Relationships Between the Microstructure and Fracture Toughness of Microalloyed (API 5L X65) Steels

The present article is intended to examine the relationships between the microstructure and the fracture mechanical properties. It uses the J integral concept for a characterization of the properties of fracture mechanics. The microalloyed steels of the X65/API-5L type were used for this purpose. Some of the individual microstructures were characterized qualitatively, others quantitatively, by means of a light microscope (LM) and a scanning electron microscope (SEM). The examinations of the size and distribution of the non-metallic inclusions and precipitates were made by means of a SEM and an LM, in some cases. Tensile tests and notch bend impact tests were performed to determine the mechanical properties. The properties of fracture mechanics were examined on three-point bending test samples at a room temperature. The multi-sample method was employed to determine the crack growth resistance curve.