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Rasulov Fuzuli Rasul, PhD, Deputy dean of the Metallurgyy Azerbaijan Technical University E-mail: [email protected]
iMPROVING THE PROPERTIES OF THE SURFACE OF THE CAST IRON BY ABSORBING INTO THE LIQUID METAL IN THE CASTING MOLD
Abstract: The article was devoted to the study of improving the surface characteristics of castings by impregnating powder of the composition of liquid iron in the casting process. Researched mechanical properties and improvement of corrosion resistance of surface alloyed castings small quantities of chromium and nickel.
Keywords: surface alloying, powder composite, casting, foundry mold, spread.
Introduction:
At present, it is sufficient to fully disclose the mechanism of corrosion of metals and alloys, and effective methods have been developed to prevent it. Despite this, nowadays the loss from corrosion is very high. Therefore, the fight against corrosion is a serious scientific and technical challenge [1].
In this case, as a rule, the bulk alloying of metals and alloys is uneconomical. Therefore, in recent years, increasing attention ofresearchers and manufacturers has been paid to various methods ofsurface alloying ofthe working surfaces of gadget [2; 6]. Surface alloying of products can be carried out by various methods: diffusion metallization, spraying, impregnation with liquid alloys, surfacing, electrospark alloying, etc. All these methods have their advantages and disadvantages. In our opinion, the most effective method of surface alloying can also be the saturation of the surface of castings with composite spreading directly during the process ofpouring the liquid metal into the casting mold.
There are known methods [2; 3] of providing the necessary complex of characteristics of the surface layer of castings by establishing the laws of their formation and structure formation in the process of impregnation of the paste from the powder composite with liquid iron directly in the mold.
A smear is applied in advance on the working surface of the mold, and then the melt is poured into it. When the interaction occurs between liquid metal and surface, which has a porosity of coating about 32-43%. It is impregnated through the pores of the spreading and, as a result of crystallization, a mixed metal-spreading framework is formed on the surface of the casting and thus not only provides corrosion resistance, but also significantly improves the quality and surface properties of the workpiece.
Content and results of the work: The work is devoted to the study of the effect of impregnation in the form of liquid iron of a layer of powder paste from an alloy based on nickel,%: 0.2-0.3 C, 1.0 Si, 0.7-1.2 Mn, 0.7-0, 8 P, 15-17 Cr, 3.0 B, < 5Fe on the formation of the structure and properties of metal castings from gray cast iron and composite coating, as well as the development of the technological process of manufacturing high-quality cast iron castings with composite coatings, obtained in single molds.
The main task of obtaining iron castings with a composite coating is to establish the optimal parameters for the impregnation of a layer of porous powder spread applied to the working surface of the mold cavity, ensuring the formation of a high-quality composite coating of the required thickness and properties.
Forresearch, gray cast iron ofCH15 brand and powder from PG-XH80CP3 alloys were used. The gran-ulometric composition of the powder corresponded to the fractions: + 50-63; + 63-100; + 100-160; 160-200 and 200-315 ym.
For this purpose, spreads with small (30-50 micron), medium (50-100 ym) and large (<1 00 ym) powders and liquid glass with densities p = 1.29, 1.35 and 1.42 g/cm3 were taken.
Investigated the relationship between the depth of impregnation with the melt of powder spread, the density of the metal of the composite coating and the pouring temperature of cast iron at various fractional and chemical compositions of the powder particles [4].
When increasing the thickness of the powder paste coating from alloy XH80CP3 from 5 to 8 and 12 mm when impregnated with liquid iron with a pouring temperature of 1380 °C, the near-surface zone is heated to 1060-1070 °C, 930-970 °C, 760-780 °C, respectively. The thermal state of the composite coating depends on the alloy of the powder and the pouring temperature, as well as on the amount of cast iron poured into the mold.
To obtain a more uniform structure in castings with composite coatings, it is advisable to increase the pouring temperature. With an increase in the thickness of powder spreads from alloy XH80CP3 from 3 to 12 mm with an interval of 3 mm (when the ratio of the thickness of spreads to the total wall thickness of the casting is 0.07 to 0.3), it is fully impregnated with liquid cast iron when the pouring temperature rises to 1440 °C.
Regardless of the thickness of the porous spreading from the powder of alloy XH80CP3, when pouring iron with a temperature below 1350 °C, its penetration is 0.5-1.5 mm. This layer is formed as a result of the penetration of liquid iron into the pores of the surface of the powder paste, covers individual powder particles and after solidification forms a composite crust, which has a very strong adhesion to the casting.
At the same time, the coefficient of hardening (C.h. = 3.22 mm/s1/2) of the casting with powder spread based on nickel alloy is slightly higher than when solidifying the casting in the form without spreading (C.h. = 2.93 mm/s1/2).
It was established that with an increase in the temperature of cast iron casting from 1360 °C to 1420 °C, the impregnation depth increases, the proportion of non-soluble powder particles in the "XH80CP3-gray cast iron" composite coating structure decreases from 67-65% to 44-47%, which is associated with an increase in the solubility of powder particles spreading in cast iron ligaments. At the same time, the porosity of the composite coating decreases by 0.91-1.33 and the interlayer between non-soluble particles increases from 0.10-0.12 mm to 0.22-0.28 mm, which contributes to the improvement of the strength properties of the material of the composite coating of iron casting.
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 powder particles in the cast iron of the bond increases and the segregation of Ni, Cr, Si and P over the cross section of the composite coating of the casting decreases.
Chemical analysis of the metal over the cross section of the composite coating revealed that the smaller the thickness of the powder coating, the greater the degree of homogeneity of the distribution of Ni, Cr, Fe and C across the coating thickness of the iron casting.
By raising the temperature of the cast iron from 1320 to 1440 °C, it is possible to achieve an increase in the rate of its filtration by 2-3 times. So, with an increase in the thickness of the spreads from the powder of the alloy XH80CP3 from 5 to 10 and 15 mm, the content of elements changes from surface to depth in 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 9.49.8%, 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%.
As can be seen, the nature of the dissolution of particles and the diffusion of Ni; Cr; Si and P in the cast iron of the ligament at the surface of the composite coating of the casting significantly depends on the thickness of the powder spread, and the content and distribution of these elements in the zones varies significantly. Thus, in experimental castings with a total thickness of 50 mm in the surface zone of the composite coating, an increase in the thickness of the powder spreads from 5 to 10 and 15 mm resulted in a decrease in the content of elements of the XH80CP3 alloy in thin interparticle interlay-ers of cast iron; namely: for nickel - from 9.64% to 4.38 and 6.51%; for chromium, from 2.2% to 0.25 and 1.31%, and also for silicon, from 2.2% to 0.34 and 0.36%. Whereas the phosphorus content in the pig iron, soaked in a porous spread, increases from 0.43% to 0.59 and 0.63%. At the same time, in all cases of casting with impregnation of powder
spreading, the metal of the contact zone of base iron with a composite coating is considerably more saturated with Ni, Cr and P compared to the peripheral zone.
At the same time, the solubility and diffusion of elements from solid particles of the powder alloy into the cast iron solubility is much greater than in the surface zone of composite coatings. However, with an increase in the thickness of the spread, the content of elements that pass into the cast iron solubility is also reduced, as in the metal surface areas of the composite coating: when the thickness of the spread is increased from 5 to 10 and 15 mm, the Ni and Cr content decreases by 2.79 and 3.71%, 0.26 and 0.40%, respectively.
Regardless of the thickness of the powder paste 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 0.33-0.38%.
Type of cast iron and composite coating castings Figure 1. Comparative mechanical properties of cast iron and composite coating of castings: I - cast iron without coating; II - composite coating XH80CP3- gray cast iron
On the basis of the research performed, a tech- The test results of composite coating materials
nological process was developed for the production "XH80CP3 gray cast iron" showed that their me-
of castings of the type ofbushings with hardening of chanical properties (&B and HB) are significantly
the working surface with a composite coating. higher than those of cast iron castings (Fig. 1.)
The maximum strength properties of the material of the composite coating "XH80CP3 gray cast iron", while ctb and HB increase 1.90-1.96 and 1.251.28 times as compared to cast iron.
To determine the qualitative changes occurring in the abrasion surface layers of metal samples from the composite coating of castings, in laboratory conditions, methods were used, a metalworking analysis, determination ofthe metal microhardness ofthe friction surfaces, speed and weight loss of the samples.
The resistance of the specimens to abrasion was tested in tandem with a diamond wheel during rotational motion on an MT-66 machine (sliding speed of 0.5 m/s; load 48H), as well as at a constant pressure of 0.015 N/m2 and sliding speed of1.25 m/s.
Compared the wear resistance of samples cut from the body of the wall of gray iron casting, with the structure consisting of a perlite-ferritic metal base with lamellar graphite; and from a composite coating based on XH80CP3 gray cast iron and austenitic nickel-chrome-plated iron, such as nirezist (monometallic).
The speed against abrasion of samples in pairs with a diamond wheel was evaluated during rotation on an MT-66 machine (sliding speed of 0.5 m/s, load 48 N). The results of testing samples cut from the reverse surface area of castings without composite coating showed high wear rates.
It is known that the wear resistance of materials depends on hardness and with its growth, the wear rate decreases.
With an increase in the thickness of the powder coating, the resistance of the metal of the composite coating to wear increases. Thus, an increase in the coating thickness from 3 to 5 and 10 mm contributes to a decrease in the wear rate from 783 ym/min to 741 and 718 ym/min, respectively.
The study of the rate of wear of the samples, depending on the conditions of formation of the composite coating of the castings, the composition of the material of the powder composites and the sliding speed (1.25 m/s) was carried out in laboratory conditions.
In the process of friction in a pair, the reference sample is a sample of a composite coating of austenitic cast iron of the nirezist type on the surface of the latter, the composite metal particles are pulled out, some ofwhich stick to the conjugate surface, and the rest are wear products.
The destruction of the friction surface of a sample of composite coatings "XH80CP3- gray cast iron" has a local character [5]. It occurs in the ce-mentite-ice buc-ryth phases of the cast iron of the bond (metal of the bond), and not in the much less durable metal of the composite coating of the brand XH80CP3, since the cementite-ledeburit phase contains less plasticity.
The metal of the composite coating "XH80CP3-gray cast iron" is characterized by spalling particles during friction, as in the process of impregnation of the spread with liquid iron in the surface zone of the powder particles are welded together particles but the area of true contact between them is significantly less than the total contact area between the metal bond and the same powder particles. Therefore, in the course of friction of samples, partially welded particles of material are cleaved from a composite coating and dyed.
Comparison of the relative wear resistance of samples from the composite coating "XH80CP3-gray cast iron" with samples from mono-castings of highly wear-resistant bulk- alloying austenitic nickel-chrome copper (nirezist) showed that the wear of the composite coating materials "XH80CP3 is 3.86 times less than in the standard sample from cast iron Fig. 2).
Thus, studies ofthe resistance ofspecimens made it possible to establish that this method of producing cast-iron castings with a composite coating can be recommended for the manufacture ofparts operating under friction conditions. The use of wear-resistant composite coatings on gray iron castings will not only increase the service life of fittings, but also recommend replacing high-chromium-nickel austenitic ni-rezist type cast iron (with a chemical composition of 18-36% Ni, 1-4% Cr and 3% Cu) in necessary cases cast iron and steel type 12X18H9TL.
Type of composite coating of cast iron Figure 2. Comparative wear resistance - composite coating of iron castings: 1 - gray cast iron CH15; 2 - nirezist; 3 - XH80CP3 - gray cast iron
It is known that in the powder from alloy XH-80CP3-chromium is an important nickel alloying element, while nickel also has the ability to dissolve many elements in large quantities, such as iron and silicon. The main advantage of nickel-chromium alloys is their high corrosion resistance in a wide range of oxidative and reducing media. The properties of nickel significantly depend on the content of impurities such as carbon, sulfur, phosphorus and oxygen.
With the thickness of composite coatings of casting "XH80CP3- gray cast iron" of 5 and 10 mm, corrosion damage spreads evenly from the peripheral surface into the sample. In a sample of composite coatings 5 mm thick, corrosion is local in nature and is localized on surface areas where it is in contact with the thin ledeburite-cementite phase of cast iron and the sintered mass of the powder composite of XH80CP alloy in the form of individual points or spots.
Since corrosion destruction always starts from the surface, there is no need to increase the thickness of the composite coating in castings.
Tests for corrosion resistance were carried out at room temperature in open glasses according to the standard technique [1]. Test time ranged from 5 to 100 hours. Corrosion resistance was estimated
by weight loss per unit of the original surface area of the samples.
It is known that iron-carbon alloys are unstable in dilute aqueous solutions of hydrochloric and sulfuric acids. The exceptions are high silicon cast iron and nickel chromium steel. Due to their high hardness, brittleness and sensitivity to temperature fluctuations, they have very limited application.
Comparative data on the corrosion resistance of the metal of the composite coating "XH80CP3- gray cast iron" castings of different thickness to the action of diluents of sulfuric and hydrochloric acids are given in the table.
From composite coatings "XH80CP3 - gray cast iron" with a thickness of 3, 5 and 10 mm, coatings with a thickness of 5 and 10 mm have the highest corrosion resistance in environments of diluted sulfuric and hydrochloric acids. At the same time, in aqueous solutions of sulfuric acid with a concentration of 15 and 20%, the corrosion rate of the material of the composite coating decreases by an average of 15 times as compared with the base iron, and in aqueous solutions of hydrochloric acid in the above concentrations, an average of 42 times.
Table 1.- The change in the corrosion resistance of cast iron, depending on the thickness of the composite coating "XH80CP3-CH15" and medium
Material Coating thickness, mm Corrosion rate, g / m2 • h
r lie concentration of sulfuric acid at 20 °C,%
1.0 3.0 5.0 10.0 15 20
CH15 - 47.8 105.8 121.8 98.98 71.97 42.9
CH15 3 0.030 0.103 0.171 0.215 0.109 0.082
CH15 5 0.001 0.024 0.032 0.091 0.005 0.004
CH15 10 0.006 0.030 0.082 0.105 0.002 0.007
Cast iron nirezist - 0.001 0.003 0.47 1.831 1.455 1.481
The concentration of hydrochloric acid at 80 °C
CH15 - 81.989 498.969 224.885 309.863 276.983 300.785
CH15 3 2.095 2.596 2.885 3.196 3.387 3.597
CH15 5 1.806 2.029 2.065 2.179 2.194 2.396
CH15 10 1.966 2.299 2.483 2.801 2.966 2.900
Cast iron nirezist - 3.717 4.892 4.776 4.611 4.705 4.601
The results of testing the corrosion resistance of the composite coating materials "XH80CP3- gray cast iron" showed that from an economic point of view it is advantageous to produce castings with the smallest thickness (~ 5 mm) of composite coating, which makes it possible to reduce the consumption of scarce powder XH80CP3 per item. However, it should be borne in mind that, with a decrease in the thickness of the spreads in the material of the composite coating, the content of cementite-ledeburitic components increases.
The conclusion: Defined rational technological parameters for the manufacture of cast iron castings with composite coating "powdered nickel alloy XH80CP3 - gray cast iron CH15" with increased strength, wear resistance and corrosion resistance. It was established that ct„ and HB material of composite coating "XH80CP3 - gray cast iron" in comparison with cast iron increases by 1.90-1.96 and 1.25-1.28 times, wear resistance - by 1.55 times, and corrosion resistance in the medium of 10% aqueous solution of sulfuric acid 42 times.
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