Microstructural Aspects of the Fracture Parameters of Controlled Rolled HSLA-steels

Abstract

<p>A combination of high strength and good fracture resistance is obtained in high strength low alloy (HSLA) steels by the use of controlled rolling and addition of micro alloying elements to refine the scale of the microstructure. In these fine grained materials traditional property-structure relationships do not adequately describe the fracture behaviour.</p> <p>This thesis is concerned with the fracture properties of HSLA-steels at various temperatures and stress states. Three modes of failure are commonly observed. At low temperatures cleavage is the predominant fracture mode, whereas ductile failure by nucleation and growth of voids occurs at higher temperatures. In the intermediate temperature range delamination fracture on planes parallel to the rolling plane is observed. The various fracture mechanisms are discussed in terms of the detailed microstructure of the materials which has been characterized by the use of standard optical and electron metallography. In addition failure criteria for the most common fracture modes have been developed.</p> <p>It is found that the condition for cleavage failure is adequately described in terms of a Griffith equation where the crack length is determined by an effective grain size of the order of twice the ferrite grain size. Further it is argued that the low temperature fracture toughness can be expressed by the cleavage stress and the size of the process lone. For fine grained materials the process zone size is found to be independent of the scale of the microstructure.</p> <p>The resistance to ductile fracture has been characterized in terms of a critical crack opening displacement (COD). It is argued that the COD value is determined by the size of the process zone which is independent of the scale of plasticity. The process zone size is related to the inclusion spacing.</p> <p>Delamination is found to occur mainly by a grain boundary tearing mechanism. However, the presence of inclusion aggregates may reduce the fracture stress substantially. Delamination by the grain boundary tearing mechanism occurs at a critical value of the maximum shear stress indicating that crack nucleation is the critical event.</p>