CC BY
45
Rasulov F. R., Ph. D., dos.
Mamedov A. T., Doctor of Technical Sciences, prof., Azerbaijan Technical University, Baku E-mail: [email protected], [email protected]
FORMiNG CHEMiCAL COMPOSiTiON OF COATiNG ON THE iRON CASTiNG
Abstract. The mechanism of formation of chemical microheterogeneity of metal of a composite covering of cast iron casting is studied. It is established, that formation of structure and properties composite covering of cast iron casting depends as on chemical and fractional structure of a powder and a thicknes of its layer and a temperature condition of impregnation powder liquid metal.
Keywords: coating, casting, impregnation, powder spread, alloy.
1. Introduction. Structural changes in the crys- the binding melt. Therefore, issues related to the for-
tallization of liquid iron, penetrating into the pores of powder spreading, occur as a result of the interaction of the melt with particles of the powder of the composite coating. At the first moment after wetting the surface of the powder particles of the spreading with coasting iron, a surface chemical reaction takes place between them, the speed of which depends on the state of the surface, the temperature in the joint zone and, above all, the heat of formation of the forming compound and is determined by the diffusion laws. The second stage is the dissolution of the metal particles in the melt. The formation of the solid solution and the intermetallic phases and the dissolution of the solid metal proceed simultaneously. Therefore, undoubtedly, the final structure will depend on the ratio of the rates of these processes. At the same time, directly in the contact part of the the powder particle paste with liquid iron, when the particles are completely melted, these changes are associated with the extraction of excess heat from the melt to heat and melt them. In the middle and peripheral parts of the spreading layer, the powder particles are partially dissolved in the cast iron. The reasons for this should be sought in the results of the heterophase interaction of the powder particles with
mation mechanism of the phase "particle boundary -metal", the near-contact zone, the severity of chemical and phase microheterogeneity of the composite metal, become of paramount importance. It is also important to ascertain the actual value of the degree of crystallographic affinity for the structural changes occurring in the binder metal under the influence of the alloy of the particles of powder spread.
It is known that in the pulverized powder of the CrNi80SiB3 alloy nickel in the overwhelming majority is in the boride (Ni3B) and in silicide (Ni3Si2), and chromium is in the boride (CrB) and in carbide (CryC3) phases.
2. Discussion of the results. The metal of cen-trifugally cast castings of bushings with dimensions of 100 x 43 x 180mm (100-outer diameter; 43-wall thickness; 180-height) with powder paste thickness xn = 5; 10 and 15mm alloy CrNi80SiB3 - gray cast iron.
As a result of the impregnation of cast iron in the layer ofpowder spread, the dispersion of the structure and chemical heterogeneity of the binder metal significantly increased. The change in the thermal conditions of the formation of the periwinkle structure was also reflected in the nature of the distribution of the excess phase; an uneven, but dispersed precipitation ofwhich
inside the grain and along its borders in the experimental metal contrasts with coarse local precipitates in the intergranular region in the metal of the control castings. Undoubtedly, these changes were the result of not only intensive heat exchange between the liquid metal and the powder particles of the spreading. The temperature conditions of the casting and the physi-cochemical constants ofthe selected powder materials (T << Tmt> Tp where T is the liquidus temperature of the alloy, Tmt is the melting point of the powder material, Tp is the pouring temperature ofthe alloy) powder in the crystallized solid solution as a separate phase [1].
Thus, the powder particles also play the role of refractory inoculators, whose catalytic influence on the process of forming the crystal structure of the alloy is largely determined by the degree of their crystallographic means with the matrix of the binder metal. Therefore, of particular interest is the analysis of the causes of structural changes under the influence of non-isomorphic powder particles with a significant discrepancy between crystallographic parameters (cast iron Class 15 a = 2.8657 A ; alloy of the powder = 3.5926 A )
The mechanism of influence may be associated with a decrease in the work of the formation and growth of crystals of the bonding alloy on the finished interface. As a result of high temperature, a regular the crystalline-structured transition zone is made on the contact between the surface of the powder particle and a crystallizing melt.
However, the reduction of the heterophase interaction only to the appearance on the surface of the powder particle of a spreading of the boundary zone with a different crystallographic orientation without taking into account possible changes in its chemical composition and uniformity of the particle material is schematic.
Further study of this issue confirmed the need for a deeper analysis of the phenomena in the contact zone. Thus, the observation of partially dissolved powder particles by means of color etching revealed heterogeneity of their color coloration, varying from
yellow - brown (corresponding to the solid solution) to green and burgundy. In addition, each color region has a clear boundary.
Colors of inhomogeneity are accompanied by different values of microparticle, the natural difference between which between the matrix and particle is complemented by an anomaly inside the particle itself in accordance with the colored areas. The values of microparticle Hy, MPa:
matrix metal binder..............2140-2190
matrix (near the contact zone)____ 2140-2920
contact area .............................3490
inner powder boundary ..................2070
powder core ............................1250
Such a distribution of microparticles is a consequence of the chemical heterogeneity of the particle and the area attached to it. This assumption is fully confirmed by the results of micro X-ray spectral analysis.
In the initial state, powder spreading alloy CrNi80SiB3 consists of 78-80% Ni; 15-17% Cr; 3% B; 0.2-0.3% C; 0.7-1.2% Mn; 1% Si; 0.75% P and 4.5% Fe, after crystallization interaction with a binder melt of gray iron of composition (Class 15) contains more carbon and iron with decreasing Ni and Cr (Fig. 1).
The type of curves of the contact layer of the partially dissolved powder microparticle components of the bases (Ni and Cr) indicates the different intensity of its flow and allows you to clearly distinguish the contact layer, the core of particles and determine their sizes (particle size distribution of powders corresponded to fractions: + 50-60; + 63-100; + 100-160; 160-200 and 200-315 ym). The change in the chemical composition of the microparticles of the powder and the contact layer has a diffusion origin [2].
The contact layer around the particle with a clearly defined boundary indicates the probability of formation of intermetallic phases by the mechanism of reactive diffusion.
Taking into account the aggregate state of the interacting particles of the powder and the bonding
alloy, we can assume that new phases is formed simultaneous dissolution of the microparticle in the liquid metal and chemical reaction at the interface, the speed ratio of which regulates the formation of a particular phase.
Under real conditions of joint crystallization of the melt with a solid multi-component particle, the variable ratio of the melt, the degree of crys-tallographic affinity between the matrix and the particle, and finally, the change in the value of the solubility of the alloy components of the solid particles with simultaneous alloying or crystallizing binder were reflected in the ratio of the rates of these processes.
Taking into account the amount of alloying components in the powder particle from alloy CrNi80S-iB3 and the fact that elements participating in the diffusion process can form intermetallic compounds, it can be assumed that a complex chemical Laves phase forms in the contact layer.
The presence in the binder metal ofdispersed particles ofvariable chemical composition and with a diffusion interlayer of a certain width from intermetallic strengthening phases that differ from the base metal of particles and cast iron in the type and parameters of the crystal lattice (aphase = 4.74 A ) explains the increased heat resistance of the test metal [3].
When reducing the powder coating layer thickness from 15 to 10 and 5 mm, as well as increasing the pouring temperature from 1360 to 1440 °C, the degree of dissolution of the powder particles in the cast iron of the bundle increases and the segregation of Ni, Cr, Si and P over the cross section of the composite coat casting decreases.
By chemical analysis of the metal over the cross section of the manual, it was established that the smaller the thickness of the powder coating, the greater the degree of uniformity of the distribution of Ni, Cr, Fe, and C across the coating thickness of the iron casting. So, with an increase in the thickness of the spreads made from the powder of the alloy CrNi80SiB3 from 5 to 10 and 15 mm, the content of the elements changes from the surface to the contact zone; namely: nickel decreases from 45.1; 55.1 and 60.2% to 44.0-44.2%, and chromium from 11.0; 14.5 and 15.2% to 9.4-9.8%, the iron content increases from 44.5-40.2 and 38.5% to 46-46.7%, and carbon content with 1.47; 1.29 and 0.96% to 1.75-1.80% [4] (Fig. 2).
The degree of saturation of liquid iron, poured into the mold at 1440 °C and penetrated into the pores of the powder composite with elements of the CrNi80SiB3 a coasting coat alloy of casting 43 mm thick, is shown in the (table 1).
Table 1. - Changes in the chemical composition, cast iron ligaments depend on the thickness of the composite coating
Thickness Number of elements,%
Powder coating, mm Powder mm in alloy Composite coating on iron casting
Ni Cr Si P
5 Ni = 78-80 Cr =15-17 Si = 1 P = 0.75 9.64 17.14 2.20 3.24 2.20 2.32 0.43 0.34
10 4.25 14.34 1.95 2.98 1.96 2.32 0.59 0.37
15 3.09 13.42 0.89 2.84 1.94 2.34 0.63 0.39
Note: In the numerator - at the surface; in multipliers - at the contact area
As can be seen, the nature of the dissolution of par- of the casting significantly depends on the thickness of ticles and the diffusion of Ni; Cr; Si and P in the cast the powder coating, and the content and distribution iron of the ligament at the surface of the coasting coat of these elements in the zones varies significantly. So,
in experimental castings with a total thickness of 50 mm in the surface zone of the gearbox, an increase in the thickness of the powder spreads from 5 to 10 and 15mm led to a decrease in the content of elements of the CrNi80SiB3 alloy in thin interparticle interlayers of cast iron: 9. 64% at 4.38 and 6.51%; for chromium, from 2.20% to 0.25 and 1.31%, and for silicon, from
2.20% to 0.34 and 0.36%. Then as the phosphorus content in the pig iron impregnated in the porous spread increases from 0.43% to 0.51 and 0.63%, while in all cases of the casting with the impregnation of the powder spread the metal ofthe contact gearbox itelno increasingly complex is saturated with Ni, Cr and P than in the peripheral regions [4; 5].
Figure 1. Chemical Inhomogeneity of Spray Particles from alloy CrNi80SiB3 and the adjacent area
Figure 2. Changes in the chemical composition of the gearbox of the casting "powder coating of alloy CrNi80SiB3 - cast iron Class 15" depending on the thickness: (figures on the curves) of spreading: 1, 2 and 3 - thickness of powder coating of 5, 10 and 15 mm, respectively; T = 1440 °C, Ô = 43 mm
At the same time, the solubility and diffusion of elements from solid particles of the powder alloy into the cast iron of the bundle is much greater than in the surface zone of the coasting coat. However, with an increase in the thickness of the spread, the content of the elements that become bonded to the cast iron composition decreases as much as in the metal surface zones of the gearbox, namely, as the thickness of the spread is increased from 5 to 10 and 15 mm, the content of Ni and Cr decreases by 2.80 and 3.72%, 0.27 and 0.40%, respectively;
Regardless of the thickness of the powder spread in the metal of the contact zone, the silicon content
remains almost unchanged; while the phosphorus content in comparison with the initial amount (0.7-0.8%), decreases almost 2 times and is 0.35-0.38%.
A coasting coat of iron casting is characterized by a layered structure, which depends on the melting temperature and the degree of interaction, the particle size distribution of the powder particles, the thickness of the spreading layer of the melt impregnation temperature of the matrix during the casting process. Part of these parameters in the casting process is maintained very approximately and cannot be strictly controlled. Since the moment of contact of liquid iron with powder spreads made from alloy CrNi80SiB3, some of
the powder particles are dissolved in the hardening cast iron and are doped with Ni and Cr. This process proceeds most efficiently, directly in the contact zone and near-contact interlayers, both on the side of the powder layer and the wall of the casting.
These elements affect the formation ofthe metallic base and the shape ofgray cast iron graphite in different ways: Ni and Mn have austenitizing effects and contribute to the formation oflamellar graphite; Cr contributing to carbidization sometimes distorts the shape of the graphite incorporation. The complex effect of alloying elements is turning into a strip of white hypoeutectic cast iron - the third zone of the transition layer.
The dispersion of the pearlite component of each zone of the transition layer is different. Redistributions of Ni, Cr, and Si are observed only within the limits of the first zone of the transition layer.
The formation of the second and third zones is associated with a change in the carbon concentration of the pig iron of the bundle.
At 3.86-22.64% Ni in the transition layer, adjacent to the cast alloy, the first zone is observed, in which, at high cooling rates, there are areas of bainite and residual austenite, as well as partially dissolved powder
particles CrNi80SiB3. In the second zone, along the grain boundary, carbides from doped cementite of the type (Fe, Cr)7C3 and cementite needles with a typemagnet orientation are observed clearly. The effect on the structure of the transition layer Cr is significantly manifested at temperatures Tb above the boiling liquid. At the same time, the first zone acquires a troost -sorbitol structure with a cementite net, which turns into a terostat - martensitic one. Redistributions of C, Ni, and Mn in the transition layer are also confirmed by the change in microparticles.
The research results indicate not only the need to take into account the effect of diffusion on the chemical homogeneity of the microparticles, but also indicate the possibility of controlling the whole complex of structural and special properties of powder materials by regulating the processes occurring in the near contact zone.
3. Conclusion. In a cast iron casting, the formation of the structure and properties of a composition coat depends not only on the chemical and fractional composition of the powder particles, but also on the temperature condition of the impregnation of the powder spread with liquid metal.
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