INCREASE OF WELDED-JOINTS STRUCTURAL STAB ILITY

The paper describes a very important problem of metallurgical technolo gy, i.e. detailed evaluation of the stability of welded joints in stee ls of different chemical composition under demanding service condition s. From the point of view of degradation of the structure and properti es of the welding joints of austenite/austenite, ferrite/austenite and ferrite/ferrite steels at the exposure to the temperature of 350 degr ees C and higher, the determination of carbon diffusion is important. It is assumed that the structural stability of austenite/austenite and ferrite/ferrite welded joints can be successfully solved by change of chemical composition of the welding metal. The study presents a new a pproach to solution of the material problems in power engineering, as well as to evaluation of the reliability of the ferrite/austenite weld ing joints. The results prove the fact that if the temperature is high er than 350 degrees C the diffusion processes of carbon between the fe rrite base material and the austenite weld metal of the type X12CrNi18 8 are caused by the high value of the carbon effective diffusion coeff icient in the welded joint. From the point of view of the service fact ors a technological procedure for using the nickel diffusion barrier a s a deposit on ferrite steel in the welded joint is proposed. Through nickel diffusion barriers the carbon redistribution from the welded bo undary to the austenite steel X12CrNi188 (weld metal) is substantially reduced. The authors present a new approach to the determination of t he diffusion width in ferrite steel, effective diffusion coefficient o f carbon and optimisation of the chemical composition of the austenite weld metal and nickel diffusion barrier and their dependence on tempe rature. The results in graphic form are suitable for estimation of the service life of the welded joints with a long term exposure to servic e temperature higher than 350 degrees C.