Study On Microstructure And Properties Regulation Of Titanium Alloy For Steam Turbine Blade In Solution Aging State

Titanium alloys are widely used in various fields of industrial production due to their excellent mechanical properties.Turbine blades are prone to failure when operating in a high temperature environment for a long time,the performance of its blades largely determines the power generation efficiency of the steam turbine,so advanced titanium alloy blade manufacturing technology has become one of the important indicators to measure the development level of steam turbines.At present,Ti-6Al-4V(TC4 titanium alloy)is widely used at home and abroad,but its further development is hindered by the lack of processability.Ti-4.5Al-3V-2Mo-2Fe(SP-700 titanium alloy)is a new type of α+β titanium alloy.On the basis of Ti-6Al-4V alloy,two β stabilizing elements Mo and Fe are added,which reduces the content of Al and V,and improves the superplasticity,corrosion resistance,strength and toughness of the alloy,so SP-700 titanium alloy is expected to replace Ti-6Al-4V alloy as a new type of turbine blade material.In this thesis,SP-700 titanium alloy is used as the research object.The effects of cooling rate(β phase zone solid solution),solid solution time and solution temperature(β+(α+β)phase zone solid solution),aging time and aging temperature(single and double stage aging)on the microstructure,hardness and compression properties of SP-700 titanium alloy were studied.The optimal strong plasticity ratio under different heat treatment conditions was determined,which provided a certain reference basis for the manufacture of SP-700 titanium alloy blades and the application in steam turbines.The main conclusions are as follows:(1)SP-700 titanium alloy is treated by solution treatment in β phase zone.When the cooling rate is slow,β phase transformation is easy to occur β phase to the a phase,and when the cooling rate is fast,β phase transformation is easy to occur β phase to martensitic.When the cooling rate is 3℃/s,the best matching between strength and plasticity can be obtained.(2)SP-700 titanium alloy for β+(α+β)phase zone solid solution treatment,when the α+βphase zone solid solution temperature is a certain,the longer the solid solution time,the more a phase precipitation.The hardness of the alloy and the solid solution time is nonlinear relationship.When the β solid solution temperature is 850℃,the solid solution time is 5min,the α+β phase area solid solution time in the range of 3-10min,SP-700 titanium alloy can obtain the best strong and plastic match.(3)SP-700 titanium alloy for β+(α+β)phase zone solid solution treatment,when the α+βphase zone solid solution time is certain,the lower the solid solution temperature,the more a phase precipitation.The hardness of the alloy and the solid solution temperature is nonlinear relationship.In the range of α+β phase zone solid solution temperature of 650-900℃,when the solid solution temperature is 650℃,the solid solution time is 5min,SP-700 titanium alloy can obtain the best strong and plastic match.(4)When SP-700 titanium alloy is treated with single-stage aging and double-stage aging,the basic structure is composed of β grains、α phases、martensitics,and the microstructure of the two-stage aging is more uniform.When the aging temperature of the single-stage aging heat treatment is 650℃ and the aging time is 90min,the strong and plasticity matching of the alloy is the best.When the aging temperature of the two-stage aging heat treatment is 650℃and the aging time is 15min,a good strong plasticity match can be obtained.