347S51 Forgings BS 347S51, UNS S34709, AISI 347H, EN 1.4912 / X7CrNiNb18-10. Forged rings, flanges, discs, shafts, tube sheets and bars.

347S51 is a chromium nickel austenitic stainless steel stabilised with niobium, designated under the British Standard system and used mainly for pressure equipment that runs hot.

Niobium has a stronger affinity for carbon than chromium does. In an unstabilised 18/8 steel, time spent between roughly 427 and 816 °C precipitates chromium carbide along the grain boundaries and leaves the metal beside them short of chromium. Corrosion then runs along those boundaries. Every weld heat affected zone passes through that band. Niobium takes the carbon out first as niobium carbide, so the chromium stays in solution where it is needed.

The second feature of 347S51 is its carbon band: 0.04 to 0.10 %, a range rather than a ceiling. A specified carbon minimum is what allows creep rupture allowables to be assigned to a steel. That is why the grade appears in BS 3059-2, which covers boiler and superheater tubes with specified elevated temperature properties, and not only in the general corrosion resisting standards.

For a forging buyer this makes 347S51 a hot service grade. It is bought for parts welded into a pressure boundary and then run at 500 °C and above: headers, refinery furnace hardware, exchanger tube sheets, hot nozzle and flange forgings. It is not a chloride grade, and section 10 sets out where it should not be used.

These two British numbers are not interchangeable, and orders are regularly placed against the wrong one.

Published mill and consumable data lists 347S31 and 347S51 together as the British designations for the niobium stabilised 18/10 family, covering UNS S34700 and S34709, with 347S51 paired specifically to S34709. A BS 1503 cross reference maps 347S51 to SUS F347H, the Japanese stainless forging grade under JIS G3214.

British Standards listing 347S51

BS 1503 has been superseded in European practice by the EN 10222 series. Most current enquiries therefore quote 347S51 together with an EN or ASTM equivalent; if your specification is legacy British, state both on the order.

International equivalents

Everything below is forged to drawing from steel melted on site. There is no closed die tooling cost, so a single prototype ring is quoted on the same basis as a production batch.

Single piece weight runs from a 10 kg minimum order up to 30 tonnes, the limit set by the 60 t melt shop heat size and the 6,300 t hydraulic press. Delivery can be as forged with allowance, proof machined for volumetric inspection, or finish machined to drawing.

Against the neighbouring grades

347S51 is supplied solution annealed. It is not hardenable by heat treatment. Strength comes from the austenitic matrix, and in cold worked product from work hardening.

Figures are published data for the grade family, given for engineering guidance. For wall or section thickness above 10 mm the proof strength can fall short of the tube figure by about 10 MPa. The values that bind on an order are those on the EN 10204 certificate issued against the heat.

Austenitic stainless has no ductile to brittle transition, and this composition keeps high toughness down to cryogenic temperature. The same grade family therefore turns up both in 600 °C superheater service and in cryogenic plant. Our laboratory runs Charpy V notch to minus 60 °C in house. Below that, testing goes to an accredited third party laboratory.

This is the data behind the decision to specify 347S51 rather than a plain 304 or 321 forging, and the reason the grade carries a carbon minimum.

Minimum proof strength at temperature

Published figures for 1.4550 are close to the Rp0.2 column: 175 MPa at 100 °C, 155 at 200, 136 at 300, 125 at 400 and 119 at 500 °C.

Creep rupture strength

The comparison with titanium stabilisation is the one that matters commercially. Published data puts 347H at roughly 10,000 hours to rupture at 650 °C and 100 MPa, against about 6,000 hours for standard 347 and 8,000 to 9,000 hours for 321H under the same conditions, and puts 347H around 20 to 25 % above 321H at 600 °C over 100,000 hours. Every extra MPa of allowable stress comes off the wall thickness, so on a large forged header or tube sheet the grade choice shows up directly in the weight you pay for.

Two things decide whether a 347S51 forging performs in service: the finish forging temperature, and whether the part was properly solution annealed afterwards.

Hot working

Published type 347 data gives 1,149 to 1,232 °C (2,100 to 2,250 °F) as the working range for forging, upsetting and other hot work, and states that the alloy should not be worked below 927 °C (1,700 °F). The same data states that material must be water quenched or fully annealed after working to regain maximum corrosion resistance. A 347S51 forging delivered as forged and air cooled through the sensitisation band has not had its stabilisation confirmed, which defeats the reason for specifying the grade.

Heat treatment cycles

1. Solution anneal1,000 to 1,100 °C with rapid cooling in air or water. Published ranges for the family also run 1,020 to 1,100 °C and, for 347H, 980 to 1,150 °C. Section thickness decides whether forced air is enough or a water quench is needed.
2. Stress relief, where specified850 to 950 °C for 10 to 15 minutes, cooling in air. Used after cold bending or heavy machining.
3. After hot bendingHot bending is carried out at 1,100 down to 850 °C and should be followed by a full solution anneal.
4. No hardening routeThe grade is austenitic and cannot be hardened by quench and temper. A specification calling for a quenched and tempered condition in this grade contains a drafting error.

Fourteen heat treatment furnaces cover this in house, including three high temperature car bottom furnaces for the upper end of the solution range, car bottom chambers to 8,000 x 2,000 x 2,000 mm, and two well type furnaces 15 m deep so long shafts can be quenched vertically without bowing. Every cycle is charted and the chart is issued with the certificate.

347S51 is an intergranular corrosion grade first and a general corrosion grade second. Being clear about the second point prevents a failed part.

Welding

Weldability is good and this is the grade's main selling point. Welding is carried out without preheat, and subsequent heat treatment is normally not required, because niobium keeps the stabilisation intact in the deposited metal. Niobium also crosses the arc with far less loss than titanium, which oxidises in the weld pool. That is the practical reason 347 type filler is the standard consumable even when the parent material is 321.

For higher carbon service ER347H is the matching filler. For dissimilar joints or service above 700 °C, ERNiCrMo-3 is often specified instead.

Machining

Austenitic, tough and work hardening. Guidance for the stabilised grades applies: the cold work hardening rate makes it less machinable than a martensitic grade such as 410, but broadly comparable to 304. Use rigid setups, sharp positive rake tooling, constant feed and plenty of coolant, and avoid dwelling in the cut, since the work hardened skin left behind is what destroys the tool on the next pass. Where the part is to be proof machined for ultrasonic inspection we do that in house on vertical lathes to 5,000 mm swing.

For a rectangular section ring, W = π/4 x (OD² − ID²) x H x ρ, with ρ = 7.90 g/cm³ for 347S51. Enter dimensions in millimetres.

Because the steel is melted here, chemistry is verified at the furnace rather than taken from another mill's certificate.

Class approvals held: CCS, BV, DNV, LR and NK. Traceability runs from the melt number through forging, heat treatment and machining to shipment, and the document file includes heat treatment charts, NDT reports, dimensional reports and PMI records. Intergranular corrosion testing and impact below minus 60 °C are scheduled with an outside laboratory, so flag them at enquiry stage rather than after the forging is finished.

Most forging shops in China buy billet. This works starts one step earlier, at the furnace, and for a stabilised grade that has a metallurgical consequence rather than only a commercial one.

347S51 turns on two composition clauses: the carbon window of 0.04 to 0.10 %, and the niobium to carbon ratio. A shop buying billet inherits both from a supplier certificate and can only accept or reject them. Melting in house means the chemistry is aimed at the forging it will become, with carbon placed deliberately inside the window rather than wherever the supplying mill happened to land it, and niobium trimmed against the actual carbon result to satisfy whichever stabilisation ratio the specification calls for.

Where 347S51 is not the right fit, these cover the neighbouring requirements: titanium stabilisation, higher chloride resistance, or higher strength.

Stabilised and austenitic stainless

• A182 F321
• AISI 321H
• AISI 330
• 904L
• Sanicro 28
• A286

Duplex and chloride resistant

• A182 F51
• A182 F53
• A182 F55
• A182 F61
• A182 F44
• Alloy 24

Higher temperature, nickel alloys

• Incoloy 800H
• Incoloy 800HT
• Inconel 625
• Inconel 617
• Inconel 718
• Haynes 230

The same shapes are forged across all of them: seamless rolled rings, forged rings, flanges, shafts, tube sheets and discs, bars and blocks.