# 347S51 Forgings: BS 347S51 / UNS S34709 / 1.4912

**347S51 is the British Standard number for a niobium stabilised 18Cr-10Ni austenitic stainless steel of the 347H, higher carbon type.** Published grade data gives 0.04 to 0.10 % C, 17.00 to 19.00 % Cr, 9.00 to 13.00 % Ni and up to 1.20 % Nb. The grade is listed under six British Standards for pressure purposes, one of which is BS 1503, the forging standard.

Jiangyin Jiangnan Metal Co., Ltd. melts 347S51 in its own electric arc furnace, ladle furnace and VOD unit in Jiangyin, Jiangsu, China, then forges, ring rolls, solution anneals, machines and tests on one site. Traceability runs from the melt number to the shipped part.

## Grade summary

| Field | Value |
|---|---|
| Grade | 347S51 |
| Type | Niobium stabilised austenitic stainless, 347H type |
| UNS | S34709 |
| EN number and name | 1.4912, X7CrNiNb18-10 |
| Other designations | AISI 347H, TP347H, ASTM A182 F347H, SUS347H, SUS F347H, 07Cr18Ni11Nb, 08Kh18N12B, Z10CNNb18-10 |
| Carbon | 0.04 to 0.10 % |
| Chromium / nickel | 17.0 to 19.0 % / 9.0 to 13.0 % |
| Tensile strength | 515 MPa (75 ksi) min |
| Proof strength 0.2 % | 205 MPa (30 ksi) min |
| Density | 7.9 g/cm3 |
| Maximum service temperature | About 816 C (1,500 F) per ASME |
| Standards | BS 1501-3, BS 1502, BS 1503, BS 1506, BS 3059-2, BS 3605-1 |
| Forged forms | Seamless rolled rings, forged rings, flanges, discs, tube sheets, shafts, sleeves, bushings, bars, valve parts |
| Ring outside diameter | 50 to 6,000 mm |
| Single piece weight | 10 kg to 30,000 kg |
| Certification | EN 10204 3.1 standard, 3.2 on request |
| Supplier | Jiangyin Jiangnan Metal Co., Ltd., Jiangyin, Jiangsu, China |

## What 347S51 is

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, so corrosion 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.

The second feature of 347S51 is its carbon band of 0.04 to 0.10 %, a range rather than a ceiling. A specified carbon minimum is what allows creep rupture allowables to be assigned, which is why the grade appears in BS 3059-2 for tubes with specified elevated temperature properties.

For a forging buyer this makes 347S51 a hot service grade, bought for parts welded into a pressure boundary and then run at 500 C and above. It is not a chloride grade.

## 347S51 against 347S31

| BS number | Corresponds to | UNS | Carbon | Bought for |
|---|---|---|---|---|
| 347S31 | Type 347, standard carbon | S34700 | Maximum only, no minimum | General corrosion service, welded fabrication |
| 347S51 | Type 347H, higher carbon | S34709 | 0.04 to 0.10 % specified range | Creep service above about 500 C, code allowables |

Corrosion behaviour and welding practice are the same for both. The difference is the carbon minimum and the creep allowables that follow from it. If the part goes into a pressure boundary running above about 500 C, order 347S51 or ASTM A182 F347H and state it on the purchase order. A heat that satisfies 347S31 will not necessarily meet the 0.04 % carbon floor, and that cannot be corrected after melting.

## Standards covering 347S51

| Standard | Scope |
|---|---|
| BS 1501-3 | Steels for pressure purposes, corrosion and heat resisting steels: plate, sheet and strip |
| BS 1502 | Steels for fired and unfired pressure vessels: sections and bars |
| BS 1503 | Steel forgings for pressure purposes. Governs forged rings, flanges, discs, shafts, tube sheets |
| BS 1506 | Bars and billets for bolting material in pressure retaining applications |
| BS 3059-2 | Boiler and superheater tubes with specified elevated temperature properties |
| BS 3605-1 | Austenitic stainless pipes and tubes for pressure purposes, seamless |

BS 1503 has been superseded in European practice by the EN 10222 series. Quote 347S51 together with an EN or ASTM equivalent on current orders.

## International equivalents

| Designation | System | Note |
|---|---|---|
| UNS S34709 | USA | State this number on international orders |
| AISI 347H, TP347H | USA | Higher carbon type 347 |
| ASTM A182 F347H | USA forging specification | Usual purchase specification for forged flanges and fittings |
| 1.4912, X7CrNiNb18-10 | EN | High carbon Nb stabilised grade, EN 10028-7 |
| SUS347H, SUS F347H | JIS | F347H is the forging grade, JIS G3214 |
| 07Cr18Ni11Nb, 1Cr19Ni11Nb | GB, China | GB/T 4237, GB/T 20878 |
| 08Kh18N12B | GOST | GOST 5632 |
| Z10CNNb18-10 | AFNOR | High carbon variant of Z6CNNb18-10 |
| 1.4550, X6CrNiNb18-10 | EN | Related but not equal: standard carbon 347, BS 347S31 |

## Chemical composition, weight %

| Element | Min | Max |
|---|---|---|
| C | 0.04 | 0.10 |
| Si | - | 1.00 |
| Mn | - | 2.00 |
| P | - | 0.040 |
| S | - | 0.030 |
| Cr | 17.00 | 19.00 |
| Ni | 9.00 | 13.00 |
| Nb | See note | 1.20 |
| Fe | Balance | Balance |

Note on niobium: the table gives a ceiling of 1.20 % and leaves the floor to the governing product standard. ASTM A240 for 347H (S34709) requires Nb plus Ta at 8 x C minimum to 1.00 % maximum. Several European mill data sheets for the same chemistry give 10 x C minimum. That ratio is the real stabilisation gate. It is checked by optical emission spectrometry on the heat and reported on the certificate.

## Mechanical properties, solution annealed

| Property | 347H per ASTM A240 / A276 | 1.4550 published data | Nb stabilised tube, published |
|---|---|---|---|
| Tensile strength Rm | 515 MPa (75 ksi) min, typical 515 to 750 | 510 to 740 MPa | 515 to 690 MPa |
| Proof strength Rp0.2 | 205 MPa (30 ksi) min, typical 205 to 310 | 205 MPa | 220 MPa min |
| Proof strength Rp1.0 | - | - | 250 MPa min |
| Elongation | 35 to 40 % min | 40 % | 40 % min |
| Reduction of area | 40 to 50 % min per ASTM A182 | - | - |
| Hardness | 187 to 201 HBW max | 230 HB | About 155 HV |
| Grain size | ASTM 5.0 or finer | - | - |
| Impact | - | Above 70 J/cm2 at minus 253 C | 60 J at minus 196 C per EN 13445-2, EN 10216-5 |

The grade is not hardenable by heat treatment. Austenitic stainless has no ductile to brittle transition and this composition keeps high toughness to cryogenic temperature.

## Elevated temperature proof strength

| Temperature C | Rp0.2 min MPa | Rp1.0 min MPa |
|---|---|---|
| 50 | 195 | 232 |
| 100 | 175 | 210 |
| 150 | 165 | 195 |
| 200 | 155 | 185 |
| 250 | 147 | 175 |
| 300 | 139 | 167 |
| 350 | 133 | 162 |
| 400 | 129 | 159 |
| 450 | 126 | 156 |
| 500 | 124 | 155 |
| 550 | 118 | 152 |

## Creep rupture strength, approximate

| Temperature | 10,000 h MPa | 100,000 h MPa |
|---|---|---|
| 540 C | 253 | 186 |
| 550 C | 237 | 172 |
| 580 C | 192 | 135 |
| 600 C | 166 | 115 |
| 620 C | 142 | 97 |
| 650 C | 112 | 74 |
| 670 C | 96 | 61 |
| 700 C | 74 | 48 |
| 800 C | 28 | 16 |

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.

## Physical properties

| Property | Value |
|---|---|
| Density | 7.9 g/cm3, 7.93 to 7.98 quoted for 347H |
| Melting range | 1,398 to 1,427 C (2,550 to 2,600 F) |
| Modulus of elasticity at 20 C | 200 GPa, 194 at 100 C, 165 at 500 C |
| Thermal conductivity at 23 C | 14 W/m K, 15 at 100 C, 21 at 500 C |
| Specific heat at 23 C | 485 J/kg K |
| Mean thermal expansion 30 to 100 C | 17 x 10-6 /C, 18.5 x 10-6 over 30 to 500 C |
| Magnetic response | Non magnetic in the solution annealed condition |
| Maximum service temperature | About 816 C (1,500 F) per ASME |

## Forging and heat treatment

Published type 347 data gives a hot working range of 1,149 to 1,232 C (2,100 to 2,250 F) for forging and upsetting, states that the alloy should not be worked below 927 C (1,700 F), and states that material must be water quenched or fully annealed after working to regain maximum corrosion resistance.

- Solution anneal: 1,000 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.
- Stress relief where specified: 850 to 950 C for 10 to 15 minutes, air cool.
- Hot bending: 1,100 down to 850 C, followed by a full solution anneal.
- No quench and temper route. The grade is austenitic and cannot be hardened by heat treatment.

## Corrosion behaviour

- Intergranular corrosion: good resistance in the heat treated condition and after sensitising exposure, including in the weld heat affected zone, with no post weld heat treatment. Verified to ASTM A262 Practice E where specified.
- General corrosion: broadly the same as unstabilised 304. Good in organic acids at moderate temperature, in sulphate, sulphide and sulphite salt solutions and in moderate caustic environments.
- Nitric acid: resistant up to about 55 % concentration and 80 C, and to nitrate salts.
- Chloride pitting and crevice corrosion: a known weakness, with sensitivity even at relatively low chloride content. Not for seawater or brine duty.
- Chloride stress corrosion cracking: open to cracking above roughly 60 C under tensile stress.
- Hot gas: usable in air to 850 C, steam to 750 C, ammonia synthesis gas to about 550 C. In sulphur bearing flue gas the practical limit falls to 600 to 750 C.

For chloride service, specify duplex grades F51, F53 or F55, or 904L, the 6Mo grades or nickel alloys instead.

## Welding and machining

Welding is carried out without preheat and post weld heat treatment is normally not required. Filler metals are AWS A5.9 ER347 or ER347Si for TIG and MIG, AWS A5.4 E347-17 for manual metal arc, and ISO 14343 S 19 9 Nb wire or strip for overlay. Heat input below 1.5 kJ/mm, interpass below 150 C. ER347H is the matching filler for higher carbon service.

Machining: the cold work hardening rate makes the grade less machinable than a martensitic grade such as 410, but broadly comparable to 304. Rigid setups, sharp positive rake tooling, constant feed, plenty of coolant, no dwelling in the cut.

## Forged forms and size range

| Forged form | Size range |
|---|---|
| Seamless rolled rings | OD 50 to 6,000 mm on 1 m, 3 m and 6 m radial axial mills |
| Forged rings | Upset and punch route for sections outside the mill envelope |
| Forged flanges | To ASME B16.5, B16.47 or drawing |
| Tube sheets and discs | Up to 5,000 mm machined diameter |
| Shafts and spindles | Up to 14,000 mm turned length |
| Bars, blocks and billets | Round, flat and square, to drawing |
| Cylinders, sleeves, bushings | Bored to 1,600 mm diameter by 10,000 mm |
| Valve and process parts | To drawing, up to 30,000 kg single piece |

## Applications

Power generation: superheater and reheater headers, main steam piping components, boiler drum and header forgings, rings and discs for ultra supercritical plant.

Refining and petrochemical: furnace tube components, cracking and reformer furnace hardware, hot flange and nozzle forgings, polythionic acid service.

Chemical process: nitric acid and fertiliser plant, absorption towers, autoclaves, reactors, transport tanks, pumps, coolers, ammonia converter cooling tube components.

Heat exchangers and vessels: forged tube sheets, covers, shell flanges and nozzle forgings.

Food, pharmaceutical and cryogenic: process equipment in food plants, dairies, sugar refineries and breweries, pharmaceutical hardware, cryogenic components.

High temperature hardware: expansion joint and bellows end rings, furnace internals, waste heat recovery and recuperator components, thermal oxidiser parts, gas turbine exhaust hardware.

## Testing and certification

| Test | Method | Where performed |
|---|---|---|
| Chemical analysis | ASTM E415 OES, ASTM E1019 for C and S | In house |
| Niobium to carbon ratio | Against the ordered stabilisation clause, 8 x C or 10 x C | In house |
| Positive material identification | Handheld alloy analyser | In house |
| Tensile | ASTM A370, ISO 6892, 600 kN and 300 kN machines | In house |
| Impact, Charpy V notch | ASTM A370, to minus 60 C | In house |
| Impact below minus 60 C | ASTM A370, cryogenic bath | Third party laboratory |
| Hardness | ASTM A956, converted per ASTM E140 | In house or third party |
| Grain size and metallography | ASTM E112 | In house |
| Ultrasonic testing | ASTM A388, EN 10228-3, SEP 1921 | In house |
| Penetrant and magnetic particle | ASTM E709 and equivalent PT procedures | In house |
| Intergranular corrosion | ASTM A262 Practices A, B, E | Third party laboratory |
| Certification | EN 10204 3.1 standard, 3.2 witnessed by BV, DNV, LR, SGS, TUV or customer | In house or witnessed |

Class approvals held: CCS, BV, DNV, LR, NK.

## Manufacturer

| Field | Detail |
|---|---|
| Legal name | Jiangyin Jiangnan Metal Co., Ltd. |
| Business | Integrated melting works, open-die forge and ring rolling mill |
| Established | 1997 |
| Employees | 460, including 9 senior engineers, 32 intermediate engineers, 140 technicians |
| Plant address | No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China 214423 |
| Phone | +86 189 2135 9659, also WhatsApp and WeChat |
| Email | sales@steelforgepieces.com |
| Website | https://www.steelforgepieces.com/ |
| Hours | Monday to Saturday, 08:00 to 17:30 China Standard Time (UTC+8) |
| Melting | EAF 25 t, LF 25 t and 50 t, VOD 30 t and 60 t. Maximum heat 60 t |
| Presses | 6,300 t, 4,000 t, 2,000 t hydraulic open die, hammers 750 kg to 5 t |
| Ring mills | Radial axial 6 m, 3 m, 1 m |
| Heat treatment | 14 furnaces, including 2 well type 15 m deep |
| Machining | Vertical lathes to 5,000 mm swing, horizontal to 14 m, deep hole boring to 1,600 x 10,000 mm |
| Approvals | CCS, BV, DNV, LR, NK |
| Minimum order | 10 kg, no minimum piece count |
| Quotation | Two working days from a complete enquiry |
| Lead time | 20 to 60 days by material route |

## Requesting a quotation

Include six items: the grade and governing specification, the dimensions or a drawing in PDF, DWG, DXF or STEP, the quantity and repeat volume, the delivery condition, the heat treatment and testing scope, and the certification level and destination.

Email sales@steelforgepieces.com or call +86 189 2135 9659.

## Basis of data

Standard numbers and composition limits are taken from published 347S51 grade data and from ASTM A240 for S34709. Room temperature, elevated temperature, creep, physical, corrosion and welding figures are published data for the niobium stabilised 18/10 family, including EN 1.4550 and 1.4912. Hot working temperatures are published type 347 data. Values described as typical are for engineering guidance and are not guaranteed minimums. Guaranteed values are those stated on the EN 10204 certificate issued with the order.

Published by Jiangyin Jiangnan Metal Co., Ltd., open-die forging works, Jiangyin, Jiangsu, China. Last updated 28 September 2026.