Z Quality plate tested through and through

"Z" quality plate is often specified for critical applications in structures such as flanges and oil rigs. What does Z Quality plate mean"

It means a low sulphur steel with a tested level of ductility through the z axis of the plate, as opposed to along the transverse or longitudinal axes.

Also referred to as through thickness tested (TTT) plate, it is usually specified where the chief design concern is high loads perpendicular to the plate surface. Plate thicknesses that can be Z tested range between 16 mm and 75 mm.

In applications such as the nodes of an offshore structure, Z testing ensures the plate can cope with the through thickness tensile forces involved. It refers to a tensile test conducted perpendicular to the surface of the plate, specifically to measure ductility rather than strength. In effect, the test replicates the forces involved in pulling the two faces of a plate apart.

Testing

Much of the work in Z testing goes into preparing the sample. Firstly, a cylindrical plug of steel is taken from a sample of the plate.

Next, in order to provide a test piece long enough to fit the tensile testing machine, stubs are welded to the ends of the cylinder (the former plate surfaces) using flash butt welding. The whole piece is then machined into the appropriate size for a tensile test.

It is this welding that limits the thickness of plate that may be Z tested, as the heat leads to changes in the plate's microstructure. If the plate was too thin, the proportion of the sample affected would be large and this would affect the result obtained in the through thickness test. Hence, a minimum plate thickness of 16 mm is set to ensure a valid test result.

Calculations

The Z test is carried out in the same manner as a tensile test except that, as already stated, it measures ductility, not strength. In Z testing the percentage reduction in cross sectional area in the Z direction, or "RAz" is the desired measure.

To obtain the RAz figure, the cross sectional area after testing (Rf) is deducted from the cross section before testing (Ro) and the result is expressed as a percentage of the original cross section
(RAz = (Ro - Rf) / Ro x 100).

Therefore, a higher RAz figure represents a greater percentage reduction in cross section before failure. This in turn reflects greater ductility of the steel in the Z direction as the steel will deform to a greater extent prior to failure. Through thickness test requirements are detailed in Australian Standards AS/NZS 3678 and AS 1548.

Most specifications for Z grade steel will require a certain minimum RAz figure, generally in the order of 15, 25 or 35 per cent. These figures appear to have been determined from practical experience.

The required RAz figures are commonly incorporated into the grade name of through thickness steels, particularly for pressure vessels, such as AS1548-PT460NRZ25. In this example the specification is calling for AS1548-PT460NR, tested to a minimum requirement of 25 percent reduction in area in the through thickness direction.

In order to ensure consistent Z test performance, it is common to perform a minimum of two tests on each plate.

Low sulphur

Clearly, not just any steel plate will pass the Z test. The key to producing Z quality plate lies in two factors. Firstly, it must be a low sulphur steel, and secondly, there must be sufficient reduction during the rolling process.

In ordinary structural steels the sulphur level is enough to form sulphides of a size that can lead to failure when subjected to very high stresses in the Z direction. Hence it is necessary to restrict the sulphur level where Z quality is required.

BlueScope Steel restricts sulphur levels to less than 0.005 per cent. This requires extra processing during steelmaking, specifically adding calcium or other "rare earth" metals which have a particular affinity for sulphur. The resulting sulphides are floated off with the slag. At BlueScope Steel, this process is generally combined with vacuum degassing to ensure the steel is as clean as possible. It is essential that specifiers discuss with BlueScope Steel the requirements of the steel to be used in applications where Z quality may be required.

Rolling reduction

Once we have a suitable low sulphur steel, the other requirement for Z quality is high reduction during rolling, generally a minimum reduction of 3:1. This limits the maximum thickness of Z quality plate to one third of the slab thickness available. With standard slab thickness at 230 mm, this limits the plate thickness to about 75 mm.

High reductions ensure that any sulphides present are rolled out to such an extent that they are broken up and as a result do not form the plane of weakness. However, the steel must be suitably low in sulphur for this process to work.

Applications

Through thickness quality plate is generally only specified where there are high stresses on the Z direction, perpendicular to the plate surface.

The high stresses may result from loading or from high restraint of welded sections, particularly where heavy plate sections are used.
As a result, Z quality plate is used mainly in offshore oil and gas platforms (particularly the node joints), ball mills, heavy flanges and some pressure vessels.

By John Dryden

* From 1 August 2002, BlueScope Steel Plate Products are known as XLERPLATE®.