Scientific journal

Modern high technologies

ISSN 1812-7320

"Перечень" ВАК

ИФ РИНЦ = 0,940

 One of promising ways of increasing mechanical properties as well as hardware lifetime is application of high structural strength steels and alloys. At the present time this problem can be solved in two ways: either by manufacturing cutting elements of tools by the powder metallurgy methods or by improving shearing high-speed steel produced by means of conventional methods. When using some powder metallurgy elements pressing articles of considerable sizes is appeared to be a major difficulty. This point has become a restraining factor. When improving conventional electric melting, for instance, alloying high-speed steel with nitrogen increasing wear resistance of the tool made of it can take place but up to a certain extent. Moreover alloying with nitrogen is a complicated and manysided process.

In our opinion the greatest effect on increasing lifetime of machines, devices and tools wearing details is possible through enhancing their hardness by artificially introduced secondary phases.

Secondary phases dispersion particles that are usually formed in the structure in the course of over-saturated solid solutions disintegration prevent dislocation movement. This fact influences directly hardening rates. Besides, secondary phases particles in steels determine the size of the ferrite and austenitic grain, type, density and character of defect distribution in crystal structure. The latter can be considered as indirect factors determining hardening. Dispersion particle formation can be observed in most processes taking place under heart treatment.

To solve this problem application of high wear-proof and hardened steel and alloys, e.g. Р6М5, Х12МФ, Х6В3М, 5Х6ВМ2, etc, increases ready-made output cost. Besides, frequent cutting of coherent precipitation by dislocation causes their thermodynamic instability. Having been dissolved in the die they result in the appearance of unhardening zones, the latter diminishing alloys hardness. The following fact should be obviously mentioned. The matter is that dispersion-strengthened steels and alloys under t⁰=(0,6...0,7) quickly lose their firmness for coagulation processes and partial dissolution of dispersion phases take place.

These last few years the problem of casting steels and alloys hardening by introducing refractory particles into liquid and solid-liquid melts is paid great attention to. The research shows that introducing different refractory particles (Al₂O_3, TiO₂, oxides; indissoluble metals Mo, W, Ti, Nb; refractory carbides TiC, VC, WC, NbC) affect properly physical properties of steel and alloys.

Applying Ti and Nb carbides, nitrides, carbonitrides as secondary phases is of great interest. However, providing secondary phase equal distribution in an ingot offers some difficulty for their lesser extent density as compared with the metal exposed to hardening.

Research analysis of applying physical-mechanical methods in melt working reveals the instability of the results received. The most effective results in the process of introducing solid multidispersion refractory particles into liquid melt when casting can be achieved by forced crystallization through bar extending oppositely directed to the gravitation forces action. This allows equal modificator distribution through the crystallization front and, as a result, the whole ingot area.

The South-Urals University chair of fundamental metallurgy and the Zlatoust steel works have worked out and produced a full-seale plant for modification under the know-how mentioned above. This plant permits containing the 10-tonn feed and ingot extending upwards with 0,1...1,0 m/min speed that meets the calculation indexes.

Experimental У7 steel casting series were realized at this plant. 0,5 ton casting (300 mm diameter) and 0,4 ton rolled hollow (300 mm outer diameter, 240 mm inner diameter) were produced. The metal quality analysis shows that the experimental ingots have denser structure through the basal area as compared with the castings produces under the conventional technology. Experimental data are given in table 1.

All the samples were subjected to 800...820 ⁰C heat treatment (water hardening) and 300...320 ⁰C tempering (am hour holding and further air cooling). Wear activity analysis are given in table 2.

Table 1. У7 steel and experimental metal mechanical properties

Table 2. Different quality steel specific wear resistance depending on heat treatment conditions

The analysis of carbide, nitride and carbonitride distribution was realized on the microsections taken from different ingot levels by МИМ-10 lens with a magnification of 630 power. The data received shows equal phase distribution through the ingot height as well as the ingot cross-sectional area.

Thus, the experimental casting has obviously demonstrated the possibility of producing high wear resistant metal hardened by aritificially introduced secondary phases.