Study On Evaluating SCC Behavior And Control Methods Of Typical Pipeline Steel

An upsurge on the building of the gas and oil transmission pipelines will be raised in china in the 21st century. Long-distance, large-diameter and high-pressure will be the new trend of building of pipelines. The safety of pipelines has an important significance in transportation. Stress corrosion cracking (SCC) is one of the most important factors of failure of pipelines, so the investigations of the SCC behaviors, evaluation methods and protection approach of pipeline steels are very necessary. However, the SCC of high-strength steel has not been known expressly at present, and the evaluating standard should be unified. At the same time, the protecting methods must be improved greatly. In this thesis, the SCC behaviors of three types of different strength pipeline steels, named as X80, 16Mn and X52, were explored in the weak acidic medium. The influences and mechanisms of SCC of pipeline steels were investigated by varying loading and sampling methods in order to establish the reasonable principles of evaluating method of SCC of pipeline steels. On the other hand, the SCC behaviors of the different strength pipeline steels under the protection by different methods, including shot penning, cathodic protection and zinc-rich powder coating, were researched. It was consulted as reference for availably controlling SCC of pipeline steels. Through series of testes and analysis, the results were shown as:The stress is the dominant factor of SCC when the specimens with slot at the center were used in bending tests. But this method only can be used to evaluate the SCC sensitivity (SCCS) of the materials which have the uniform component and organization (such as the based pipeline steel metal). The four-point bend loading method was suggested as more superior than other methods.The three-point bend loading method was inappropriate to the sample with weld zone. However, the four-point bending was proposed as a suitable method to the specimen without the hole. In this condition, the weld seam, the heat-affected zone and the partial based metal were located in the uniform stress zone between two loading point.Because the surface residual stress was introduced in the crushed progress, the SCCS of the specimens were changed. Thus, the natural SCC behavior of the pipeline was different with the experiment results. The degree of difference was inverse ratio to the pipeline diameter. This result was especially serious for small diameter pipeline. So C-ring specimens should be firstly chosen to evaluate the SCCS of small-diameter pipeline.The SCC resistance (SCCR) of the welded joints in pipeline steel could be effectively improved by Shot peening. The SCCR could be more improved if the shot...