PHYSICAL AND MECHANICAL PROPERTIES OF MARINE ENGINE PARTS RESTORED BY DIFFUSION CHROME PLATING

ABSTRACT

The development of the industry of the Republic of Azerbaijan implies the transition to a market economy with various forms of ownership and management. The development of entrepreneurship and the process of privatization of state property stimulate the development and strengthening of service enterprises. At the same time, it is envisaged to significantly increase the reliability and resource of the machines, the development of the firm repair of the special complex equipment in operation.

Saving fuel and energy resources of the country by any means is one of the important problems. The main consumer of petroleum products is internal combustion engines. Having certain advantages, they will retain their importance as mobile energy devices in the near future.

АННОТАЦИЯ

Развитие промышленности Азербайджанской Республики предполагает переход к рыночной экономике с различными формами собственности и управления. Развитие предпринимательства и процесс приватизации государственной собственности стимулируют развитие и укрепление предприятий сферы услуг. При этом предусматривается существенное повышение надежности и ресурса машин, развитие фирменного ремонта специальной сложной техники, находящейся в эксплуатации.

Экономия топливно-энергетических ресурсов страны любыми способами является одной из важных задач. Основным потребителем нефтепродуктов являются двигатели внутреннего сгорания. Имея определенные преимущества, они сохранят свое значение в качестве мобильных энергетических устройств и в ближайшем будущем.

Keywords: diffusion chroming, mechanical processing, plunger pair, microhardness.

Ключевые слова: диффузионное хромирование, механическая обработка, плунжерная пара, микротвердость

Main part

As a result of the analysis of the results of the experiments, the following mode of diffusion chroming was selected: temperature 1150-1200°C, storage time 6 hours. The variation of the roughness and microhardness of the linear dimensions of the samples is given in table 1. After diffusion metallization, diagrams and profilograms of precession surfaces were produced, the results of measurements showed that the non-circularity of the details remains at the same level after the process and before it, and the increase in linear dimensions after the process is about R_a=1.6-2 μm, which gives the required increase. This increase is taken into account in the subsequent mechanical processing [1-6].

Table 1.

Physical and mechanical properties of steel grades

In the process of experimental studies, it was confirmed that the microhardness of chromed layers of details and samples is around 16000-22700MPA.

The maximum microhardness of the layer for all steels was 22100-22700MPa.

The surface hardness of the diffusion layer is uneven. Thus, when the hardness in gas-phase chroming at 1150 °C temperature is around 17600-18000 MPa, the average number was 17800 MPa as a result of numerous measurements [2-7].

In vapor phase chroming, these figures were 19600-20000 MPa, and the average figure was 19800 MPa. Accordingly, the surface hardness is 2-3 times higher than the initial samples.

For a more in-depth study of the processes taking place in diffusion chroming, X-ray analysis of the phase composition of the base and surface layers of the plunger under chromium irradiation was carried out in the ДРОН-1 diffractometer.

The results of X-ray analysis of the surface phase composition of ХВГ steel samples and details after diffusion vapor phase chroming in different regimes. As can be seen, in vapor phase vacuum chroming, Cr₇C₃ and TiC type carbide phase is formed on the surface of the part. The appearance of one or another phase depends on the composition of the steel and the modes of diffusion metallization [3-8].

The analysis of the phase composition of the layer showed that, the layer obtained by diffusion metallization with the specified mode consists of Cr₇C₃ and TiC trigonal carbide, its thickness is 35 μm, and it was single-phase, the same in the diffractogram the presence of α-iron and carbide indicates that X-rays penetrate this layer.

The results of the spectral analysis of carbon in the surface layers of the plungers (serial and diffusion chromed) are shown in Fig. 1 [8, p.56-59]. Traces of different layer thicknesses were shown by polishing.

From the analysis, it can be seen that if the carbon in the serial plunger is evenly distributed in the entire volume, the amount of carbon on the surface of the coating in diffusion chromed ones is greater than in the core (49 μm from the surface to the core). This factor shows that in the process of diffusion of chromium, mutual diffusion of carbon from the core of the metal to the reaction zone takes place. the amount of carbon in the layer increases [4-9].

The microstructure was studied in transverse sections under a MIM-8 microscope at 50 to 500 times magnification. The microstructure was determined by etching the slices in a 4% solution of nitric acid in ethyl alcohol.

Samples and details subjected to vapor phase diffusion chroming shown in Fig. 1 were taken for research.

The initial structure of the XВГ steel plunger after factory heat treatment represents martensite with Vickers hardness of 750-770 units. After diffusion metallization with modes 1 and 2, a "triple" structure is observed on the surface when examining the cross section of the plungers (Fig. 1).

Figure 1. Microstructure of XВГ steel after diffusion metallization:

a-series tabbed (x450); b-diffusion chromed in vacuum (1180°C, τ= 4 hours) (x350); c- diffusion chromed in vacuum (1180°C, τ= 4 hours) (x450); d- after diffusion chrome plating and machining (x450)

A carbide layer of equal thickness is added to the surface itself. The hardness of the layer along the circumference of the plunger was 1500-1800 HV. Under this layer, a pearlite structure layer with a hardness of 400 HV, corresponding to the hardness of the base metal, is found. A decarburized layer with a hardness of 190 HV and pure ferrite appears between the transition layer and the base metal. Plunger 5 (mode 3) has no decarbonized layer (Fig. 1).

The study of the microstructures of samples and cores of details from steel XВГ after diffusion chroming showed that it is frostite sorbite with a hardness of 350-370 HV. The absence of residual austenite in the core and also under its layer was determined.

After gas chroming of low-carbon steel by the contact method, a thin layer of Cr₂₃C₆ and TiC carbide phase, distinguished by its high hardness, is formed. Below this layer is the transition (electrode) zone, which appears as a dark strip in the microstructure. This zone has a frostite structure and consists of a mixture of two phases: Cr₂₃C₆ and TiC near ferrite and pearlite, and there is a decarburized zone that gradually transitions to the typical structure of the steel under consideration. After contact chroming of decarbonized steel, X-ray analysis shows the presence of nitride phases on the surface. Cr₂N and CrN (0.02 mm deep), Cr₂N and CrN (0.02-0.06 mm deep) and Cr₂N (0.06-0.14 mm deep), surface thickness microhardness was 11800-14200 MPa . Below this layer is a thick light zone of the solid layer, below it is a dark (eutectoid) zone with a frostite-type structure. This zone transitions to a broad decarbonized light zone near the core [5-10].

In non-contact gas chroming of high carbon steels, a double carbide layer is formed: Cr₂₃C₆ and TIC on the surface and Cr₇C₃ closer to the core. It should be noted that in conventional tanning methods, a carbon-reduced zone is not observed in the structure.

In contact chroming in a non-hermetic container, a thin layer of carbonitride containing Cr₂(N.C) is formed in the brittle zones of the steel, apart from the TiC, Cr₂₃C₆ and Cr₇C₃ phases.

When chromium diffuses, there is a continuous sharing of carbon and chromium in austenite. In the process of diffusion of chromium in steel, the structure of carbon in austenite changes as a result of diffusion of carbon from the core zones to the surface.

In Figure 1, ШX15, XВГ, etc. The effect of temperature and chroming time on the chroming of steels is shown. Steel XВГ is chromed better than steel ШX15, because it also contains tungsten in addition to chromium. The microhardness of the chromed layer of these steels is approximately the same as in diffusion vapor phase chrome plating and is around 17600-18620 MPa. The obtained results confirm that vanadium and chromium accelerate diffusion chromization, while manganese inhibits it.

The hardness of the base metal immediately close to the surface layers is close to the hardness of the core and is 350-370 HB (modes 1,2,3).

Fig. 1 Microstructure of XВГ and ШX15 steel (x350) after diffusion chromed in vacuum (a) and after subsequent plating (b). The obtained results show that under the carbide layer with a thickness of 12 mm, there is a layer enriched with carbon in the process of diffusion metallization. This can be explained by the flow of carbon from the lower layers.

The surface layers of medium and high carbon alloy steels, as a rule, have a carbide nature and differ in high hardness around 18000-20000 MPa. The surface of low-carbon structural steel is dominated by a solid solution phase with chromium carbide additives. The microhardness of this layer (17000-17500MPa) is 107 n/m² or around 2000-10000 MPa, and for carbon steels it is 16000-16500 MPa.

The results of metallographic, X-ray and spectral studies allow us to present the mechanism of formation of the following coatings.

When steel is impregnated with carbide-forming elements, carbides are formed in the carbon surface layers. This can be explained by the mutual diffusion of metal and carbon diffusing from the core. As a result, a full layer of carbides can be formed on the surface of the steel. From this layer to the core, there is a triple electrode (Fe, Cr, C) transition zone, and then a carbonized zone. In the presence of carbon, chromium forms trigonal carbon Cr₇C₃ and TiC.

It is not always possible to achieve successful chrome plating of existing alloy steels. In some cases, it appears that special steels must be machined, taking into account the requirements presented by the manufacturer for chrome plating.

Considering the high cost of many steels, it should be noted that; in cases where the use of alloy steel is not strictly necessary, it should be replaced with carbon steel.

Thus, analyzing the research results, it can be concluded that the optimal mode of diffusion chroming for precession details of fuel pumps of marine engines is as follows: process temperature T=1200°C, storage time τ=6 hours

Conclusions

1. As a result of studies on the effect of mechanical processing modes on various indicators of the process, the optimal modes for each repair operation of carving and plungers were determined. It is advisable to polish the plunger in the following mode: circle rotation frequency - 1890 min-1; rotation frequency of the driving circle – 50 min-1; transverse movement - 0.42 mm/min; processing time – 6...8 seconds.

2. It is appropriate to carry out rough processing of the slot in the groove in the following mode: cutting speed - 20.01 m/min; average forward-backward speed of the grinding tool – 9.52 m/min; pressure of the clamp of the rubbing tool - 0.7...0.8 MPa, delivery time 0.35 min.

3. Parameters of the initial finishing mode of carving and plungers: peripheral speed of the rubbing tool – 13.35 m/min; average forward-backward speed of the grinding tool – 7.56 m/min; clamp pressure - 0.2...0.22 MPa, completion time - 0.75 min; For the final completion, these figures are respectively: 8.41 m/min; 6.12 m/min, 0.16..0.18 MPA; It should be equal to 0.5 minutes.

4. Analysis of statistical data on the accuracy of parameters of mechanical processing of plungers and grooves showed that after each operation they met the technical conditions of the manufacturing plant.

References:

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