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to concrete. It was the goal of this research project to develop Pretreatment and Lubrication
such methods.
A more complete description of the history of strand bond- The character and quantity of the residual film on the
related research is given in Appendix A. prestressing strand is governed by the pretreatment of the
rod, lubricants used during manufacture, and postdrawing
processes including stress relieving. The purpose of the pre-
Manufacture and Surface Condition treatment, which typically is conducted on the spooled rod
of Prestressing Strand stock, is to provide a foundation for the drawing lubricants.
The production of prestressing strand, as observed at one The drawing lubricants are applied to minimize friction,
manufacturing facility, includes the following basic compo- which dictates the amount of energy required for drawing,
nents (Figure 1): and to prolong the life of the dies.
The cleaning and pretreatment performed on the rod stock
· Coils of nominal 1/2-in. AISI C1080 steel rod stock are are critical and influence all remaining steps in the produc-
cleaned and pretreated. This can be achieved using a variety tion process. This is because the surface quality of the result-
of methods; in one example, the raw steel coils are cleaned ant steel that must be subsequently drawn through the dies
by pickling (dipped in acid, then rinsed with water) and governs the lubricant selection and effectiveness.
then phosphate treated (coils are submerged in a zinc phos- For strand production, the most common coating applied
phate solution, then rinsed in water and dried). to rod stock during the pretreatment process is zinc phosphate,
· Rods from several coils are butt welded end to end, and which serves as a carrier for the lubricants applied during
then fed into the wiredrawing machine. the wiredrawing process. The phosphating process often con-
· The wire drawing machine consists of a series of eight sists of some of the following steps: (1) mechanical cleaning,
successively smaller dies that draw down the rod stock to (2) pickling--cleaning in acid, (3) rinsing--using neutralizing
wire with a diameter of about one-third the strand diame- lime solution or water to remove all chloride if hydrochlo-
ter. For 1/2-in. diameter strand, the center, or "king" wire, ric acid solution (HCl) is used for pickling, (4) activating
has a diameter of 0.174 in., approximately 5% greater prerinse--a dip in a solution or suspension of titanium phos-
than that of the six outer wires. Integral with each die is phate to produce a thinner zinc phosphate layer, and (5) zinc
a box containing wiredrawing lubricant that the wire phosphating in a solution that may also contain orthophos-
passes through before entering the die. (This arrange- phoric acids and sodium nitrate (Wire Industry 1992, Liberti
ment allows for different lubricants to be used with dif- 1994).
ferent dies, and it is not uncommon to use a different The cleaning (i.e., descaling) of the wire may be done me-
lubricant for the first ["ripper"] die than for subsequent chanically or chemically. Mechanical methods of descaling
dies.) Both the die and the capstan, which pulls the wire include reverse bending, belt sanding, or shot blasting.
through the die, are water cooled since the performance of Chemical descaling usually involves a soak in an acid solution
the lubricant and properties of the wire are very sensitive and is typically more effective than mechanical methods.
to temperature. At the end of the machine, the individual As a result, acid baths are most common in strand plants, since
wire is spooled. high carbon steel is difficult to clean using other methods.
· The spools of wire are installed in a skip strander. In this Zinc phosphating deposits a thin layer of zinc phosphate
machine, the six outer wires are helically wound around crystals on clean wire to act as substrate for dry lubricating
the king wire to form the seven-wire strand. soaps and to provide a barrier to metal-to-metal contact. The
· The strand is drawn under tension through an induction best results are achieved with a dense layer of fine crystals.
furnace. This stage imparts the stress relieving and low- The phosphating process is influenced by temperature and
relaxation properties to the strand. The plant visited has a acidity of the phosphate bath, since these factors influence the
box installed at the end of the induction furnace that con- phosphate solubilities. A longer immersion time will produce
tains equipment to wash and cool the strand. a heavier coating (Liberti 1994). It has been observed that zinc
· A venting hood is situated between the furnace and the phosphate/lubricant coated wires have better corrosion per-
final cooling bath to draw off any vapors created by treat- formance suggesting that a residual film may be left on the wire
ment in the induction furnace. produced with such a pretreatment (Rutledge 1974). Phosphate
· The strand is cooled in a spray chamber with recirculated coatings are themselves difficult to remove (Wire Industry
water. This process may remove some additional wire- 1992).
drawing lubricants if they are sufficiently water soluble. Borax and lime may also be used for pretreatment, either
· The strand is spooled, packaged, warehoused, and shipped alone or in combination with zinc phosphate. A fundamental
to customers. difference between phosphate coatings and other coatings
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I
II
III
IV
Figure 1. Schematic of strand manufacturing process.
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such as borax is that the phosphate reacts with the steel surface is applied to the wire and how quickly it is removed during
to provide the foundation for lubricant, but the borax does not. drawing. If the lubricant is too soft at operating temperatures,
Borax is more likely to be removed during processing than it may come off before the drawing is complete. If too hard,
phosphate coatings but is less effective at aiding lubrication the lubricant film is not applied uniformly and scratching or
(Hajare 1998). Therefore, although a non-phosphate-based feathering (flaking) will result (Gzesh and Colvin 1999). The
process is used in at least one North American manufac- temperatures generated at the strand surface are determined
turer's plant for pollution control reasons, that approach is not by the plant configuration, including the drawing speed, die
common. The most common combination of pretreatments geometry, and area reduction in each die. As a result, the de-
is to follow the zinc phosphate treatment with a bath in hot sirable lubricant properties vary from plant to plant and even
borax solution prior to drying. The alkalinity of borax serves from die to die. The softening point is determined by the
to neutralize acid from the pickling process not removed by alkali and fatty acids (such as stearic acid or tallow acids) on
washing. However, borax has some disadvantages including which the lubricant is based. The viscosity of the lubricant,
its highly hygroscopic nature (i.e., it absorbs water) (Wire In- which affects the thickness of the coating applied to the steel,
dustry 1992). It should also be noted that both zinc and borax is determined by the fat content and filler materials. To min-
can retard Portland cement hydration. imize the amount of residual film, the ideal lubricant system
Following the pretreatment processes, lubricant is applied to would be one that, at the plant die operating temperatures,
the wire at each die during the drawing process. Dry lubricants provides a coating of just sufficient thickness to facilitate
are used exclusively by strand manufacturers in the United drawing, but which would be nearly all removed from the
States in the wiredrawing process for strand. The lubricity wire by the dies.
agent in such lubricants is typically a chemical compound of As previously stated, the two most common lubricants are
a metallic element (calcium, sodium, aluminum, potassium, sodium and calcium stearate-based materials. These lubricants
barium, or combinations of these) plus a fatty acid (such as are compounds made from sodium hydroxide or calcium
stearic acid). The dry lubricants may also contain borates. hydroxide and a fatty acid (stearic acid) in combination with
It is commonly felt that calcium-based dry lubricants provide additives to impart special properties to the lubricant. These
the lubricating properties needed for wiredrawing more cheaply materials are soaps (Ivory soap, for instance, is 99% sodium
and effectively than any other material. stearate) and are supplied in dry form. Calcium stearate typ-
Powdered wiredrawing lubricants are usually classified by ically has a lower softening point than sodium stearate (Gzesh
their solubility in water. Insoluble lubricants are usually cal- and Colvin 1999).
cium based (e.g., calcium stearates; partially soluble lubricants Calcium stearate may have been more appealing to strand
are usually mixtures of sodium stearates and calcium stearates; manufacturers at one time because its lower softening point
and soluble lubricants are typically sodium stearates). Within permits calcium stearate to produce a more effective coat-
each classification, additives are used to modify the proper- ing on the wire early in the drawing sequence, when the rate
ties of the lubricant to a considerable extent. Thickeners or of draw is slower, and the wire is at lower temperatures.
fillers are usually unreactive, fine powders blended into the In addition, it is typically cheaper than sodium stearate. As a
lubricant base to increase its viscosity, or resistance to flow result, it is not uncommon for calcium stearate to be used
under pressure at a given temperature. Although lime (possibly for the first one to three drafts (dies) and then for sodium
limestone dust in quantities of 30% to 70%) is the most pop- stearate to be used for the remainder (Wire Industry 1991).
ular thickener additive in general wiredrawing lubricants, The drawback in the use of calcium stearate-based lubri-
the choice of thickener depends on the application and the cants is that they are more difficult to remove from the
end use of the wire; for example, coatings that must be easily drawn wire, since they are water insoluble. In fact, calcium
cleaned should contain soda ash, borax, or other soluble stearate lubricants may be chosen in certain applications
material. Calcium and sodium sulfate compounds are also (such as nails or coat hangers) because they leave a residue
potential fillers. Extreme pressure additives are used in dry film that makes certain subsequent wire processing proce-
lubricants to reduce friction and increase die life. Molybdenum dures easier (Platt 1991). On the other hand, sodium soaps
disulfide is the most popular of such agents; however, it is or lubricants are "generally used when subsequent operations
relatively expensive and may leave a very slippery and difficult- demand wire that may be readily cleaned" (Wire Association
to-clean surface on finished wire. Graphite, sulfur, chlorine, 1965, p. 285).
and phosphates are also possible additives (Gzesh and Colvin The strategy of using calcium stearate in only the first
1999). die(s) does not necessarily limit the residual film of the final
One of the key properties that determines which lubricants product. This is because 80% of the lubricant needed is applied
are most suitable in a given operation is the softening point in the ripper box (the first die). Subsequent applications of
(related to melting point) since this governs how the lubricant lubricant retard loss, but typically do not significantly add
