Orientation effects in metals can result in unexpected consequences.

SEPTEMBER 2017

Although it is normally assumed that engineering materials are isotropic this assumption is often not valid.

In many engineering components much effort is often taken in ensuring that the correct grade of steel or alloy is specified, often with the assumption that once acquired its properties will be isotropic and universal. Unfortunately this is not always, or even generally, true and there is a preponderance of metals systems and structures to retain evidence of their fabrication and manufacture, particularly in the form of microstructural features. Metals that have been significantly worked or forged display a preferred grain orientation which can be both beneficial, as in the case of a forged crane hook/connecting rod, or detrimental, as in severely laminated steel failing like pages in a book. These ‘preferred orientation’ features in rolled plate or forged materials can lead to substantial property differences, as a function of orientation, which are often neglected in design or application.

 

The effects are quite widespread. The grain structure in rolled plate, for example, is most often elongated parallel to the surface which can be of benefit (as in forgings) but also be of concern if components are machined from slabs or blocks of such materials without cognisance of the underlying micro-structure. In some steels, highly alloyed to obtain high strength properties or hard wearing characteristics, the distribution of the alloying elements are not well dispersed and the steel often highly oriented, such as manganese sulphides or carbides. In such cases, the mechanical properties are far from isotropic with equal strength and particularly equal fracture toughness in all three directions not being achieved, and laminations are common. The analogy is steel with a grain structure not unlike timber, where it is well known that splitting a piece of braai wood with the grain is much easier than across the grain. In some cases the reduction in fracture toughness is reduced by a factor of five or six, which can have disastrous consequences for the particular engineering application. For such a steel, instead of a fracture toughness of about 100 MPa√m (typical for tough steels) one obtains (as a result of the laminated inclusion effects) fracture toughness of between 15 and 20 MPa√m (a value consistent with that of cast iron!).

 

Such laminated steels not only exhibit weakness in the plane of the steel plate but affect the inter-plate cracking if subject to welding and /or corrosion. The fracture surfaces of these steels are often not unlike a split piece of wood, and welded attachments to such plate can readily become detached. Unfortunately, laminations not only limited to highly alloyed steels and there are numerous examples in ship plates and structural applications where this effect is manifest and ‘preferred orientation’ effects from steel manufacture lead to some steels performing as if they were made of thin papers stuck together like a book. Unfortunately, such laminated steels are not readily detected even by ultrasonic NDT (Non Destructive Testing) as the laminar plates are bonded just sufficiently so that sound waves do not exhibit reflection on the raw plate, but only when these same plates are under stress, particularly bending stress, do they separate and exhibit longitudinal cracking.

 

Additional structures or joints welded onto the surface of such steels readily de-bond such laminations or lines of weakness in the steel which can then detach completely, compromising the structural integrity of the component. This can also happen not only from welding but also plastic deformation where bending stresses can lead to delamination between the lines of weakness layers (from rolling or forging) with unhappy consequences for the inter-laminar strength. In components that have been rolled this is particularly prevalent, but can also occur in extruded materials, and even in the high tech emerging field of 3D printing, where properties of strength and toughness depend critically on how well bonded one micro-layer is to the next. Weakness here can lead to delamination cracking and is also compromised by the high residual stress induced by the rapid cooling down from molten temperatures, albeit on a microscopic scale. Variations in the mechanical properties as a function of orientation of over 30% are not uncommon.

 

Heat treatment and careful attention to fabrication conditions is vital to overcome such difficulties. Such metals and indeed any steel exhibiting preferred orientation discrepancies is also more susceptible to corrosion at such sites and consequent delamination as is not uncommon in the shipping industry. The message then, is to be aware that not only is the correct steel specified and identified but its orientation characteristics are well known and suitable for the application despite such possible lamination effects.


Published in Technical Tips by Origen Engineering Solutions on 1 September 2017