Degradation of Microstructure and Mechanical Properties of 15HM Steel after 420 000 Hours of Service

The results of a microstructure examination and mechanical properties of 15HM (13CrMo4-5) steel are presented in the article. The examined elements are the samples taken from the live steam pipeline serviced for about 420 000 hours at the temperature of about 510°C, and pressure of 11 MPa. It has been shown that after long-term operation the examined steel has a ferritic-pearlitic microstructure with a dominant content of quasipolygonal ferrite. The processes of fragmentation of lamellar precipitates and their spheroidization were observed in pearlite. On the grain boundaries, single lamellar precipitations were observed. Moreover, numerous precipitations at the interface of three grain boundaries were revealed. The examined steel, despite its long-term service time, was characterized by the strength properties (YS, TS) slightly lower than the required minimum, the impact energy value KV equal to 20 J, and the transition temperature shifted to a temperature above zero. Relatively low level of degradation of the microstructure and mechanical properties of the investigated steel can result from high stability of the ferritic-pearlitic microstructure.