Where phosphating is used for the cold drawing of
seamless tube, the requirements are somewhat different from those
applicable to welded steel tube. Until about 30 years ago, phosphate
coating weights of 20-40 g/m²
were used allowing the largest possible number of successive draws.
Such coatings were formed with nitrate-accelerated systems at 90-95°C.
Over the years, in search for increased
operational efficiency, the number of draws for a given reduction of
seamless tubing, has decreased. In order to maintain the dimensional
tolerances and quality of surface finish, tubes in the mid-range of
available sizes are now reduced in a single or, at most, two draws.
At the same time, drawing rates have been increased. For these
reasons phosphate coating for drawing of seamless tubing are now
formed with weights of 4-10 g/m².
This has improved the efficiency of the surface treatment and, at
the same time, avoided the adverse effects which act in the firs
drawing stage where coarser-crystalline phosphate coating are found.
The most suitable coating is based on nitrate/nitrite accelerated
zinc phosphate, formed at 40-75°C.
At the upper end of this temperature range, the option exists to use
self dosing nitrate type systems. Chlorate accelerated zinc
phosphate baths are also found. In all cases, the preferred form of
the phosphate for cold drawing of tube and section is strongly
adherent but soft structured.
In the drawing of welded tubing, the seam must
first be ground down. In the case of smaller diameter tubing, this
is not possible inside the welding machine. In some cases, there may
be a deformation to give a particular cross-section. Since, as a
rule, less severe deformations can be tolerated by welded, as
opposed to seamless tubing, the use of phosphating is widespread,
coating weights being of the order 1.5 - 5 g/m².
These are mostly based on zinc phosphate baths operated between 50
and 75 °C
with additives used to promote thinner coatings.
Phosphating is also used for tubing of un-alloyed
or low-alloyed steel with chromium content up to 4-6%. Such
coatings offer a number of advantages, all arising from reduced
metal-to-metal contact between tube and die. Thus, cold welding
damage, leading to grooving or crack formation, is minimised, tool
and die life is extended and higher drawing rates may be used.
Zinc phosphate coating also allows a greater degree of reduction per pass and
an increased number of passes without intermediate heat-treatment.
Phosphating is carried out by immersion along the
Pickling in sulphuric or hydrochloric acid.
Drying and storage.
In earlier times, the surface treatment of tubes
took place using bundled tubes lifted by a manually-operated hoist
from one tank to the next. More recently, automatic conveyors have
been introduced and, while retaining the sequence of operations
listed above, the whole operation takes place automatically.