Effect Of Nitrogen, Manganese, Nitrogen-Chromium On Welded Pipe Fittings
Nitrogen acts as a hardener in butt welded pipe fittings. The mechanical properties (yield strength) of butt welded pipe steels with higher manganese, nitrogen and chromium contents are also relatively higher, sometimes than the hardness of 304 chrome-nickel steel. The strength is 30% higher. However, these high-nitrogen steel butt-welded pipe parts are difficult to form, thus increasing the processing cost.
Softer steel grades can be easily “deep-drawn”, with less energy consumption in processing, and on the other hand, processing equipment that was originally designed to process the extensible 300 series of steel grades.
This softer steel grade can be obtained simply by reducing nitrogen, which inevitably reduces chromium. Copper addition is a solution that keeps the nickel-chromium content stable while reducing nitrogen. The austenite phase forming ability of copper can be further utilized to replace and reduce the nickel content in the steel (when nitrogen is fixed).
The more copper content, the easier it is to be stamped, and the less nickel, the cheaper the steel.
These relative improvements do not mean that the "200" steel grade can always replace the "300" steel grade, because the corrosion resistance of the "200" steel grade requires a very narrow range of applications.
The chromium-manganese series is sensitive to martensitic transformations, a microstructural change that can occur during material deformation, especially for low-nickel steels.
The consequence of the phase change is that 4% nickel steel, especially 1% nickel butt welded pipe fittings, will clearly exhibit delayed cracking after deep drawing. Although the addition of a heat treatment after the processing step can avoid the above problems, it increases the manufacturing cost.