Stainless Steel Grade AISI 904L | EN 1.4539 | DIN X1NiCrMoCu25-20-5

Steel AISI 904L | EN 1.4539 | DIN X1NiCrMoCu25-20-5 is a super austenitic chromium-nickel stainless steel. It is characterized by increased corrosion resistance in highly aggressive environments. It is particularly resistant to sea water, has high resistance to pitting, crevice and intercrystalline corrosion.

It should be noted that the level of corrosion resistance of the AISI 904 L grade is an order of magnitude higher than that of metals of the chromium-nickel-molybdenum group.

The high nickel content in its composition provides the steel with excellent resistance to stress corrosion cracking. Copper contributes to better resistance to acids, especially in the most aggressive range of medium concentrations.

This super austenitic stainless steel is sometimes classified as a nickel alloy. It is most often produced in the form of rolled products and flat products, which are mainly used in the chemical, energy and petrochemical industries. This metal is resistant to sulfuric acid, phosphoric acid, nitric acid and orthophosphoric acid in various concentrations at high temperatures.

AISI 904L can be used in sea water up to 70 °C, in sulphuric or phosphoric acid and in areas containing chlorides. Other areas of application for EN 1.4539 material are the construction industry, the chemical industry, machines used for the production of artificial fertilisers, and the medical and pharmaceutical industry.

In general, AISI 904L can work well in cold conditions. However, higher forming forces are required to strengthen the material. AISI 904L can be used in the temperature range from - 60 °C to 400 °C.

Hot forming

Hot forming of AISI 904L is possible at 1200 °C - 900 °C. Cooling takes place in air. To avoid the formation of brittle and intermetallic phases, the rest time in the range of 600 °C - 900 °C should be kept to a minimum.

Welding

The steel is easily welded using standard methods. No heat treatment is required after welding, and the weld should be cleaned of scale and passivated.

When selecting a filler alloy, corrosion stress should also be considered. The use of a high-alloy filler metal may be necessary due to the cast structure of the weld metal.

Preheating is not necessary for this steel. Post weld heat treatment is usually not common. Austenitic steels have only 30% of the thermal conductivity of unalloyed steels. Their melting point is lower than that of unalloyed steel, so austenitic steels must be welded with less heat input than unalloyed steels. Higher welding speeds should be used to avoid overheating or burning through thinner sheets. Copper backing plates are functional for faster heat dissipation, while surface fusion of the copper backing plate is not allowed to avoid cracks in the solder metal. This steel has a significantly higher coefficient of thermal expansion than unalloyed steel. Due to the poorer thermal conductivity, more distortion should be expected. When welding in accordance with EN 1.4539, all procedures to counteract this distortion must be observed (e.g. welding in reverse order, welding alternately on opposite sides with double V-butt welding, assignment of two welders if the components are correspondingly large). For products thicker than 12 mm, double V-butt welding should be preferred to single V-butt welding. The included angle should be 60° - 70°, when using MIG welding, about 50° is sufficient. Accumulation of welds should be avoided. Tack welds should be attached at a relatively shorter distance from each other (significantly shorter than in the case of unalloyed steels) to prevent severe deformation, shrinkage or delamination of the tack welds. The tack welds must subsequently be ground or at least freed from crater cracks. EN 1.4539 in connection with the austenitic weld metal and excessive heating there is a dependence on the formation of thermal cracks. The tendency to thermal cracks can be limited if the weld metal has a lower ferrite content (delta ferrite). A ferrite content of up to 10% has a beneficial effect and generally does not affect the corrosion resistance. The thinnest possible layer should be welded (stringer technology), since a higher cooling rate reduces the tendency to hot cracks. During welding, rapid cooling should be aimed at to avoid vulnerability to intercrystalline corrosion and brittleness. EN 1.4539 is very suitable for laser beam welding. For a weld groove width of less than 0.3 mm corresponding to a product thickness of 0.1 mm, the use of filler metals is not required. With larger weld grooves, a similar filler metal can be used. To avoid oxidation on the weld surface, weld with a laser beam using reverse welding, such as helium as an inert gas, the weld seam is as corrosion-resistant as the base metal. There is no danger of hot cracking for the weld seam when selecting the appropriate process. EN 1.4539 is also suitable for laser cutting with nitrogen or gas cutting with oxygen. The cut edges have only small heat-affected zones and are generally free of microcracks, so they are well formed. When selecting the applicable processes, edges cut by fusion can be converted directly. In particular, they can be welded without additional preparation.

During processing, only stainless steel tools such as steel brushes, air cutters, etc. may be used to avoid the risk of passivation. Marking the weld area with oil bolts or temperature indicator pencils should be neglected. The high corrosion resistance of this stainless steel is based on the formation of a homogeneous, compact passive layer on the surface. Annealing paints, scale, slag residues, cast iron, spatter, etc. must be removed so as not to destroy the passive layer. Brushing, grinding, digestion or blasting (iron-free quartz sand or glass spheres) can be used to clean the surface. Only stainless steel brushes may be used for cleaning. Pickling of the pre-treated weld area is carried out by immersion and spraying, but pickling pastes or solutions are often used. After pickling, rinse thoroughly with water.

Mechanical properties

Physical properties

Heat treatment

The recommended annealing temperature for deformed AISI 904L steel is 1095 °C.

Peculiarities

Due to the high content of nickel and chromium, under severe conditions, steel significantly reduces the development of corrosion, and the high density of chromium and molybdenum promotes passivation of metal surfaces and improves the mechanical properties of products. In addition, the high concentration of copper helps to increase the resistance of steel to reducing gases, increase plasticity and resistance to acids.

EN 1.4539 is one of the best acid-resistant austenitic steel grades, an alternative to EN 1.4547 and EN 1.4529 steels. Duplex steels with a ferritic-austenitic structure EN 1.4462 and S32205 have comparative corrosion resistance to AISI 904L, but their improved mechanical properties can limit the use of materials in certain situations, for example, reduced ductility. In some applications, the austenitic structure is irreplaceable.

In addition to the above applications and environmental resistance, the grade tolerates well the presence of formic acid, acetic acid, oxalic acid, citric acid, lactic acid, high concentrations of salt solutions, nitrates, chlorides, hydrogen sulfide, sea water, acetate. The material with a given chemical composition can work at elevated temperatures up to approximately 400 ℃.

Application

Stainless steel AISI 904L has excellent technical characteristics that make this grade suitable for use in many areas. It is especially recommended for use in highly aggressive environments, including radioactive ones. Stainless steel AISI 904L EN 1.4539 | DIN X1NiCrMoCu25-20-5 is used in:

- chemical, oil and gas, food, pulp and paper industries;
- medicine and pharmaceuticals;
- construction, watchmaking, mineral fertilizer production;
- creation of welded structures, heat exchangers, pipelines, tunnels, reactor structures;
- manufacturing of parts for operation in solutions containing chlorine ions, sulfuric and phosphoric acid;
- manufacturing of parts for operation in sea water;
- welding machines, flue gas cleaning equipment;
- manufacturing of containers for operation at high temperatures.