National Standard: GB/T9112-2010 (GB9113.1-2010～GB9123.4-2010)
Chemical Industry Standards: HG5010-52～HG5028-58, HGJ44-91～HGJ65-91, HG20592-2009 series, HG20615-2009 series
Machinery Industry Standards: JB81-59～JB86-59, JB/T79-94～JB/T86-94, JB/T74-1994
Pressure Vessel Standards: JB1157-82～JB1160-82, NB/T47020-2012～NB/T47027-2012, B16.47A/B, B16.39, B16.
Another meaning of "national standard flange" is: flanges manufactured according to the dimensions and tolerance range required by national standards. This is different from flanges not manufactured according to standard dimensions, also known as "second-standard flanges" (it is incorrect to call them non-standard flanges). Some unscrupulous merchants will reduce the thickness and outer diameter of the flange to save materials. Some also use scrap steel or offcut steel to process flanges. Usually, this kind of steel is waste material that does not meet the standards for chemical composition and mechanical properties. Even worse, some use steel illegally smelted by black steel mills to produce flanges. The steelmaking technology used in this illegal steelmaking is outdated and cannot guarantee mechanical properties and weldability. When using it, it may not be able to be welded to steel pipes, or the steel itself may have cracks or pores, which will cause water leakage after welding. Therefore, when purchasing flanges, try to choose national standard flanges.
