RU2454466C1 - Steel and alloy modification method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method involves obtaining of molten metal or alloy in melting unit, its tapping to ladle, and addition of modifying agents to molten metal. As modifying agents there used are nanopowders of titanium carbonitride and tungsten carbide in equal proportions (5-10 wt %), which are mixed with nickel powder. Mixture is compacted and added to molten metal prior to the completion of melting process or to molten metal stream at its tapping in the quantity of 0.03-0.45% of molten metal weight.

EFFECT: improvement of alloy characteristics.

4 dwg, 1 tbl

Description

The invention relates to the field of metallurgy, in particular to foundry, and can be used to produce cast high alloy steels and heat-resistant alloys.

It is known from the prior art that in order to refine the structure and increase the mechanical properties of cast parts during the smelting process or when casting into molds, special additives (modifiers) are introduced into the melt, which ensure the formation of the crystalline structure of the casting. Modifiers are usually administered within hundredths or tenths of a percent of the mass of the melt.

Recently, ultrafine powders and nanopowders of chemical compounds, which are ultrafine crystalline formations, have been used as modifiers. These compounds have unique physicochemical and mechanical properties that differ significantly from the properties of materials of the same chemical composition, but larger structures.

Modification of melts with such modifiers can significantly improve the operational characteristics of products obtained from castings.

A known method of modifying steel with nitrides, including melting the charge, introducing into the melt a ferroalloy containing niobium nitrides, deoxidation and casting of steel, moreover, together with niobium nitrides, zirconium nitrides are additionally introduced at a ratio in the ferroalloy Zr: Nb 1: 10-20, and the ferroalloy is introduced into in the form of a powder with a particle size of 0.5-3.0 mm immediately after steel deoxidation (see published application No. 94037421, class C21C 5/52, 1996).

As a result of the analysis of the known method, it should be noted that the use of zirconium nitrides with particle sizes of 0.5-3.0 mm as modifiers does not allow to obtain a uniformly distributed fine-grained structure of castings, which negatively affects the properties of cast parts.

A known method of producing single crystal castings mainly from heat-resistant nickel alloys, including the preparation of the melt, modifying it with a mixture of powders of the refractory compound and one of the metals forming stable chemical compounds with it, in an amount of not more than 0.1% by weight of the processed melt, pouring the melt into a foundry a form heated to a temperature above the liquidus temperature of the alloy, and directed crystallization, moreover, the melt is modified with a mixture of powders in an amount of not less than 0.03% of m Assy of the melt being processed, then it is crystallized to form a billet, reheated to 1500-1740 ° C and additional modification with the same mixture of powders is carried out in the amount of 0.01 0.1% of the mass of the processed melt, after which the melt temperature is brought to 1650- 1750 ° C and maintained for 0.5-10.0 minutes before pouring into the mold, while carbides and / or nitrides and / or carbonitrides and / or titanium or niobium oxycarbonitrides are used as powders of the refractory compound.

RF patent No. 2068317, cl. B22D 27/04, 1994 - the closest analogue.

As a result of the analysis of the known method, it should be noted that it is very difficult to implement and does not allow to obtain high-quality castings due to the uneven distribution of modifiers in the volume of the casting.

The technical result of the claimed invention is to improve the method of modifying steels and alloys, which allows to obtain castings with desired high mechanical properties through the use of certain modifiers and the formation of their uniform distribution in the melt volume and preservation of the obtained properties at high temperatures.

The specified technical result is ensured due to the fact that in the method of modifying a heat-resistant nickel-based alloy, which includes obtaining a molten metal or alloy in a melting unit, releasing it into a ladle, introducing modifiers into the melt, it is new that titanium carbonitride nanopowders are used as modifiers and tungsten carbide in equal proportions in the range of 5-10 wt.%, which are mixed with nickel matrix powders, after which the mixture is compacted and introduced into the melt before melting or into the melt stream when it is released in an amount of 0.03-0.45% by weight of the melt.

The essence of the claimed method is illustrated by the materials attached to the application materials, on which:

figure 1 - compacted modifiers;

figure 2 - dendritic structure of the alloy alloy ZhS3-DK after its modification;

figure 3 - macrostructure of the cross section of castings, (above - casting without modifiers (No. 2), below - casting with modifiers (No. 1), (a) - section of castings; (b) - solid castings;

figure 4 - electron scanning microscopy of the cast alloy ZhS3-DK after etching with Marble's reagent. Cellular matrix structure and carbide inclusion;

table - the results of comparing the structures of unmodified and modified castings shown in figure 3.

The method is as follows

To implement the method, modifiers are initially prepared, which are fed into the melt in a compacted form (Fig. 1).

Modifiers are compacted by vacuum sintering or cold pressing. Before compacting, a mixture of matrix metal powders and nanopowder modifiers is prepared, in which the amount of the latter is in equal proportions in the range of 5-10% by weight. As a matrix powder, nickel powder of the NPE brand with a particle size of 10-50 microns is used.

As modifiers, ultrafine powders and nanopowders obtained by plasma chemical synthesis are used, namely, tungsten carbide (WC) and titanium carbonitride (Ti _x C _y N _z ).

The use of titanium carbonitride and tungsten carbide in a mixture with a nickel matrix provides a high modifying effect at a low cost of raw materials.

The use of modifiers in a compact form is due to the fact that particles of nanopowders with sizes less than 60-70 nm easily stick together, their oxidation begins at relatively low temperatures, and they are poorly wetted by a liquid metal. Despite the high density (from 1380 kg / m ³ ), nanopowders easily form a dusty suspension in the air, which under certain conditions spontaneously ignites and explodes. All this makes the introduction of modifiers into melts in the form of nanopowders undesirable.

The mixture is melted in a steelmaking unit. They conduct melting in accordance with the technological process of melting. When releasing the melt or before the end of melting (3-7 minutes before its end), modifiers — titanium cubitride and tungsten carbide in equal proportions in an amount of 0.03 ... 0.45% by weight — are introduced into the melt stream or melt in a compact form, respectively. melt.

As experiments have shown, the introduction of modifiers into the melt in an amount of 0.03-0.45% by weight of the melt leads to a significant change in both the resulting structure and the morphology and topography of the carbide phase. In this case, a sharp grinding of the macrograin occurs, the columnarization of the grains and the heterogeneity are eliminated. The dendritic structure of the cast metal (figure 2) is thin and uniform in cross section of the ingot. Carbides in heat-resistant alloys of the ZhS3-DK type acquire a compact equiaxial shape and are uniformly distributed over the grain volume.

With an increase in the amount of modifier above the specified upper limit, it is crushed macro-grain, the total length of grain boundaries increases, the boundary concentration of grain boundary hardening elements decreases, and, as a result, the heat resistance of the casting can decrease.

With a decrease in the number of modifiers less than 0.03%, the modifying effect sharply decreases.

The essence of the claimed method will be further understood from the example below.

Test melts of the material — ZhS3-DK alloy with and without a modifier were carried out.

The charge was loaded into a crucible. The charge was loaded into the crucible in the following sequence. Nickel (50-60% of the calculated weighted amount), tungsten, molybdenum, chromium, cobalt, and the rest nickel were placed at the bottom of the crucible.

Packed in aluminum foil (carbon, titanium sponge, aluminum, nickel-boron ligature, mischmetal, nanoligature) were placed in the dispenser, hanging them in the indicated sequence from the bottom up.

Melting was carried out with a discharge of 10 ^-3 mm RT.article. and maximum power for a specific load.

During melting, heating of the molds in the chamber to a temperature of 500-700 ° C was included.

After complete melting of the charge loaded into the crucible, the melt was heated to a temperature of 1590-1650 ° C and refining was performed. After that, carbon was placed on a metal mirror and the melt was held.

The melt temperature was adjusted to 1450-1500 ° C (temperature was measured with an optical pyrometer) and titanium was added. Withstood the melt.

At a melt temperature of 1450-1490 ° C, aluminum was added, and the melt was held.

The melt temperature was adjusted to 1550-1570 ° C and nickel-boron and mischmetal ligatures were introduced from the dispenser. The melt was held for 3-5 minutes, after which the melt was poured into a mold.

In principle, this is a standard technological process for producing this alloy.

To obtain a modified material, all actions were carried out as described above. Additionally, after holding the melt during casting, modifiers were introduced into the melt stream in an amount of 0.23% by weight of the melt in the form of compressed tablets.

After casting, heating was turned off. The exposure of the chill molds (forms) in vacuum was 2 hours. The resulting castings were examined.

The results of microstructural studies showed that the main phases in the structure of the alloy are ZhS3-DK carbides and the γ phase. In an unmodified alloy, carbide inclusions in the form of skeletons are located mainly along the grain boundaries and have a large extent. Carbides of a similar morphology have a negative effect on the properties of steels and alloys. As a result of the modification, carbides acquire a compact polyhedral shape and are uniformly distributed over the grain volume.

The table and figure 3 presents in comparison the structure of unmodified and modified alloy.

By comparing the structures of the modified (1) and unmodified (2) castings shown in figure 3, it was found:

- the grain size in the casting with modifiers (1) is less than that of the serial casting for all the studied zones (table);

- the macrostructure of the modified casting is dense, casting defects are absent.

Confirmation of the effect of the modification of the ZhS3-DK alloy by the introduction of TiC + WC and WC + Ti (CN) modifiers is a higher level of strength properties of the material of the modified castings compared to unmodified ones.

When testing the obtained materials, an increase in the mechanical characteristics of the material of the modified castings was established. Thus, the introduction of modifiers (WC + TiCN), in an amount of 0.45% of the metal filling, increased the casting strength by 15%, elongation by 138%, impact strength by 100% compared with the corresponding characteristics of unmodified castings.

Figure 4 presents scanning electron microscopy of a cast modified alloy ZhS3-DK. The cellular structure of the matrix is clearly visible with the cavities located in it, having a shape close to a square, with a cell size of less than 500x500 nm, from which we can conclude that these particles are a γ'-phase of the type (Ni, Co) ₃ , coherently connected with the matrix.

Thus, the introduction of modifiers into a metal or alloy does not lead to a change in the chemical composition of the castings of ZhS3-DK alloy. Strength and plastic characteristics as a result of the introduction of modifiers into the melt increase.

Table Casting number Grain size, point Rim (section 2 mm thick) Shovel (cross section 5 mm thick) Disc (20 mm thick section) Hub (cross section 30 mm thick) one 3 four 5 5 2 5 6 7 7

Claims (1)

A method of modifying a heat-resistant nickel-based alloy, including producing a molten metal or alloy in a melting unit, releasing it into a ladle, introducing modifiers into the melt, characterized in that nanopowders of titanium carbonitride and tungsten carbide in equal proportions in the range of 5-10 are used as modifiers wt.%, which are mixed with nickel powder, after which the mixture is compacted and introduced into the melt before the end of melting or into the stream of the melt when it is released in an amount of 0.03-0.45% by weight of the melt.

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