Development of a Steel Resistant to Lamellar Tearing-Z-Steel
Authors: Norring Kjell , Nilsson Krister 
Published: October 2008

1. Introduction

The problem of lamellar tearing is old and most designers and constructors of heavy welded structures have struggled with it. Cracks of this kind are known to occur quite frequently at fabrication of drilling rigs and other offshore platforms.

The main condition for this type of fissures to occur is that there are strains in the through thickness direction of a plate (1), (2). The strains from the contraction occurring during solidification and subsequent cooling of the weld may be sufficient to induce these cracks. External load or bad design may, of course, locally raise the strains further. Other important facts to be considered are the properties of the plate subjected to the strains. To be non susceptible to lamellar tearing the through thickness ductility must be sufficiently high (1), (2), (3), (4).

The low ductility of conventional plates is due to nonmetallic inclusions which are flattened out during grilling. Initially small cracks form at the inclusions in various plane sill-the plate and propagate by means of inter connecting ductile shear fractures into a single large crack, having a step-like appearances illustrated in figure 1.

In some reports (5), (6) the influence of hydrogen has been discussed. At the edge of an inclusion there will be a triaxial stress state with the maximum tensile stress in front of the tip of the inclusion at the edge of the elastic-plastic zone (7). In this stress field the equilibrium concentration of hydrogen will be higher than in the unstressed state due to the stress dependent chemical potential (8). This could lead to hydrogen embitter areas at critical points in the vicinity of inclusions near welded joints.

2. Tensile testing

To detect susceptibility to lamellar tearing the reduction of area (RA) of tensile test pieces Tilden in the through thickness direction seems to be the most relevant measure (1). Therefore this parameter has been used in the following discussion concerning the influence of different parameters on the susceptibility to lamellar tearing.

If stress-strain diagrams from testing in the rolling (x) and through thickness (z) directions of a conventional plate are compared, figure 2, it is obvious that the curves are fairly close to each other until the z-direction specimen fractures rapidly, while in this case the x-direction specimen is still stable - in this case before the point of maximum load in the x-direction.

The corresponding diorama from a plate with improves through thickness ductility, Z- plate, is shown in figure 3. The yield point (yp) and ultimate tensile strength (UTS) in the two directions are about equal. Our investigation have shown that the yield points are nearly always equal and if the RA in the z-direction is above 15-20% the UTS values are very close to each other (ratio 1.00 + .02). But it is important to note that this does not mean that the inverse statements are true. The only difference between the two curves in figure 3 is that after reaching maximum load the z-direction curve drops faster, i.e. the crack propagates faster, due to the orientation of the inclusions.