AISI 347

What is AISI 347?

AISI 347 is a niobium-stabilised 18/8 austenitic stainless steel. ASTM A240 specifies 17.0 to 19.0 percent chromium, 9.0 to 13.0 percent nickel, and niobium from a minimum of ten times the carbon content to a maximum of 1.00 percent. Niobium binds carbon as niobium carbide before chromium carbide forms. This mechanism preserves resistance to intergranular corrosion after exposure to the carbide precipitation range without reducing the carbon content.

AISI 347 has chemistry and functions similar to those of AISI 321. Welding requirements determine the choice between them. Titanium oxidises readily and does not transfer reliably across a welding arc. A titanium-stabilised covered electrode is therefore unavailable. Niobium transfers across the arc, so AISI 347 is the standard consumable for welding AISI 321. Atlas Steels reports only occasional use of AISI 347 as parent plate material.

AISI 347 serves as a parent metal mainly in high-temperature applications. Niobium carbide remains stable at higher temperatures than titanium carbide. The grade therefore provides more dependable stabilisation near the upper operating range and appears in high-temperature power and refinery practice.

Laxcon Steels lists AISI 347 in its grades reference as an austenitic grade. The same steel is written 347, SUS 347, 1.4550 and S34700.

Nb minimum 10xC per ASTM A240, maximum 1.00

What is the chemical composition of AISI 347?

Composition limits in weight percent are carbon 0.08 maximum, manganese 2.0 maximum, sulphur 0.03 maximum, phosphorus 0.045 maximum, silicon 0.75 maximum, chromium 17.0 to 19.0, nickel 9.0 to 13.0 and niobium 1.0 maximum.

What is AISI 347 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of AISI 347?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Mechanical properties

ASTM A276 includes type 347 with types 304, 316 and 321 in Condition A. Hot-finished annealed bar requires a minimum tensile strength of 515 MPa, a minimum 0.2 percent yield strength of 205 MPa, elongation of 40 percent in 50 mm and reduction of area of 50 percent. ASTM A240 specifies the same tensile and yield strengths for plate, with 40 percent elongation. It limits hardness to 92 HRB or 201 HB. The corresponding limits for AISI 321 are 95 HRB and 217 HB. Cold-finished Condition A bar up to 12.70 mm diameter requires 620 MPa tensile strength and 310 MPa yield strength.

The principal mechanical distinction appears above 500 degrees Celsius. A fine dispersion of niobium carbide restricts movement at dislocations and grain boundaries. This structure gives AISI 347 creep and stress-rupture strength that solution-annealed unstabilised steel cannot match. The controlled-carbon 347H variant applies when this strength must be specified rather than only present.

Niobium and titanium stabilisation

Niobium and titanium both compete with chromium for carbon. This reaction leaves chromium available to maintain the passive film. Niobium carbide dissolves at a higher temperature than titanium carbide, so AISI 347 retains protection nearer the upper operating range. Niobium also transfers across a welding arc and can form part of a consumable. Titanium is lighter and cheaper. It requires about half of the carbon multiple required by niobium. The stabilisation ratio is five times carbon plus nitrogen for titanium in AISI 321 and ten times carbon for niobium in AISI 347.

AISI 304L controls sensitisation by limiting the available carbon. This approach is effective during the seconds that a weld remains in the critical temperature band, but not during years of service within that band. The reduced carbon content also lowers elevated-temperature strength. AISI 347 retains its carbon and strength while directing the carbon into niobium carbide.

Corrosion and elevated-temperature behaviour

The pitting resistance equivalent number of AISI 347 ranges from 17.0 at the specified minima to 19.0 at the maxima. The grade contains no molybdenum and has no specified nitrogen, so chromium alone determines this value. Its aqueous corrosion behaviour resembles that of AISI 304. Its advantage follows thermal exposure. An unstabilised 18/8 steel loses resistance to intergranular corrosion after exposure between 425 and 860 degrees Celsius, while AISI 347 retains it.

Oxidation resistance follows the 18/8 pattern. The grade is useful in intermittent service to about 870 degrees Celsius and in continuous service to about 925 degrees Celsius. Extended service above about 600 degrees Celsius eventually produces sigma phase and reduces toughness. A full solution anneal can reverse this change.

Welding and hot cracking

Rods and electrodes of the 347 and 347Si classes are pre-qualified consumables for welding AISI 321 and AISI 347 parent metals. Niobium in the weld metal increases susceptibility to solidification hot cracking and to microfissuring in reheated passes of a multi-run joint. Contents above roughly 0.3 percent measurably increase this susceptibility in fully austenitic deposits.

Controlled delta ferrite limits this cracking. Phosphorus, sulphur, silicon and niobium form low-melting compounds that segregate at grain boundaries during solidification. These compounds are much more soluble in ferrite than in austenite. A few percent of retained delta ferrite therefore absorbs them and prevents cracks from opening. AISI 347 consumables deposit a controlled ferrite number instead of a fully austenitic weld. Welding procedures also limit heat input and interpass temperature. Nuclear and cryogenic service may restrict ferrite, so the filler selection must resolve the competing requirements.

Applications and designations

AISI 347 is used primarily for welding consumables. Other applications include high-temperature refinery and petrochemical piping, superheater and boiler components, exhaust manifolds, aircraft collector rings, expansion joints in hot gas service, and heat exchangers operating through the sensitisation band. The European designation is material number 1.4550, with the name X6CrNiNb18-10. The UNS designation is S34700. The Japanese designation is SUS 347, also written without a space as SUS347 or SS347. Laxcon supplies the grade under these designations.

Where AISI 347 is not the right choice

AISI 347 is unsuitable when chloride exposure governs material selection. Its PREN range of 17.0 to 19.0 is the lowest among the common 18/8 derivatives because the grade contains neither molybdenum nor specified nitrogen. Its resistance to pitting in seawater, coastal atmospheres and chlorinated water is no better than that of AISI 304. AISI 316 or AISI 316Ti applies when service combines high temperatures with chloride exposure.

AISI 347 is also unsuitable for fabrication without procedure control. Niobium-bearing weld metal cracks more readily than an unstabilised deposit, especially in restrained multi-pass joints and thick sections. Control requires a consumable with a specified ferrite number and disciplined heat input. Welded AISI 347 remains susceptible to knife-line attack in strongly oxidising acids. This attack occurs in the narrow intergranular band beside the fusion line, where niobium carbides dissolved and did not re-form. A post-weld stabilising anneal provides the remedy.

Below about 425 degrees Celsius, stabilisation provides no benefit. AISI 304L is more available and less expensive for the same corrosion result. Above about 900 degrees Celsius, AISI 347 lacks sufficient oxidation and creep resistance. AISI 310S is the higher-temperature alternative.

Other austenitic grades