Технические науки/1. Металлургия

Каnaev А.T., Baibossynova L.А.

Eurasian national university named after L.N.Gumilev, Kazakhstan

ANALYSES OF EXPLOITATION TERMS INFLUENCE

ON MICROSTRUCTURE AND WORKING EFFICIENCY OF PLASMA-IRONCLAD LAYER

The most important aspect of the usage of local, plasma heat hardening for railway wheels is consideration of possible influence of exploitation terms on microstructure and working efficiency of ironclad layer. The thermo, appearing during braking and slide of wheels over the rail is perceived by outside part of surface rim and flange, which depending on breaking regulation can be heated till the phasic and structural transformation temperature.

It is known, that for carbonic steels, reinforced with heat hardening, admissible temperature of exploitation must not exceed 200°C, as by heating in the higher temperature breakdown of martensite with sharp abating of ironclad layer is happening. Also, we know that on exploitation in various regimes of braking, there can be much higher temperatures than 200°C. So, heat calculation, done in the work, witness that in the end of braking maximum of wheel driving surface heating temperature can exceed 500°C, and most part of wheel band heats in the higher temperature than 250°C. It leads to plasma ironclad effect taking off.

That’s why, it will be efficient to use low-alloy steel with vanadium, instead of carbon, for the production of wheels and wheel bands, it will allow us significantly increase heat stability. It is explained so, that, diffusion mobility of alloy elements’ carbon atoms, saluted by the way of substitution is much lower, than carbon atoms’ diffusion mobility, which is saluted in α – ferrum by way of insertion.

In abatement temperatures lower than 400°C in matrices diffusion redistribution of alloy elements do not happen, from α – dilution at the beginning ε – carbide is plated out, and then iron carbide, in which concentration of alloy elements the same with those as it was in martensite. Alloy elements’ atoms in beaker plate of transitional ε – carbide and iron carbide, appearing in temperature lower than 400 °C, often replace ferrum atoms – (Fe, V)₃.

Analyses of literary facts shows, that for the first stage of martensite dissection, that is for “biphasic” dissection in temperatures lower than 150°C, alloy elements, in particular V, doesn’t make much influence on practice. It conforms that on the first stage of martensite dissection, the main process is the origination of carbide parts, and it depends, mainly, on congestion of α – solution by carbon, and diffusion growth of carbide extraction, also in carbon steel is developed very flimsy.

Many alloy elements influent very strong on the dissection of martensite, slowing down the growth of carbide parts and maintaining congestion of α – carbon solution, that is maintaining condition of loosen martensite till the temperature 450-500°C. For example, adding V, Cr, Si, act this way. Latency of martensite dissection by alloy elements at this stage is explained by two reasons:

At first, such alloy elements as V, Cr, Mo decrease speed of carbon diffusion in α – solution;

Another reason is elevating of interatomic bond’s durability in breaker plate

α – solution under the influence of such elements, as V, Si, Cz and others, in which transition of atoms through bound α – carbide solution is made difficult and consequently, martensite dissection is made difficult.

Alloy elements influent much on carbide transformations during letting down in the temperature higher than 450°C, when their diffusion redistribution is becoming possible. As a result of this, special carbides can be produced. There are two ways of their appearing. At first, concentration of carbide-forming, alloying element as a result of its diffusion redistribution between α – solution and iron carbide is increasing to such dimension in cementite, that it is turning into special carbide, for example, alloyed chrome-cementite (Fe, Cr)₃C is turning into carbide chrome

(Cr, Fe)₇C₃.

Secondly, special carbide can be produced right in congested alloyed element

α – solution.

At the beginning, clusters out of atoms of alloying element and carbon with dimension of 3-5 Nm is forming near to dislocation. On increasing the temperature till 550°C, they are forming in the parts of special carbide, for example, VC. It is very important, that extraction of such carbides, as VC is much smaller than dissolving parts of cementite, one of the reasons of this is little diffusion mobility of alloy elements’ atoms.

Alloying elements influent on speed of carbide parts’ coagulation. Cr, V, Mo and some other elements make coagulation difficult. Elements, enforcing interatomic connection in breaker plate α – solution and carbide and carbon diffusions, decreasing the speed in α – solution, make atom transition through bound carbide - α – solution and transit of carbon through solution difficult. Such elements hold back dissolving of little and increase of big parts during coagulation. Because of little speed of alloy elements’ diffusion, carbides’ coagulation is going slowly (for example, VC).

Certainly, the speed of these processes is increasing with rise of temperature and rise of drawing length.

If in the capacity of plasma forming gas, nitrogen of particular cleanliness is used, in this way there will appear formation in case of plasma heat hardening in wheel steel of nitride constituents, also firm against dissolution and coagulation possible. On heating temperatures till 600°, formation of nitride constituents is becoming possible due to enrichment of steel with nitrogen in the process of plasma processing. Herewith, nitrogen concentration in surface layers so big, that in the result of process formation of structures, not corresponding to simple carbon steel is happening.

Affirmation, that wheels, appearing during braking down of heating, totally take off effect of wheel band’s plasmic hardening is not convincing and require concrete metallographic researches with measurements of microhardness of structural and phasic components of heat-treated zone surface. Conversion of scientifically founded technology of crown’s hardening, require cardinal reconsideration of accepted apprehension concerning surface firmness.

Today, it is considered, that crowns are eroding during gripping, that is leading mechanism of side wear is binding. It can be prevented, only by increase of firmness and change of contact surface structure material. It is recommended to set the correlation of wheels and rails firmness as 1:1,2.

Because of plastic deformation’s suppression heat-hardening of even one element of contacting system to high firmness 600 HV and higher not only influent positively on other element, but also improve its condition.

Differences in indicators of hardened wheel pairs’ durability improvement, mentioned in various sources, probably, conditioned by parameter spread of hardened wheels’ layer. Parameter spread of layers is defined only by accuracy and stability of heat-hardening parameters. That’s why, it is very important to provide with accuracy and positioning precision of hardening process.

Conclusion

1. High concentration of carbon in wheel band steel (0,57-0,65%), on the one hand, provides with durability and back-to-back endurance, on the other hand, decrease heat endurance. That’s why, vanadium is included to the content of wheel band steel of the mark 2, which by increasing heat resistance, contribute to the improvement of resistance to thermic and thermothermic influences.

2. Thermal analyses, done in the work, confirm that in the end of breaking down, maximal surface heat temperature can exceed 500°C, and most part of wheel band can be heated till 200-250°C. It leads to relief of effect from plasmic hardening.

3. In plasma-flaming technology, in the capacity of plasma forming gas, nitrogen of special cleanliness is used, leading to the formation of nitride constituents, heatproof against dissolution and coagulation till 500°C.

References

1. Киселев С. Н. О работоспособности колес с плазменным упрочнением. Локомотив, 2000, №1, стр. 28-29.

2. Башнин Ю. А., Ушаков Б. К., Секей А. Г. Технология термической обработки. М.: Металлургия, 1996, 423 с.

3. Новиков И. И. Теория термической обработки металлов. М.: Металлургия, 1996, 479 с.

4. Лыков А.К., Редькин Ю.Г., Глибина Л.А. Различные методы плазменной закалки. Локомотив, 2000, №1, стр.27-28.

5. Канаев А. Т., Кусаинова К. Т. Влияние соотношений твердости рельсовой и бандажной стали на износостойкость пары трения «колесо-рельс». Вестник Акмолинского аграрно-технического университета им. С. Сейфуллина.