WO2024067985A1

WO2024067985A1 - WC-9.0FeNi-[0.5-1.0]Cr3C2-0.5NbC HARD METAL WITH IMPROVED MECHANICAL PROPERTIES AND CORROSION RESISTANCE

Info

Publication number: WO2024067985A1
Application number: PCT/EP2022/077241
Authority: WO
Inventors: Tamara ALEKSANDROV FABIJANIC; Johannes POETSCHKE; Ivan Jeren
Original assignee: ALFA TIM d.o.o.
Priority date: 2022-09-29
Filing date: 2022-09-29
Publication date: 2024-04-04

Abstract

Disclosed is a method for the preparation of hard metal products based on powders composition consisting of tungsten carbide (WC), 9 wt.% FeNi, 0.5-1 wt.% Cr₃C₂ and up to 0.5 wt.% NbC, where WC, Cr₃C₂ and NbC wt. percentages are selected to produce 91 wt.% of the said powder mixture. The method consists of the following pre-steps; preparation of the starting powders mixture, mixing and homogenization for 48 hours with the addition of heptane and optionally paraffin wax, drying by vacuum distillation, sieve granulation and shaping. Pre-sintering is performed by optional vax removal, heating the product to 1200°C in a vacuum and subsequently to 1400°C - 1450°C under an argon (Ar) atmosphere. Finally, the sinter-HIP is performed at the same temperature, in a 60-100 bar Ar atmosphere. The obtained WC-9.0FeNi-[0.5-1.0]Cr3C2-0.5NbC hardmetal has improved mechanical properties and corrosion resistance, a density of 14.45 g/cm³, and hardness of about 1930 HV2.

Description

WC-9. OFeNi- [0.5-1.0] Cr3C2-0.5NbC HARD METAL WITH IMPROVED MECHANICAL

PROPERTIES AND CORROSION RESISTANCE

DESCRIPTION

Technical Field

A method for the preparation of hard metal products based on powders composition is disclosed. The composition is selected to be tungsten carbide (WC) powder, iron-nickel (FeNi) powder as the binder, chromium carbide (CrsC2) powder as a grain growth inhibitor, and niobium carbide (NbC) powder. The main technical field of the present disclosure is the formation of alloys based on carbides, oxides, nitrides, borides, or silicides, e.g., cermets, or other metal compounds, e.g. , oxynitrides, sulfides; more specifically based on tungsten carbide as the main constituent. Regarding the way of preparation, the present disclosure belongs to powder metallurgy, where mixtures of metal powders with non-metallic powders are used.

Technical Problem

Cemented carbide is an alloy material made by a powder metallurgy process where a hard compound of refractory metal and a selected binder are used. Carbide, particularly WC, has a series of excellent properties such as high hardness, wear resistance, strength and toughness, heat resistance, corrosion resistance, etc. It is well known that the high hardness and wear resistance remain basically unchanged even at a temperature of 500°C. So, in general, cemented carbides are widely used as tool material, such as turning tools, milling cutters, planers, drills, boring tools, etc. It is common in the art to use carbides for cutting cast iron, non-ferrous metals, plastics, chemical fiber, graphite, glass, stone, and ordinary steel, as well as heat-resistant steel , stainless steel , high manganese steel , tool steel and other dif f icult-to-machine materials .

With recent developments , there are many demands for the tools to operate under harsh environmental conditions , such as acid and alkali corrosion conditions accompanied by significant material wear, high- pressure sealing , and similar conditions . The application range of cemented carbide is continuously expanding , and in addition to hardness and strength, there is a frequent demand for the corrosion resistance and high-temperature oxidation resistance of cemented carbide to be met .

Therefore , the technical problem solved with the present disclosure is to improve the tungsten carbide (WC ) compound in a way to retain its mechanical properties and acquire good corrosion resistance , which tungsten carbide - cobalt (WC- 9Co ) hardmetals with coarser grain structure cannot achieve .

State of the Art

According to the best applicant' s knowledge , the following prior art documents seem to be relevant for the present disclosure .

Chinese patent application CN107964620A for TUNGSTEN-NICKEL-BASED ENHANCED HARD ALLOY AND PREPARATION METHOD THEREOF, filed in the name of Zhuzhou Sanxin Cemented Carbide Production Co . Ltd . The cited invention relates to the technical field of cemented carbide materials , and more particularly, to WC- 9Ni-based tungsten-nickel- based enhanced cemented carbide and a preparation method thereof . Examples 1 and 3 reveal the material with 9 wt . % Ni , 0 . 3 wt . % CrsC2 and 0 . 02 wt . % AIN . It is reported that the degree of oxidation and weight loss of alloys containing chromium carbide and aluminium nitride in WC-9Ni are lower than those of alloys without such additives . Chinese patent application CN109652695A for WC- 9Ni-0 . 57Cr HARD ALLOY , filed in the name of Yang Jing . The cited invention relates to a cemented carbide material , particularly WC- 9Ni-0 . 57Cr cemented carbide . It is reported that this material has good corrosion resistance , and the corrosion rates of WC-9Ni-0 . 57Cr alloy versus WC- 6Co alloy are 0 . 003 mm/y and 0 . 009 mm/y, respectively, so WC-9Ni- 0 . 57Cr has stronger corrosion resistance . The microscopic corrosion morphology is manifested by the corrosion of the binder Nickel phase . Also , it is reported that the corrosion of the cemented carbide binder phase decreases the material ' s mechanical properties .

Chinese patent application CN109652704A for WC-9Ni-lCr FINE GRAIN HARD ALLOY , is filed in the name of Yang Xiaodong . It teaches about cemented carbide material , particularly a method for obtaining WC-9Ni-lCr finegrained cemented carbide . The described procedure includes powders ball milling for 36 hours , and, after ball milling , the composite was dried in a vacuum drying cabinet at a drying temperature of 65 ° C for 100 min . Then, 50 microns mesh sieve was used to obtain a WC-9Ni-lCr . mixed powder . The prepared powder mixture was added to a single-column hydraulic press for press forming, where the applied pressure was 80 MPa, and the dwell time was 10 s . Subsequently, the obtained compact is subj ected to low-pressure sintering in a hydrogen dewaxing low- pressure sintering integrated furnace . Finally, the maximum sintering temperature is 1470 ° C; the temperature is maintained for 90 min with the sintering pressure of 5 MPa .

A very similar procedure was applied in Chinese patent application CN109652705A for WC- 9Ni-lCr FINE GRAIN HARD ALLOY , filed by the same inventor .

Chinese patent application CN109652706A for CeO2 -CONTAINING ULTRAFINE HARD ALLOY , filed in the name of Yang Xiaodong . The cited invention teaches about WC-9Ni-0 . 4CrsC2 cemented carbide containing CeO2 . According to the disclosure , it seems that CeO2 can inhibit the growth of cemented carbide grains during sintering, resulting in a more uniform internal structure , a higher degree of densif ication and a more complex phase composition. The addition of Ce02 has greatly improved hardness, densif ication degree and flexural strength.

Chinese patent application CN111636024A for WC-Ni-Cr/Ta COMPOSITE HARD ALLOY AND PREPARING METHOD AND APPLICATION THEREOF, filed in the name of Hohai University. The invention discloses WC-Ni-Cr/Ta composite hard alloy and a preparation method and application thereof. The material comprises, by weight percent, 8 to 12% of Ni, 0.2 to 1% of Cr/Ta and the balance WC. The disclosed composite has a good high- temperature wear performance and high fracture toughness performance . It seems that the higher hardness and corrosion resistance performance can be achieved with the said material.

Balbino, N.A.N., Correa, E.O., and de Carvalho Valeriano, L. (2018) . Development of the 90WC-8Ni-2Cr3C2 cemented carbide for engineering applications . The International Journal of Advanced Manufacturing Technology, doi : 10.1007/s00170-018-2511-y. This document discusses the addition of 2 wt% CrsC2 in the WC-Ni alloy system using the conventional adding method. Surprisingly, it was not favourable to densif ication . The porosity was characterized by the presence of relatively large, elongated pores distributed in the microstructure. As a result, there was no significant improvement in the hardness and flexural strength of the WC-8Ni-2Cr3C2 cemented carbide in comparison with WC-lONi cemented carbides without CrsC2 addition, obtained from the literature .

Aleksandrov Fabijanic, T. , Kurtela, M. , Skrinjaric, I. , Pdtschke, J., & Mayer, M. (2020) . Electrochemical Corrosion Resistance of Ni and Co Bonded Near-Nano and Nanostructured Cemented Carbides. Metals, 10(2) , 224. doi : 10.3390/metl0020224. The document discusses the development of cemented carbides with alternative binders to increase the corrosion resistance and retain mechanical properties. Described is the use of grain growth inhibitors, such as VC and CrsC2, and an identical binder amount of ll-wt.% was prepared, using Cobalt (Co) and Nickel (Ni) , respectively. It is reported that the corrosion rate of Ni-bonded cemented carbides is approximately four times lower compared to Co-bounded cemented carbides.

In the above cited documents, hard metal WC-9. OFeNi- [0.5-1.0] Cr3C₂- 0.5NbC was not mentioned. The closest document reveals ultrafine WC with 9 wt% Ni and 0.4 wt% Cr3C₂ composition (CN109652706A) , with an extra addition of Cerium dioxide (CeO₂) where improved hardness, densif ication degree and flexural strength are achieved, but the corrosion is not mentioned in the document. It seems that the role of 9 wt% FeNi with 0.5 wt% NbC was never reported or tested.

Summary of the Invention

A method for the preparation of hard metal products based on powders composition selected to be tungsten carbide (WC) powder, iron-nickel (FeNi) powder as the binder, chromium carbide (Cr₃C₂) powder as a grain growth inhibitor and niobium carbide (NbC) as a cubic carbide is disclosed. The composition is selected to be WC, 9 wt . % FeNi, 0.5-1.0 wt . % Cr3C₂ and up to 0.5 wt . % NbC, where WC wt.%, Cr3C₂ wt . % and NbC wt . % is balanced to form 91 wt.% of the powder mixtures. The said process consists of the following steps:

A. preparation of the starting powders mixture,

B. mixing and homogenization in a ball mill to obtain a homogeneous microstructure, with an addition of heptane (CH3 (CH2) 5CH3) as a liquid medium, and optionally paraffin wax addition up to 2 wt . % of the mixture,

C. drying the mixture to eliminate the liquid heptane phase by vacuum distillation,

D. sieve granulation to bring the mixture of powders into a flowing condition, and

E. shaping the product by extrusion, or by compacting the mixture into the mold at a temperature within the range of 15-30° C, The product thus formed is subjected to a sintering process in several steps :

F. debindering process, where removal of optional paraffin wax is performed at 400°-800° C in hydrogen (H2) atmosphere with a dwell time of 60 minutes,

G. heating the product to 1200°C in a vacuum,

H. heating the product up to temperature between 1400°C - 1450°C in argon (Ar) atmosphere at a pressure of 50 mbar with a dwell time of 30 minutes, and

I. sintering the product by the sinter-HIP process at a temperature between 1400°C - 1450°C, with the applied pressure of 60-100 bar, in Ar atmosphere, for at least 45 minutes.

The hard metal of the composition WC-9. OFeNi- [ 0.5-1.0 ] Cr3C2-0.5NbC is obtained. The said material is substantially without pores, with improved corrosion resistance, with a density of 14.45 g/cm³, and a Vickers hardness of about 1930 HV2.

Description of Figures

Figure 1A shows the hard metal surface of C-9Ni-lCr3C2-0.5NbC composition, obtained with the disclosed method and characterized by optical microscopy. Approximately the surface area of 0.6 mm² is visible .

Figure IB shows the preferred hard metal surface of C-9FeNi-lCr3C2- 0.5NbC composition, obtained with the disclosed method and characterized by optical microscopy. Approximately the surface area of 1 mm² is visible.

Figure 1C shows the hard metal surface of C-9FeNiCo-lCr3C2-0.5NbC composition, obtained with the disclosed method and characterized via optical microscopy. Approximately the surface area of 1 mm² is visible. Figure 2A shows the hard metal surface of C-9Ni-lCr3C2-0.5NbC composition, obtained with the disclosed method and characterized via the Field Emission Scanning Electron Microscopy (FESEM) .

Figure 2B shows the preferred hard metal surface of C-9FeNi-lCr3C2- 0.5NbC composition, obtained with the disclosed method and characterized via the FESEM.

Figure 2C shows the hard metal surface of C-9FeNiCo-lCr3C2-0.5NbC composition, obtained with the disclosed method and characterized via the FESEM.

Figure 3 shows the phase diagram calculation for the composition WC- 9. OFeNi-O .5Cr3C2~0.25NbC which confirms the complete solution of cubic carbides/grain growth inhibitors within the binder phase.

Detailed Description of the Invention

A method for the preparation of hard metal products based on powders composition selected to be tungsten carbide (WC) powder, iron-nickel (FeNi) powder as the binder, chromium carbide (CrsC2) powder as a grain growth inhibitor and niobium carbide (NbC) as a cubic carbide is disclosed .

For the comparative analysis, three samples were prepared, sintered, and measured. The weight percentage was identical in all cases, but the binder powder was altered to be {Ni, FeNi, FeNiCo}, as presented below :

(51) WC-9Ni-lCr₃C₂-0.5NbC

(52) WC-9FeNi-lCr₃C₂-0.5NbC

(53) WC-9FeNiCo-lCr₃C₂-0.5NbC

A. Preparation of the starting powders mixtures The compositions are selected to be WC, 9 wt . % Ni/FeNi/FeNiCo, 1.0 wt . % CrsC2 and 0.5 wt . % NbC.

In all three samples, the selected tungsten carbide powder (WC) was WC DN 3-0 powder produced by H.C. Starck Tungsten, Germany. This powder has an average BET grain size of about 130 nm, and a BET specific surface of 2.9 m²/g. The WC powder exhibits a homogeneous microstructure suitable for powder metallurgy.

Chromium carbide CrsC2 160 powder used in all three samples was produced by Hdganas, where the BET grain size was about 450 pm or, according to FSSS (Fisher sub-sieve sizer) 1.5 pm. BET specific surface is measured to be 2.0 m²/g.

Niobium carbide (NbC) powder used in all three samples was produced by H. C. Starck Tungsten, where the FSSS grain size is estimated to be 0.9 pm.

For the first sample, Ampersint® MAP Ni, produced by Hdganas, was used. The average grain size according to FSSS was 2.6 pm.

For the second sample, Ampersint® MAP A 8500 Fe/Ni=15/85 wt%, produced by Hdganas, developed by H.C. Starck, was used. The average grain size according to FSSS less than 2.6 pm.

For the third sample, Ampersint® MAP A 6050 HT Fe/Co/Ni=40/20/40 wt%, produced by Hdganas, developed by H.C. Starck, was used. The average grain size according to FSSS was 2.0 pm.

It is known in the art that the Ampersint® powders are obtained via gas or water atomization, with low content of impurities and with high uniformity of the grains - which render them suitable for the powder metallurgy . The powders were weighted in a standard manner and prepared to be mixed/homogenized in a powder mixture, where each mixture had in total 200 g.

B. Mixing and homogenization

The starting powder mixture preparation was carried out in a horizontal ball mill equipped with a stainless-steel vessel with hard metal balls to prevent the contamination of the powder mixture with the balls' material due to their surface damage. The primary goal of the said procedure is to mix and homogenize the powders from step A. As already mentioned, 200 g of the powder mixture was put into the ball mill, and paraffin wax was added up to 2 wt . % of the mixture. The used balls were 4.5 mm in diameter, where a total weight ratio of the said balls mass vs. the powder mixture mass was close to 10:1, i.e. , the total mass of the balls was close to 2 kg. The mill vessel's volume, where the mixing was carried out, was approximately 1 dm³. For the mixing and homogenization ZOZ GmbH, Germany, a laboratory ball mill was used.

200 mL (135 g) of liquid medium for mixing and homogenization, i.e. , heptane (CH3 (CH2) 5CH3) , was added to the milling chamber, which is a common procedure in the art.

All three samples were treated equally in the ball mill; they were milled for 48 hours, with the same milling speed of 70 revolutions per minute (rpm) . After that period, the mixing and homogenization steps were finished.

If the paraffin wax was added, the paraffin removing process, i.e. , the pre-sintering, should be executed before the sintering. In the preferred embodiment, in all three samples, the wax was added.

C . Drying When mixing and homogenization are finished, it is necessary to remove the liquid phase from the mixture, i.e. , to remove heptane. The heptane removal is performed by vacuum distillation, i.e. , by heating the mixture to a temperature above the boiling temperature in a vacuum, which is around 80°C. For reference, the heptane boiling point at atmospheric pressure is about 98°C. The drying procedure can be carried out as long as there is a "pressure" that indicates the heptane evaporation from the mixture, or via empirically determined drying time, for example, through approximately 8 hours in the present cases.

D. Sieve granulation

Dried powders' mixture is subjected to sieve granulation to bring the mixture of said powders into a flowing condition. Furthermore, the said procedure retains the balls used in milling and separates them from the powders' mixture. In the preferred embodiment, the sieve with a mesh size of approximately 315 pm was used, and the resulting mixture of powders had uniform granulation and ability to flow.

E. Shaping the products

Shaping the product into desired form or geometry is carried on by compacting the mixture into the mould.

Compacting is usually carried out at a temperature of 15-30°C and a pressure of 150-350 MPa, preferably at room temperature and a pressure of approximately 200 MPa.

For the purposes of the experimentation, all three samples were compacted into the molds with dimensions of 60.8 mm x 8.8 mm, where the average thickness of the compacted green body (sample) was about 7.5 mm, yielding samples of about 35 g. The uniaxial compacting was used .

The extrusion process is a frequently used process in the art, especially for forming more complex shapes, such as water-jet nozzles or tool parts. Any standard available extruder with the appropriate mold is suitable for shaping the products in the desired manner.

F. Debindering process / pre-sintering process

If the samples were formed with paraffin wax, before the actual sintering process, the wax must be removed by the pre-sintering process. There are many ways to perform said debindering process.

One technique called "vacuum dewax" uses a low pressure of approximately 30 mbar of a hydrogen atmosphere and a temperature of approx. 800°C for 60 minutes, while samples "drain" and "evaporate" the wax.

For the procedure mentioned above, it is good to see the reference: https : / /vacuumfurnaces . com/ images/ debindingand%2 Osinteringofmetalsceramics 062001. pdf

Alternatively, the debindering process, according to the preferred embodiment, was carried at 400° C in a hydrogen (H2) atmosphere with a dwell time of 60 minutes. The said process achieves the same or similar technical effect as in the procedure described above.

The debindering process causes no wax residue in the mentioned samples, which are then ready to be sintered.

G.-I. Three step sintering process

The sintering process is carried out in three steps, using the sinter- HIP furnace model FPW280/600-3-2200-100 PS ( FCT Anlagenbau GmbH, Sonneberg, Germany.

Firstly, in step G, the products were heated to 1200°C in a vacuum where solid-state sintering and reduction of stable oxides occurred. In this phase, maximum shrinkage of the hardmetal samples is achieved. Densif ication begins before the eutectic occurs. More than half of hardmetals densif ication is thought to occur at temperatures below the eutectic, in the solid state. The consequence of sintering in the solid state is a considerable shrinkage of hard metals, recorded in the temperature interval from 800 to 1250 °C; see for instance Gopal S. Upadhyaya : Cemented Tungsten Carbide: Production, Properties and Testing, Noyes Publications, 1998.

In ultra-fine and nanostructured hardmetals, up to 90% of the total densif ication can be achieved; see for instance G. Gille, B. Szesny, K. Dreyer, H. van den Berg, J. Schmidt, T. Gestrich. G. Leitner: Submicron and ultrafine grained hardmetals for microdrills and metal cutting inserts, Internal Journal of Refractory Metals and Hard Materials 20, 2002, 3-22.

Then, in step H, the products are heated up to a temperature between 1400°C - 1450°C in an argon (Ar) atmosphere at a pressure of 50 mbar with a dwell time of 30 minutes, where the liquid phase - binder occurs. This phase is the isothermal sintering phase or liquid phase sintering consequence of which is the accelerated diffusion of atoms in the presence of a liquid phase, responsible for material transfer and complete densif ication without external pressure. According to thermodynamic calculations performed by Fraunhofer IKTS, Dresden, group Hardmetals and Cermets using software Factsage 8.0 , the first liquid is formed in the range 1240 - 1265°C.

This phase of the sintering is performed at very low pressure, below atmospheric pressure, and is still considered as vacuum sintering.

Finally, in step I, the products are subjected to the sinter-HIP process at a temperature between 1400°C - 1450°C, with the applied pressure of 60-100 bar, in Ar atmosphere, for at least 45 minutes. The pressure is applied using an inert Ar gas from all directions equally. Under the mentioned conditions of temperature and pressure, the internal pores and irregularities break up, and diffusion bonding occurs at the grain boundaries. It is considered that during the HIP process, the stress caused by pressure at a specific temperature exceeds the value of the material's tensile strength. Plastic yielding occurs at the microscopic level, which causes the formation of isolated pores that just disintegrate, allowing contact between the two surfaces . The aforementioned enables connection at the points of contact by diffusion of atoms in both directions. The plastic deformation of the powder results in the elimination of porosity and achieving 100% theoretical density of the samples.

All three samples:

(51) WC-9Ni-lCr₃C₂-0.5NbC

(52) WC-9FeNi-lCr₃C₂-0.5NbC

(53) WC-9FeNiCo-lCr₃C₂-0.5NbC were equally treated and sintered, while the sample (S2) is the selection invention, for the reasons discussed below.

Samples characterization - mechanical parameters

Samples characterization was performed to analyze microstructural characteristics and mechanical properties, such as the actual densities of consolidated samples, relative densities, surface analyses for estimation of porosity and unbounded carbon in given samples, grain size estimation and hardness according to the Vickers method. The obtained data are summarized in the table below:

Table 1

where A, B and C values are surface characterization according to ISO 4499-1:2020 standard. For the determination of porosity, carbon defects and eta-phase content, the surfaces of the obtained samples were polished, cleaned and etched in accordance with the ISO 4499-1:2020 and ISO 4499-4:2016 standards. The microstructure characterization was based on different reactions of Murakami's solution with different phases of the examined microstructures. The etching procedure has led to classifying microstructure constituent etching reactions using Murakami's solution. The Murakami's solution consists of 10 g of K3Fe(CN)e, 10 g of KOH and 100 ml of H2O in accordance with ISO 4499-4:2016 standard. Usually, several cycles of etching, rinsing with water and cleaning the surface using a 96% ethanol solution are carried out. It is well- known in the art that such a procedure makes the microstructure more visible under an optical microscope.

The microstructures of the obtained samples were analyzed with an optical microscope and on a Field Emission Scanning Electron Microscopy (FESEM) .

The surfaces prepared in the above-described way were photographed with an optical microscope, and the results are visible in Figures 1A, IB, and 1C . All three pictures show the absence of pores. The degree of porosity can be classified as less than A02 or A00, and no unbound carbon or any other irregularities are found after the sintering process. It is also evident that there are no large pores or cracks, as well as no unbound carbon/carbon defects, and therefore the samples are also classified as BOO and COO, in accordance with the above-mentioned standard.

Figures 2A, 2B and 2C show the samples' surfaces taken with a Field Emission Scanning Electron Microscopy (FESEM) . It is evident that the microstructures of all samples are homogeneous; no irregularities such as abnormal grain growth, carbide clustering or the appearance of the eta phase, i.e., carbon defects, were found. The microstructure investigation, therefore, indicates adequately selected process parameters for obtaining samples (SI) , (S2) and (S3) , resulting in excellent quality samples that are pores free.

In addition, the hardness test was carried out according to the Vickers method using different loads. The well-known Vickers method consists of indenting a diamond indenter, shaped as a regular four-sided pyramid, into the surface subjected to the test - by applying the prescribed force F(N) in accordance with HRN EN ISO 6507-1:2018 standard. The hardness of samples (S2) with FeNi and (S3) with FeNiCo binder is comparable to the hardness of standard WC-Co hard metals, with a Cobalt (Co) binder of 9 wt . % see for instance T. Aleksandrov Fabijanic, Z. Alar, D Coric: Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano-and nano-structured WC-Co cemented carbides, International Journal of Refractory Metals and Hard Materials 54, 82-89.

It is known in the art that one of the main disadvantages of using alternative binders, such as Ni, FeNi, and FeNiCo, results in poorer mechanical properties and lower hardness compared to hard metals with Co binder.

Surprisingly, the above findings show that the application of nano powders in the starting mixtures, especially in the case of FeNi and FeNiCo binders, removes the observed mechanical (hardness) disadvantage known in the art. On the other hand, alternative binders are characterized by better corrosion resistance in acidic media compared to hard metals with Co binder, which is demonstrated in the next section.

Samples characterization - electrochemical corrosion resistance

The samples (S2) with FeNi binder and (S3) with FeNiCo binder were used to investigate electrochemical corrosion resistance, having in mind that sample (SI) with Ni binder shows inferior mechanical properties. The surface of the samples was placed into the corrosion cell filled with IM/1 or 96% H2SO4 + CO2 (pH=0.6) . As a reference electrode was selected, saturated calomel electrode SCE (SCHOTT Instruments GmbH, Mainz, Germany) with a potential of + 0.242 V according to the standard hydrogen electrode. Graphite wires were used as a counter electrode. The samples were researched by direct current techniques DC, the open-circuit potential Ecorr, the linear polarization resistance (LPR) , and the Taffel extrapolation method. Corrosion potential Ecorr versus SCE was recorded for 30 min. LPR was carried out in the potential range from - 0.02 V vs. open circuit potential to 0.02 V vs. open circuit potential with a scan rate of 0.167 mV/s. Tafel extrapolation was conducted in the potential range from - 0.25 V vs. open circuit potential to 0.25 V vs. open circuit potential, total points 1001 with the scan rate of 0.167 mV/s. Immediately after the DC techniques, the samples were researched by alternating current techniques AC, more precisely by electrochemical impedance spectroscopy (EIS) .

The electrochemical corrosion resistance is summarized in Table 2, showing the results of the direct current DC technique:

Table 2

where the measured and estimated values are:

T_s - measured temperature, Ecorr - open circuit potential, R_p - polarisation resistance,

- slope of anodic Tafel curve c - slope of cathodic Tafel curve, Lorr - corrosion current density, and Vcorr - corrosion rate. The experimentations were performed in acid medium, IM/1 or 96% sulfuric acid H2SO4 + C02 (pH = 0.6) .

The used technique and parameters are more than hundred years known and well established among the corrosion investigators; see the reference below; see for instance Burstein, G. T. (2005) . A hundred years of Tafel' s Equation: 1905-2005. Corrosion Science, 47 (12) , 2858- 2870. doi: 10.1016/j . corsci .2005.07.002

From Table 2, it is easy to conclude that sample (S2) has 2.5 lower corrosion rate under the same acidic environment.

It is interesting to check the mechanical properties before and after the exposure to the acidic medium (AM) (IM/1 or 96% sulfuric acid H2SO4 + CO2 (pH = 0.6) . Instrumented indentation test was conducted using Anton Paar micro combi tester MCT³ with Vickers indenter to determine the micro-mechanical properties of both samples. The force of 500 mN was applied, and a matrix of 24 indentations was made on each sample type. The results are summarised in Table 3:

Table 3

where the measured and estimated values are:

Fmax - contact force,

Hit - indentation hardness,

Eit - indentation modulus,

Cit - indentation creep, S - stiffness,

Welast - elastic work,

Wplast - plastic work, and

HV - Vickers instrumented hardness .

The conducted experiment clearly shows that the sample (S2) with FeNi binder is superior to (S3) sample, containing FeNiCo, and remains almost unaffected once being exposed to the above defined acidic medium, which is not the case with the (S3) sample which deteriorated its mechanical properties, i.e., its hardness.

Therefore, the experimentation results selected (S2) sample WC- 9. OFeNi-1.0Cr3C2-0.5NbC over the (SI) sample C-9Ni-lCr3C2-0.5NbC and (S3) sample C-9FeNiCo-lCr3C2-0.5NbC, as the sample with improved mechanical properties and corrosion resistance.

WC-9. OFeNi- [0.5-1.0] Cr3C2-0.5NbC

Further experimentations were performed to establish the ideal content of cubic carbides/grain growth inhibitors CrsC2 and NbC in the starting mixtures. It was found that the total amount of 0.5 - 1.0 wt . % can be safely used, preferably 0.5-0.75 wt . % , where said range of CrsC2 enables that grain growth inhibitor to be fully dissolved in the FeNi- binder phase .

Since the etching with Murakami for 2 sec. shows an additional third phase with star-like structures in light optical microscopy. Further investigations revealed that these structures are the precipitation of the used carbide additions. The results show that for alternative binder bonded hardmetals, already etching for 2 sec. is enough for revealing cubic carbide precipitations, in contrast to Co bonded hardmetals where such carbide precipitations are only expected after > 5 sec.; see for example reference: G. Petzow, Materialkundlich- technische Reihe, Vol. 1: Metallographisches , keramographisches , plastographisches Atzen (Borntraeger , Berlin, Stuttgart, 1994) [ger] . The use of Cr₃C2 and NbC addition in total of 1.5 wt . % in the starting mixtures of WC-9 Ni, WC-FeNi and WC-FeNiCo is thus too high for a complete solution within the binder phase and leads to the f ormation/precipitation of cubic lattice calculated by Fraunhofer IKTS, Dresden, group Hardmetals and Cermets, leader Johannes Pdtschke using software Factsage 8.0.

Furthermore, the calculations with lower weight percentage of cubic carbides, e.g. , WC-9. OFeNi-O .5Cr3C2-0.25NbC showed complete solution within the binder phase, as clearly visible in Figure 3.

Therefore, the assumed role of niobium carbide (NbC) as a cubic carbide is to limit grain growth during sintering and to improve the mechanical properties and the performance of hardmetal tools.

Regardless of the binder material used, adding different types of cubic carbides such as Cr₃C₂, TaC, TiC, and VC is necessary to limit grain growth during sintering in grades with WC grain sizes below 1 pm, which is the case here.

Previous studies indicate that adding NbC to conventional WC-Co hardmetals significantly refines WC grain size and limits grain growth, which is described in the reference below:

Huang, S.; Liu, R.L. ; Lib, L.;Van der Biest, O.; Vleugels, J. (2008) . NbC as grain growth inhibitor and carbide in WC-Co hardmetals. Int J Refract Hard Met., 26. 389-395.

Besides grain refinement, NbC also improves mechanical properties such as transverse rupture strength, hardness, and wear resistance, that was reported in reference:

Acchar, W. ; Revoredo de Macedo, H. (2005) . Influence of NbC-Addition on Mechanical Properties of WC-Co. Materials Science Forum. (Vols. 498-499, pp. 363-368) . Trans Tech Publications. Industrial Applicability

The present invention discloses a method for the preparation of hard metal products based on powders composition consisting of tungsten carbide (WC) , 9 wt . % FeNi, 0.5-1 wt . % CrsC2 and up to 0.5 wt . % NbC, where WC, CrsC2 and NbC wt . percentages are selected to produce 90 wt . % of the said powder mixture. The industrial applicability of the said invention is obvious, having in mind that such hard metal, having composition WC-9. OFeNi- [0.5-1.0] Cr3C2-0.5NbC is useful for forming the products with high hardness and strength, capable of operating in the harsh environment where the corrosion resistance has to be met.

Claims

CLAIMS A method for the preparation of hardmetal products based on powders composition selected to be tungsten carbide (WC) powder, iron-nickel (FeNi) powder as the binder, chromium carbide (CrsC2) powder as a grain growth inhibitor and niobium carbide (NbC) as a cubic carbide, where the composition is selected to be WC, 9 wt . % FeNi, 0.5-1.0 wt . % CrsC2 and up to 0.5 wt . % NbC, where a WC wt.%, CrsC2 wt . % and NbC wt . % together are selected to produce 91 wt . % of the powder mixtures, where the said process consists of the following steps:

A. preparation of the starting powders mixture,

B. mixing and homogenization in a ball mill to obtain a homogeneous microstructure, with an addition of heptane (CH3 (CH2) 5CH3) as a liquid medium, and optionally paraffin wax addition up to 2 wt.% of the mixture,

D. sieve granulation to bring the mixture of powders into a flowing condition,

E. shaping the product by extrusion, or by compacting the mixture into the mold at a temperature within the range of 15-30° C, where the product thus formed is subjected to a sintering process in several steps :

F. debindering process, where removal of optional paraffin wax is performed at 400° -800° C in hydrogen (H2) atmosphere with a dwell time of 60 minutes,

G. heating the product to 1200°C in a vacuum,

I. sintering the product by the sinter-HIP process at a temperature between 1400°C - 1450°C, with the applied pressure of 60-100 bar, in Ar atmosphere, for at least 45 minutes , whereby the hardmetal of the composition WC-9. OFeNi- [0.5- 1.0 ] Cr3C2~0.5NbC is obtained, and where the said material is substantially without pores, with the density 14.45 g/cm³, and hardness about 1930 HV2 according to the Vickers method. The method for the preparation of hardmetal products, according to claim 1, where CrsC2 content is selected to be 0.5 -0.75 wt . % which is entirely dissolved in the FeNi-binder phase. The method for the preparation of hardmetal products, according to any of previous claims, wherein step B is performed in a horizontal ball mill for 48 hours, at 70 rpm, and where the mass ratio of powder mixture vs. balls is close to 10:1. The method for the preparation of hardmetal products according to any of previous claims, wherein step C is carried on 80 °C. The method for the preparation of hardmetal products according to any of previous claims, wherein step D is carried out by a sieve mesh of approximately 300 pm. The method for the preparation of hardmetal products, according to any of previous claims, wherein step E is performed by using the pressure 150-350 MPa, preferably 200 MPa. Product of the composition WC-9. OFeNi- [ 0.5-1.0 ] Cr3C2~0.5NbC, which is obtained by any of the claims 1-6.