| High strength and super-high strength stainless steels have been applied into high-tech industries widely due to good matching between strength and toughness, good corrosion-resistance, processablity and weldability. Among them, maraging stainless steels with high strength are often used as airfoil beam, corrosion-resistant parts of submarine, impellers of compressor engine and so on. Such typical steel brands were17-4PH, FV520and Custom465, formed by a small amount of different elements, such as C, Mo, Ti, Nb and Cu, alloying of Fe-Cr-Ni ternary basic system. High strengths of these alloys were resulted from their structure with martensite transformation after quenching and secondary strengthening phases (carbides, intermetallics) dispersed on the martensitic matrix. The maraging stainless strengthing by carbides is also called martensitic precipitated hardening stainless steel with the C content being about0.1wt.%. Materials properties relied greatly on the types of alloying elements and their contents, so that some inconvenience will be brought about in composition design of multi-component alloys. Therefore, the present work will take the Fe-Ni-Cr-based maraging stainless steel as the research object, and use the cluster-plus-glue-atom structural model to design compositions. Firstly, the composition rule of maraging stainless steels is given by analyzing the typical steel brands using the caluter model to construct an unifom cluster formula; then new multi-component alloy compositions are designed according to the cluster formla; last, a series of new cluster formula alloys would be obtained through experiment verifications.The designed cluster alloy ingots and rods with a diameter of6mm were prepared by using arc-melting and copper-mould suction-casting under argon atmosphere. The carbide-strenthened{Fe-Ni-Cr}-C serial alloy ingots and rods were solid-solutioned at1323K for2h followed by water quenching, then aged at753K for4h, and the intermetallics-strengthened{Fe-Ni-Cr}-M serial alloys rods were solid-solutioned at1273K for1h followed by water quenching, then aged at783K for3h. Structural identification and microstructure were carried out by means of BRUKER X-ray diffractometer (XRD), OLYMPUS optical microscopy (OM) and SEM (Zeiss Supra55), respectively. The property measurements, such as micro-hardness, tensile test and corrosion-resistance were also done.. finally, the radom forest algorithm was used to calculate and predict the microhardness of the designed alloys based on the experimental data. The main results are listed as follows. (1) The the basic cluster formula [NiFe12]Cr3of maraging stainless steel is determined by the cluster structure model as well as the the enthalpy of mixing ΔHM-Fe between alloying element M and the base Fe, where [NiFe12] cluster is a cubocthedron in FCC structure centered by a Ni atom and surrounded by twelve Fe atoms and Cr atoms serve as glue atoms to link clustersThis cluster formula corresponds exactly to the lowsest limit of austenite in Fe-rich cornor. In order to incarnate the minor alloying elements into the basic cluster formula [NiFe12]Cr3, it is self-magnified from [NiFe12]Cr3with16atoms to [Ni16Fe192]Cr48with256atoms. Then the cluster formula{[(Ni,Cu)16Fe192](Cr,Mo,Nb48}C1in{Fe-Ni-Cr}-C system, and [(Ni,Cu)16Fe192](Cr32Ni16-x-y-z-m-nTixMoyVzNbmAln) in{Fe-Ni-Cr}-M system are achieved respectively, after similar element substitution.(2) The microstructure and properties of the solutioned and aged{Fe-Ni-Cr}-C alloys strengthened by carbides are changed with the different types and amounts of adding elements. Among them, the{[(Ni13Cu3)Fe192](Cr45Mo2.5Nb0.5)}C1alloy (Fe74.92Ni5.33Cu1.33Cr16.35Mo1.67Nb0.32C0.08wt.%) after aging treatment has a highest hardness and tensile strength (Hv=397,σ0.2=971MPa and σb=1093MPa), as well as a good corrosion-resistant property in3.5%NaCl solution. In the Ni3M-strengthened{Fe-Ni-Cr}-M systems, the aged alloy [(Cu4Ni12)Fe192](Cr32(Ni8.5Mo2Ti2Nb0.5Al1V1))(Fe74.91Ni8.82Cr11.62Mo1.34Ti0.67Nb0.32Al0.19V0.36Cu1.78wt.%) has a good combined performance between superhigh strength and corrosion-resistant properties, where the characteristic parameters are Hv=488,σ0.2=1417Mpa,σb=1494MPa,Ecorr=-0.334V,Eb=0.318V,icorr=0.804uA/cm2, respectively.(3) There exists a linner relationship among atom size, value of outmost electron in a cluster formula (VEC) and macro-performace (microhardness Hv) of design alloy. This relationship could not only procast the hardness difference between difference heat treatments, but also determine the total optimal added amount of minor-alloying elemets M (M=Ti, Nb, Al, V), being about1.8wt.%, which can provide a direct reference for large-scale industrial production.(4) The microhardness of new maraing stainless steel alloys could be simulated and predicted using a Random Forest algorithm which is constructed based on the experimental results. Thus the trial-and-error tests could be lessened during the alloy design presoss.. |
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