Study On Microstructure And Properties Of Solid Solution Strengthened Heat Resistant Alloy C-HRA-2

At present,the world's most advanced commercial ultra-supercritical coal-fired power station has a reheat steam temperature of 620?.China has approved the construction of the world's first ultra-supercritical coal-fired thermal power unit demonstration project with a reheating temperature of 630?.In order to further improve the thermal efficiency and reduce the coal consumption,the steam temperature of thermal power generation still needs to be continuously improved.China is developing coal-fired power generation technology with reheated steam above 630? and thermal efficiency exceeding 50%.The most difficult one is the development of heat-resistant materials for thick-walled components such as main steam pipes and headers.When the metal temperature exceeds 650?,martensitic heat resistant steel can not meet the design requirements.Austenitic heat-resistant steel has high thermal expansion coefficient and high aging brittleness.And the ?'phase in iron-nickel-based alloys tends to cause long-term instability of structure and properties,especially in the heat affected zone(HAZ)liquefaction problem.Both of them are not suitable for the selection of large-diameter thick-walled pipes such as boiler headers and main steam pipes.Therefore,it is urgent to develop a new type of solid solution strengthened nickel-based heat resistant alloy,which meets the engineering requirements of high temperature strength and field welding.Heat-resistant alloy C-HRA-2 is independently researched and developed in China.It has been developed in multiple rounds of engineering.It is one of the candidate materials for the 650-680? temperature boiler header and main steam pipeline in China.In this investigation,a series of studies of heat-resistant alloy C-HRA-2 were carried out on the composition optimization,hot deformation behavior,heat treatment,long-term aging and long-term strength performance.The results showed that:(1)The equilibrium precipitation phase of the alloy C-HRA-2 at 650-700?was studied by Thermo-Calc software.It was found that the C content had a significant effect on the precipitation of M23C6 particles.The content of M23C6 particles would increase by about 0.2 wt.%for every 0.01 wt.%C content.;Cr content had a dominant influence on the temperature at which M23C6 transformed into M6C phase.When the Cr content was higher than 22 wt.%,the M6C phase didn't precipitate in the temperature range from 650-700?.(2)In the research of thermal deformation process,the thermal deformation equation and processing map of alloy C-HRA-2 were obtained.The recommended hot working temperature of the alloy was 1050-1150?.The average thermal deformation activation energy of alloy C-HRA-2 is 446 kJ/mol.(3)The grain growth process of alloy C-HRA-2 could be well fitted by Anelli model.According to the remelting temperature of the carbide,the model needed to be divided into two parts for calculation.The yield strength of the alloy after solid solution was mainly composed of solution strengthening and grain-boundary strengthening.The value of solution strengthening was about 232 MPa and the k value of grain-boundary strengthening was about 20.27 MPa·mm1/2.(4)The strength of alloy C-HRA-2 increased rapidly during the aging at 675-700? within 500 h,and then remained stable.The precipitates in the aging process were mainly M23C6 particles,and its amount increased with the increase of aging time.After aging for 3000 h,it reached the maximum with the content of 1.077-1.108 wt.%.The impact toughness of the alloy maintained over 110 J after aging at 675-700? for 8000 h.(5)The creep strength of alloy C-HRA-2 was calculated by single logarithm method.The creep strength at temperatures of 650?,675? and 700“C was respectively 171 MPa,139 MPa and 103 MPa after extrapolation for 100,000 hours.Under low temperature and high stress,the fracture of the creep specimen was crystal sugar fracture along the fracture.The crack source was mainly oxide inclusions containing Al and Ca element.Under high temperature and low stress,the creep specimens had obvious necking.The crack source was mainly void crack at triple points of grain boundaries.