Study On The Solid Solution Behavior Of δ-ferrite In Martensitic Heat-resistant Stee

Ultra(ultra)critical technology is currently the most promising power generation technology for improving thermal power generation efficiency and energy utilization,among which heat-resistant steel is the key material for ultra(ultra)critical units.9%-12%Cr martensitic heat-resistant steel is widely used in ultra(ultra)critical unit boiler,steam turbine manufacturing,aviation,petrochemical and other fields because of its good oxidation resistance,strong corrosion resistance,high creep strength and good thermal parameters.However,martensitic heat-resistant steel is prone to produce harmful segregation structure during production delta ferrite,its existence will adversely affect the mechanical properties and corrosion resistance of the material,therefore should be eliminated.However,it is very difficult to eliminate delta ferrite,that is caused by the segregation of alloy components during the solidification process,which is difficult to completely eliminate.In addition to long-term high-temperature heating,it is difficult to remove the delta ferrite that has already appeared in the ingot through conventional heat treatment.However,the dissolution rate of delta ferrite in martensitic heat-resistant steel is very low and enters a "stable state" with the extension of the dissolution time.When reaches a certain level,the dissolution rate of delta ferrite becomes extremely low,making it more difficult to eliminate through dissolution,long-term high-temperature dissolution brings many disadvantages such as energy consumption,coarse grain size,and material loss.Therefore,how to improve the dissolution rate and quickly eliminate delta ferrite is the key and difficult point in the field of martensite heat-resistant steel.In this paper,taking martensitic heat-resistant steel as the research object,the dissolution behavior of delta ferrite in martensitic heat-resistant steel was studied by changing the alloy system,cyclic heat treatment,microalloying and other means,and reveal the mechanism of the influence of dissolution rate of delta ferrite,which laid a theoretical foundation for the rapid dissolution of delta ferrite in martensitic heat-resistant steel,and proposed a new technology to improve the dissolution rate of delta ferrite in martensitic heat-resistant steel.The effect of Ni,Mo,Mn,Si and V conventional alloy elements on the dissolution behavior and the mechanism of low dissolution rate of delta ferrite in martensitic heatresistant steel was revealed.The experimental results show that alloy elements have a significant impact on the formation and dissolution rate of delta ferrite in experimental steel.Ni and Mo elements can significantly promote the formation of delta ferrite in the cast microstructure of experimental steel,while the addition of Mn,Si,and V can also promote the formation of delta ferrite,but the effect is relatively small.By comparing the dissolution rate of delta ferrite in experimental steel with different alloy compositions,it can be seen that Ni and Mo significantly reduce the dissolution rate of delta ferrite,the addition of Mn,Si,and V elements also reduces the dissolution rate of delta ferrite,but its impact is relatively small.The influence of alloy elements on the dissolution behavior of delta ferrite is complex and coupled.On the one hand,the diffusion rate of alloy elements itself inhibits the dissolution of delta ferrite,on the other hand,the addition of alloy elements leads to increased segregation in the as-cast structure,resulting in the formation of more delta ferrite,in particular,promotes the formation of large size delta ferrite,which is the main reason for the low comprehensive dissolution rate of delta ferrite in martensite heat-resistant steel.The effect of heat treatment process on the dissolution behavior of delta ferrite in martensitic heat-resistant steel was studied through heat treatment experiments,revealing the mechanism by which the dissolution rate of delta ferrite decreases with the dissolution time,a new technology to improve the dissolution rate of delta ferrite through cyclic heat treatment has been proposed,and the mechanism of its action has been revealed.The experimental results indicate that the morphology and distribution position have a significant impact on the dissolution rate of delta ferrite.Under a single heating process,with dissolution time increases,the austenite grains grow rapidly,and the position distribution of delta ferrite migrates from the austenite grain boundaries to the interior of the austenite grains,the morphology of delta ferrite transforms from irregular shapes to spherical shapes,and the decrease in specific surface area of delta ferrite is an important reason for the decrease in its dissolution rate.Under the cyclic heat treatment process,with dissolution time increases,the austenite grains are always in a smaller size state,and delta ferrite is always in an irregular state at the austenite grain boundary,with a higher specific surface area.Therefore,the dissolution rate of ferrite decreases more slowly with the extension of dissolution time and always maintains a higher dissolution rate.This is the main reason why the dissolution rate of delta ferrite under the cyclic heat treatment process is higher than that under single heating.The effect of micro alloyed element Nb and Ti on the dissolution behavior and mechanical properties of delta ferrite in martensitic heat-resistant steel was studied,a new technology to improve the dissolution rate of delta ferrite through Nb and Ti micro alloyed has been proposed,and the mechanism of its action has been revealed.The experimental results show that the addition of Nb and Ti elements will promote the formation of delta ferrite in the cast microstructure,but the morphology of delta ferrite is relatively small,and the addition of Nb and Ti elements will increase the dissolution rate of delta ferrite in martensitic heat-resistant steel.Analysis suggests that Nb and Ti as ferrite forming elements,aggravate the segregation of alloy components,leading to an increase of delta ferrite in the as-cast structure,after the addition of Nb and Ti elements,Nb C and Ti C precipitates will be formed in the experimental steel,and the grain boundary will be pinned to inhibit the growth of austenite grains.During the heating process,more delta ferrite will be maintained at the austenite grain boundary in an irregular shape for a longer time,maintaining a larger specific surface area,thus improving the dissolution rate of delta ferrite.In addition,smallsized Nb C will be formed after Nb micro alloyed,resulting in precipitation strengthening and fine grain strengthening to ehance mechanical properties of experimental steel.Largesized Ti C will be formed after Ti micro alloyed,promoting the generation of microcracks to reduce the plasticity of the material.