CHARACTERISATION OF UNDERMATCH WELDED JOINT OF X20CrMoV121 STEEL AFTER PROLONGED SERVICE
Gordana Bakić1, Vera Šijački Žeravčić1, Miloš Djukić1, Bratislav Rajičić1, Miladin Radović2, Ivan Gajić3, Aleksandar Maslarević4, Aleksandar Jakovljević5
1)University of Belgrade, Faculty of Mechanical Engineering, Serbia,
2)Department of Mechanical Engineering, Texas A&M, College Station, TX, U.S.A.
3)TPP “Nikola Tesla B”, Ušće, Obrenovac, Serbia
4)University of Belgrade, Faculty of Mechanical Engineering – Innovation Centre, Serbia
5)Electric Power Industry of Serbia, Belgrade, Serbia
Heat resistant martensitic steel X20CrMoV121 (DIN) is extensively used in the last few decades as a material for tubing systems and pipelines in thermal power plants. Long term behaviour of this material is well known and understood. X20CrMoV121 is found to be reliable for prolonged service at elevated temperatures. Main disadvantage is poor weldability, apart from other properties of great importance for tubing systems. Failure of the tubing system during service requires quick replacement of damaged parts. In situ repair welding of martensitic steels is always problematic since it requires special welding techniques usually difficult to perform in short time during forced outages. Here we report on the repair welding procedure for the outlet superheater in two 600 MW lignite power plant units. The super heater is made of X20CrMoV121 (DIN). Outlet temperature and pressure in the super heater system are 540°C and 18.6 MPa. The local thinning of tube walls is found to be the main damage mechanism of the super heater during prolonged service. Highly erosive lignite combustion particles are identified as the main reason for wall thinning.
A serious problem is the long required period for repair welding of damaged martensitic X20CrMoV121 steel. The solution was cold welding involving repair welding by austenitic electrode with high nickel and chromium content. This technique has many advantages as short welding time, no need for special atmosphere and preheating treatment, relatively easy welding process, etc. Repaired joints are usually replaced during next overhaul, but for investigation purposes some are maintained in service over 10 years. Intensive characterization of base and filler material of repair welds after 10 years service is reported in this paper.
Keywords: heat resistant martensitic steel, repair welding