Fe-Cr-Al-Nb-W Alloy High Temperature Creep Strengthening Method Based On Element Control

Due to its outstanding creep resistance,good corrosion resistance and high temperature oxidation resistance,ferritic heat-resistant steel is widely used in key parts such as steam pipes of supercritical thermal power units,high-temperature section superheaters and reheaters.In the course of long-term service,"organization degradation"(such as:martensite lath coarsening,precipitation and coarsening of second phase particles)will occur,which will lead to a decrease in creep strength and life,which will affect safe service.The Laves phase is an intermetallic compound phase,which is gradually precipitated during the service of the ferritic heat-resistant steel,and its certain structure will cause the ferritic heat-resistant steel to creep strengthening.Therefore,this study intends to explore the strengthening mechanism of Laves relative to Fe-Cr-Al-Nb-W new ferritic heat-resistant steel.First,a new type of ferrite heat-resistant steel containing two eutectic structures of ferrite and Laves phase was designed and prepared by eutectic reaction,and the histological characteristics of ferrite and Laves and the main effects of each element on them were investigated.,And then prepare a new type of ferritic heat-resistant steel under eutectoid reaction to strengthen it through element control.By analyzing the microstructure,microhardness and high temperature creep properties of the new ferritic heat-resistant steel,the following conclusions are drawn:(1)Based on literature review and JMat Pro software for thermodynamic simulation,a new type of Fe-9Cr-6.5Al-Nb-W ferritic heat-resistant steel with Laves phase precipitated before service was successfully designed and prepared.Through research,it was found that:The new Fe-Cr-Al-Nb-W ferritic heat-resistant steel with different W/(Nb+W)values contains two eutectic structures of ferrite and Laves phase in the as-cast state.The samples containing only Nb are different from others Samples with W/(Nb+W)values have significantly different tissue structures,and the Laves phase is in a branch-like morphology;but due to the larger volume fraction of Laves phase and the higher hardness of heat-resistant steel,Failed to show excellent high temperature creep performance.(2)Aiming at the heat resistance of Fe-9Cr-6.5Al-Nb-W ferrite,the content of Al,Nb,W element was adjusted and the content of Ce element was added to design and prepare Fe-9Cr-2.5Al-0.5Nb-2W-Ce ferritic heat-resistant steel:The eutectic heat-resistant steel is transformed into an eutectoid reaction,which successfully reduces the Laves phase volume fraction,improves the high-temperature creep properties of the first batch of materials,but compared with the traditional ferritic heat-resistant steel,the precipitation phase ingle,short rupture time,poor high-temperature creep performance,hardness lower;in addition,Ce element has no obvious effect on the microstructure and mechanical properties of ferritic heat-resistant steel.(3)In order to further improve the high temperature creep performance of Fe-9Cr-2.5Al-0.5Nb-2W-Ce ferritic heat-resistant steel,the Ce element is removed under its elemental composition,and a small amount of C and N elements are added.Content control,Fe-9Cr-Al-0.5Nb-2W-C-N ferritic heat-resistant steel was designed and prepared,and M₂₃C₆and MX phases were successfully introduced,and the high-temperature creep properties of heat-resistant steel were significantly improved:C,N and Al affected microstructure and creep of heat-resistant steel.As the Al content increased or as N content decrease and C element increased,the average diameter of the Laves,M₂₃C₆and MX phases at grain interior decreased,the unit density increased,the phases pacing decreased and Orowan stress increased.This led to the minimum creep rate decrease and creep rupture time increase;Compared to M₂₃C₆,Laves and MX phases mainly affected the alloy creep strength;The ferritic steel hardness was not affected by contents of C and N,but it was affected by Al content.As the Al content increased,the hardness of heat-resistant steel increased first and subsequently decreased.When the Al content was 2%,the hardness as the lowest;The continuous coefficient F_C=d/?decrease of the Laves phase at the grain boundary was beneficial to the plasticity improvement of the heat-resistant steel.