Abstract
The paper describes a very important problem of metallurgical technology, i.e. detailed evaluation of the stability of welded joints in steels of different chemical composition under demanding service conditions. From the point of view of degradation of the structure and properties of the welding joints of austenite/austenite, ferrite/austenite and ferrite/ferrite steels at the exposure to the temperature of 350 °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 approach to solution of the material problems in power engineering, as well as to evaluation of the reliability of the ferrite/austenite welding joints. The results prove the fact that if the temperature is higher than 350 °C the diffusion processes of carbon between the ferrite base material and the austenite weld metal of the type X12CrNi188 are caused by the high value of the carbon effective diffusion coefficient in the welded joint. From the point of view of the service factors a technological procedure for using the nickel diffusion barrier as a deposit on ferrite steel in the welded joint is proposed. Through nickel diffusion barriers the carbon redistribution from the welded boundary to the austenite steel X12CrNil88 (weld metal) is substantially reduced. The authors present a new approach to the determination of the diffusion width in ferrite steel, effective diffusion coefficient of carbon and optimisation of the chemical composition of the austenite weld metal and nickel diffusion barrier and their dependence on temperature. The results in graphic form are suitable for estimation of the service life of the welded joints with a long term exposure to service temperature higher than 350 °C.

Key words
metallurgical technology, stability of welded joints, ferrite/austenite and ferrite/ferrite steels, weld metal of the type X12CrNi188

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