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The austenitic steel with designation 1.4404 contains about 2% molybdenum in addition to chromium and nickel. The difference against 1.4307 apart from the presence of molybdenum is that the nickel content is greater. This type of stainless steel is often referred to as acid resistant which is to some degree a misnomer since the material resists attack only by weak acids. 1.4404 has low carbon content which means that it can be welded without risking impairment of corrosion resistance in the vicinity of the weld. In terms of strength, 1.4404 is more or less equivalent to the lower alloyed grade 1.4307.
In the form of welded tubes, 1.4404 is standardised in EN 10296-2. Tibnor’s stock-standard programme covers tubes with outside diameters in the range 8-204 mm and wall thicknesses between 1.0 and 3.6 mm. The tube manufacturing process starts from sheet that has been annealed, pickled and cold rolled (usual finish is 2B) which after slitting is bent into a round shape and longitudinally welded. The welding process can vary but for thin-walled tubes, resistance butt welding is the usual method.
The presence of molybdenum along with chromium renders 1.4404 resistant to corrosion in weak acids and water containing chlorides so long as the chloride concentration is not too high; an example of an environment where the steel works well is salt-laden coastal atmosphere. The material is also a common choice for equipment in professional kitchens and for large-scale processing of food and pharmaceuticals.
1.4404 has good formability and weldability and these favourable characteristics are retained even when the product form is a welded tube. In the context of cold formability, welded tubes are subjected to extensive testing in order to ensure that there is no risk for cracking at or in the vicinity of the weld in the event the tubes need to be processed via cold forming.
Welded tubes that are stocked in grade 1.4404 have after welding been annealed and pickled once again or bright annealed. The surface finishes in these instances, which are designated in EN 10296-2 as W2A or W2R respectively, are metallically clean/bright and without oxide but with the weld still discernible. It is permitted that the weld bead is raised somewhat. The tolerances on outer diameter (D) and wall thickness (T) are stipulated in detail in EN 10296-2 and EN-ISO 1127 but are at most ±1% of D and ±10% of T. Straightness is good with the allowed height of arc at most 0.002 times length.