Researchs On Effect Of Al Element On Microstructures And Properties Of Heat-resistant Casting Alloy HP40

HP40 alloy has been extensively used for high temperature alloy tubes such as ethylene cracking furnace tubes and steam reformer tubes.During running the material is often destroyed by coking and carburization,high temperature oxidation,squirm invalidation and so on.Therefore we attempt to develop HP40 steel with Al element in displacement of Ni to improve elevated temperature mechanical properties,oxidation resistance and coking & carbonization resistance as well as decrease cost of the steel.By vacuum arc melting furnace in copper mold cooling with water,HP40 steel with 5-20 wt.%Al in displacement of Ni were prepared.Microstructures,mechanical properties,oxidation resistance of the steels were investigated.The results showed that the matrix of the steels transformed fromγ-Fe solution to a-Fe solution phase and lots of FeNiAl phase appeared in the microstructure with further increased Al content.The FeNiAl phase shape changed from sphere to dendrites.Yield strength and hardness of the alloys were improved markedly by Al element.With increased Al content, brittleness of the alloys was increased.The oxide film on the surface of the alloys mainly consisted of Al₂O₃,thus the oxidation resistance was increased.When Al content was 5 and 10wt%,the oxidation resistance was the best.But when Al content further increaseed,the oxidation resistance was decreased.Due to increase of the alloy's oxide film thickness,the film was broken off easily and the oxidation resistance was decreased.Then,HP40 based steels with 2.5-15 wt.%Al in displacement of Ni were prepared by frequency inductive furnace melting at atomosphere.Microstructures,mechanical properties,oxidation resistance of the steels were also investigated.Because of different cooling conditions,the microstructures of the alloys made by frequency inductive furnace were different from that of made by vacuum arc melting furnace observably.The Al element was all dissolved in the matrix of the alloy with 2.5 wt.%Al.There were a few particles of Ni₃Al precipitated in theγmatrix of the alloy with 5 wt.%Al.When the Al element added above 7.5wt.%,the FeNiAl phase solidified directly from the melting body.The matrix of the steels transformed fromγ-Fe solution to a-Fe solution phase and the amount of FeNiAl phase in the alloys increased with further increased Al content.Their shape changed from grain to dendrite.When the Al element was added up to15wt.%,the eutectic structure was made from FeCrNiAl and carbide.The room temperature and high temperature mechanical properties of the alloy with 2.5 wt.%Al were not increased.The alloy with 5 wt.%Al had the best room temperature and high temperature mechanicai properties.The mechanical properties of the alloys with 7.5,10 wt.%Al were lower than the HP40.The ductilities of the alloys decreased with further increased Al content.The high-temperature oxidation resistance of the alloys with 2.5-10wt.%Al at 1200℃for 30h in air was studied by using weight gain method.Experiment results indicated that addition of aluminum element had improved high temperature oxidation resistance of the alloys.The alloys' oxidation behaviour was approximately parabolaic. The oxidation speed was decreased with time.The phases of the oxidation film varied with aluminum content.The oxidation film of the alloy with 5 wt.%Al was more compacted.With increased Al content,the oxidation film became very thick and flaked off when outside force processed.Influence of heattreatment on the microstructure of HP40 alloys with 5-10wt.%Al was studied by solution with two-step aging treatment.The results showed that the precipitation phases of as-cast alloys were changed.The Ni₃Al phase shape of 5wt.%Al became bigger from particle to strip,and the shape of 7.5,10wt.%Al became homogeneous and small from block to strip and particle.Between the solution temperature 1180℃and 1220℃,the upper temperature was more reasonable for optimizeing microstructure.The weldability of HP40 and the alloy with Al 5wt.%was investigated.The mechanical properties of welded joint were same to that of HP40.The welded joint had excellent high-temperature mechanical properties.The alloy with 5 wt.%Al had good weldability.