UGI® 4539

UGI® 4539 is quite ductile and forms easily. The high strength of the high nitrogen and molybdenum containing UGI® 4539 is clearly demonstrated when the stress-strain curves of this grades is compared with the standard austenitic product 304/304L grades.

Although originally developed for its uniform corrosion resistance to sulphuric acid, it also has a very high resistance to a wide range of environments. Its value of PREN* between 33 and 38 indicates that the material has good pitting corrosion resistance to warm sea water and other high chloride environments.

*The temperature limit can be influenced too by the surface: roughness …

Localized corrosion

Pitting corrosion: Chloride ions in a neutral or acidic environment facilitate local breakdown of the passive layer. As a result, pitting and crevice corrosion can propagate, causing corrosion failures. The PREN value can be used for rough comparisons of different grades. A more reliable means, however, is to rank the grades according to the critical pitting temperature (CPT). There are several different methods available, for example ASTMG 150 with a 1M NaCl solution (35 000 ppm chloride ions). The CPT values are shown in the table below:

*(%Cr+3.3*%Mo+16*%N) **The CPT can be influenced too by the surface: roughness …

Intergranular corrosion: UGI® 4539 has very low carbon content. This means that there is very little risk of carbide precipitation in connection with ordinary heat treatment and welding. The risk of intergranular corrosion after ordinary heat treatment and welding is thereby eliminated.

Stress corrosion: Owing to its high nickel and molybdenum contents, UGI® 4539 is highly resistant to stress corrosion cracking in chloride solutions, or concentrated hydroxide solutions and in environments rich in hydrogen sulphide (oil and gas market). The graph below represents for different grades the range of cracking resistance in function of temperature and H₂S pressure: the limit for UGI® 4539 is in blue on the graph on the right side of the material page.

Like other austenitic steels, UGI® 4539 is tough and is prone to develop cold work-hardening. Moreover its sulphur content is kept low. All of that makes it less machinable than 410 and 304 grades. However with the right choice of tool and machine data, fully satisfactory machining results can be obtained on it.

Solution annealing

Optimal mechanical and fabrication properties are realized after solution annealing in the temperature range 1060°C – 1150°C followed by rapid cooling in air or water. Exposure to temperatures in the range 600°C – 900°C must be minimized to prevent the formation of brittle and less corrosion resistant intermetallic phases.

Hardening

UGI® 4539 cannot be hardened by heat treatment but it can be hardened by cold working.

Forging

UGI® 4539 is usually pre-heated to between 1150 – 1180°C with forging taking place between 1180 and 950°C. After forging, the forged component must be rapidly cooled in air or in water when no danger of distortion exists.

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Pickling

This process is necessary when oxidized surfaces are present during heating. An acid bath must be used: hydrochloric acid, or sulphuric acid, or mixed nitro-hydrofluoric acids. Warming the bath (e.g. to 50°C) reduces immersion time. Finally, it is absolutely essential to rinse the surfaces thoroughly.

Passivation

Stainless steels, especially those with high Chromium/Molybdenum content, do not generally need to be passivated as the passive film forms spontaneously in air and, after 24 hours, it achieves a certain stability. On the other hand, stainless steels can be contaminated by contact with carbon steels or low-alloy steels: this phenomenon is known as ferrous contamination. In this case, nitric acid passivation for 30 minutes at ambient temperature is required. Surface rinsing or even surface neutralization is essential as a final treatment.

Electropolishing

The usual molybdenum austenitic steel electropolishing conditions can be used for UGI® 4539

UGI® 4539 during welding displays a tendency towards hot cracking due to its full austenitic solidification structure. Nevertheless it is readily weldable using all standard welding processes on SS Austenitics. Welding without the use of a filler metal is not recommended since this will promote the tendency for hot crack formation. When using matching filler materials, care must be taken to limit the heat input to low values which could result in poor fusion.

Weaving during welding must also be avoided in order to minimize the heat input. Heat treatment after welding is not necessary, and even large sections are resistant to inter-crystalline corrosion after welding, due to the low carbon content.