| In order to meet the requirements of environmental protection and energy conservation,reducing CO2emissions and continuously improving thermal efficiency have become the technological development direction of transcending thermal power units.Higher temperatures and pressures place higher demands on the development of heat-resistant materials and welding technologies for high-temperature and high-pressure sections.9Cr-3W-3Co steel not only has better structure stability and higher permanent creep strength in the temperature range of 630±20?,but also has good resistance to high temperature steam oxidation and higher.At present,the research on the welding process,welded joint structure and mechanical properties of the new 9Cr-3W-3Co martensitic heat-resistant steel is still insufficient,especially on the high-temperature creep properties and fatigue crack growth of welded joints.This thesis takes the new martensitic heat-resistant steel 9Cr-3W-3Co as the research object,adopts ZD150-60C CV60M high-pressure vacuum electron beam equipment.This thesis optimizes the focusing current,electron beam current,welding speed and scanning frequency through orthogonal experiments parameters,then obtained high-quality welded joints,and controlled the microstructure and properties of welded joints through high-temperature tempering treatment.This thesis studies the high-temperature creep behavior of welded joints under different temperatures and stress conditions through a series of high-temperature creep tests,then analyzes the evolution of the microstructure of the weld metal under the same creep temperature and different stress conditions.This thesis discussed the effect of temperature on the fatigue crack growth rate and life of each region of the welded joint,then analyzes the influence of the microstructure on the fatigue crack growth.(1)Research on the microstructure and mechanical properties of the welded joints.This thesis uses metallographic microscopy,scanning electron microscopy,transmission electron microscopy,X-ray diffraction and other methods to study and compare the welded joints in the as-welded state and the heat-treated state.The mechanical properties of the welded joints are compared and analyzed by methods of microhardness test,normal temperature tensile test and impact test.The results show that the structure of the welded joint in the as-welded state is mainly composed of lath martensite,as well as the M23C6and MX second phase precipitates distributed at the edges of the martensite lath and the lath interface.There is a big difference in the grain size of the weld zone,fusion zone,the coarse-grain zone and the fine-grain zone in the heat-affected zone.The weld zone is composed of equiaxed grains and a small amount of columnar grains.The content of the second phase in the welded weld is less.Due to the rapid and unbalanced cooling during welding,martensite transformation occurs in the weld and the heat-affected zone,causing high-density dislocations in the martensite lath in the weld.After high temperature tempering at 730?and holding for 8hours,each region of the welded joint transforms from lath martensite to lath tempered martensite structure.Compared with the base metal,the grains of the weld metal are smaller,part of the martensite lath in the weld metal has an insufficient polygonal transformation.At the same time,there are more carbides M23C6and MX phases near the lath interface,and there is a tendency to aggregate and grow up,and the precipitated phases are distributed continuously along the interface.Due to the high dislocation density of welded joints,the tensile strength at room temperature(average value:735MPa),yield strength(average value:698MPa)and microhardness(average value of weld metal:442HV0.98Nand average value of thermal influence Zone:381HV0.98N)are higher than tempered welded joints,but the impact toughness is lower(average value of weld metal:7.6J;HAZ:12.2J).After high temperature tempering heat treatment,the dislocation density in the welded joint is significantly reduced,making the microhardness of the weld zone significantly reduced(average value of weld metal:273HV0.98N,average value of heat affected zone:274HV0.98N),but it still maintained at a level slightly higher than that of the base metal(average value:240HV0.98N)without softening.The toughness of the weld metal and heat-affected zone metal is improved.At the same time,the presence of a large number of second phase particles in the weld and heat affected zone hindering the movement of dislocations,leads to the satisfying mechanical properties of the welds,including tensile strength(average value:750MPa),yield strength(average value:707MPa),impact toughness(average value of weld metal:25.1J;HAZ:23.3J)of the welded joints,etc.(2)Research on high temperature creep behavior of welded joints.The high temperature creep properties of the welded joints were tested at 630?,650?,670?and different stress conditions.The results show that under the same temperature condition,the creep rupture time becomes significantly shorter with the increase of the test stress.Under the same stress condition,the creep rupture time of the joint becomes significantly shorter as the creep temperature rises.Through the BMD equation,the threshold stress values of welded joints at630?,650?and 670?are obtained as 115.28MPa,104.73MPa and 96.98MPa respectively,and the true stress index is about 6.Through derivation,the true creep activation energy of the joint is 288.15 k J/mol,which is very close to the self-diffusion activation energy of Fe(250-270k J/mol).It confirms that the creep deformation mechanism of the welded joint is the dislocation climbing control mechanism.In the necking and fracture stage of the creep process,the damage caused by the smaller cross-sectional area of the sample exceeds 75%of the total damage factor,but the creep damage caused by the microstructure accounts for a relatively small proportion.In the process of creep rupture,the cavities continue to gather and grow.When the cavities reach a critical size,the local plastic instability between them leads to the final fracture.Under the effect of stress-temperature coupling,most of the M23C6in the joint precipitates on the large-angle grain boundary of about 50°,and only a small part of the M23C6carbide precipitates on the small-angle grain boundary of about 10°,while the MX carbonitrides distributed in the matrix are relatively stable.The analytical formula between the creep rupture time and the creep temperature and stress of the vacuum electron beam welded joint of the new martensitic heat-resistant steel of 9Cr-3W-3Co steel is:tf=10(1/T)(98229.0857-20171.71187lg?)-55.46772.(3)Research on fatigue crack growth behavior of welded joints.The fatigue crack growth behavior of the metal in different regions of the welded joint was studied at room temperature and 630?.The results show that based on the Paris formula,the expressions of the steady-state crack growth rate in the weld zone and the heat-affected zone at room temperature are da/d N=2.809×10-8(?K)1.948and da/d N=2.437×10-8(?K)1.994.The expressions of the steady-state crack growth rate in the weld zone and heat-affected zone at630?are da/d N=3.43×10-7(?K)2.39and da/d N=2.68×10-6(?K)1.76.The fatigue crack growth rate of the base metal is relatively highest,and the weld zone is the lowest,and the thermal effect is somewhere in between.As the temperature increases,the crack growth rate increases,while the fatigue crack growth period decreases.The fatigue crack propagation fracture surface of the welded joint is a transgranular fracture morphology,which is composed of a relatively flat near-threshold zone,a stable propagation zone and a rough final instantaneous fracture.Clear fatigue bands can be observed at room temperature,but due to severe oxidation at 630?,the fatigue bands on the fracture surface are not clear.There are a large number of secondary microcracks in the weld zone and the heat-affected zone's stable crack propagation.The secondary cracks are mainly formed and propagated by the separation of carbon(nitride)compound/matrix interface and the cracking of carbon(nitride)itself.The occurrence of secondary cracks can reduce the stress concentration at the crack tip,leading to slower crack propagation.The fatigue crack propagation path in the weld zone is a tortuous propagation.The fatigue cracks in the heat-affected zone propagate along the boundary of the slab block,and the boundary of the slab block is bent,the crack propagation path is deflected here.EBSD analysis shows that the grain orientation difference near the crack path in the weld zone and the heat-affected zone also showed the same trend,mainly in the range of less than 10°and 50°-60°.The cracks mainly propagate forward through the slat block,and only partially extend along the boundary between the slat block and the slat bundle. |
PDF Full Text Request