Z-QUALITIES AND LAMELLAR TEARING


The mechanical properties of steel are most of the time only specified for the x- (rolling direction) and y-direction (the width), while the properties of the z-direction (through-thickness) are seldom specified.

In situations where stress is applied in the z-direction it is important to ensure that the z-qualities are tested and correct. If the z-qualities is not correct lamellar tearing can occur. Lamellar tearing refers to the cracks parallel to a steel plate's surface which usually occurs beneath the weld and is commonly seen in T and L joints of plates. The cracks, known as lamellar tearing, can already occur during the cooling of a steel plate. Steel does not always have the ability to shrink evenly causing breaks or cracks without deforming the steel which in turn can rip the steel plate apart especially when extra stress is applied. The chance of lamellar tearing appearing is higher in low quality steel plates with low ductility.

Transverse strain, weld orientation and material susceptibility are the three conditions under which lamellar tearing can occur. The presence of welding stresses in the z-direction of the steel plate increases the risk of lamellar tearing.

The possibility of lamellar tearing is kept to a minimum with specific z-direction properties and better steel qualities. A test sample is used for testing the z-qualities. The axis of the sample is placed 90 degrees to the steel plate's surface. Tensile forces are then placed on this test sample during which the fracture area the better the z-quality of the steel.