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 ...
Welded turbine rotors are commonly produced from steels 34KhMA and 32KhMIA. Since the unit power of the rotors has increased from 300 to 1200 MW, these steels no longer satisfy the requirements for rotor materials where their strength and hardenability are concerned. It became necessary to develop new deeply hardening and well weldable high-strength steels. The present paper concerns the results of an all-around analysis of the mechanical properties and weldability of industrially molten metals.Novel steels 25Kh2NMFA, 20KhN2MFA, 20KhN2MFASh, and 20KhN3MFASh for forged-and-welded rotors have been introduced after an investigation of the effect of the method of steel making (including electroslag remelting) on the set of their properties. In the present work we studied their strength and ductile properties, impact toughness, and other characteristics and devoted much attention to the resistance to brittle fracture. For this purpose we determined the semibrittleness temperature 7"50, the work of nucleation and propagation of a crack in impact tests, and the critical coefficient of stress intensity. We tested specimens 16 and 45 mm in diameter with ground and toughened notches for multi-cycle fatigue in order to estimate the weldability of the new steels, i.e., to determine the temperature range of brittleness in a treatment corresponding to the welding thermal cycle (WTC), the semibrittleness temperature after the WTC and tempering, and the critical stresses in delayed fracture.We studied the quality of the metal in the following pilot forgings:(1) 1st, 2nd, and 3rd stage disks of a K-500-65/3000 turbine from steel 20KhN2MFA. The chemical composition of the steel in the studied heat included 0.23% C, 1.29% Cr, 2.18% Ni, 0.50% Mo, 0.10% V, 0.26% Si, 0.63% Mn, 0.015% S, and 0.02% P.(2) A 5th stage disk of a K 1200-240/3000 turbine 1800 mm in diameter with a boss 600 mm high produced by electroslag remelting (ESR) from a 40-ton ingot of steel 20KhN2MFASh. The chemical composition of the steel in Reseach and Production Association of Heavy Machine Engineering (TsNIITMASh), Moscow, Russia. the heat included 0.23% C, 1.28% Cr, 2.04% Ni, 0.41% Mo, 0.08% V, 0.34% Si, 0.52% Mn, 0.013% S, 0.010% P.The forging process included billeting of the ingot with subsequent upsetting. The final heat treatment of the disks consisted of double quenching from 880 and 850~ with cooling in the second quenching first in water (45 min) and then in oil (300 min) followed by tempering at 640-650~ for 25 h.An analysis of the results of the approval tests and statistical processing of the mechanical properties of the disks determined in specimens cut from cross sections of the forgings has shown (Tables !-3) that the new steels have a set of properties exceeding considerably the level of the properties of steel 32KhM1A.Comparing the quality of two commensurable disks from steel 20KhN2MFA produced by different methods we established that the use of ESR and double quenching with cooling in the process of the second quenching first in water and then...
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