RU2684523C1 - METHOD OF PRODUCING Ni-Cr-C ALLOYS OF INCREASED HARDNESS CONTAINING CHROMIUM CARBIDE WHISKERS - Google Patents

Abstract

FIELD: chemistry.SUBSTANCE: invention relates to production of ternary alloy Ni-Cr-C. Method involves heating the initial mixture of powders of micron size consisting of 25–45 wt% of chromium, 3–5 wt% of graphite and the rest of nickel, and its subsequent cooling. Heating of powders mixture is carried out in air at temperature 1,200–1,300 °C for 30–60 minutes, and cooling – in vapor of liquid nitrogen immediately after completion of heating to form Ni-Cr-C alloy containing chromium carbide microwhiskers CrC.EFFECT: higher hardness.1 cl, 2 tbl, 4 dwg

Description

The invention relates to the field of powder metallurgy, in particular to the production of materials of increased hardness containing chromium carbide, intended for use in conditions requiring increased wear resistance.

Hard alloys containing nickel-bonded chromium carbide, due to their high hardness, resistance to wear and corrosion, are among the materials that can be used under conditions of simultaneous exposure to factors such as friction, abrasion, corrosive environment and elevated temperatures. These materials are relatively inexpensive and affordable due to the low cost of the starting components. Chromium carbide - nickel (KHN) alloys are used in industry for the manufacture of machine parts and mechanisms designed to work in extreme conditions, for the formation of durable coatings on metal and other surfaces. They also find application for the manufacture of ceramic-metal dentures. Despite the long-known popularity of these materials, the task of increasing their hardness and wear resistance does not lose relevance at the present time. Research in the field of nano- and microtechnologies opens up new opportunities in this direction.

Known methods for producing KHN alloys include co-thermal processing of powders of chromium carbide and nickel or Ni-containing alloys. Ready-made chromium carbide is used [see, for example, SU 1163990, publ. 06/30/1985, SU 1818873, publ. 07.20.1995, SU 1818874, publ. 05/10/1995] or get it, for example, from chromium oxide in the first stage of a multi-stage process [SU 1096034, publ. 06/07/1984].

In the application [CN 101760684 A, publ. 06/30/2010] describes a method for producing KHN alloy containing mass. %: carbon 3-23, nickel 2-65, chromium - the rest, intended for use at temperatures above 850 ° C. The method includes: mixing in a ball mill taken in the right quantities of chromium carbide and NiCr alloy, agglomeration and granulation of the obtained powder, melting of the granules and subsequent sintering in vacuum at a temperature of 1100-1400 ° C for more than an hour. A material with a low porosity that is resistant to wear and oxidation is obtained.

In US patent [US 6537343 B2, publ. 03/25/2003] described a wear-resistant, corrosion-resistant composition based on chromium carbide, including chromium carbide (preferably 83-87 wt.%) In the form of small grains distributed and cemented in a Ni-containing binder. The method for preparing the composition includes mixing in a ball mill the starting powders of chromium carbide and nickel, drying and pressing the mixture using a mechanical press, and subsequent sintering in vacuum at 1200 ° C for one hour. Under these conditions, according to the authors, the grain size of chromium carbide in the composition obtained does not exceed 10 μm, which, combined with its high quantitative content, ensures high hardness of the obtained material (Rockwell hardness is 89.5 RA).

The described methods relate to the classical technologies of powder metallurgy and are quite laborious since they include a sequence of several operations carried out, including in vacuum, in an atmosphere of hydrogen or inert gas using high pressure, special modes and equipment. The high hardness of the materials obtained is mainly due to the high mass content of chromium carbides of various structures.

Another approach to the production of QCN alloys involves the formation of chromium carbide directly during the heat treatment of powder mixtures containing elemental nickel, chromium and carbon. We have not found patent documents related to the production of Ni-Cr-C alloys directly from elemental components, however, we know of a series of publications of the results of scientific studies carried out in France under the guidance of P. Berthod, devoted to the study of the microstructure and properties of materials obtained in this way, depending from the quantitative content of the components in the initial powder mixtures and the conditions of their high-temperature processing. In particular, in the work of [O. Hestin, E. Souaillat, A. Dia, M. Ba, and P. Berthod ISRN Thermodynamics V. 2012, ID Article 587584, (7 p.) Doi: 10.5402 / 2012/587584] describes the preparation of ternary Me-Cr-C alloys including the method for producing the Ni-Cr-C alloy, which we took as a prototype, including fusion in a high-frequency induction furnace (frequency of about 100 kHz) of a mixture of pure powders of chromium, nickel and graphite taken in the required amount, followed by curing of the material in cooled water copper crucible. The temperature and fusion time are not reported in the work, however, it is indicated that to prevent oxidation of the material, all operations are carried out in an argon atmosphere (300 mbar). The initial mixture contains from 25.5 to 37 mass. % Cr and from 3.0 to 5.0 wt. % C, the rest is Ni. The average hardness H _{c of the} obtained samples, measured on a Testwell Wolpert instrument with a load of 30 kg, is, according to the authors, from 322 to 379 HV, depending on the quantitative ratio of the components of the initial powder mixture. In [P. Berthod Materials and Corrosion 2012, 63, No. 9999. P. 1-11] it is shown that prolonged heating of the obtained materials in air at temperatures of 1000-1200 ° C leads to a decrease in their hardness as a result of oxidation processes in different phase microstructures of the alloys.

The technical problem solved by the present invention is to optimize the composition and processing conditions of the initial powder mixtures, including nickel, chromium and carbon, in order to obtain ternary alloys with high hardness.

The problem is solved by the proposed method for producing ternary Ni-Cr-C alloys of increased hardness, including heating a powder mixture containing elemental nickel, chromium and graphite, and its subsequent cooling, characterized in that the heating is carried out in air at a temperature of 1200-1300 ° C for 30-60 minutes, and cooling is carried out in liquid nitrogen vapors immediately after completion of heating, while the initial powder mixture contains (% wt.):

chrome: 25-45

graphite: 3-5

nickel: the rest.

The list of graphic materials explaining the invention:

In FIG. 1 shows micrographs of the powders of chromium (a) and nickel (b) introduced into the composition of the initial powder mixture.

In FIG. 2 presents microphotographs of chips of alloy samples obtained by the claimed method containing micro rods (whiskers) of chromium carbide. FIG. 2a - the sample obtained in example 1, FIG. 2b - the sample obtained in example 2.

In FIG. 3 presents micrographs of the sample obtained by the inventive method in example 3. 3a - the sample is prepared by grinding and etching the alloy obtained in example 3. 3b is an enlarged fragment (site 01) of the sample shown in FIG. 3a.

In FIG. 4a is a micrograph of a fragment of a thin section prepared from an alloy sample obtained by the inventive method in Example 2. Microhardness measurements were performed in sections 1, 2, and 3 with different phase microstructures: 1 — sections containing crystalline whiskers of chromium carbide Cr ₃ C ₂ ; 2 - sections containing inclusions Ni-C, Ni-Cr; 3 - sections containing mainly nickel.

In FIG. 4b, the distribution on the section of sections with different phase world structures (see Fig. 4a) is highlighted for clarity: 1 (blue), 2 (black) and 3 (white).

Microphotographs were taken with a JEOL JSM-700F microscope connected to an energy dispersion X-ray-EDX system.

The inventive method for producing ternary alloys Ni-Cr-C of increased hardness is carried out by sintering in air a powder mixture containing 25-40 mass. % chromium, 3-5 mass. % graphite, the rest is nickel at a temperature of from 1200 to 1300 ° C for 30-60 minutes and subsequent immediate rapid cooling of the melt in liquid nitrogen vapor. To achieve a technical result - to obtain a ternary alloy Ni-Cr-C with high hardness characteristics, the use of an inert gas atmosphere or other special conditions is not required.

The essence of the invention lies in the fact that in the process of sintering a mixture of micro-sized powders of graphite, nickel and chromium under the conditions of the proposed method, a multiphase structure is formed in the material, which, along with a relatively soft matrix containing nickel, its compounds with chromium and carbon, contains graphite, contains characterized by high hardness, crystalline regions formed mainly by chromium carbide Cr ₃ C ₂ . Chromium carbide is present in the form of clusters of filamentary crystalline structures - whiskers, which are micro rods up to 100 or more micrometers in length from about 1 to several hundred microns in diameter. The higher the volume fraction of crystalline regions and the smaller the geometric dimensions of the whiskers, the higher the hardness of the resulting alloy. The volume fraction of crystalline structures in the sample, as well as their geometric dimensions, depend on the mass quantitative ratio and particle sizes of the components in the initial powder mixture, sintering and cooling conditions.

There is every reason to believe that the formation of chromium carbide micro-rods (whiskers) in the described method occurs according to the well-known “vapor-liquid-crystal” mechanism described in [RS Wagner, WC Ellis, Appl. Phys. Lett. 4, 89 (1964)] for the growth of semiconductor whiskers. In a three-component system, including chromium, carbon and nickel, the latter not only plays the role of a binder, but also is a catalyst for the formation of microcrystalline chromium carbide. Under the conditions of the method, carbon atoms diffuse into a drop of molten nickel, which, in contact with a solid chromium particle, causes a chemical reaction with the formation of a crystallizing, energetically advantageous Cr ₃ C ₂ micro rod under a drop. This is how the growth of chromium carbide whisker occurs. Thus, the transverse dimensions of the forming whisker should be determined by the diameter of the Ni-C melt drop, which, in turn, depends on the size of the nickel particles in the initial powder mixture. As experiments show, the use of nickel particles in the micrometer size range, in particular, the use of a powder in which the bulk of the particles corresponds to a range of 30-70 μm in diameter, allows to obtain whiskers, the geometrical dimensions of which provide high local microhardness. The length of the micro rod is determined by its growth rate and depends on the heating time and the quantitative content of chromium in the environment of the growing whisker. The shorter the sintering time and the smaller the size of nickel particles in the initial powder mixture, the smaller the geometric dimensions of the chromium carbide micro rods and the higher their microhardness and the average hardness of the sample as a whole. Immediate rapid cooling in liquid nitrogen vapor (T = 100K) immediately after heating leads to “freezing” (fixation) of the form of microcrystals formed in micropores formed due to the transition of graphite and chromium particles to the composition of whiskers, which ensures, as a result, high hardness indicators of the material at room temperature. Upon smooth cooling, the micro rods would have to lose their shape due to the high diffusion of the atoms surrounding them and the filling of the pores due to sintering of the micro rods with the softer nickel compounds with carbon and chromium surrounding them. Apparently, this can explain the relatively low hardness indices of the ternary alloys Ni-Cr-C obtained by the prototype method.

To obtain the desired result, it is advisable to use chromium and graphite powders with micron particles in the initial mixture, which ensures a quick reaction of the formation of micro-whiskers in the intergranular regions in the selected temperature range and determines the formation of an alloy uniform at the micro level due to the maximum mutual contact of the alloyed microparticles.

In FIG. 1 shows micrographs of the powders of chromium and nickel introduced into the composition of the initial powder mixture. The chromium powder (Fig. 1a) of the PH1M brand contains at least 99.1% Cr, particle size less than 125 microns. PNE-1 grade nickel powder (Fig. 1b) with a purity of more than 99% contains at least 30% of particles up to 45 microns in size and no more than 4% of particles larger than 71 microns. As can be seen in the photograph, nickel particles form conglomerates, which decompose with the formation of smaller droplets during heating of the powder mixture. Graphite with a purity of more than 99% is crushed to particles with a size of 1-30 microns.

In Tab. 1 shows examples of the invention - the composition of the initial powder mixtures, temperature and sintering time. In all examples, the melts are cooled as quickly as possible in liquid nitrogen vapor immediately after completion of heating. The values of the average hardness of the obtained samples, the values of microhardness, and the quantitative fraction of the sections of the samples containing clusters of microviskers are also given.

To determine the morphology, chips and thin sections of the obtained samples were studied with a 1000-fold magnification Versamet-2 metallographic microscope. Microphotographs were obtained using a JEOL JSM-700F microscope connected to an X-ray analysis of dispersion. By way of illustration in FIG. 2a and 2b show micrographs of chips of samples of materials obtained in examples 1 and 2, respectively. In a nickel matrix containing mainly metallic nickel, chromium, nichrome and graphite, whiskers are distributed in the form of whiskers with a length of up to 100 or more micrometers and a cross section of about 1 to several hundred microns ² . An increase in the chromium content, an increase in temperature, and a decrease in the sintering time (examples 2 and 3) lead to the formation of smaller crystalline structures and an increase in their number. This is clearly seen in FIG. 3a, which shows a micrograph of a sample obtained by grinding the alloy of example 3 and etching the metal components from the obtained powder as described in [M. Beckert und N. Klemm, Handbuch der metallographischen Atzverfahren. VEB Deutscher Verlag fur Grundstoff Industrie, Leipzig, 1976]. An enlarged fragment (site 01) is shown in FIG. 3b, which shows the “forest” of thin micro-whiskers of chromium carbide with a cross section of about 1 μm.

Phase analysis of micro-whisker powder (Empyrean Panalytical (Netherlands) diffractometer compared to the standard spectrum of Cr ₃ C ₂ [A. Garcia-Marquez, D. Portehault, C. Giordano, J. Mater. Chem. 21, 2136 (2011)] shows that whiskers are more than 90% composed of crystallites of chromium carbide Cr ₃ C ₂ .

The microhardness of the samples was determined by the standard Vickers method of measuring the hardness of metals on a PMT-3 microhardness tester (NERKON Co. https://nerkon.ru/) with loads of 50 g and 100 g (measurement error 5-7%). It is shown that areas containing mainly nickel with inclusions of nichrome, chromium and carbon (sections 2 and 3 in Fig. 4) are characterized by a relatively low microhardness of 300-400 HV. At the same time, the areas corresponding to the accumulation of whiskers show extremely high averaged microhardness indices (see Table 1). For example, sections of the sample of example 3, occupying about a third of the thin section, have a microhardness of about 3200 HV, exceeding the microhardness of previously obtained materials with crystals of chromium carbide [R. Chattopadhay. Surface Wear: Analysis, Treatment, and Prevention, Materials Park, OH: ASM International, p.p. 228-229 (2001)] and exceeding the microhardness of chromium carbide nanowhiskers [S. Motojima, S. Kuzuya, J. Cryst. Growth, 1985, 71, 682].

The average hardness of the samples obtained in examples 1-3, is determined as described in [Z. Dong, L. Zhang, W. Chen, Mater. Sci. Eng., 2012, 552, 24], no by measuring the microhardness of H ₁ , H _2, and H ₃ in sections 1, 2, and 3 with different phase microstructures.

As an example, in Tab. 2 shows the calculation of the average hardness of the sample obtained in example 2. In FIG. 4a shows the selection of points in sections 1, 2, and 3 with different phase microstructures (2 points for each phase), on which microhardness measurements were performed. In FIG. 4b, highlighting shows the quantitative distribution of areas with a characteristic microstructure over the area of the thin section of the test sample.

The calculation of the average hardness N _{with is} carried out according to the formula

N _s = H ₁ × V ₁ + H ₂ × V ₂ + H ₃ × V ₃ ,

where H ₁ H ₂ and H ₃ - values of microhardness at points 1, 2 and 3, averaged over 3 measurements at each point; V ₁ , V ₂ and V ₃ - the proportion of the corresponding phases in the thin section. The area of the thin section is 80 × 120 μm ² .

Similarly determined the average hardness values of the samples obtained in examples 1 and 3, are given in Table. 1. Comparison of these data with the hardness indices of the samples obtained by the prototype shows that the conditions of the proposed method allow to obtain ternary alloys Ni-Cr-C, the average hardness of which is several times higher than the average hardness of the similar in chemical composition of the samples obtained by the prototype method. The maximum average value of hardness (about 1300 HV) is characterized by a sample containing 45 mass. % chromium and 5 mass. % carbon. A further increase in the content of chromium and graphite and, as a consequence, a decrease in the nickel content in the initial powder mixture is below 50 mass. %, does not lead to an increase in the content of whiskers of chromium carbide. A decrease in the content of chromium and graphite in the initial powder mixture leads to a decrease in the hardness of the obtained ternary alloys due to a decrease in the fraction of the crystalline phase in the alloy structure. Sintering in the temperature range of 1200-1300 ° C allows you to quickly, within 30-60 minutes to obtain a triple alloy with high hardness. An increase in sintering time is undesirable, since, as was shown in [P. Berthod Materials and Corrosion 2012, 63, No. 9999], prolonged heating in air can lead to a decrease in the hardness of ternary Ni-Cr-C alloys due to oxidative degradation in various phase structures of the alloy.

As noted above, in order to achieve a technical result — obtaining a material of high hardness, the cooling mode of the obtained alloy is essential. In accordance with the invention, the fastest cooling is carried out immediately after sintering, for which the sample is placed in a pair of liquid nitrogen. Such "quenching" prevents undesirable diffusion processes, which, with slow smooth cooling, could lead to a decrease in the hardness of the material due to the destruction of the surface of crystalline structures and the formation of a mixture of chromium carbides and NiCr with mixed stoichiometry.

It is of interest to compare the characteristics of the alloys obtained by the claimed method, with the characteristics of the compositions described in the patent [US 6537343 B2, publ. 03/25/2003] obtained by sintering at a temperature of 1200-1225 ° C of the initial powder mixture containing chromium carbide Cr ₃ C ₂ and nickel. Despite the high hardness indicators of the obtained composition, the microphotographs of the samples presented in the description show the absence of crystalline whiskers in the microstructure of the alloy — a large number of small grains of chromium carbide of various shapes are visible in the section. This suggests that in this case there are no conditions for the formation of whiskers, and the high hardness of the material is mainly due to the high (up to 90 wt.%) Content of chromium carbide in the initial powder mixture. In contrast to this analogue, under the conditions of our proposed method, it is possible to obtain a material similar in hardness with a chromium carbide content of about 30% (see Example 3 in Table 1).

It can be expected that the use of nanosized nickel in the initial powder mixture will make it possible to obtain even harder alloys due to the formation of chromium carbide nanowhiskers characterized by an even higher microhardness.

Claims (3)

1. A method of producing a ternary alloy Ni-Cr-C, comprising heating the initial mixture of powders of elemental nickel, chromium and graphite and its subsequent cooling, characterized in that the heating of the mixture of powders is carried out in air at a temperature of 1200-1300 ° C for 30-60 min, and cooling in vapor of liquid nitrogen immediately after heating is completed with the formation of an Ni-Cr-C alloy containing chromium carbide microviscr Cr ₃ C ₂ , while the initial mixture of powders consists of micron particles with the following components, wt.%: Chromium                          25-45 Graphite                  3-5 Nickel      rest 2. The method according to p. 1, characterized in that they use nickel powder with a particle size not exceeding 71 microns.

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