【作者】 刘致远；

【导师】 郝远； 喇培清； 李克雄；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2008， 博士

【摘要】 针对石化装置用高温合金炉管（如乙烯裂解炉管及制氢转化炉管等）材料HP40易受高温氧化、蠕变失效以及结焦渗碳等问题的影响,通过在该合金中加Al降Ni改性,尝试解决以上问题。研究了加入不同Al含量以及不同冶炼条件下合金的组织、常高温力学性能、抗氧化性能以及热处理对合金组织的影响和新合金的焊接性能等。目标是提高合金在高温下的力学性能、抗蠕变能力、抗氧化能力以及防渗碳结焦能力,实现延长该材质高温合金炉管的清焦周期和使用寿命;减少Ni含量以降低材料成本。以HP40为基础,分别加入5、10、15、20wt.%的Al含量、相应减少5、10、15、20wt.%的Ni含量,将各粉体材料在真空电弧炉中熔炼并在水冷坩埚中凝固后制备成试样,进行微观组织、力学性能和抗氧化性能的研究。结果表明,当Al加入量为5wt.%时,合金中沉淀析出了少量片条状Ni₃Al金属间化合物,其余组织与HP40相近;当Al加入量增加到10%时,凝固析出的FeNiAl相显著增加,基体转变为以铁素体组织为主;当Al加入量在15 wt.%以上时,合金中形成大量树枝状的FeNiAl金属间化合物,基体完全转变成为铁素体。碳化物的形态随着Al的加入由以前的网状变成了片条状,并随Al元素的增加而变得更加细小,在基体中的分布更趋离散。随着Al加入量的增加,合金的强度和硬度得到了显著提高。但随着Al含量的提高,由于析出金属间化合物数量的增多以及形态的进一步粗化,脆性也随之增加。1200℃的高温抗氧化性能研究发现,Al加入后在合金表面形成了以氧化铝为主的氧化膜,从而显著增加了合金的高温抗氧化性能。在Al含量为5 wt.%和10 wt.%时,合金的高温抗氧化性能最好。将工业炉料在中频无芯感应炉中冶炼、浇入树脂砂型中制备出HP40和以此合金为基础加Al2.5、5、7.5、10和15 wt.%、降Ni2.5、5、7.5、10和15 wt.%的合金试样,并进行微观组织、室温及高温下的机械性能和抗氧化性的研究。从组织上比较,中频炉组织与真空电弧炉组织总的变化规律基本一致,但在相同成分对应的组织上,由于冷却条件不同,使得组织的组成各相有着显著的差异。加Al量为2.5%时,Al主要固溶于基体中,未析出沉淀强化相。加Al量为5wt.%时,沉淀析出的Ni₃Al金属间沉淀相数量相对显著增多,而且呈颗粒状弥散分布。加Al量增加到7.5wt.%以上时,基体开始向铁素体转变,FeNiAl金属间化合物相大量凝固析出,形状从颗粒状转变成树枝状、大块状。加Al量增加到15wt.%时,合金组织最终变成了FeCrNiAl固溶体和碳化物的共晶组织。加Al2.5 wt.%的改性合金在室温、高温下的屈服、抗拉强度没有提高。加Al5wt.%的改性合金在室温、高温下的屈服、抗拉强度得到了显著提高,也具有较好的塑韧性。而含Al7.5、10wt.%的改性合金在室温、高温下的屈服、抗拉强度反而降低了。合金的延伸率也随着Al元素的增加而显著降低。改性合金的氧化动力学曲线符合抛物线规律,斜率随Al含量的增加而降低,抗氧化性在逐步提高。但从形貌上来看,加铝2.5wt.%的合金氧化后表面较HP40合金有所改善,但有明显的烧损,氧化膜不致密。加铝5wt.%的合金氧化后表面生成了较致密的氧化膜,局部地方有坑蚀;而在加Al7.5wt.%和10wt.%的合金表面上,由于Al氧化膜太厚,表面发生了隆起,增加了从基体表面剥落的可能性。对改性合金热处理的初步研究表明,经过固溶+两次时效处理,使加Al5wt.%改性合金的Ni₃Al金属间化合物析出相增多,形态由铸态下的弥散颗粒状变成了链条状。对加Al7.5和10wt.%的改性合金,金属间化合物沉淀相的形态较铸态明显改善,从块状转向条状、颗粒状。较高的固溶处理温度对组织形态的改进更有效。通过对HP40和加Al 5wt.%合金焊接接头的组织和力学性能的对比分析,发现加Al5wt.%合金焊接后的力学性能与母材基本相当,焊接后的合金接头和母材一样具有优良的高温力学性能。研究结果表明,加Al5wt.%改性合金具有良好的焊接性。更多还原

【Abstract】 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.更多还原