Method of preparing steel powder

The invention relates to the field of powder metallurgy, namely to methods for manufacturing alloyed steel powders by the method of reducing metal halides in salt melts.

Alloyed steel powders are widely used in the manufacture of corrosion-resistant and wear-resistant products using traditional powder metallurgy, 3D printing, MIM-technologies.

In the process of obtaining doped powders by the method of reduction in salt melts, there is a problem of incomplete assimilation and uneven distribution of alloying components in powders, which makes it difficult to guarantee the required properties of steels. In most cases, the incompleteness and unevenness of doping is due to the physicochemical properties of the alloying components themselves. For example, the cause of incomplete assimilation of alloying components can be their increased volatility, as a result of which they quickly evaporate from the reaction zone, not having time to recover to the metal. Chlorides Ti, Si, Mo, Nb, V are highly volatile in alkali metal chloride melts, and chloride chlorides Fe, Cr, Ni, Cu, Co, La, Zr, Mn have extremely low volatility.

It is actual to find ways to stabilize the chemical composition of alloyed powders.

A method is known for obtaining powders of metals (alloys, steels) [Ru No. 2423557 dated March 18, 2009], which includes electrochemical dissolution of metal anodes-metal bars (alloys, steels) and the reduction of the metal chlorides formed in the electrolyte volume by an equivalent amount of a reducing agent , cooling the melt, washing the powder from the electrolyte with water, drying the powder. When the powdered steel 12X18H10T is obtained by this method, the electrochemical dissolution and reduction process is carried out in molten chlorides of alkali or alkaline earth metals at 800 ° C for 0.6 hours.

The disadvantage of the method is the impossibility of obtaining powders of steels with the required chemical composition, which is associated with the evaporation of the volatile chlorides Ti and Si from the melt, since the reduction process is carried out at a high temperature of 800 ° C for a sufficiently long time of -0.6 h.

A method for obtaining powders of rare metals and their alloys is known [RU No. 2416493 dated December 15, 2009], which includes the preparation of charge from complex salts of rare metal chlorides and potassium chloride, heating and melting of the charge at 750-800 ° C, a one-time magnesium-thermal reduction of rare metal chlorides, holding the melt under isothermal conditions for 15 ± 5 minutes, cooling the melt, separating the powder from the melt by rinsing with acid and water, drying the powder. When the Zr-5% 1Nb alloy powder is obtained by this method, the reduction process is carried out at 750 ° C.

The drawback of the method is the problematic nature of obtaining Zr-Nb alloy powders with the exact chemical composition due to the evaporation of Nb chlorides during melting of the charge at 750 ° C. Depending on the duration of the melting operation, the amount of evaporated niobium, and hence the composition of the alloy powder, changes.

The technology of obtaining powders of 12Х18Н10Т steel, based on the calcium-thermal reduction of a mixture of metal chlorides in the melt of potassium chloride, adopted for the prototype is the closest in technical essence to the proposed invention ["A new technology for producing powders of stainless steels suitable for use in additive technologies" / A.V. Ivakin, OA Arzhatkina, A.V. Samokhin, O.V. Tokarev, V.D. Fedorov [Electronic resource]: II International Conference "Additive Technologies: Present and Future": VIAM, 16.03.2016. - Mode of access to the collection of proceedings:<u>admin@viam.ru]</u>. The technology includes chlorination of steel waste 12X18H10T (composition,%: 17-19 Cr, 8-13 Ni, 0.4-1.0 Ti, 0.2-0.8 Si) in the melt of potassium chloride at 850 ° C, calcification reduction chlorides of metals in the melt at 800-850 ° C, cooling of the melt, washing powder of steel, drying of the powder.

The disadvantage of this technology is the loss of Ti and Si alloying components at the stage of chlorination of steel waste due to their evaporation in the form of volatile chlorides.

The technical result of the proposed method is to produce powders of steels with a given content of alloying components and to achieve an even distribution of these components in the reconstituted powders.

The technical result is achieved by the fact that in a process for producing a powder of steel including chlorination of steel waste in a melt of potassium chloride, calcining the reduction of metal chlorides in the melt with stirring, cooling the melt, washing the powder of steel, drying the powder, the alloying metals are introduced into the melt form of fluorine-containing salts.

The technical result is achieved by the fact that the introduction of fluorine-containing salts into the melt is carried out at a time. Achieving the technical result is facilitated by the introduction of fluorine-containing salts into the melt at a temperature of 780-800 ° C and stirring at a speed of 300-360 rpm, as well as holding the melt under isothermal conditions with stirring for 10-20 minutes.

The essence of the method lies in the combination of distinctive features of the process of introducing alloying metals into the salt melt.

The first significant difference is the introduction into the melt of alloying metals in the form of complex fluorine-containing salts of the K type₂TiF_6th, K₂SiF_6th, or fluorides of lower valencies MoF₃, VF₃ in an amount corresponding to the steel grade. These fluorine-containing salts, on the one hand, are soluble in the melt of potassium chloride, and on the other hand, do not evaporate from the melt, which ensures an even distribution of all the alloying components in the melt, and hence in the resulting powder.

The second significant difference is the introduction of fluorine-containing salts into the melt at a temperature of 780-800 ° C. This temperature is sufficient to complete the process of dissolving fluoride-containing salts of alloying metals in the melt of potassium chloride. A decrease in temperature of less than 780 ° C leads to the crystallization of a melt of potassium chloride and inhibition of the dissolution of fluorine-containing salts. An increase in the process temperature above 800 ° C provokes an undesirable process of anion exchange between reagents, namely between the fluorides of the alloying metals and potassium chloride with the formation of volatile chlorides of the alloying components, which is described by the reactions:

K₂TiF_6th + 4KCl = 6KF + TiCl₄↑

K₂SiF_6th + 4KCl = 6KF + SiCl₄↑

Evaporation from the melt of the alloying components leads to a decrease in their content in the powder of steel.

The third essential difference is the one-time input of fluorine-containing salts with subsequent exposure of the melt for 10-20 minutes with stirring under isothermal conditions. This time is sufficient to complete the process of dissolving the fluorine-containing salts.

Reduction of the holding time of less than 10 minutes leads to incompleteness of dissolution of fluorine-containing salts and, consequently, to an uneven distribution of alloying components in the melt, which negatively affects the homogeneity of the chemical composition of the powders obtained. The increase in the time of high-temperature exposure of the melt for more than 20 minutes stimulates the course of exchange reactions with the formation of volatile metal chlorides, which leads to a decrease in the content of these metals in the powder of steel. One-time input of fluorine-containing salts creates the same time conditions for the dissolution of all introduced alloying components. Sequential input of salts leads to an undesirable increase in the residence time in the reaction system of the components introduced first, which stimulates the flow of exchange reactions with them to form volatile chlorides.

The fourth significant difference is the mixing of the melt at a speed of 300-360 rpm. This rate of mixing is sufficient to ensure a good homogenization of the melt, a uniform distribution of fluorine-containing salts in its volume, and the production of a powder of steel with a uniform chemical composition. A decrease in the stirring rate of less than 300 rpm results in insufficient homogenization of the fluorine-containing salts in the melt, and an increase in the mixing rate of more than 360 rpm to melt spraying.

The process of introducing alloying components in the form of fluorine-containing salts is simple in instrumental design. The introduction of alloying metals into the melt immediately before the reduction operation allows not only to accurately correct the composition of the powders, but also to create powders with new compositions.

In general, the method for producing the alloyed steel powder according to the invention is carried out in the following manner.

Degreased waste steel in the form of chips size 1 × 0.5 × 0.5 mm and the dried potassium chloride is loaded into a quartz retort equipped with a sealed lid with gas flues, a stirrer and a siphon. The retort is placed in a protective metal beaker and an electrically heated melting furnace is placed. Preheat the retort to a temperature of 800-850 ° C, at which melting of potassium chloride occurs. Switch on the agitator. Gaseous chlorine is bubbled through the melt for 2 hours to chlorinate the steel chips. In the process of chlorination, non-volatile chlorides are accumulated in the melt (FeCl₃, CrCl₃, NiCl₂ etc.), and volatile chlorides (TiCl₄, SiCl₄ etc.) evaporate from the melt and are removed to the ventilation system. The resulting melt is drained by a vacuum siphon into a crucible of silicon carbide, equipped with a stirrer and a sealed stainless steel lid, mounted in a shaft electrically heated melting furnace. The temperature of the melt is maintained at 780-800 ° C. The melt stirring speed is 300-360 rpm. In the melt is introduced at a time "volley" those alloying components that evaporated during chlorination, and they are introduced as fluorine-containing salts (for example, K₂TiF_6th and K₂SiF_6th). The melt is aged for 10-20 minutes with stirring at a speed of 300-360 rpm. Then a reducing agent is introduced into the melt - metal calcium to reduce the metal chlorides. The melt is kept under isothermal conditions for 15 ± 5 minutes and poured out with a vacuum siphon into the receiver for cooling. Cooled melt is extracted from the receiver, crushed and treated with a solution of 0.1-0.3 molar hydrochloric acid and water. The obtained powder of steel is dried at a temperature of no more than 40 ° C. The content of the components in the powder is analyzed by an atomic-emission method with inductively coupled plasma on a Vista-Pro device from Varian.

The claimed invention is illustrated by examples.

Example 1. In the chlorinator, degrease wastes of steel 12X18H10T (composition,%: Cr-18, Ni-11, Ti-0.8, Si-0.5) are loaded in the form of chips 1 × 0.5 × 0.5 mm, weighing 0.1 kg and drained potassium chloride in an amount of 0.8 kg. Chlorination of the chips is carried out at 800 ° C for 2 hours. The melt is poured with a siphon from the chlorinator into the crucible-reactor of the reduction furnace. The melt temperature in the crucible-reactor is set at 800 ° C. The melt agitation rate was 300 rpm. The melt is injected at a time with a "volley" of 0.004 g K₂TiF_6th and 0.0039 g of K₂SiF_6th. The melt is kept under isothermal conditions for 10 minutes under stirring. Then the reductant is introduced into the melt - metal calcium in an amount of 0.12 kg (at the rate of 110% for the reduction of chlorides and fluorides of metals). The melt temperature rises by 30 ° C to 830 ° C due to the thermal effect of the reduction reaction. The melt is kept with stirring for 10 minutes, after which it is poured into a vacuum siphon. Cool the melt in the receiver to room temperature. The melt is removed from the receiver, crushed to pieces of 5-10 mm in size and treated with a solution of hydrochloric acid and water for washing from an excess of reducing agent and CaCl salts₂, CaF₂, KCl. The powder is dried in air. The specific composition of the powder is,%: Cr - 18, Ni - 11, Ti - 0.8, Si - 0.5. The composition of the powder steel corresponds to the brand 12H18N10T.

Example 2. In this example, the process for obtaining steel powder from waste (chips) of grade 08X17N15M3T steel (composition,%: Cr-17, Ni-14, Mo-3.3, Ti-0.5) is similar to the process described in Example 1 The difference lies in the fact that the chlorination of the chips is carried out at 850 ° C, and the introduction of alloying components and calcification reduction at 780 and 810 ° C, respectively. The melt agitation rate is 360 rpm. The alloying components are introduced in the form of 0.0025 g K₂TiF_6th and 0.0053 g of MoF₃. The melt is kept under stirring under isothermal conditions for 20 minutes. Then the reductant is introduced into the melt - metal calcium in an amount of 0.13 kg (at the rate of 120% for the reduction of chlorides and metal fluorides). The melt is kept with stirring for 10 minutes, then it is poured into a vacuum siphon and cooled. The melt is removed from the receiver, crushed, treated with a solution of hydrochloric acid and water. The resulting powder was dried in air. A certain composition of the powder is,%: Cr-17, Ni-14, Mo-3.4, Ti-0.6. The composition of the steel powder corresponds to the brand 08Х17Н15М3Т.

Example 3. In this example, the possibility of changing the grade of steel powder from 12Х18Н10Т to 09Х16Н15М3Б is shown. The process for obtaining the powder is similar to the process described in Example 1 and 2. In this example, the chlorination of the chips is carried out at 800 ° C, and the introduction of alloying components and calcification reduction at 800 and 830 ° C, respectively. The melt agitation rate was 300 rpm. Before calcicothermic reduction, a single "volley" of 0.0049 g NiF₂, 0.0053 g of MoF₃ and 0.0021 g K₂NbF_6th. The melt is kept under isothermal conditions for 15 minutes under stirring. Then the reductant is introduced into the melt - metal calcium in an amount of 0.13 kg (at a rate of 120%<i></i>on the reduction of chlorides and fluorides of metals). The melt is kept with stirring for 10 minutes, then it is poured into a vacuum siphon and cooled. The melt is removed from the receiver, crushed and treated with a solution of hydrochloric acid and water. The powder is dried in air. The specific composition of the powder is,%: Cr-16, Ni-14, Mo-2.7, Nb-0.7. The composition of powder steel corresponds to the brand 09H16N15M3B.

Analyzing the data of the examples, it can be seen that the proposed method makes it possible to obtain powders of steel with a given composition, while using the known technology, powders without part of the alloying components are obtained. An additional advantage is the possibility of changing the composition of the powders of steel by introducing additional alloying components.