Investigation on Nickel base superalloy steam turbine bolts fractured at high temperatures, a case history

摘要

Ni-base superalloys have been often used as material for bolts of the steam turbine valvesrnand of the steam admission areas of the turbine casings, thanks to their high-temperaturernresistance properties. In order to withstand high creep and fatigue loading these alloys arernstrengthened by u0001’ and/or u0001” precipitates.rnAmong them, the most used are Incoloy 901, Refractalloy 26, Inconel X750, Nimonic 80Arnand Nimonic 90 (PER 2 B). Although these alloys are designed specifically for employingrnin high temperature applications (up to 600 ℃) some brittle intergranular cracking havernbeen observed to be a consequence of high temperature exposure and high tensile loads.rnIn the present paper a case study on some fractured Refractalloy 26 bolts, after aboutrn31.000 h service at 600℃ has been reported.rnMechanical and microstructural analysis have been performed on fractured, as-receivedrnand serviced (not fractured) bolts. Fractographic analysis have shown an intergranularrnbrittle fracture in all the fracture surface. SEM observations of secondary cracks havernrevealed an oxides penetration at grain boundary without evidence of impurities from thernenvironment other than oxygen. A moderate microstructural degradation of used andrnfractured bolts have been highlighted by the presence of “eta” phase. Nevertheless, thisrnkind of microstructural degradation seems not to be responsible of the observed fracturernboth because the tensile tests on as-received and exercised samples have not shownrnsignificant differences and because the eta phase has not been observed in the regionrnclose to the cracks.rnMoreover, no creep-induced microstructural damages (such as grain boundariesrnmicrovoids) have been observed in failured and serviced samples.rnFrom all the experimental evidence, the most probable governing failure mechanism is thernStress Accelerated Grain Boundary Oxidation (SAGBO), the only one compatible with thernintergranular brittle appearance of fracture surface and the presence of oxygen penetrationrnat grain boundaries in regions close to the fracture surface.