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Austenitic SS 347

Austenitic SS 347

ALLOY 347 / ALLOY 347H | 1.4550 | X6CRNINB18-10 | SUS 347 / SUS F 347 | S34700 / S34709

Austenitic SS 347 has a similar composition to grade 321 with a slightly improved corrosion resistance. This grade is widely used at elevated temperatures because of their property of retaining strength and corrosion resistance. Carbide precipitation in the grain structure of 347 is curbed during its treatment and welding with the help of columbium-tantalum.

The grade is highly stabilized because of its chemical composition that includes chromium (18%) for general corrosion resistance, nitrogen (11%) for strength at room and elevated temperatures, carbon (0.08%) for ductility and columbium-tantalum for stabilization of carbides at the grain boundary.

In the case of non-stabilized alloys, the carbides accumulate at the grain boundary and weaken the metal, resulting in inter-granular corrosion. The columbium-tantalum addition (at least 10 times carbon content) stabilizes the grain structure. The chromium content in the alloy is used to restrict carbide precipitation in the microstructure, but the alloy 347 has a greater affinity towards carbon rather than chromium. Columbium-tantalum are strong carbide formers and precipitate only within the grains and not at the grain boundary, this helps the alloy to efficiently restrict inter-granular corrosion.

The microstructure of the grade is face-centered cubic and the grade is typically nonmagnetic due to its microstructure. However, magnetic properties might change after cold working and welding. Grade 347 are readily considered for applications which involve processing between 430- 900ºC. The grade 347H has a higher carbon content and is generally used at elevated temperatures with better creep resistance properties. The grade 347 can be suitably used for high-temperature service applications. The grade resists hardening and can be used at cryogenic temperatures efficiently. It has suitable use under aqueous solutions as well as low temperatures with a fine stability of grain structure. In petrochemical industries and refineries, the presence of polythionic acid is common. Polythionic acid readily forms in the presence of sulphur, moisture and oxygen. Depending on the source of sulphur, a pipe might fail from any position. The grade 347 efficiently resists polythionic acid, and hence it is widely used in refineries as well as reactor systems.

  • Chemical composition
  • Physical properties
  • General data
  • Standards Specifications
  • Forms of Supply
  • Corrosion Resistance
  • Heat Treatment
  • Weldability
  • Machining
  • Applications
Grade   Ni C Cr Mn Si P CB S Fe
347 Min. 9.0 - 17.0 - - - - - 62.74
Max. 13.0 0.08 20.0 2.00 1.00 0.045 10Xc-1.10 0.030 -
347H Min. 9.0 0.04 17.0 - - - - - 63.72
Max. 13.0 0.10 19.0 2.00 1.00 0.045 8Xc-1.10 0.030  
Density 0.288 lbs / in3 7.96 g /cm3
Melting Point 2550 – 2635°F 1398 – 1446°C
Specific Heat 0.12 BTU/lb-°F (32 – 212°F) 500 J/kg-°K (0 – 100°C)
Thermal Conductivity @ 212°F 133 BTU/hr/ ft2/ft -°F 16.3 W/m-°K
Elastic Modulus 28.0 x 106 psi 193 GPa
Electrical Resistivity 72 Microhm-cm at 20°C
Annealing 1010 to 1193°C (1850 to 2000°F)
Poisson Ratio 0.27-0.30
Standard 347 347H
UNS S31700 S31703
WERKSTOFF NR. X6CRNINB18-10 X6CRNINB18-10
EN 1.4550 -
BS - -
Swedish - -
JIS SUS 347 SUS 347
Pipes & Tubes
Summary Standards
Standard Specification for Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater, and Heat-Exchanger Tubes ASTM A213
Standard Specification for Welded Austenitic Steel Boiler, Superheater, Heat-Exchanger, and Condenser Tubes ASTM A249
Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for General Service ASTM A269
Standard Specification for Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes ASTM A312
Standard Specification for Electric-Fusion-Welded Austenitic Chromium-Nickel Stainless Steel Pipe for High-Temperature Service and General Applications ASTM A358
Standard Specification for Welded Large Diameter Austenitic Steel Pipe for Corrosive or High-Temperature Service ASTM A409
Standard Specification For Seamless Stainless Steel Mechanical Tubing ASTM A511
Fittings & Flanges
Summary Standards
Standard Specification for Wrought Austenitic Stainless Steel Piping Fittings ASTM A403
Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service ASTM A182

Shalco industries supply grade 347 in large quantities worldwide. It is available in different forms, particularly in pipes/tubes, fittings, flanges, washers.

The grade has a highly stabilized grain structure, which gives a commendable resistance from intergranular corrosion. The grade resists corrosion after welding due to columbium-tantalum stabilization from carbide precipitation at the heat affected zones.

Hence, it is widely used for heavy welded equipment which do not go for post-weld annealing. The oxidation resistance of the grade is similar to other austenitic stainless steels viz., 18/8 (18% chromium, 8% nickel) In an oxidation test for an exposure for 500 hours at 700 ºC, the grade showed weight change of 0.045 mg/cm^2. Since the grade is suitably resistant to polythionic acid, it is highly used in petrochemical industries, nuclear reactor systems, and refineries. The grade is generally preferred over grade 304 and 321 for general corrosion resistance, aqueous solution as well as low-temperature surroundings. Thermal cyclic load can be resisted by this alloy. It is also less resistant to chloride stress corrosion cracking and it can suitably resist nitric acid and phosphoric acid solutions.

The grade like previous austenitic grade is non-hardenable by heat treatment. Annealing can be used for removal of internal stresses. The process can be done by heating the metal about 980-2000 ºC, soaking it appropriately across the length and then water quenching.

347 is one of the most weldable grades since it has stabilization elements for the heat affected zone as well as fine mechanical properties post-weld. Normally, post weld annealing is not required but it can be welded by common fusion or electric resistance welding methods. For heavy welded structures, this grade is efficiently used.

This grade can be suitably machined by using standards machining techniques with slow speeds, powerful machinery, positive feeds and rigid tooling.

Chemical processing, aircraft collector rings, boiler casings, cabin heaters, pharmaceutical production, petroleum refining, waste heat recovery, furnace heating elements, heat exchanger tubes, radiant superheaters, heavy wall welded equipment, oil and gas refinery piping, reactor components, thermowell.

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