PREDICTION OF THE MECHANICAL PROPERTIES AND FRACTURE MECHANICAL VALUES OF WELDED JOINTS OUT OF PIPELINE-STEELS (X70 AND X80)

摘要

Nowadays the pipeline-steels X70 and X80 are mainly used for onshore-pipelines and a lot of money is spent worldwide in finding the right joining process for the circumferential welds of these pipes. A worldwide central collection of welding variables and efficient processing could result into a prediction of the mechanical properties and fracture mechanical values out of the data of the preceding joining process and would save a lot of trial and error and therefore costs. This paper deals with the prediction of the mechanical properties, as yield strength, tensile strength, impact energy, and hardness, and the fracture mechanical values, as CTOD- (Crack Tip Opening Displacement-) values according to BS 7448 using three point bend specimens and CA (Crack-Arrest Fracture-Toughness) values using three-point bend specimens (test proposal of TVFA), compact-crack-arrest specimens (ASTM E 1221), and full-thickness compact-crack-arrest specimens (test proposal of Crosley and Ripling), out of the parameter, as for example heat input, of the joining process. The problems in determining and predicting the yield strength, the tensile strength, the impact energy, the hardness, the crack tip opening displacement, and the crack-arrest fracture-toughness are discussed in detail, the results are compared. Finally the different resulting values are compared with each other, with the values of the base material, and with the values out of numerous references. The tested materials were the base material, the weld metal, and the heat-affected zone of welds, using different welding processes, as for example manual metal arc welding, gas metal arc welding, or submerged arc welding, of the pipeline steels X70 according to API 5L (StE 480.7 TM according to DIN 17172 or L 485MB according to OENORM EN 10208-2) and X80 according to API 5L (L 555MB according to OENORM EN 10208-2), which are nowadays mainly used for building onshore-pipeline systems.