Stainless steel 347 AISI - X6CrNiNb18-10 - Z6CNNb18-11

The role of niobium and stabilization

The key feature of AISI 347 is its niobium stabilization (also called columbium in the United States). This addition addresses a major issue encountered with non-stabilized stainless steels (such as AISI 304): intergranular corrosion.

• The issue addressed: when conventional stainless steels are heated between 500 and 800 °C (for example during welding), carbon reacts with chromium to form chromium carbides at grain boundaries. This phenomenon locally depletes chromium, breaks down the protective layer, and opens the door to corrosion and cracking.
• The stabilization treatment: to be effective, the steel undergoes a stabilization anneal (typically around 885 °C). This treatment drives the reaction between carbon and niobium. Unlike non-stabilized grades, 347 can then be exposed for long periods to critical temperatures (427–816 °C) without risk of sensitization.

Corrosion resistance and service limitations

In general corrosion, alloy 347 offers performance comparable to 304, with excellent resistance in many oxidizing environments and atmospheric conditions. Its main advantage remains its resistance to intergranular corrosion after thermal exposure, significantly outperforming non-stabilized grades.

Limitations: 347 is not intended for reducing or marine environments. It performs poorly in chloride-containing solutions (even at low concentrations) and in sulfuric acid. Use in seawater should be avoided, as it is susceptible to pitting and crevice corrosion. For these environments, molybdenum-bearing grades such as 316L are preferred, as they are more resistant to localized attack.

Hot working (forging and rolling)

Hot working of AISI 347 (forging, rolling, drawing) requires strict temperature control, because the material combines good ductility with high resistance to deformation.

Typically, the recommended process is as follows:

• Heating: the steel should be heated uniformly to between 1 150 and 1 200 °C to make it workable.
• Deformation: it should be carried out progressively and stopped once the metal temperature drops below 950 °C, a threshold below which the material becomes too hard and may crack.
• Cooling: thanks to niobium stabilization, slow cooling through the critical range (500–800 °C) does not create a risk of subsequent corrosion, unlike 304. Cooling can therefore be done in air.

After hot working, a solution anneal (heating to 1 020–1 100 °C followed by rapid quenching in water or forced air) is recommended to restore a homogeneous austenitic structure.

High-temperature behavior and mechanical properties

AISI 347 is specifically selected for its high-temperature mechanical performance. It resists oxidation and scale formation up to about 800 °C. Above that, although oxidation becomes a limiting factor, it retains higher mechanical strength than common stainless steels.

For long-term service at temperatures close to 900 °C, the “H” version, 347H, is preferred. This grade specifies a higher carbon content (0.04–0.10 %) to maximize creep strength and stress-rupture resistance.

Change in mechanical properties

At room temperature (20 °C), in the annealed condition, the alloy typically shows an ultimate tensile strength (Rm) of 640 MPa and a 0.2% proof strength (Rp0.2) of 250 MPa. The table below shows how these values change with temperature:

Note: a marked drop in mechanical properties is observed beyond 500 °C. At 816 °C, the tensile strength is only about one quarter of its initial value, which generally represents the upper limit for structural use.

Industrial applications

It is suitable for pressure vessels operating at high temperature, for example: boilers and heat exchangers.

Aerospace industry: components exposed to thermal cycling. It is used in gas turbine systems (recuperators, piping, combustor parts) due to its stability at high temperature and under pressure. It is also used in aircraft exhaust systems, including ring manifolds.

Petrochemical industry: high-temperature processing equipment in the oil and gas sector, such as cracking units, reactors, and distillation columns.

Power generation industry: steam piping, heat shields, and turbine system components (recuperators, piping, etc.).