Z Quality Test

The Z Quality Test (through-thickness tensile test, also known as the short transverse tensile test) is a destructive testing method that determines a steel product's ductility and capacity for cross-sectional contraction under a tensile load applied perpendicular to the rolling plane, that is, along the thickness (Z) axis. In hot-rolled plates and sections, manganese sulphide (MnS) inclusions flattened during rolling and the resulting layered internal structure weaken the material significantly in the thickness direction compared with the other two axes (rolling and transverse directions). This anisotropy creates the risk of "lamellar tearing" in thick welded joints under high restraint. The Z quality test ties that risk to a measurable acceptance criterion at the production stage.

The physical principle of the test is to pull a cylindrical tensile specimen machined in the thickness direction uniaxially until fracture and then measure the contraction of the cross-section at the fracture. The governing quantity in the evaluation is the reduction of the cross-sectional area after fracture, that is, the "reduction of area" Z value: Z = (S₀ − Sᵤ) / S₀ × 100. Here S₀ is the original cross-sectional area before the test and Sᵤ is the smallest cross-sectional area after fracture. The higher this Z value, expressed as a percentage, the better the material's through-thickness ductility and its resistance to lamellar tearing. Unlike tensile strength or yield strength, the decisive property here is not strength but the ductility reserve in the thickness direction.

In practice the most critical point is specimen preparation. Because the plate thickness is often insufficient to provide the length of a standard tensile specimen, extension pieces are usually attached to the ends of the specimen extracted in the thickness direction, by friction welding or another suitable welding process; this yields a specimen long enough to be gripped between the jaws of the testing machine. Specimens are taken from defined positions across the product width and generally in sets of three. The test is carried out on a universal tensile testing machine with traceable calibration, at room temperature and at a controlled strain rate, with the extension-piece weld positioned outside the test section.

The results of the Z quality test allow the product to be assigned to one of the quality classes defined in TS EN 10164. These are the Z15, Z25 and Z35 classes, determined from the average Z value of a set of three specimens and from the lowest individual value; the class number expresses the minimum average reduction of area that must be achieved (for example, Z35 means higher through-thickness ductility and a more severe lamellar tearing risk). Which class is required is set by the designer according to the thickness of the member to be joined, the weld volume, the degree of restraint and the design rules (for example, the through-thickness assessment within Eurocode).

Reporting includes the original and fracture cross-sectional dimensions of each specimen, the calculated individual Z values, the set average, any specimens deemed invalid, and the final quality class decision. The report also documents the calibration traceability of the tensile machine and measuring equipment, the sampling position and, where applicable, information on the extension weld. This output makes it possible to declare the Z class on the material certificate (EN 10204 inspection document) and to design thick-section, heavily welded structures safely against lamellar tearing. TÜV AUSTRIA SILA KALİTE performs the through-thickness tensile test as a TS EN ISO/IEC 17025 accredited laboratory service and presents the results in reports based on traceable calibration.