Three main stages are involved in the manufacture of pipes and fittings:
- metal preparation: blast furnace, cupola, metal treatment,
- pipe spinning/foundry casting,
Molten metal can be obtained directly by reduction of iron ore in a blast furnace, or by melting pig iron and scrap in a cupola (or electric furnace). In all cases the materials have to be selected and checked carefully, in order to produce a very high purity base metal suitable for the treatments described below. After desulfurization, the iron temperature is adjusted in an electric furnace, to provide the optimum casting temperature.
At this stage, corrections can be made to the chemical composition by additions of scrap metal, or specific ferro-alloys. Magnesium is introduced into the molten metal, to render it ductile.
The pipe spinning process consists in spreading a layer of molten iron inside a rapidly rotating cylindrical mould, and solidification of the metal by continuous mould cooling. The principal methods used are the “de LAVAUD” process and the “WET SPRAY” process. In the “de LAVAUD” process, molten metal is poured into an uncoated steel mould and is subjected to rapid cooling. A graphitizing, then ferritizing heat treatment is necessary to obtain pipes with the required structure and mechanical properties.
In the “WET SPRAY” process, before the iron is poured, the internal surface of the mould is coated with a fine layer of powdered refractory silica, which reduces the thermal conductivity of the molten metal / mould interface. The pipe therefore cools at a slower rate than in the “de LAVAUD” process and consequently only a ferritizing heat treatment is required.
On leaving the heat treatment furnace, the pipes receive an external coating of pure metallic zinc, applied by electric arc melting zinc wire and then by spraying
Many types of inspections and tests are carried out to guarantee quality : checking the structure and mechanical properties of the metal, visual inspection, dimensional checks, individual hydrostatic tests. Particular attention is paid to spigots and sockets because of their importance in joint sealing. The mortar lining is centrifugally applied. In the method used by PAM, the mortar is poured into the pipe and then spun at high speed, which has the effect of giving the lining good compaction.
The cement mortar is then heated under controlled temperature and humidity conditions. After maturation of the mortar, the pipes move on to the coating lines. A layer of bituminous paint is then applied by spraying on top of the zinc. The pipes are then bundled (DN ≤ 300) and put into stock to await dispatch.
Production of ductile iron fittings and accessory items follows the same pattern (metal preparation, casting, finishing and coating), except heat treatment.
Various moulding processes are used in casting, depending on the dimensions and type of casting being made.
The principal processes used by are:
- compacted sand moulding (flask less or box) on high output automatic lines, for small diameters,
- vacuum sand moulding (V-Process) on an automatic line for medium diameters,
- self-setting sand moulding, for large diameters.
On leaving the casting shop, the castings have their running systems removed, then are shot blasted and fettled. Finally they are air tested before being given a dipped or sprayed bituminous or epoxy coating.