Investigation of 5 Cr- 0.5 Mo steel welded joints metal degradation in the process of intensive heat treatment

Abstract

The article deals with high temperature steel degradation in the process of long life service. 5Cr-0.5Mo grade steel pipes are widely used in the reforming and visbreaking units of the oil refinery. Long life service at the temperature of 400- 500°C is a typical work conditions of the pipelines made from the particular grade of steel. When operated at high temperatures for a long time, the metal of the pipelines is subject to degradation process. Refinery welded structures manufacturers may unintentionally accelerate the process of degradation of metal properties even before a pipeline is put into operation. Welding of the particular steels requires post weld heat treatment (PWHT) During the research, PWHT was carried out for welded joints of ASTM A335 grade P5 steel under different parameters. The PWHT temperature varied from 500 to 800"C, and the holding time from 1 to 8 hours. After the joints were tested mechanically and the microstructure thereof was analyzed, optimal parameters for PWHT of investigated steel were determined. Additionally, it was determined that, when using intensive PWHT parameters, carbides tend to form within metal grain boundaries of welded joints just like in the parent metal (with the already started degradation process) of pipelines from chromium-molybdenum steel when such are operated at high temperatures for a long time. This means that intensive PWHT accelerates the process of metal degradation.