Effect Of Cold Deformation And Heat Treatment On Precipitation Behavior And Properties Of Sanicro25 Steel Under High Temperature Aging

Nowadays,the demand for electric energy is increasing day by day,and thermal power generation has always been the most important way of power generation.Still,thermal power generation is facing increasingly serious environmental pollution and energy shortage,which leads to improving the efficiency of thermal power generation,and improving the steam parameters of thermal power unit is one of the most effective ways to improve the power generation efficiency.However,the higher the steam parameters,the higher the requirements for the steel used in the unit.As a new type of austenitic heat-resistant steel,sanicro25 steel has good resistance to steam oxidation corrosion,high temperature endurance strength and excellent machining performance,and has become one of the candidate materials for 700℃ultra supercritical units.At present,sanicro25 steel has been in service for a short time at home and abroad,and its impact toughness decreases severely with aging time in the experimental process,which will become a hidden danger for the operation of ultra-supercritical unit.The reason for the change of properties（such as the decrease of toughness,etc.）of the steel at high temperature must be related to the change of microstructure（such as precipitation behavior,etc.）in the process of high temperature.Therefore,the research on the precipitation behavior of the steel under high temperature and the relationship between the precipitation behavior change and the performance change is of great significance for the safety of sanicro25 steel in the service of ultra-supercritical unit.In this paper,the size and type of the precipitation of sanicro25 steel were studied by using the precipitation powder obtained from the electrochemical extraction experiment.The precipitation distribution of sanicro25 steel was analyzed by means of optical microscope（OM）and scanning electron microscope（SEM）,etc.The cold deformation treatment was carried out on the as-received sanicro25 steel pipe,and the influence of deformation and heat treatment on its precipitation behavior was studied with the solution treated sample of sanicro25 steel after deformation treatment.Furthermore,the change in mechanical properties was analyzed.The results show that:During the aging process,the number and size of the precipitation in the grain boundary and inside the grain of the cold deformed sample and the undeformed sample of sanicro25 steel increase continuously.The width of the precipitation in the grain boundary of the cold deformed sample is smaller than that of the undeformed sample,and the width of the precipitation in the grain boundary decreases with the increase of the deformation;During the aging process,M₂₃C₆,NbCrN and Laves phases are precipitated from both cold deformed and undeformed samples of sanicro25 steel.After cold deformation,the particle size of the precipitation powder is greatly increased,and an additional coarse phase rich in Cr and W is precipitated,composition of which is similar to that ofσphase;The larger Nb rich phase and NbCrN phase in sanicro25 steel will reduce the internal bonding force of the material and facilitate the crack initiation and propagation;The effect of the second phase strengthening makes the hardness of sanicro25 steel increase gradually in the aging process;After cold deformation treatment,the hardness of cold deformed material will be greatly improved by the work hardening effect.The hardness of cold deformed materials in aging process will be affected by two aspects:the hardening effect caused by the second phase strengthening and the softening effect caused by the elimination of work hardening at high temperature.The hardness of cold deformed samples after long term high temperature aging is higher than that of undeformed samples.High hardness will affect the impact toughness of materials to a certain extent.After aging,the impact toughness of the cold deformed sample of sanicro25 steel has been improved to some extent.The impact fracture has changed from intergranular fracture to mixed fracture.However,the way of grain boundary bending by solution treatment with furnace cooling cannot change the fracture mode or improve the impact toughness.That is to say,changing the fracture mode is conducive to improving the impact brittleness.