CN111378857B - Preparation method of high-performance ultrafine-grained hard alloy - Google Patents

Abstract

The invention relates to a preparation method of high-performance ultrafine crystal hard alloy, which comprises the following steps of 1: preparing materials; step 2: wet grinding and mixing; and step 3: molding; and 4, step 4: sintering by low pressure hot isostatic pressing at a final sintering temperature T_SIs 1400 ℃ to 1450 ℃; keeping the temperature for 5-10 minutes, filling inert gas, pressurizing to 1-3 MPa, keeping the pressure for 5-10 minutes, filling inert gas, pressurizing to 3-6MPa, and simultaneously cooling to T_SAfter the temperature is between 20 and 50 ℃, the mixture is subjected to heat preservation and pressure maintaining sintering for 5 to 10 minutes again, inert gas is continuously filled into the mixture to be pressurized to 5 to 10MPa, and the temperature is continuously reduced to T_SAnd after the temperature is between 40 and 90 ℃, carrying out heat preservation and pressure maintaining sintering for the third time for 20 to 30 minutes, and then cutting off the power. The ultra-fine grain hard alloy end mill or drill prepared by the method has excellent service performance in high-speed or high-efficiency cutting processing of difficult-to-process materials such as stainless steel, titanium alloy, PCB (printed circuit board), acrylic, glass fiber, hardwood, resin, CFRP (carbon fiber reinforced plastics) and the like.

Description

Preparation method of high-performance ultrafine-grained hard alloy

Technical Field

The invention belongs to the technical field of hard alloy manufacturing, relates to an ultrafine grain hard alloy and a preparation method thereof, and particularly relates to a WC-Co hard alloy with specific components and tissue structure characteristics and excellent performance, wherein the WC average grain size of the WC hard alloy is within the range of 0.2-0.6 mu m, and a preparation method thereof.

Background

WC-Co hard alloy takes WC as hard phase, Co as binder phase and a small amount of Cr₃C₂Metal composite materials containing carbides such as VC, TaC and NbC as functional components and having excellent wear resistance, heat resistance and corrosion resistanceCan be widely applied to various fields of national economy, and is an indispensable tool and die and part material in modern industry. Currently, the industrial preparation of hard alloy generally adopts a powder metallurgy process, which comprises main process flows of ball milling, molding, sintering and the like.

(1) Ball milling, and the industrial preparation of hard alloy mixture generally adopts a wet ball milling process, and generally adopts organic solvents such as alcohol, hexane and the like as wet milling media. Rolling ball milling is adopted by most hard alloy manufacturers, and stirring ball milling is adopted by few manufacturers. The purpose of ball milling is to crush the raw material WC powder to the required granularity and to mix the crushed raw material WC powder with Co powder and other additive component powder uniformly to obtain a mixture meeting the requirements of the designed component proportion and granularity. For ultra-fine grain cemented carbide, high-strength ball milling for a long time (60-100 hours) is usually required to fully crush the raw material WC powder to obtain sufficiently fine initial powder. However, the long-time ball milling causes the problems of serious oxygenation, WC crystal lattice distortion, sub-crystal defects, overhigh activity, excessive superfine powder and the like, the subsequent compression molding and sintering densification are seriously influenced, and the tendency of grain growth during sintering is further increased. In this regard, many efforts have been made by the predecessors to avoid these disadvantages. For example, in order to reduce fine particle powder generated by grinding and avoid the growth of WC grains through a dissolution-precipitation mechanism in the subsequent sintering process to obtain an alloy with more uniform WC grain size, the invention US9777349B2 adopts an ultrasonic mixing method to prepare a mixture; in order to prevent the mixture from being oxidized in the ball milling process, the CN1207121C of the invention adds Tween 60 into a wet milling medium; in order to solve the problem of uneven dispersion of components such as a grain growth inhibitor, the CN101768678A of the invention adopts a method of adding a powder dispersing agent consisting of Tween 80 and oleic acid, and the CN105734321A of the invention carries out pre-dispersion treatment on the grain growth inhibitor, namely, the inhibitor and Co powder are firstly subjected to ball milling and mixing treatment and then added with superfine WC for material blending and wet milling.

(2) And (3) forming, namely obtaining a green body with a certain shape from the mixed material of the grade by a method of rigid mold uniaxial pressing, cold isostatic pressing or powder extrusion.

(3) The sintering is a technological process for carrying out liquid phase sintering on a hard alloy product green body obtained by powder forming at a temperature higher than a liquid phase appearance temperature (namely, an eutectic temperature, which is about 1280-1330 ℃, and depends on factors such as components, initial powder granularity and the like, for WC-Co alloy, the Co content is high, the carbon content is high, the initial powder is finer, the eutectic temperature is lower) by 20-120 ℃ (depending on the components, the WC grain size requirement of the alloy to be prepared and the like. Vacuum sintering and low pressure hot isostatic pressing sintering (Sinter-HIP) are currently the predominant sintering techniques for the commercial production of cemented carbides.

When the chemical composition is the same, the performance of the hard alloy mainly depends on the microstructure of the hard alloy, and the grain size distribution of WC, the chemical composition of a binding phase, the thickness (mean free path) of the binding phase, the WC adjoining degree and the like are main structural parameters influencing the performance of the alloy. According to Hall-Petch theory, the WC grain size is reduced to be less than micron to nanometer size, and the hardness and the toughness of the hard alloy can be improved simultaneously. In general, the industry defines the average grain size of WC in the range of 0.6-0.8 μm as a Sub-fine (Sub-micron) crystal cemented carbide, the average grain size of WC in the range of 0.2-0.6 μm as an Ultra-fine (Ultra-fine) crystal cemented carbide, and the average grain size of WC below 0.2 μm as a Nano-crystal (Nano-grained) cemented carbide. The preparation of ultra-fine grain (less than or equal to 0.6 μm) or nano-grain (less than or equal to 0.2 μm) hard alloy is the main technical development trend and research and development hot spot in the current hard alloy field. However, the finer the particle size of the WC powder, the more easily it is agglomerated due to its large surface energy and extremely high activity, and the grains grow up during sintering, making it difficult to keep the WC grains fine and uniform. The inhibition of grain growth in the sintering process is a technical problem which must be solved for preparing high-performance ultrafine-grained hard alloy. Adding a certain amount (usually 0.05-0.10 of Cr in weight ratio to Co)₃C₂The metal carbides such as VC, TaC, NbC and the like as Grain Growth Inhibitors (GGI) are effective technical means commonly used for preparing sub-fine Grain and ultra-fine Grain hard alloys at present (Silkana Luyckx 2001). The existing research shows that the type, the addition amount and the uniform dispersion of GGI in the alloySex is a major factor affecting the inhibitory effect. It is generally believed that the finer grain size requires the addition of more GGI (Seegopaul, Mccandish et al 1997), but that the addition of GGI significantly reduces the toughness of the alloy while inhibiting grain growth (Toufar, Schubert et al 2010), and in particular, when the amount of GGI added is too large or is not uniformly dispersed, a brittle third phase precipitates, severely weakening the alloy properties. Therefore, it is the key to prepare high performance ultra-fine grain cemented carbide to add a proper amount of GGI and disperse it as uniformly as possible to obtain grains as fine and uniform as possible in size.

US5918102A discloses an ultra-fine grain cemented carbide for a magnetic tape cutter, which takes Co, Ni or their alloy with the content of 6-15 wt.% as a binding phase, and TiC, TaC, NbC, HfC, ZrC, Mo and less than or equal to 1.0 wt.% are added₂One or more of C, VC and the like are used as grain growth inhibitors, and the superfine hard alloy with the grain size less than or equal to 0.6 mu m (about 0.4 mu m) is obtained by low-pressure hot isostatic pressing sintering at the temperature of about 1400 ℃.

US6511551B2 discloses a method for adding a grain growth inhibitor during the preparation of an ultra-fine grain WC-Co cemented carbide, which introduces the components of the grain growth inhibitor by adding water-soluble salts of V, Ta and Cr elements into W, Co water-soluble salts, and the method can greatly improve the dispersion uniformity of the grain growth inhibitor, thereby more effectively inhibiting the uneven growth of grains and further greatly improving the mechanical properties of the alloy.

US9005329B2 discloses a fine grain WC-Co cemented carbide and a method for manufacturing the same, wherein the content of Co + Cr is 3-15 wt.%, the Cr/Co weight ratio is 0.05-0.15, and simultaneously Ti, V, Zr, Ta or Nb, etc., which are extremely small in amount (ppm level), are added singly or compositely as grain growth inhibitors.

CN1207416C discloses a preparation method of ultra-fine grain cemented carbide: comprises the working procedures of material preparation, wet grinding, drying, press forming and sintering, wherein WC powder with HCP value of 38.5-41 kA/m and Co powder with the weight percentage of 11-13 percent are selected during material preparation, and VC with the weight percentage of 0.3-0.5 percent and Cr with the weight percentage of 0.8-1.0 percent are added₃C₂When the ingredients are calculated, the carbon balance value is (+ 0.18- + 0.21)%, and the average crystal of the hard alloy WCThe particle size is 0.30-0.35 μm.

CN1207417C discloses a method for preparing ultra-fine grain cemented carbide for manufacturing micro-drill, which comprises the working procedures of material mixing, wet grinding, drying, press forming and sintering, wherein, WC powder with HCP value of (38-41) kA/m and Co powder with the weight percentage of 7.8-8.2% are selected during the material mixing, and VC with the weight percentage of 0.28-0.30%, TaC with the weight percentage of 0.31-0.35% and Cr with the weight percentage of 0.31-0.35% are added₃C₂VC, TaC and Cr₃C₂The total weight percentage of the alloy is 0.95-1.0%, when the ingredients are calculated, the carbon balance value is (+ 0.26-0.28)%, the average grain size of WC in the hard alloy reaches (0.3-0.4) mu m, the hardness HV3 reaches 1900-1980, and the minimum axial diameter of a PCB micro drill manufactured by the superfine hard alloy reaches 0.3 mm.

CN1207418C discloses a method for preparing ultra-fine grain cemented carbide, which comprises the working procedures of burdening, wet grinding, drying, press forming and sintering, wherein WC powder with HCP value of (26.5-28.5) kA/m and Co powder with weight percentage of 5.7-6.2% are selected during burdening, VC with weight percentage of 0.18-0.22% is added, during burdening calculation, the carbon balance value is (+ 0.18- + 0.20)%, and the average grain size of WC in the cemented carbide is (0.4-0.6) mu m.

CN1544675A discloses an ultra-fine grain hard alloy and a preparation method thereof: the ingredients are selected from superfine WC powder with Fisher's particle size of 0.3-0.7 μm, Co powder (5.5-7.0) wt%, VC powder (0.2-0.5) wt% and Cr powder (0.2-0.8) wt%₃C₂Powder of Cr₃C₂VC is 2.2 to 1.1; ball-milling for 80-90 hours under the conditions that the ball-material ratio is 4.5-5.5: 1 and the liquid-solid ratio is 500 ml/kg; sintering at 1380-1410 ℃ under 5-10MPa to finally obtain the superfine crystal hard alloy, wherein the average grain size of WC of the superfine crystal hard alloy is (0.2-0.4) mu m, the bending strength is 3000-3800 MPa, and the hardness HV30 is 2000-2200.

EP1803830B1 discloses a cemented carbide with a grain size of 0.3 μm or less and a binder phase content of 5.5 to 15 wt.%, with addition of 0.005 to 0.06 wt.% (preferably 0.01 to 0.04 wt.%) Ti and Cr (the ratio of the amount of Cr to the binder phase content is 0.04 to 0.2) as grain growth inhibitors, without addition of Ta.

CN101629263B discloses an ultra-fine grain hard alloy, which comprises 8.0-9.0 wt% of Co, 0.5-1.0 wt% of TaC and the balance of 0.3-2.0 wt% of Cr₃C₂The WC of (1). 3-6 of ball material: 1, using alcohol as a wet grinding medium, carrying out wet grinding for 60-80 hours under the condition that the liquid-solid ratio is 300-500 ml/kg, and sintering at 1390-1425 ℃ and 3-8 MPa for 40-100 minutes under low pressure to obtain the ultrafine grain hard alloy with the WC average grain size of 0.2-0.4 mu m, the transverse rupture strength of more than or equal to 2800MPa, the hardness of more than or equal to 92.5HRa and the coercive force of 26-35 kA/m.

CN101665881A discloses a method for preparing ultra-fine grain hard alloy for PCB tool, which comprises the working procedures of material mixing, wet grinding, drying, press forming and low-pressure sintering, and is characterized in that WC-Co composite powder with the sub-grain size of 50-100 nm is selected as raw material during the material mixing, then Co powder is mixed or not mixed to reach the Co content of 5-10% by weight, and Cr with the weight percentage of 0.1-1.0% is also mixed₃C₂VC in 0.1-1.0 wt%, Co powder and Cr₃C₂The FSSS granularity of the powder and the VC powder is less than 1.5 mu m, and the carbon balance value calculated by the ingredients is between +0.10 and +0.30 percent. Wet grinding the materials for 60-100 hours according to a ball material ratio of 5-8: 1. The low-pressure sintering temperature is 1380-1420 ℃, and the pressure is 7.5-10 MPa.

CN103042257A discloses a micro drill for PCB processing and a preparation method thereof: selecting 95-86 parts of WC powder with a Fisher particle size of 0.3-0.8 mu m, 4-8 parts of Co powder with a Fisher particle size of 0.6-1.0 mu m, 0.1-0.5 part of VC powder, and Cr₃C₂0.1-1.2 parts of powder, 0.04-1.6 parts of Ti powder, 0.01-0.03 part of oleic acid and 1-3 parts of polyethylene glycol are mixed, stirred, ground, filtered by a 20-200 mesh sieve and dried at 50-250 ℃ to form hard alloy mixed particles, the hard alloy mixed particles are molded by die pressing or extrusion or injection molding, the sintering temperature during molding is 1360-1450 ℃, the Ar pressure is 8-10 MPa, the heat preservation time is 30-150 min, and finally the hard alloy mixed particles are processed to form the micro drill for the PCB.

CN103627942A discloses a method for preparing high-performance WC-Co nanocrystalline hard alloy, which is prepared from nano WC powder, superfine Co powder, superfine VC powder and superfine Cr powder₃C₂The powder is used as raw material powder, and the WC-Co nanocrystalline hard alloy is prepared by utilizing an alcohol wet milling process and through vacuum sintering and hot isostatic pressing sintering.

CN104451217A discloses a preparation method of ultra-fine grain hard alloy, which comprises the working procedures of material preparation, ball milling, press forming and sintering; the material is prepared from superfine WC with a Fisher particle size of 0.4-0.6 mu m, Co powder with a Fisher particle size of 6-10.5 wt% and a Fisher particle size of 1.0-1.5 mu m, wherein the superfine WC contains 0.16-0.28 wt% of V and 0.42-0.58 wt% of Cr; the ratio of Cr to V is 2.6-2.0; controlling the carbon balance value to be + 0.06- + 0.11%; the sintering adopts vacuum sintering and positive pressure sintering in a fast cooling furnace. The average value of the grain size of the WC crystal grain of the superfine crystal hard alloy prepared by the method of the invention by a line cutting method is 0.42-0.49 mu m.

CN103639406A discloses a preparation method of a hard alloy bar for a PCB milling cutter, which selects 4-8% of Co powder with Fisher granularity of 0.5-0.8 mu m, 0.2-0.5% of VC powder, and Cr according to the total weight percentage₃C₂0.3-1.0% of powder, 0.005-0.01% of TiN powder and the balance of WC powder with the Fisher size of 0.3-0.8 mu m.

CN103667757A discloses a preparation method of a hard alloy bar for PCB micro-drilling, which comprises the following steps: selecting 4-8% of Co powder with the grain size of 0.6-1.0 mu m, 0.1-0.5% of VC powder and Cr according to the total weight percentage₃C₂0.1-1.2% of powder, 0.005-0.01% of TiN powder, Y₂O₃0.01-0.06% of powder, and the balance of WC powder with the FSSS granularity of 0.3-0.8 μm.

CN104087790A discloses an adding method of grain growth inhibitor in the preparation of ultra-fine grain hard alloy, which comprises designing the components of Co-based alloy powder; and weighing corresponding tungsten carbide powder and Co-based alloy powder according to a formula ratio, mixing, adding a wet grinding medium, and carrying out wet grinding to ensure that the Co-based alloy containing the grain growth inhibitor element uniformly coats the particle surface of the tungsten carbide powder. In addition, an ultra-fine grain hard alloy with grain size below submicron (including but not limited to nano-scale range) is disclosed, the carbonization temperature of the ultra-fine grain hard alloy is not limited by Co, WC is completely carbonized, so that the grain is more complete and the defects are few.

CN104480334A discloses a method for preparing nanocrystalline hard alloy, which takes WC powder with FSSS granularity smaller than 0.2 μm, Co powder with FSSS granularity smaller than 0.8 μm as raw materials, and vanadium carbide and chromium carbide as inhibitors, and prepares nanocrystalline hard alloy with grain size smaller than 0.4 μm through the working procedures of material preparation, ball milling, spray drying, extrusion forming and pressure sintering.

CN106513670A discloses a sintering method of ultra-fine grain hard alloy, which comprises the steps of preparing WC powder, binding phase powder, a forming agent and an inhibitor into pressed compact; removing the forming agent from the green compact in a non-oxidizing atmosphere and sintering the green compact in two steps: in the first step, the temperature is increased to 1450-1500 ℃ under the vacuum condition at the temperature increase rate of 10-20 ℃/min, and then the temperature is kept; secondly, cooling to 1390-1410 ℃ at a cooling rate of 10-15 ℃/min, and carrying out heat preservation sintering in an inert atmosphere; and then cooled.

CN103801746A discloses an ultra-fine grain hard alloy coating blade suitable for a numerical control machining center, which comprises (by weight percent) tantalum carbide 0.5-2%, chromium carbide 0.5-2%, niobium carbide 0-0.5%, cobalt 6-10%, and the balance of ultra-fine tungsten carbide with the particle size of FSSS 0.3-1.2 μm.

(Stanciu, vitamin et al, Optimization of the particle growth inhibitor inhibited carbides, Europm2017) Co + GGI composite powder prepared by MA method, the dispersion uniformity of GGI was improved by adding Co + GGI composite powder.

Disclosure of Invention

The invention aims to provide a preparation method of high-performance ultrafine grain WC-Co hard alloy, which comprises the following steps:

step 1: selecting a commercially available FSSS particle size

About the average grain size of the target alloy

1-2 times of the total carbon content, 6.14% -6.20% (preferably 6.14%; E;)6.18%) WC powder, fine-grained Co powder, fine-grained Cr₃C₂VC, TiC or Ti (C, N) powder (with FSSS particle size less than or equal to 2.0 μm, preferably less than or equal to 1.5 μm) as raw material;

step 2: firstly, putting the powder raw materials into a ball mill according to a set proportion for wet milling and mixing, simultaneously adding paraffin wax accounting for 2-3 percent of the weight of the materials, taking hard alloy balls with the diameter of 4-6 mm as a grinding body, wherein the ball-material ratio is (5-10): 1, and the ball milling rotating speed is critical rotating speed ((5-10))

D is the inner diameter of the grinding cylinder) of 60-75 percent, and the mixture is obtained by ball milling for 64-76 hours, discharging and drying (spray granulation).

The purpose of this step is to fully crush and mix the raw materials WC and GGI powder, eliminate coarse particles and obtain the initial powder with the desired particle size. The ball milling for a longer time is favorable for fully and uniformly mixing all the components, but the time is not longer, and the longer time can cause the widening of the powder particle size distribution (the deterioration of the particle size uniformity).

And step 3: and forming a green product of the required product according to a conventional powder metallurgy process.

And 4, step 4: and (3) carrying out low-pressure hot isostatic pressing sintering on the green body obtained in the step (3), wherein the process procedure of the low-pressure hot isostatic pressing sintering has the following characteristics:

(1) final sintering temperature T_S1400 ℃ to 1450 ℃ (determined by binder phase content and other factors, generally eutectic temperature T_EAbove 20 deg.C to 120 deg.C.

(2) Heating to 500-550 deg.c by conventional process to eliminate forming agent (paraffin, etc.).

(3) The time of the solid phase sintering stage is controlled for 8-12 hours (determined according to the volume size of the green compact) within 500 ℃→ 1320 ℃ (preferably 550 ℃→ 1300 ℃).

The purpose of this step is to recover the lattice distortion caused by ball milling, fully reduce and remove oxygen, and reduce the activity of the powder.

(4) The temperature rises to the final sintering temperature T_SKeeping the temperature for 5 to 10 minutes, and filling inert gas (Ar andor N)₂) Pressurizing to 1-3 MPa and maintaining the pressure for 5-10 minFilling inert gas into the bell, pressurizing to 3-6MPa, and simultaneously cooling to T_SAfter the temperature is between 20 and 50 ℃, the mixture is subjected to heat preservation and pressure maintaining sintering for 5 to 10 minutes again, inert gas is continuously filled into the mixture to be pressurized to 5 to 10MPa, and the temperature is continuously reduced to T_SAnd after the temperature is between 40 and 90 ℃, carrying out heat preservation and pressure maintaining sintering for the third time for 20 to 30 minutes, and then cutting off the power.

The further improvement is that: after the temperature is raised to the final sintering temperature TS 1420-1450 ℃, the temperature is kept for 10 minutes, inert gas is filled in to pressurize to 3-6MPa, then the pressure is kept for 10 minutes, the inert gas is filled in to pressurize to 6-9MPa, and the temperature is reduced to 1390-1400 ℃ at the same time, the temperature is kept and the pressure is kept again to sinter for 20 minutes, and then the power is cut off.

Comprehensive analysis of the prior known technology shows that the technical key point of preparing the ultrafine grain WC-Co hard alloy is to adopt a powder raw material with fine grain size, add a proper amount of GGI, adopt a proper ball milling process to fully crush WC powder particles to obtain a mixture with fine and uniform grain size and uniformly mixed with each component powder, and finally obtain a two-phase tissue alloy with uniform size, fineness, perfect development and less sub-grain defects, wherein WC grains are uniformly distributed in a binding phase through a proper sintering process.

A large number of researches show that VC and Cr are added in small amount (the weight ratio of VC to Co is usually 0.05-0.15)₃C₂、Mo₂C. One or more carbides of transition metals such as TaC, NbC, TiC, ZrC, HfC and the like can effectively inhibit WC grains from growing unevenly during the sintering of WC-Co hard alloy, and the method is a general technology for industrially producing the sub-fine grain or ultra-fine grain hard alloy with the WC average grain size less than or equal to 1.0 mu m at present. The existing research is generally considered that: the sequence of the inhibition effect when the grain growth inhibitor is added independently is as follows: VC is greater than Mo₂C＞Cr₃C₂＞NbC\TaC＞TiC＞ZrC\HfC；VC、Mo₂C. The addition of TiC can seriously reduce the toughness of the alloy, and Cr is added₃C₂TaC, NbC have minimal effect on toughness of the alloy. Therefore, the current commercial sub-fine grain and ultra-fine grain hard alloy is generally prepared by adding Cr independently₃C₂Or combined addition of Cr₃C₂+VC，Cr₃C₂+ TaC/NbC. The inventor finds in practice that the technical scheme is good in effect when the hard alloy with the grain size of more than 0.4 mu m is prepared, but when the alloy with the grain size of less than 0.4 mu m is prepared, various problems such as coarse grains, uneven structure, high porosity, low strength and the like are easy to occur. This is probably because finer starting powders are required to produce finer grain sizes, and finer starting powders require finer raw powders or higher intensity milling, both of which result in higher oxygen content, more severe processing distortion, higher powder activity, and further increased grain growth tendency during subsequent sintering; in order to control the growth of crystal grains, the final sintering temperature needs to be reduced, the sintering time is shortened, the ultra-fine grain hard alloy can only be sintered at a low temperature which is only slightly higher than the eutectic temperature by 10-30 ℃, the finer the grain size requirement of the alloy to be prepared is, the lower the allowable sintering temperature is, the nanocrystalline or near nanocrystalline hard alloy is prepared, even solid phase sintering needs to be adopted, the too low sintering temperature is not beneficial to the compactness of the alloy, particularly low-bonding phase alloy, so that the micro-porosity residue is caused, the full development of WC crystal grains is not beneficial, the sub-crystal defect residue is caused, and the alloy performance is reduced; in addition, in order to fully inhibit the grain growth during sintering, more GGI needs to be added, which causes the problems that the segregation and aggregation of GGI are more likely to occur and a harmful third phase is generated. In order to overcome these technical difficulties, a plurality of technical solutions are listed in the aforementioned background art. The inventor finds in long-term experimental research that the problems can be solved more economically and effectively by optimized component proportion and improved sintering process, and puts forward the invention.

It is known that the main mechanism of WC grain growth during the sintering of WC-Co cemented carbide is dissolution-precipitation mechanism, i.e. fine WC grains are dissolved in the binder phase and then precipitated on the surface of coarser WC grains, resulting in the disappearance of fine WC and the growth of coarse WC. It is thus clear that inhibition of this process (inhibition of dissolution or inhibition of precipitation, or both) is effective in inhibiting the growth of WC grains, which is the principle of inhibition of grain growth by GGI as described above. It is generally considered that Cr₃C₂The main inhibition mechanism of (2) is to reduce WC in the liquid phaseSolubility; VC is mainly deposited on the surface of WC to reduce the surface energy of WC crystal grains, prevent WC crystal grains from growing together, and prevent dissolved WC from being separated out on the surface of larger WC crystal grains to play a role. Thus, when Cr is present₃C₂When the addition amount of the Cr reaches the solid solubility of Cr in a Co phase at the final sintering temperature, the inhibition effect reaches the maximum; similarly, when the amount of VC added is sufficient to fill the WC grain boundaries, the inhibition effect is maximized. Therefore, an appropriate amount of Cr is added₃C₂The alloy performance is basically harmless, and the alloy has the beneficial effects of alloy strengthening and solid solution strengthening; VC is mainly segregated at grain boundaries, so that the VC is harmful to the toughness of the alloy. But no matter whether it is Cr₃C₂And VC or VC with excessive addition can precipitate a third phase with harmful alloy performance. Based on the principle, Cr is added in a composite way₃C₂+ VC becomes the most common solution used.

The inventor finds that for WC-Co hard alloy with Co content of 3-15 wt%, Cr is present₃C₂The addition amount of (B) is 0.3-2.0% (weight percentage), namely the weight ratio of Cr to Co is about 0.08-0.12; the addition amount of VC is 0.15-0.35% (weight percentage), namely the ratio of VC to WC is about 0.002-0.004, and the alloy has the best comprehensive performance. Cr (chromium) component₃C₂If the addition amount is too small, the solid solubility of W in the binder phase cannot be sufficiently reduced, and if the addition amount is too large, a third phase containing Cr is precipitated; the addition amount of VC is too low to sufficiently cover the surface of WC particles, and too large an amount causes VC segregation and severely weakens grain boundaries.

The inventors of the present invention have surprisingly found in experiments that Cr is added to the above-mentioned alloy₃C₂On the basis of VC, 0.04 to 0.20 weight percent of TiC is added, namely the weight ratio of TiC to WC is about 0.0005 to 0.002, so that the non-uniform growth of WC crystal grains can be further inhibited, and the alloy hardness can be obviously improved. This effect is probably caused because, although TiC alone has the worst inhibitory effect among GGIs, TiC and WC undergo a solid-solution reaction in the solid-phase sintering stage (800 ℃ C. to 1300 ℃ C.), a (W, Ti) C film is formed on the WC crystal grain surface, and the dissolution-precipitation of WC in the Co phase can be effectively inhibitedAnd (6) taking out the process. The larger the WC content, the finer the grains (i.e. the larger the number of WC grains), the more TiC needs to be added, but the too much TiC is added, which results in too thick (W, Ti) C film and the toughness of the alloy is reduced. Meanwhile, TiC has higher hardness compared with WC, so that the alloy hardness can be obviously improved. In addition, the alloy containing TiC can bear higher final sintering temperature, so that the alloy can be sintered at higher temperature, the development of WC crystal grains is improved, and the intragranular defects are reduced.

Further experiments of the inventor also find that Co is replaced by trace Ni and/or Fe, the total addition amount of Ni and/or Fe accounts for no more than 15% (preferably no more than 10%) of the total amount of the Co + Ni/Fe binding phase, grains can be further refined, the allowable sintering temperature is increased, and the alloy strength and hardness are improved, which is probably because the addition of Ni and/or Fe generates an alloy strengthening effect on the Co phase, and simultaneously, the solid solubility of elements such as W, Cr, V, Ti, C and the like in the binding phase is reduced, and the eutectic temperature is improved.

The inventor of the invention finds that the conventional low-pressure hot isostatic pressing sintering process is adopted for the WC-Co alloy with low binder phase content, namely the final sintering temperature (T)_S) And (3) filling gas, pressurizing to 3-10 MPa, and sintering for 30-60 minutes under the condition of heat preservation and pressure maintaining, so that the problems of easy residue of A-type and B-type microscopic pores or more coarse crystals and the like exist. This is because the low binder phase content alloy has a small amount of liquid phase and is difficult to densify, and in particular, the finer the grain size is, the higher the capillary force is, and a higher pressure is required to sufficiently densify the alloy. However, increasing the pressure, i.e. equivalently increasing the temperature, will increase the dissolution of WC in the liquid phase, and more WC will be precipitated during the cooling process, promoting grain growth. To overcome this problem, the final sintering temperature T can be lowered_SThe dissolving amount of WC in the liquid phase and/or the amount of WC precipitated in the cooling process are minimized by methods of shortening the sintering time, rapidly cooling and the like. However, lowering the temperature and shortening the sintering time are not favorable for alloy densification, and the rapid cooling requires a special sintering furnace, so that the method is not an optimal technical scheme. The experiment of the invention finds that the solid phase sintering stage time is prolonged, the dissolving amount of GGI in the liquid phase is increased, the WC dissolution is inhibited, and the special high-temperature high-pressure sintering process is adopted to effectively realize the purposeIn order to overcome this problem, it is necessary to,

the medium-high temperature and high-pressure sintering process is characterized in that 2-3 times of heat preservation and pressure maintaining sintering is adopted, lower pressure is adopted at the highest temperature, higher pressure is adopted at the lower temperature, and the temperature is reduced while cold air is supplemented for pressurization, so that the temperature is reduced, namely, the temperature is accelerated, and the precipitation is reduced; but also saves energy consumption, and can also adopt high temperature and high pressure to ensure the alloy to be fully compact.

The alloy prepared by the method adopts a microscopic image line cutting method to quantitatively measure the WC average grain size according to ISO 4499

Testing the bending strength TRS according to the ISO3327 standard; vickers hardness HV30 as measured in GB/T7997 (or ISO 3878); determination of the fracture toughness K according to JB/T12616 (or ISO28079)_ICThe value is obtained. The invention adopts the crack resistance index Rc ═ HV30 xK_ICTo characterize the ability of the alloy to resist cracking and crack propagation. The results show that the ultra-fine grain WC-Co hard alloy has the average grain size of WC

Is (0.2-0.6) mu m, the grain size deviation coefficient K of WC grains is less than or equal to 0.6, and the grain size is more than 3 times

The percentage of WC crystal grains is not more than 5 percent, and the granularity is more than 5 times

The percentage of WC crystal grains is not more than 1 percent, the WC crystal grains have excellent comprehensive mechanical properties, and the crack resistance index value Rc of the WC crystal grains is generally more than 16.5 multiplied by 10³(MPa)².m^1/2. The end mill and the drill made of the ultra-fine grain hard alloy have excellent use performance in high-speed or high-efficiency cutting processing of difficult-to-process materials such as stainless steel, titanium alloy, pcb, acrylic, glass fiber, hardwood, resin, CFRP and the like.

Drawings

FIG. 1 is a 1500-fold metallographic photograph of the microstructure of example 2C of the present invention

FIG. 2 is an SEM photograph at 8000 magnification of the microstructure of example 2C of the present invention

FIG. 3 is a 1500-fold metallographic photograph of comparative example 2a prepared in the background art (sintering temperature 1420 ℃ C.) showing the presence of coarse WC grains (large white particles).

FIG. 4 is a 1500-fold metallographic photograph of comparative example 2b prepared in the background art (sintering temperature 1420 ℃ C.), showing the apparent presence of coarse WC grains (large white particles).

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

Example 1:

the weight percentage of Co is 3.4%, and the average grain size is prepared

The superfine crystal WC-Co hard alloy.

Step 1: selecting FSSS particle size

About 0.73 μm (about the average grain size of the target alloy)

2 times of that of the powder), WC powder with a total carbon content of 6.20%, fine-grained nearly spherical Co powder and fine-grained GGI powder (Cr)₃C₂VC and TiC) (the FSSS particle size is 1.76 mu m, 1.85 mu m, 1.33 mu m and 1.42 mu m in sequence) as raw materials;

step 2: putting raw material powder raw materials into a ball mill according to the proportion of table 1-1 for wet milling and mixing, simultaneously adding paraffin wax accounting for 3.0 percent of the weight of the raw materials for wet milling and mixing, taking hard alloy balls with the diameter of 6mm as a grinding body, taking the ball-material ratio as 5:1, the ball milling rotation speed as 75 percent of the critical rotation speed, discharging, drying and carrying out spray granulation after ball milling for 64 hours to obtain a mixture.

And step 3: obtained by single-shaft pressing and forming according to a common rigid mold

And (5) compacting.

And 4, step 4: and (4) respectively carrying out low-pressure hot isostatic pressing sintering on the pressed compact obtained in the step (3) according to the following 3 processes.

And (3) sintering process A:

and (3) carrying out a conventional low-pressure hot isostatic pressing sintering process, wherein the final sintering temperature TS is 1450 ℃, the pressure is 10MPa, and the high-temperature high-pressure sintering is carried out for 40 minutes.

And (3) sintering process B:

(1) final sintering temperature T_SIt was 1450 ℃.

(2) Heating to 500-550 deg.c by conventional process to eliminate forming agent (paraffin, etc.).

(3) The time for the solid phase sintering stage was 10 hours at 550 ℃→ 1300 ℃.

(4) Heating to 1450 deg.C, keeping the temperature for 10 min, and filling inert gas (Ar and/or N)₂) Pressurizing to 6MPa, maintaining the pressure for 10 minutes, filling inert gas, pressurizing to 10MPa, simultaneously cooling to 1400 ℃, performing heat preservation, pressure maintaining and sintering for 20 minutes, cutting off the power, cooling and discharging.

And (3) a sintering process C:

(1) final sintering temperature T_SIt was 1450 ℃.

(2) Heating to 500-550 deg.c by conventional process to eliminate forming agent (paraffin, etc.).

(3) The time for the solid phase sintering stage was 10 hours at 550 ℃→ 1300 ℃.

(4) Heating to 1450 deg.C, keeping the temperature for 10 min, and filling inert gas (Ar and/or N)₂) Pressurizing to 3MPa, maintaining the pressure for 10 minutes, filling inert gas, pressurizing to 6MPa, simultaneously cooling to 1400 ℃, performing heat preservation and pressure preservation sintering again for 10 minutes, continuously filling inert gas, pressurizing to 10MPa, simultaneously continuously cooling to 1360 ℃, performing heat preservation and pressure preservation sintering for the third time for 20 minutes, then cutting off the power, cooling and discharging.

And 5: the size obtained in step 4 is about

Preparing alloy sample according to relevant standard, testing its hardness and bending strength, and using metallographic microscope or scanningThe microscopic structure was analyzed by observing the scanning electrode, and the results are shown in tables 1-2.

TABLE 1-1

Test No	Co	Cr3C2	VC	TiC	Cr:Co	VC:WC	TiC:WC
								1A	3.4	0.30	0.35	0.05	0.08	0.0036	0.00052
1B	3.4	0.40	0.25	0.05	0.10	0.0026	0.0005
								1C	3.4	0.45	0.15	0.15	0.12	0.0016	0.0016
Comparative example 1a	3.4	0.50	0.25	—	—	—	—
								Comparative example 1b	3.4	0.35	0.35	—	—	—	—

Note: comparative examples 1a, 1b preparation of samples with FSSS particle size

About 0.78 μm (about the average grain size of the target alloy)

2 times of the total carbon content), using WC powder with 6.17 percent of total carbon as a raw material, and adopting a conventional mixture preparation process, namely putting the selected raw materials into a ball mill according to the mixture ratio of table 1-1, simultaneously adding paraffin wax accounting for 3.0 percent of the weight of the materials for wet milling and mixing, using a hard alloy ball with the diameter of 6mm as a grinding body, wherein the ball-material ratio is 5:1, the ball milling rotating speed is about 70 percent of the critical rotating speed, discharging, drying and spraying for granulation after ball milling for 72 hours, and obtaining a mixture.

Tables 1 to 2

Note 1: the bending strength test adopts ISO C test samples, and the statistical value of 25 test samples is tested.

Note 2: Rc-HV 30 XK_ICAnd is used for characterizing the crack resistance of the alloy.

Example 2:

the weight percentage of the prepared Co is 6.3 percent, and the average grain size

The superfine crystal WC-Co hard alloy.

Step 1: selecting FSSS particle size

About 0.58 μm (about the average grain size of the target alloy)

1.7 times of) of WC powder having a total carbon of 6.18%, fine-grained nearly spherical Co powder and fine-grained GGI powder (Cr)₃C₂VC, TiC or Ti (C, N)) (the FSSS particle size is 1.76 mu m, 1.77 mu m, 1.53 mu m, 1.18 mu m and 0.86 mu m in sequence) as raw materials;

step 2: putting raw material powder raw materials into a ball mill according to the proportion of table 2-1 for wet milling and mixing, simultaneously adding paraffin wax accounting for 3.0 percent of the weight of the raw materials for wet milling and mixing, taking hard alloy balls with the diameter of 6mm as a grinding body, wherein the ball-material ratio is 5.5:1, the ball milling rotating speed is 70 percent of the critical rotating speed, discharging and drying after ball milling for 68 hours to obtain a mixture.

And step 3: obtained by powder metallurgy extrusion forming

And (5) compacting.

And 4, step 4: and (4) respectively carrying out low-pressure hot isostatic pressing sintering on the pressed compact obtained in the step (3) according to the following three processes.

And (3) sintering process A:

conventional low pressure hot isostatic pressing sintering process, wherein the final sintering temperature T_SAnd (3) sintering at 1400 deg.C (or 1420 deg.C) and 6MPa for 40 min.

And (3) sintering process B:

(1) final sintering temperature T_SIs 1420 ℃.

(2) Heating to 500-550 deg.c by conventional process to eliminate forming agent (paraffin, etc.).

(3) The time for the solid phase sintering stage was 8 hours at 550 ℃→ 1300 ℃.

(4) Heating to 1420 deg.C, keeping the temperature for 10 min, and filling inert gas (Ar and/or N)₂) Pressurizing to 3MPa (pressurizing time is 10 minutes), maintaining the pressure for 10 minutes, filling inert gas, pressurizing to 6MPa, simultaneously cooling to 1390 ℃, performing heat preservation and pressure maintaining sintering again for 20 minutes, then cutting off the power, cooling and discharging.

And (3) a sintering process C:

(1) final sintering temperature T_SIs 1420 ℃.

(2) Heating to 500-550 deg.c by conventional process to eliminate forming agent (paraffin, etc.).

(3) The time for the solid phase sintering stage was 8 hours at 550 ℃→ 1300 ℃.

(4) Heating to 1420 deg.C, keeping the temperature for 10 min, and filling inert gas (Ar and/or N)₂) Pressurizing to 3MPa, maintaining the pressure for 10 minutes, filling inert gas, pressurizing to 6MPa, and simultaneously cooling to T_SKeeping the temperature and pressure below 1390 ℃, sintering for 10 minutes again, continuously filling inert gas, pressurizing to 9MPa, and simultaneously continuously cooling to T_SAfter 1360 ℃ below, carrying out heat preservation and pressure maintaining for the third timeAfter 20 minutes, the furnace is cut off, cooled and discharged.

And 5: the size obtained in step 4 is about

The alloy samples are prepared according to the relevant standards, the hardness, the bending strength and other properties of the alloy samples are tested, and the microstructure of the alloy samples is observed and analyzed by adopting a metallographic microscope or a scanning electrode, and the results are shown in tables 2-2.

TABLE 2-1

Note: the WC raw materials of comparative examples 2a and 2b were selected to have FSSS particle sizes

About 0.56 μm (about the average grain size of the target alloy)

1.6 times of the total carbon content of the raw materials), 6.14 percent of WC powder, and adopting a conventional mixture preparation process, namely putting the selected raw materials into a ball mill according to the mixture ratio of table 2-1, simultaneously adding 3.0 percent of paraffin wax of the weight of the materials for wet milling and mixing, adopting hard alloy balls with the diameter of 6mm as grinding bodies, wherein the ball-material ratio is 5:1, the ball milling rotating speed is about 70 percent of the critical rotating speed, discharging, drying and spraying for granulation after ball milling for 76 hours to obtain a mixture.

Tables 2 to 2

Note 1: the bending strength test adopts ISO C test samples, and the statistical value of 25 test samples is tested.

Note 2: RcHV30 XK_ICFor characterizing the cracking resistance of the alloyCapability.

Example 3:

the weight percentage of Co is 14.6%, and the average grain size is prepared

The superfine crystal WC-Co hard alloy.

Step 1: selecting FSSS particle size

About 0.54 μm (about the average grain size of the target alloy)

1.1 times of) of WC powder having a total carbon of 6.14%, fine-grained nearly spherical Co powder and fine-grained GGI powder (Cr)₃C₂VC and TiC) (the FSSS particle size is 1.98 mu m, 1.85 mu m, 1.33 mu m and 1.35 mu m in sequence) as raw materials;

step 2: putting the powder raw materials into a ball mill according to the proportion of table 3-1 for wet milling and mixing, simultaneously adding paraffin wax accounting for 2.2 percent of the weight of the materials for wet milling and mixing, taking hard alloy balls with the diameter of 4mm as grinding bodies, wherein the ball-material ratio is 9:1, the ball milling rotating speed is 60 percent of the critical rotating speed, discharging, drying and granulating after ball milling for 76 hours to obtain a mixture.

And step 3: obtained by single-shaft pressing and forming according to a common rigid mold

And (5) compacting.

And 4, step 4: and (4) respectively carrying out low-pressure hot isostatic pressing sintering on the pressed compact obtained in the step (3) according to the following two processes.

And (3) sintering process A:

conventional low pressure hot isostatic pressing sintering process, wherein the final sintering temperature T_SAnd (3) sintering at 1380 ℃ and 5MPa for 40 minutes at high temperature and high pressure.

And (3) sintering process B:

(1) final sintering temperature T_SIt was 1400 ℃.

(2) Heating to 500-550 deg.c by conventional process to eliminate forming agent (paraffin, etc.).

(3) The time for the solid phase sintering stage was 10 hours at 550 ℃→ 1300 ℃.

(4) Heating to 1400 deg.C, keeping the temperature for 5 min, and filling inert gas (Ar and/or N)₂) Pressurizing to 1MPa, maintaining the pressure for 5 minutes, filling inert gas, pressurizing to 3MPa, and simultaneously cooling to T_SAfter the temperature is 1380 ℃, the mixture is subjected to heat preservation and pressure maintaining sintering for 5 minutes again, inert gas is continuously filled to pressurize to 5MPa, and the temperature is continuously reduced to T_SAnd after the temperature is 1340 ℃, carrying out heat preservation and pressure maintaining sintering for the third time for 30 minutes, and then cutting off the power, reducing the temperature and discharging the product from the furnace.

And 5: the size obtained in step 4 is about

The alloy samples are prepared according to the relevant standards, the hardness, the bending strength and other properties of the alloy samples are tested, and the microstructure of the alloy samples is observed and analyzed by adopting a metallographic microscope or a scanning electrode, and the results are shown in a table 3-2.

TABLE 3-1

Note: preparation of samples of comparative examples 3a, 3b FSSS particle size was selected

About 0.58 μm (about the average grain size of the target alloy)

1.2 times of the total carbon content of the raw materials), 6.11 percent of WC powder, and adopting a conventional mixture preparation process, namely, putting the raw materials into a ball mill according to the mixture ratio of table 3-1, simultaneously adding 2.2 percent of paraffin wax of the weight of the materials for wet milling and mixing, adopting hard alloy balls with the diameter of 6mm as grinding bodies, wherein the ball-to-material ratio is 6:1, the ball milling rotating speed is about 70 percent of the critical rotating speed, discharging, drying and granulating after ball milling is carried out for 72 hours, and obtaining the mixture.

TABLE 3-2

Note 1: the bending strength test adopts ISO C test samples, and the statistical value of 25 test samples is tested.

Note 2: Rc-HV 30 XK_ICAnd is used for characterizing the crack resistance of the alloy.

The embodiments of the specific embodiments described above are merely to illustrate the present invention and are not intended to limit the present invention. Changes and variations may be made to the particular embodiments without departing from the spirit of the invention as set forth in the claims. For example, in the technical scheme of the invention, the Cr, V and Ti can be added into the WC powder raw materials with the required contents of Cr, V and Ti according to the designed proportion before the WC powder raw materials are carbonized, and then, for example, the Ni and the Fe can be added, or the Cr, V and Ti can be added in a Co-Ni or Co-Ni-Fe alloy powder mode according to the requirement. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (3)

1. The preparation method of the high-performance ultrafine grain hard alloy is characterized by comprising the following steps:

step 1: the raw materials are mixed according to the following weight percentage:

WC with weight percentage of 83-96 percent and FSSS granularity

Is the average grain size of the target alloy

1-2 times of the total carbon content, 6.14% -6.20% of the total carbon content;

3 to 15 weight percent of binder phase Co,

Cr₃C₂0.3 to 2.0 percent of the weight percentage content,

VC accounts for 0.15 to 0.35 weight percent,

0.05 to 0.20 weight percent of TiC or Ti (C, N),

above, Cr₃C₂VC, TiC or Ti (C, N), the FSSS particle size is less than or equal to 2.0 μm;

step 2: putting the raw materials into a ball mill for wet grinding and mixing, adding a forming agent accounting for 2-3% of the total weight of the raw materials, and drying to obtain a mixture; the ball milling process comprises the following steps: hard alloy balls with the diameter of 4-6 mm are used as grinding bodies, the ball-material ratio is (5-9): 1, and the ball milling rotating speed is critical rotating speed (C

D is the inner diameter of the grinding cylinder) for 64-76 hours;

and step 3: forming a required product green body according to a conventional powder metallurgy process;

and 4, step 4: and (3) carrying out low-pressure hot isostatic pressing sintering on the green body obtained in the step (3), wherein the low-pressure hot isostatic pressing sintering process comprises the following steps:

(1) removing the forming agent according to a conventional process;

(2) a solid phase sintering stage at 550 ℃→ 1300 ℃ for 8-10 hours;

(3) final sintering temperature T_SIs 1400 ℃ to 1450 ℃; keeping the temperature for 5-10 minutes, filling inert gas, pressurizing to 1-3 MPa, keeping the pressure for 5-10 minutes, filling inert gas, pressurizing to 3-6MPa, and simultaneously cooling to T_SAfter the temperature is between 20 and 50 ℃, the mixture is subjected to heat preservation and pressure maintaining sintering for 5 to 10 minutes again, inert gas is continuously filled into the mixture to be pressurized to 5 to 10MPa, and the temperature is continuously reduced to T_SAnd after the temperature is between 40 and 90 ℃, carrying out heat preservation and pressure maintaining sintering for the third time for 20 to 30 minutes, and then cutting off the power.

2. The method for preparing the ultra-fine grained cemented carbide according to claim 1, wherein in step 1, the content of WC in percentage by weight is 83-93%; the binder phase comprises Co, and Ni or Ni and Fe; the weight percentage content of the binding phase is 6-15%; the total addition of Ni or Ni and Fe accounts for less than or equal to 18 percent of the total amount of the binder phase.

3. A process as claimed in claim 1 or 2The method for preparing the ultra-fine grain cemented carbide of (1), characterized in that the step 4- (3): the temperature rises to the final sintering temperature T_SKeeping the temperature of 1420-1450 ℃ for 10 minutes, filling inert gas, pressurizing to 3-6MPa, keeping the pressure for 10 minutes, filling inert gas, pressurizing to 6-9MPa, cooling to 1390-1400 ℃ at the most, keeping the temperature, keeping the pressure, sintering for 20 minutes again, and then cutting off the power.

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