But with good experience from Ductile Pipe in service, preference for Ductile Iron progressively increased. In the mid seventies, all major producers went totally to Ductile Iron and abandoned gray iron pipe. Those producers who resisted Ductile Iron were eventually closed or purchased. Processes and Properties With conversion to Ductile Iron, pressure pipe production became an even more specialized process. Pipe producers generally melt in large water-cooled cupolas with the latest accessory equipment.
Some producers duplex in electric furnaces for more positive supply and control.
In the early years, some used a basic slag to obtain low sulfur from the cupola. In recent years, general practice has been to melt with an acid to neutral slag and desulfurize externally using on of several available agitation methods. Low sulfur iron is then treated with magnesium by several of the available treatment processes. Magnesium treated iron with closely controlled chemistry and temperature is poured at a controlled rate into molds rotation at proper centrifugal speed with post inoculant added in the stream and mold. After stripping, the pipes are fully annealed through long annealing furnaces, some as long as 240 feet.