Cyclic crack Resistance of the Steels of Bends of Steam Pipelines After Long-Term Operation
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CYCLIC CRACK RESISTANCE OF THE STEELS OF BENDS OF STEAM PIPELINES AFTER LONG-TERM OPERATION O. P. Ostash,1, 2 O. V. Vol’demarov,1 and P. V. Hladysh3
UDC 539.43.620.191.33:620.034.15
The operational degradation of 12Kh1MF and 15Kh1M1F steels decreases their resistance to the nucleation and subcritical growth of a fatigue crack, especially near the external surface of tension zone of the bends of steam pipelines. Durability in the stage of nucleation of an initial macrocrack is the most sensitive to the structural-mechanical damaging of degraded steels. We show that MnS-type sulfide inclusions decrease the serviceability of such steels due to the formation of wedge-like microhollows near them. We have developed a method for the accelerated degradation of the steels of steam pipelines under laboratory conditions at a temperature of 540 to 630°C and under cyclic loading with a high load ratio ( R = 0.6), which models the maneuver conditions of operation of steam pipelines. Keywords: heat-resistant steels, operational and model degradation, characteristics of cyclic crack resistance.
Bends of the steam pipelines of thermal power plants in the course of their high-temperature operation are subjected to the action of significant loads caused by the thermal expansion of the metal. The temperature stresses are also induced by the nonuniform heating of a pipe over its wall thickness or length. All these loads change in the course of work of a steam pipeline and, especially strongly, during start-stops of power generating units (under maneuver conditions) [1, 2]. Repair operations and hydraulic tests are also connected with changes in the loads. The severe conditions of operation (the action of thermal and force factors and working medium) affect, first of all, the technical state of bent parts, whose durability, as a rule, is lower than that of the straight parts of pipes, especially under cyclic loads. In addition, this is caused by the specific features of the stressed state of bent pipes and worsening of the mechanical properties of the material in the course of their manufacture [3]. The imperfections of technological processes in the manufacture of the bends of steam pipelines and, especially, thermal and force factors in the course of their long-term operation lead to structural-phase changes in the metal as well as to the initiation and accumulation of microdamages (pores and their chains along grain boundaries) [2, 4]. They often serve as nuclei of cracks that can lead to the uncontrolled fracture of steam pipelines. To prevent such phenomena, it is necessary to diagnose the state of the metal of steam pipelines in the stage of structural-phase changes and damage initiation under the influence of operational factors, especially of the cyclicity of loads. Therefore, it is important to establish relationships between the structural-phase state and microdamaging of the steels of steam pipelines after long-term operation and their fracture strength in the stages of macrocrack nucleation and subcritical growth under cycl
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