. Mechanical and physical data

The figures below apply to the solution annealed condition at room temperature, for bar up to 250 mm diameter, per EN 10088-3. Forgings are verified on prolongations from the same heat and heat treatment batch.

5. Corrosion behaviour and limits

In low chloride environments a passive film forms in oxygenated water and gives good corrosion resistance for as long as the film stays intact. The grade resists water vapour up to 400 °C, and nitric acid and formic acid in low concentrations at room temperature, with high resistance to a broad range of chemical media. Its defining property remains resistance to intergranular attack, retained through the weld heat affected zone.

The limit is equally worth stating. Z6CNT18-10 is not resistant to seawater or salt-laden environments. At a PREN of 17 to 19, long contact with seawater or sodium chloride carries a real pitting risk. For seawater, brine, produced water or high chloride duty the answer is a different alloy, normally A182 F44, 904L, A182 F53 or A182 F55, all of which we also forge. Better to settle that before the order than after.

6. Forging and heat treatment

1.4541 / Z6CNT18-10 forges well, open die or closed die. Temperature control is what separates a sound forging from a cracked one.

Working window

Billets are soaked evenly at 1150 to 1180 °C. An even soak matters, since a thermal gradient puts in stresses that appear later as cracks or distortion. Deformation runs from about 1180 °C down to 950 °C, after which the piece is cooled quickly in air or water. Hot work has to finish above the 600 to 900 °C band, because sigma phase forms there and takes corrosion resistance and toughness with it. Surfaces are cleaned of contaminants before and during heating, as residues attack the material during the soak.

Solution annealing

The standard final condition is solution annealing at 1000 to 1100 °C, cooled in air or water, which dissolves precipitates and restores the austenitic structure and corrosion resistance. Where a customer specification calls for a narrower or different window, for instance a stabilising anneal for polythionic acid service in refinery duty, we follow the specification and keep the furnace chart on file.

Melting route

Depending on cleanliness requirements, material is sourced through EAF plus AOD or VOD for standard pressure equipment work, with ESR remelt where tighter inclusion ratings or improved transverse properties are called for. The melting route is stated on the certificate.

7. Welding and machining

Weldability is one of the main reasons this grade gets specified. TIG, MIG/MAG and manual arc welding all suit it. Preheating is not normally needed. Interpass temperature is kept as low as practical, around 150 °C, to limit hot cracking risk and heat tint. Recommended fillers are 1.4316 (G 19 9 L / ER308L) or the niobium-stabilised 1.4551 (G 19 9 Nb / ER347). Post-weld heat treatment is not usually required, but heat tint and scale are removed afterwards to keep corrosion resistance.

Machining is the weak side. Like most austenitic stainless steels the grade work hardens quickly and conducts heat poorly, so the cut surface hardens, tool load climbs and chips form badly. Rigid setups, positive rake, plenty of coolant, low to medium cutting speeds, no dwelling. We allow for this when quoting rough or finish machined forgings.

8. Forged product forms

All of the following are produced to customer drawing or specification, as single pieces or repeat series.

See also open-die forging pieces, forged parts capability and all stainless steel grades.

9. Applications

Aerospace and engine exhaust. Type 321 is specified for aircraft exhaust manifolds, stacks, bellows, flexible joints and heat shields, and for piston engine manifolds, where repeated cycling through the sensitisation range would ruin an unstabilised grade.

Chemical and petrochemical plant. Reactor internals, expansion joints, separator and heat exchanger parts, nozzles and piping components. In refining, stabilised grades are the conventional answer for polythionic acid stress corrosion cracking service.

Power generation and rotating equipment. Turbine components, gas compressor parts, gearbox and power transmission forgings, generator and nitrogen generator parts.

Pulp and paper. Digesters, liquor tanks, paper machine components and the associated pipework.

Food, pharmaceutical and biochemical. Processing equipment and storage, thermal oxidisers in waste treatment, general hygienic process hardware.

Onshore oil and gas. Compressor parts, valve components, heat exchanger internals. For subsea, seawater injection or chloride-rich duty, see the limits in section 5.

10. Testing and certification

Non-destructive testing

Ultrasonic testing of austenitic and austenitic-ferritic stainless steel forgings is covered by EN 10228-4, which is the correct part of the series for this grade. EN 10228-3 applies to ferritic and martensitic forgings and should not be called up here. Where an American specification governs, testing is to ASTM A388. Penetrant examination to ASTM E165 or EN ISO 3452 is available for surface work, to customer acceptance classes.

Material certification

Inspection documents follow EN 10204. A type 3.1 certificate is issued by the manufacturer on specific inspection, validated by the manufacturer's authorised inspection representative, who is independent of the manufacturing department. A type 3.2 certificate adds countersignature and verification by an independent third party, either the purchaser's nominated inspector or an officially designated body such as TUV, SGS, BV, DNV or Lloyd's Register, and is normally driven by the purchase order or project requirement.

Typical certificate package

Heat number and melting route, full chemical analysis, heat treatment records with furnace charts, room temperature mechanical results from forging prolongations, hardness, intergranular corrosion test to ASTM A262 Practice E where required, NDT reports, dimensional report, PMI results, marking and traceability.

11. Enquiry checklist

Send a drawing in PDF, STEP or DWG, or the basic dimensions. The details that let us price correctly the first time are the governing specification, whether Z6CNT18-10 to NF A 35-578, 1.4541 to EN 10222-5 or EN 10250-4, A182 F321, A965 F321 or your own; finished dimensions with machining allowance, or whether we machine; quantity and delivery schedule; supply condition and heat treatment; NDT requirement and acceptance class; certificate type, 3.1 or 3.2, with the third party named if applicable; and any client-specific points such as ferrite content, grain size, IGC testing or marking rules.

12. Questions and answers

Related grades

• AISI 321H
• A182-F321
• A182-F44
• 904L
• A182-F51
• A182-F53
• A182-F55
• A182-F61
• Sanicro 28
• Alloy 24
• A286
• Nitronic 50
• Nitronic 60
• 17-4PH

Standards referenced