Experience gained with carrying out restorative thermal treatment of stamped-and-welded elbows made of grade 15Kh1M1F steel after long-term operation

Abstract

At present, the power industry faces serious prob lems due to the fact that a large number of main pipe lines remain in operation that had worked out their fleet life long ago. Since it is impossible to fully replace them in the nearest years, one possible solution of this problem may consist in extending the service life of equipment by carrying out a restorative thermal treat ment (RTT) in accordance with the instruction [1] that is presently in force. However, this instruction does not recommend to carry out RTT of steam lines made of Grade 15Kh1M1F steel together with their welded connections because the RTT schedules ensuring the optimal ratio between the properties of main metal and weld metal have not been studied to a sufficient extent. At the same time, some of hot reheat steam lines include stamped and welded elbows (SWEs) made of Grade 15Kh1M1F steel, whose welded joints are integral components of their struc ture. Re welding of such connections under power station conditions is impossible, and the individual cost of such an elbow is very high. This paper presents the results of a research work aimed at determining whether it is possible to carry out RTT of SWEs made of Grade 15Kh1M1F steel after their long term oper ation.Stamped and welded elbows made of tubes with the outer diameter D out = 750 mm, wall thickness S = 45 mm and radius R = 1000 mm that had been in oper ation for 143 500 h in a hot reheat steam line at steam parameters t = 545°С and р = 4 MPa served as a subject of the study. As is well known, the fleet life of such elbows was specified equal to 100 000 h.A primary examination of the elbows showed that the metal did not have inadmissible flaws; the short term mechanical properties of metal were estimated to be satisfactory, and the level of microdamage inflicted to their longitudinal welded joints made of Grade 09Kh1MF steel and to the parts of the thermal influ ence zone (TIZ) with degraded strength properties did not exceed 3 points according to [2].The program of investigation for solving the prob lem that was set forth included two stages:(i) making a comparison of different schedules for carrying out RTT on samples under laboratory condi tions and determining the optimal RTT schedule based on the results of this comparison, 1 and (ii) subjecting the check SWE to thermal treatment in accordance with the optimal schedule followed by carrying out long term strength tests and studying the metal structure and other properties of samples cut from it.In working out the program of studies, we took into account a considerable difference between the struc ture and properties of the cast welded joint metal (Grade 09Kh1MF steel) and the main metal (Grade 15Kh1M1F steel) due to the difference in their com positions, structure, and strength. The longitudinal welded joints of the elbow are parts of metal with degraded strength, which just determines the SWE service life in terms of the heat resistance criterion.The aim of this work is to select an RTT schedule that would make it possible to achi...