Study On The Key Basic Technical Problems Of Microstructure And Property Of Chromium Wear-resistant Alloy

Chromium cast iron is used widely as a wear-resistant materials. The service life ofcomposite system with high hardness carbides and different matrix was dominated bythe structure,size, morphology, distribution of carbides. The studies for many years havebeen foucus on the structure, size, morphology, distribution of carbides and thecomposition and microstructure control of the matrix. Based on solidification theoryand liquid structure and property of chromium cast iron, the nucleation and growthmechanism of carbides and solidification behavior of matrix were discussed. Theproperty characteristics was further studied on the basics of microstructure in order tolay the foundations for the future development of chromium cast iron with highproperty.The innovative achievements of the paper were as follows:（1） Guided by the metal solidification nucleation theory and started with carbonatoms in tetrahedral or octahedral interstice of carbides, the nucleation model of primarycarbides of hypereutectic high chromium cast iron was builded by calculation andderivation. The primary carbide crystal structure of high chromium cast iron ishexagonal lattice, the tetrahedral and octahedral clusters might merge to six polyhedracrystal nucleus from thermodynamic consideration. The driving force of tetrahedronatomic cluster combined into six polyhedra crystal nucleus was much larger than that ofeight polyhedron atomic cluster. However, the nucleation was influence by kineticsconditions. The fact of octahedral interstice greater than tetrahedral interstice providedadvantageous conditions to Fe and Cr clusters trapping carbon atom.The melt existed incompetitive nucleation between tetrahedral and octahedral. （2） The thermodynamic behavior of carbide precipitation and growthmorphology of carbides were thoroughly investigated. The microstructure ofcarbides-liquid interface had an important influence on carbide type changes and growthpatterns. Jackson factor increases with the increase of Cr content, the smoothnessdegree of the carbide in the carbides-liquid interface was increased accordingly, leadingto the decrease of net M3C carbides soaring level. Added Cr changed the liquid atomicstructure of cast iron, raised the eutectic transition temperature and decreased thethermal conductivity in iron liquid. Which aggravated the carbide lattice distortiondegree.The intrinsic reasons for carbide type critical transition in different solidificationconditions were explained. The growth process of primary carbide of hypereutectic highchromium cast iron was elementarily revealed. The growth mechanism of primarycarbides was explained by periodic bond chain theory, heat and mass transmissionprinciples. It was believed that the primary carbides grew in form of planar interface inthe initial growth stage in liquid. Carbides outflanked inward growth when latent heat ofsolidification quickly dissipated along certain sides of carbides and C atoms wasdifficult to diffuse to the carbides-liquid interface. The carbides morphology not onlyhad relationship with internal bond of carbides, the relative growth rate of variousplanes and crystal defects, but also largely depended on the growth environment. Rapidcooling rate led to small area of outflanking inward carbides, the carbides morphologywas easy to grow into a solid shape. Slow cooling rate led to L-shaped, U-shaped orhexagonal shape carbides.（3） It is useful to ascertain whether there existed a larger number of Cr₂O₃filmunder H2SO4（PH=3.5）and quartz sand erosion-corrosion wear conditions. The presenceof a larger number of Cr₂O₃film was associated with Cr contents. The number of Cr₂O₃film had a marked influence on the improvement of corrosion wear resistance. Underthe experiment conditions, Cr₂O₃was hardly detected as for the samples of white castirons containing10wt.%Cr,12wt.%Cr and15wt.%Cr after erosion-corrosion wearexperiments, which had a poor corrosion wear resistance. As for high chromium castirons containing23wt.%Cr and28wt.%Cr, a larger number of Cr₂O₃existed ingrinding chips, which improved corrosion wear resistance of the two kinds of alloys.（4） The influence of solidification rate on the corrosion resistance was mainly depended on the competition between potential difference between carbides and matrixand grain-boundary energy under the alkali static corrosion resistance conditions. Theincrease of solidification rate led to elements homogeneously distributed in matrix andfiner size. Correspondingly, potential difference between carbides and matrix wasdecreased and the corrosion resistance of alloys was improved. Meanwhile, the finermicrostructure increased the grain boundary area and grain boundary energies, soincreased the sensitivity of grain boundary, which resulted in the increase of corrosionresistance. Under the experiment conditions, the increase of grain boundary energiesowing to the increase of solidification rate was mainly recognized in15wt.%Cr whitecast iron with the experiment going on, which decreased the corrosion resistance. As for10wt.%Cr,12wt.%Cr,23wt.%Cr and28wt.%Cr white cast iron, the decrease ofpotential difference between carbides and matrix was dominant for the whole corrosionprocess with increase of solidification rate, so the corrosion resistance was improved.