VARIATION IN PROPERTIES OF METAL OF STEAM PIPES, OF 12Cr1MoV STEEL, IN THE PROCESS OF LONG SERVICE.

12Cr1MoV Steel is extensively used for manufacturing steam pipes of 150 and 200 MW units operating at temperatures 565-570 degree C and pressures 140 kgf/cm**2. The properties of metal of 14 steam pipes of 12Cr1MoV steel after operation for different times (36-92 thous. h) at steam temperatures 565-570 degree C and pressures 130-140 kgf/cm**2 were investigated with the aid of an electron scanning microscope and X-ray structural analysis. The tests have shown considerable structural changes in the process of service associated with carbide transitions, coagulation of carbide phases, variation in the structure and in the degree of alloying of the solid solution. These changes lead to a great decrease in strength characteristics. It is concluded that for pipes operating at a temperature of 565-570 degree C their service life can be increased beyond 100,000 h either by improving their properties by means of repeated heat treatment or by considerably reducing operational parameters.