UGI®347H / UGI®4550: Nb‑Stabilized Austenitic Steel for 450–850 °C Applications

Drawing – Profiling – Forming

UGI®347H / UGI®4550 is suitable for transformation via the usual cold-working processes, in the same way as the majority of austenitic stainless steels. It can easily be stretched and drawn until its cross-section is significantly reduced by as much as 80%.

Cold heading

Due to the presence of Niobium, UGI®347H has a fine-grained structure that makes this material particularly suitable for cold or warm heading. Its fine, regular structure makes it possible to obtain uniform deformation during cold heading.

UGI®347H has excellent intergranular corrosion resistance, better than that of 1.4301 (AISI 304), it is particularly suitable for applications in the temperature range between 450°C and 850°C.

Localized corrosion

Pitting corrosion: Pitting potential measurement tests are carried out in our laboratory in a 5% NaCl environment (50g/L 0.86M) at 6.6 pH and at a temperature of 35°C, in accordance with operating standard ISO 15158. The pitting potential of UGI®347H is similar to that of a standard austenitic steel (for 5 mm diameter wire rod in the longitudinal direction):

Intergranular corrosion: Due to the presence of Niobium, there is significantly less risk of sensitization to intergranular corrosion compared with standard 1.4301 austenitic steels; its maximum working temperature in dry air is 850°C, but this decreases with the presence of corrosive compounds in the air, such as water vapour or sulphur compounds.

Stress corrosion

As for the family of standard 1.4301 / 1.4307 austenitics, UGI®347H is susceptible to stress corrosion caused by chloride ions from approximately 50°C.

Turning

The machinability of UGI®347H / UGI®4550 is similar to that of a low S 304 / 1.4301 steel. In terms of tool wear, compared with a 304 / 1.4301 low S steel, the cutting conditions (speed and/or feed) of UGI®347H / UGI®4550 must be reduced by approximately 10 to 15% to maintain the same useful tool life. In terms of chip breakability, UGI®347H / UGI®4550 is very similar to a low S 304 / 1.4301.

Solution annealing

This heat treatment involves maintaining a high temperature of between 1000 and 1100°C, followed by air cooling or water cooling. This treatment, which is called solution annealing, removes all trace of hardening, while giving the steel its lowest level of mechanical properties.

Stabilization

A treatment known as stabilization can be performed by completely precipitating the carbon and nitrogen in the form of Nb (C,N), for example for welding. This makes it possible to improve the intergranular corrosion resistance. The treatment can be performed at between 815°C and 870°C for a minimum of 5 minutes for each mm of thickness and followed by oven cooling or air cooling (ASTM standard A182 recommendation).

Forging

UGI®347H / UGI®4550 can be forged without the need for any special precautions. A heating temperature between 1150°C and 1250°C and an end-of-forging temperature, preferably above 950°C are recommended. As with all austenitic stainless steel grades, the force required to deform the metal is high (far higher than that required for carbon steels).

Components can be air- or water-cooled.

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As with many austenitic grades, there are no particular difficulties with welding UGI®347H / UGI®4550 via most arc welding processes (with or without filler metal): resistance welding, electron beam welding, friction welding, LASER beam welding, etc. Oxy-acetylene welding is not recommended.

If filler metal is required, a filler product homogeneous with UGI®347H / UGI®4550, i.e. UGIWELDTM 347 (TIG/EE) or 347M (MIG), should be used, which will give the weld metal zone the same corrosion resistance qualities as those of UGI®347H / UGI®4550.

In the case of MIG or TIG welding, the argon-based shielding gas (which can partly be replaced with helium), must not contain nitrogen, otherwise there is a risk that the weld metal zone will no longer be stabilized with Nb and its intergranular corrosion resistance will be lost when assemblies obtained are used at high temperature.

Niobium carbonitride precipitation heat treatment is recommended (about 850°C: see the "Stabilization" section below) for welded areas when assemblies are used at high temperature.