| Sanicro25 austenitic heat-resistant steel is the ideal material for superheater/reheater in ultra-supercritical units,with good microstructure stability,high temperature performance,excellent corrosion resistance and excellent cold working performance.Excellent cold working performance enables superheater/reheater to save steel during manufacturing and reduce construction costs.However,the microstructure,crystal structure and dislocation density of the metal will change after cold processing,which will directly affect the mechanical and corrosion resistance of Sanicro25 austenitic heat-resistant steel under high temperature service,and it is difficult to ensure the stable operation of thermal power units.In this paper,Sanicro25 heat-resistant steel in cold working state is taken as the research object.The effects of cold working on microstructure and properties of Sanicro25 heat-resistant steel,cold working and annealing treatment(GBE treatment)on grain boundary characteristic distribution and corrosion resistance of Sanicro25 heat-resistant steel,cold working and high temperature aging treatment on precipitation and creep properties of the second phase of Sanicro25 heat-resistant steel were studied.The results are as follows:1.The Sanicro25 austenitic heat-resistant steel was cold-processed with different compression ratios by using cold-stretching equipment.The effects of cold-processing on the microstructure,texture evolution,mechanical properties and working hardening mechanism of Sanicro25 austenitic heat-resistant steel were studied.The results are as follows:The tensile strength,yield strength and hardness of Sanicro25 austenitic heat-resistant steel are enhanced and the elongation decreases after cold processing.The work hardening behavior becomes more obvious with the increase of cold working amount,and the enhancement of work hardening behavior is due to the transformation of plastic deformation mechanism from dislocation slip gradually to twin deformation.The transformation of deformation mechanism leads to the accumulation of dislocations near the deformation twins,which leads to the enhancement of material work hardening.At the same time,with the further increase of deformation,deformation twins reduce the strength of Goss texture and Brass.R texture.2.On the basis of cold processing,the samples were annealed under different conditions,namely GBE treatment of grain boundary engineering.The effect of GBE treatment on special grain boundary distribution and corrosion resistance of Sanicro25 austenitic heat resistant steel was studied.The results are as follows:?CSL content of GBE treated sample is higher than that of solid solution sample.The single low strain long time low temperature annealing treatment has no obvious effect on the improvement of the ?CSL content of Sanicro25 austenitic heat resistant steel,while the multiple strain recrystallization annealing treatment has the most obvious improvement on the ?CSL content of the material,and the pitting corrosion resistance of the material is improved with the increase of the?CSL content.The higher the ?CSL content is,the better the corrosion performance is.3.On the basis of cold processing,high temperature aging treatment at 700°C was carried out to study the effects of cold processing and aging on the microstructure morphology,second phase precipitation and mechanical properties of Sanicro25 austenitic heat resistant steel.The results are as follows:With the increase of aging time,the grain size and twin size of cold-processed Sanicro25 austenitic heat-resistant steel gradually increase,but the increasing trend is gradually flat.The second phase of M23C6 and Nb Cr N(Z phase)is precipitated during aging.The coarsening effect of M23C6 is more serious with the increase of cold processing.After 3000 h aging,the creep properties of the cold processed sample decreases gradually with the increase of processing amount,which is due to the decrease of grain boundary binding force caused by the precipitation of M23C6 between grains.Meanwhile,the decrease of dislocation density leads to the decrease of creep property. |
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