Development of the technonological process and investigation of the quality of forged-and-welded turbine rotors made on steel 25Kh2NMFA

Abstract

The quality of the metal of rotors of high-power turbines and generators is a very important factor that determines their reliability and service life. The rotor quality, in turn, depends on the correct choice of the material and the process of rotor production. The paper concerns the properties of steel 25Kh2NMFA and the choice of an optimum heat treatment for forged-and-welded parts of turbine rotors.In estimating the quality of the metal [1] of test and industrial preforms of forged-and-welded, seamless forged, and welded-and-forged turbine and generator rotors we based ourselves on existing industrial specifications and tentative specifications developed by the consumer, the producer, and TsNIITMASh.In correspondence with the specifications we determined the chemical composition, mechanical properties, macrostructure, and residual stresses in the preforms. The preforms were subjected to ultrasonic inspection. In addition, the properties of the test preforms were examined by the methods of metallographic, dilatometric, x-ray diffraction, magnetic, and carbide analysis.Much attention was devoted to the study of the brittle strength. For this purpose we determined the semibrittleness temperature from 50% tough component in the fracture surface, the work of crack nucleation and propagation in impact tests on specimens of types I and II , and the critical coefficient of stress intensity in a plane stress state IK c ) on model disks and off-center tensile specimens.For the first time in heavy-machinery and power mechanical engineering we used the method of mathematical statistics [2-4] for developing requirements on parts made of basic steels, estimating the correspondence of the properties of existing and new materials to the specifications, estimating the quality of the metal over the cross section of the part, and choosing the process technology for large forgings. We also detemlined the probability of correspondence of the properties of rotor parts to the requirements of the specifications (at P = 0.95).On the basis of earlier studies we chose steel 25Kh2NMFA for production of welded rotors of steam turbines.In order to test the heat-treatment process we fabricated the following preforms (representative parts) of a new modification of rotors:(1) A solid disk for the 1st, 2nd, and 3rd stages of a K 500-65/3000 turbine 1630 mm in diameter with a 630-ramhigh boss, made from a 52-ton ingot of an acid vacuum-degassed open-hearth steel (0.22% C, 0.31% Si, 0.6% Mn, 0.15% S, 0.020% P, 1.86% Cr, 1.38% Ni, 0.41% Mo, 0.08% V). The ingot was forged with removal of the central defective zone. The final heat treatment consisted of a single hardening from 900~ with cooling first in water (45 min) and then in oil (80 rain) and tempering at 650-660~ for 20 h [5].(2) A solid disk of the 5th stage ofa K 500-60//1500 turbine 2740 mm in diameter with a 750-ram-high boss, made from a 75-ton ingot (60 tons of acid open-hearth steel containing 0.22% C, 0.24% Si, 0.58% Mn, 0.016% S, 0.010% P, 1.85% Cr, 1.51% Ni, 0.50% Mo, 0.09% V and ...