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 hollow-bar format, 1.4404 is standardised in EN 10297-2. Tibnor’s programme covers tubes with outside diameters in the range 32-224 mm and inside diameters between 16 and 180 mm. The tubes are manufactured by hot rolling in a tube mill or by hot extrusion. The hot-formed tubes are then finished by annealing and pickling.
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.
1.4404 is characterised by excellent formability and weldability so that if needed, welding or cold-forming are seldom an issue. On the other hand, austenitic stainless grades like 1.4404 are generally troublesome to machine but working is facilitated through bespoke tooling. If good machinability is an important consideration for the application at hand, variants of 1.4404 are available for which the steel has been metallurgically treated for improved machinability.
Hollow bars on offer in grade 1.4404 have after hot forming been annealed and pickled. The surface finish, designated HFD in EN 10297-2, is metallically clean without oxide but some minor imperfections from hot forming can remain. The tolerances on outer diameter (D) and wall thickness (T) are stipulated in detail in EN 10297-2 and EN-ISO 1127 but are at most ±1.5% of D and ±15% for T. Straightness is good with the allowed height of arc at most 0.002 times length.