The yield strength (0.2% proof stress) of prestressing steels should be high to make their use effective and economical. Depending on the product type, different treatments can be used to increase the strength. Prestressing steels of up to 16mm diameter are produced in wire rolling mills, wound into coils and used as wires or stranded wires. Diameters between 16 and 36mm are also rolled and used as bars. For wires, there are two different methods for obtaining high yield strength values: quenching and tempering of low-alloyed steels, and cold straining by drawing unalloyed steel.
For the first method, the desired strength is reached by quenching and tempering wires hot-rolled into smooth or ribbed products. Continuous welded coils made of smooth or ribbed wires are austenitised, and subsequently quenched and tempered to achieve a fine-grained microstructure with finely dispersed precipitates. In the cold-straining method, steel wires hot-rolled in a smooth groove and with a ferritic±pearlitic microstructure are drawn through a die plate at ambient temperature. The reduction in cross-section leads to an increase of the dislocation density, which results in an increase of tensile and yield strength (or 0.2% proof stress), respectively. Some of the wires are subsequently cold-shaped. Finally, all cold-drawn wires are heated to moderate temperatures (annealed wires), some under tensile stress (stabilised wires). Stranded wires are made from cold-drawn smooth wires using stranding machines with specified lengths of twist. The necessary heating (annealing) takes place after stranding, partly under tensile stress (stabilised stranded wires). Using offset welding of single wires, stranded wires can be produced in nearly unlimited length without reducing the load capacity.
The high yield strength of bars (usually >16mm diameter) is achieved by a combination of two measures related to chemical composition and cold straining. The chemical composition of these steels leads to a pearlitic micro- structure after cooling from the rolling temperature; adding vanadium results in strengthening precipitations which lead to increasing yield strength values. Cold-straining by drawing and subsequent heating to temperatures of 250 to 350ëC, results in further improvements in the elastic limit and yield strength when compared to the as-rolled condition.
In addition to the methods for achieving high yield strength values, all cold deformed prestressing steels are annealed to moderate temperatures in order to improve long-term behaviour under fatigue loading conditions. For drawn wires and stranded wires, this process is mostly done under tensile stresses to modify unfavourable relaxation behaviour.
To achieve sufficient fatigue strength in ribbed and shaped bars, the geometry of the ribs is also significant, and there are special requirements, depending on the dimensions of the bars, in particular for maintaining minimum radii at the transition of the cross-sections to avoid significant stress concentrations in these areas. Table 6.1 gives an overview of standard approved prestressing steels while Table 6.2 shows examples of the chemical composition of standard prestressing steels.