The Effects Of Alloying And Process Optimization On Microstructure And Performance Of Austenitic Heat-Resistant Steel Strengthed By L1₂-Orded Phase

Thermal power plants use lots of coals which are fired generating CO₂greenhouse gas and other harmful substances.It causes much attention.Although in recent years the proportion of other forms of power generation such as nuclear power and wind power has risen a lot,thermal power generation still will be the most important in a long period.Improving the efficiency of thermal power generation becomes the matter.The key to improve the power generation efficiency is to improve the working parameters of the thermal power unit?working pressure and working temperature?.At present,the European Union,the United States and others have increased the target working parameters to 700^oC/35MPa or more.However,the improvement of the operation parameters of ultra supercritical thermal power units brings greater challenges to the performance of various key parts,such as high temperature strength and oxidation resistance.In recent years a new type of alumina-forming austenitic?AFA?heat-resistant steel has been developed which can form compact Al₂O₃ oxide film on the surface making excellent oxidation resistance at high temperature.In this paper the high temperature performance of AFA alloy?The analyzed composition of master alloy is Fe-20.22Ni-14.80Cr-1.26Mn-1.89Al-2.36Mo-2.8Cu-0.18Si-0.49Nb-0.01B-0.04C,wt.%,denoted as 20Ni?has been further improved by alloying and process optimization under 700 ^oC and 750 ^oC.The effects of alloying and process optimization on performance and microstructure of steel have been analyzed and the micromechanism of the improvement of high temperature performance was revealed.The main research results are as follows:1)Pre-strain can introduce appropriate dislocations into alloys to improve the high temperature creep rupture behaviors.Based on general mechanical treatments the effects of pre-strain on the creep resistance were studied by increasing the amount of pre-strain.This study showed that the creep life of 20Ni alloy increased from 401 h to 1215 h at 700 ^oC/200 MPa when the amount of pre-strain increased from 0%to 15%.The introduction of dislocations could not only play the role of dislocation strengthening but also provide the nucleating sites of nano-size second phase precipitates.Further increase of the pre-strain made the creep life decline significantly.2)The effects of the contents of Ni?Al?Ti and Cu on the amounts of the second phases of AFA alloy were simulated by the nickle based superalloy database of thermodynamic software JMatPro7.0.The increase of the content of Ni could effectively inhibit the precipitation of harmful phases?such as coarse Laves phase and?phase?and increase the amount of L1₂ phase,excessive increase of Al was bad for the precipitation of L1₂ phase but increased amount of?phase,the increase of Ti could increase the amount of L1₂ phases and harmful phases at the same time,Adding appropriate amount of Cu could increase the amount of L1₂ phase and reduce the amount of harmful phases.3)Adding 7%or 15%Ni into 20Ni was helpful for the high temperature creep.When adding 0.5 wt.%Ti into 27Ni or 35Ni alloy the improvement of the high temperature creep was the most obvious.However,further adding Ti to 1.0%would reduce the oxidation resistance of 27Ni alloy and make the main precipitation phase L1₂-Ni₃Al phase of 35Ni alloy coarse at 750 ^oC.6%or 10%Cu caused the degradation of plasticity and high temperature creep of 27Ni alloy.The results showed that the changes of element contents changed the amounts and sizes of second phases during the process of creep tests,especially the size of L1₂ phase had the greatest influence on creep resistance.4)Adding 0.5%and 1%Ti to 27Ni alloy made the degradation of oxidation resistance at 750 ^oC significantly.However,further add 8%Ni into the Ti-containing AFA steel could restore its oxidation resistance to a certain extent.