Studies On Microstucture And Properties Based On HP40 With High Al And Low Ni

Aiming to the problems existing in the materials of HP40 used in ethylene cracking furnace,new types of alloys in which part of Ni was replace by Al were designed,the alloys according to the design composition were febricated in arc melting furnace and intermediate frequency furnace,respectively.The microstructure variations of the alloys that melted in arc and intermediate frequency furnace were discussed.We found that a little(Ni,Fe)₃Al precipitates was formed in theγ-Fe solution matrix in the HP40 steel alloyed with 5 wt.%Al;(Ni, Fe)Al dendrite phase appeared in the alloy with 10 wt.%Al and the matrix wasγ-Fe solution.A lot of(Ni,Fe)Al dendrite phase and a littleσphase with dissolved Al appeared in the steel with 15 wt.%Al and the matrix wasα-Fe solution.Microstructure of the steel with 20 wt.%Al was consisted of a lot of equiaxed(Fe,Ni)Al andαandσphase solution and chromium carbide.The microstructure of alloy melted in intermediate frequency furnace wereγ-Fe and eutectic ledeburite without adding Al.A little of particle intermetallic compound were distributed around carbide in the HP40 steel with 5 wt.%Al in displacement of Ni.With aluminum content in steel increasing, the matrix of the alloy was transformed fromγ-Fe toα-Fe.With further increasing the aluminum content,the amount of FeNiAl precipitation increased and distributed with featherlibe.the matrix of the alloy was stillα-Fe.Compared with the alloy in arc furnace, the microstructure has a big change with 10 wt.%Al and a little change with 5 wt.% Al.The high-temperature oxidation resistance of alloy were studied in two different melting conditions.We found that the surfaces of the alloy were covered with a dense oxidation layer,which improved high-temperature oxidation resistance of the alloy.The high-temperature oxidation resistance of the alloys with 5 wt.%Al were the best. Component of oxide film transformed from Cr₂O₃ and spinel structure(FeCr₂O₄,NiFe₂O₄,NiCrO₃)to Al₂O₃ with the increasing content of Al.Finally,the alloys melted in intermediate frequency furnace with different aluminum contents at 1180℃and 1220℃,and aging 28 hours were investigated.The results indicated that for the alloy with 5 wt.%Al at 1180℃,a little of particle intermetallic compound precipitated from the matrix;with aluminum content increasing,there was uniform intermetallic precipitation in alloys with 7.5 wt.% Al.The precipitation were dendritic intermetallics and fine intermetallic precipitation in interdendritic with 10 wt.%Al.The temperature increase to 1220℃,the needle-like and flocculent intermetallic compound precipitated from the matrix with 5 wt.%Al, they distributed more uniform.Comparing with the former,the shape of the precipitation was changed;There was fine intermetallic precipitation in alloys with 7.5 wt.%Al.The distribution and size of particles become more perfect.After high temperature aging,the varieties of yield strength and hardness with two different conditions test samples are small.