阅读说明：本技术 一种功能梯度结构硬质合金及其制备方法和应用 (Functionally gradient structure hard alloy and preparation method and application thereof ) 是由 张颢 赵玉玲 徐涛 郭建中 于 2021-07-02 设计创作，主要内容包括：本发明涉及一种功能梯度结构硬质合金及其制备方法和应用。该合金由硬质合金基体的芯部和表层组成,所述表层为梯度层,所述硬质合金基体的芯部和梯度层的组织结构中均不含缺碳相和游离石墨,所述梯度层的厚度大于2毫米,所述梯度层的厚度为2～6毫米,且梯度层内的粘结相含量呈梯度分布。该合金的制备方法采用了低温沉积+高温渗碳的两步渗碳烧结工艺,获得的功能梯度结构硬质合金的综合性能良好,满足了硬质合金尤其是矿用凿岩工具用梯度结构硬质合金的使用需求,其市场价值及应用前景良好。(The invention relates to a functionally gradient structure hard alloy and a preparation method and application thereof. The alloy consists of a core part and a surface layer of a hard alloy matrix, wherein the surface layer is a gradient layer, the core part of the hard alloy matrix and the organization structure of the gradient layer do not contain carbon-deficient phase and free graphite, the thickness of the gradient layer is more than 2 mm, the thickness of the gradient layer is 2-6 mm, and the content of a binding phase in the gradient layer is in gradient distribution. The preparation method of the alloy adopts a two-step carburizing and sintering process of low-temperature deposition and high-temperature carburization, the obtained functional gradient structure hard alloy has good comprehensive performance, the use requirements of the hard alloy, particularly the gradient structure hard alloy for the mining rock drilling tool, are met, and the market value and the application prospect are good.)

1. The utility model provides a function gradient structure carbide, its core and the top layer by the carbide base member are constituteed, the top layer is the gradient layer, all do not contain in the core of carbide base member and the gradient layer and lack carbon phase and free graphite, the thickness of gradient layer is 2 ~ 6 millimeters, and the intraformational binder phase content of gradient is the gradient distribution.

2. The functionally graded structure cemented carbide according to claim 1, wherein the binder phase content in the functionally graded structure cemented carbide gradually increases from below the binder phase content in the cemented carbide matrix from the surface to the inside and then decreases to the binder phase content in the core of the cemented carbide matrix.

3. Functionally graded structural cemented carbide according to claim 1 or 2, characterised in that the binder phase content of the surface of the graded layer is below 85% of the binder phase content in the cemented carbide matrix;

preferably, the content of the binder phase in the hard alloy matrix is 2-20 wt%, and the balance is carbide;

further preferably, the binding phase comprises cobalt, or a mixture of cobalt and at least one of nickel and iron; and/or the carbide comprises tungsten carbide, or a mixture of tungsten carbide and at least one of titanium carbide, tantalum carbide, chromium carbide, niobium carbide and vanadium carbide.

4. The functionally graded structure cemented carbide according to any one of claims 1 to 3, wherein the hardness of the surface layer is higher than that of the core, and the toughness of the core is higher than that of the surface layer; preferably, under the load of 30 kilograms of force, the hardness of the surface layer of the hard alloy is higher than the hardness of the core part by 60-300 Vickers hardness values.

5. A method for producing the functionally graded structural cemented carbide according to any one of claims 1 to 4, comprising: providing hard alloy, and carrying out carburizing heat treatment on the hard alloy in a furnace with a carburizing atmosphere to obtain the hard alloy with a functional gradient structure;

wherein the carburizing heat treatment comprises the following steps:

s1, introducing methane in a carburizing atmosphere into a furnace, and carrying out first heat treatment on the hard alloy to obtain the hard alloy after the first heat treatment;

and S2, introducing inert gas into the furnace for gas replacement, introducing carbon dioxide in a carburizing atmosphere after vacuumizing, and performing second heat treatment on the hard alloy after the first heat treatment to obtain the hard alloy with the functionally graded structure.

6. The method according to claim 5, wherein in step S1, the temperature of the first heat treatment is 900-1150 ℃, the time is 60-180 minutes, and the gas partial pressure is 500-1000 mbar; and/or

In step S2, the temperature of the second heat treatment is 1150-1500 ℃, the time is 60-180 minutes, and the gas partial pressure is 100-500 mbar.

7. A method according to claim 5 or 6, wherein cemented carbide is provided by:

t1, mixing the carbide, the binder phase and the forming agent, wet grinding and drying to obtain mixed powder of the carbide and the binder phase;

and T2, pressing and sintering the mixed powder of the carbide and the binding phase to obtain the hard alloy.

8. The method of claim 7, wherein the mixed powder of carbide-binder phase has a sub-stoichiometric carbon content; and/or

The carbide comprises tungsten carbide, or a mixture of tungsten carbide and at least one of titanium carbide, tantalum carbide, chromium carbide, niobium carbide and vanadium carbide; the binding phase comprises cobalt or a mixture of cobalt and at least one of nickel and iron; the forming agent is selected from any one of polyethylene glycol and paraffin.

9. The method according to claim 7 or 8, characterized in that the sintering and the carburizing heat treatment are performed in the same furnace or in two separate furnaces.

10. Use of a functionally graded structural cemented carbide according to any one of claims 1-4 or prepared by a method according to any one of claims 5-9 in a rock drilling tool and a machining tool for mining.

Technical Field

The invention belongs to the technical field of hard alloy, and particularly relates to a functional gradient structure hard alloy and a preparation method and application thereof.

Background

Functionally graded cemented carbide (Functionally graded carbide) refers to a cemented carbide with a graded composition or structure. The functional gradient structure hard alloy has different properties at different parts due to the special structure or component gradient distribution change, and the alloy integral product has excellent comprehensive mechanical properties. The functionally gradient structure hard alloy can well solve the contradiction between the wear resistance and the toughness in the hard alloy with a uniform structure, thereby improving the comprehensive performance and the service life of the hard alloy.

The hard alloy with the high surface hardness/poor Co functional gradient structure has the advantages of good surface wear resistance and high core toughness, and can be used as one of materials of mining rock drilling tools and machining tools. At present, the research on preparing the hard alloy with the gradient structure by adopting a controlled atmosphere method is the most active. The common method is to prepare a carbon-poor (eta-phase-containing) alloy matrix, and then prepare the gradient structure hard alloy with high surface hardness, good wear resistance and good core impact toughness by adopting a carburizing atmosphere sintering method. However, the gradient structure alloy prepared by the method still has eta phase in the core part, so that the improvement of the overall performance of the alloy is influenced. Another type of disclosed method is to prepare a cemented carbide material having a normal microstructure (with a sub-stoichiometric carbon content prior to sintering) and then perform a carburizing heat treatment (e.g., introducing CH)₄And H₂Mixed gas of (B) produced gradient structure cemented carbide without η phase with hardened surface layer, as in patent CN 101724760B. Although the patent avoids the problem that the eta phase still exists in the core part of the hard alloy with the gradient structure, the patent adopts a one-step carburizing heat treatment method (introduces CH)₄And H₂The mixed gas is subjected to carburizing heat treatment in the temperature range 1260 ℃ + 1300 ℃ where three phases of solid-phase tungsten carbide (WC), liquid-phase cobalt and solid-phase cobalt coexist), the hardness value of the surface layer of the gradient structure hard alloy prepared is lower, and is only higher than the hardness of the inner center of the material by at least 30 Vickers hardness value; meanwhile, the surface layer is very thin and is only larger than 10 microns, so that the use requirement of the hard alloy, particularly the hard alloy with the gradient structure for the mining rock drilling tool, can not be met. Further, the carburizing atmosphere introduced in the above patent is CH₄And H₂Of mixed gases, especially H₂Industrial safety in hard alloy industryThe use of the fertilizer in production should be avoided as much as possible.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a functional gradient structure hard alloy and a preparation method and application thereof. The functionally gradient structure hard alloy has the characteristics of high surface hardness, good wear resistance, high core strength and toughness, and excellent overall mechanical properties. The surface hardness value of the functional gradient structure hard alloy is higher, and the surface thickness is thicker, so that the use requirements of the hard alloy, particularly the gradient structure hard alloy for a mining rock drilling tool, are met.

Therefore, the invention provides a functionally graded cemented carbide, which consists of a core part and a surface layer of a cemented carbide substrate, wherein the surface layer is a graded layer, the core part and the graded layer of the cemented carbide substrate do not contain carbon-deficient phase and free graphite, the thickness of the graded layer is 2-6 mm, and the content of a binding phase in the graded layer is in graded distribution.

Specifically, the microstructure of the core part and the gradient layer of the hard alloy matrix do not contain carbon-deficient phase and free graphite.

In the present invention, the term "carbon-deficient phase" is also referred to as "η phase".

In some embodiments of the invention, the binder phase content in the gradient layer increases from the outside to the inside. Specifically, the content of the binding phase in the gradient layer is gradually increased from the outside to the inside from the lower content than that in the cemented carbide matrix.

In some embodiments of the present invention, the content of the binder phase in the functionally graded cemented carbide gradually increases from the lower content of the binder phase in the cemented carbide matrix to the lower content of the binder phase in the core of the cemented carbide matrix.

In some embodiments of the invention, the binder phase content of the surface of the gradient layer is less than 85% of the binder phase content in the cemented carbide matrix.

In some embodiments of the invention, the binder phase content in the cemented carbide matrix is 2 to 20 wt%, the balance being carbides.

In some preferred embodiments of the invention, the binding phase comprises cobalt, or a mixture of cobalt and at least one of nickel and iron.

In the present invention, when the binder phase comprises a mixture of cobalt and at least one of nickel and iron, that is, at least one of nickel and iron is used in place of part of the cobalt.

In other embodiments of the invention, the carbide comprises tungsten carbide, or a mixture of tungsten carbide and at least one of titanium carbide, tantalum carbide, chromium carbide, niobium carbide, and vanadium carbide.

In the present invention, the term "binder phase content in the cemented carbide substrate" is the average content of binder phase in the cemented carbide substrate, also referred to as "nominal composition average binder phase content of the cemented carbide substrate".

In the invention, the functionally graded cemented carbide has a surface layer with high hardness and a core with high toughness.

In some embodiments of the present invention, the surface layer of the functionally graded cemented carbide has a hardness higher than that of the core, and the toughness of the core is higher than that of the surface layer.

In some preferred embodiments of the present invention, the hardness of the surface layer of the cemented carbide is higher than the hardness of the core by 60 to 300 vickers hardness under a load of 30 kg force.

In a second aspect, the present invention provides a method for preparing the cemented carbide according to the first aspect of the present invention, comprising: providing hard alloy, and carrying out carburizing heat treatment on the hard alloy in a furnace with a carburizing atmosphere to obtain the hard alloy with a functional gradient structure;

wherein the carburizing heat treatment comprises the following steps:

s1, introducing methane in a carburizing atmosphere into a furnace, and carrying out first heat treatment on the hard alloy to obtain the hard alloy after the first heat treatment;

and S2, introducing inert gas into the furnace for gas replacement, introducing carbon dioxide in a carburizing atmosphere after vacuumizing, and performing second heat treatment on the hard alloy after the first heat treatment to obtain the hard alloy with the functionally graded structure.

In the present invention, the above step S1 is referred to as a deposition process, and the above step S2 is referred to as a carburization process.

In some embodiments of the invention, the inert gas may be argon (Ar).

In some embodiments of the present invention, in step S1, the first heat treatment is performed at 900 to 1150 ℃ for 60 to 180 minutes at a gas partial pressure of 500 to 1000 mbar.

In other embodiments of the present invention, in step S2, the second heat treatment temperature is 1150-1500 ℃, the time is 60-180 minutes, and the gas partial pressure is 100-500 mbar.

In some embodiments of the invention, cemented carbide is provided by:

t1, mixing the carbide, the binder phase and the forming agent, wet grinding and drying to obtain mixed powder of the carbide and the binder phase;

and T2, pressing and sintering the mixed powder of the carbide and the binding phase to obtain the hard alloy.

In some embodiments of the invention, the mixed powder of carbide-binder phase has a sub-stoichiometric carbon content. The carbon content is high, so that the mixed powder of the carbide-binder phase does not have eta phase and free graphite at any temperature and time during or after the sintering step or the carburizing heat treatment step.

In other embodiments of the invention, the carbide comprises tungsten carbide, or a mixture of tungsten carbide and at least one of titanium carbide, tantalum carbide, chromium carbide, niobium carbide, and vanadium carbide; the binding phase comprises cobalt or a mixture of cobalt and at least one of nickel and iron; the forming agent is selected from any one of polyethylene glycol (PEG) and paraffin.

In some embodiments of the present invention, the amount of the forming agent may be 1 to 3 wt% of the weight of the raw material.

In some embodiments of the present invention, in step T1, the ball milling medium for wet milling is alcohol, and preferably, the addition amount of the alcohol is 250 to 350 ml/kg.

In other embodiments of the invention, in step T1, the ball-to-material ratio of the wet grinding is 2:1 to 5:1, the rotation speed of the wet grinding is 50 to 100 rpm, and the time of the wet grinding is 15 to 30 hours.

In some embodiments of the invention, the sintering and carburizing heat treatments are performed in the same furnace thermal cycle, and there is no need to remove the sintered alloy blank in the furnace after sintering.

In other embodiments of the invention, the sintering and carburizing heat treatments are performed in two separate furnaces, so that there are two separate thermal cycles.

In some embodiments of the present invention, the sintering temperature may be 1400 to 1500 ℃.

In some embodiments of the present invention, the functionally graded structural cemented carbide comprises in particular the following steps:

(1) mixing carbide (tungsten carbide or a mixture of tungsten carbide and other metal carbides), a binding phase (cobalt powder or a mixture of cobalt powder and nickel/iron powder) and a forming agent, and then performing alcohol ball milling and drying to obtain uniform mixed powder of carbide and the binding phase;

(2) pressing the uniform carbide-binder phase mixed powder to obtain a formed compact;

(3) sintering the formed compact to obtain a hard alloy with a uniform structure (compact), wherein free graphite and a carbon-deficient phase (eta phase) do not exist in the alloy;

(4) carrying out carburizing heat treatment on the hard alloy with the uniform structure in a furnace with a carburizing atmosphere to obtain the hard alloy with the functionally gradient structure; the carburizing heat treatment specifically comprises two main steps, which are respectively: the first step is as follows: deposition, i.e. introduction of a carburizing atmosphere of methane (CH) in a furnace₄) Carrying out heat treatment at the temperature of 900-1150 ℃ for 60-180 minutes, wherein the gas partial pressure is 500-1000 mbar; the second step is that: carburizing, i.e. by introducing argon (Ar) into the furnace at the end of the first step for gas replacementAfter vacuum pumping, carbon dioxide (CO) in carburizing atmosphere is introduced₂) And carrying out heat treatment at the temperature of 1150-1500 ℃ for 60-180 minutes, wherein the gas partial pressure is 100-500 mbar. Then furnace cooling to room temperature.

The functionally graded cemented carbide obtained after the two-step carburization heat treatment has a cobalt-poor (binder phase) surface layer, the cobalt content (binder phase content) of the surface layer is lower than that of the nominal composition value of the cemented carbide substrate, and the functionally graded cemented carbide does not contain free graphite nor eta phase.

In a third aspect, the invention provides a functionally graded cemented carbide according to the first aspect or a functionally graded cemented carbide prepared by the method of the second aspect for use in rock drilling tools and machining tools for mines.

The invention has the beneficial effects that: the preparation method of the functionally gradient structure hard alloy avoids H from the aspect of safe production₂Introduction of carburizing atmosphere CO₂Carburizing atmosphere is CH₄+Ar+CO₂The combination mode is more suitable for industrial safe production, popularization and application in the hard alloy industry; the method adopts a two-step carburizing and sintering process of low-temperature deposition and high-temperature carburization, namely the carburizing and sintering process is divided into two main steps (deposition and carburization), and a carburizing atmosphere CH is selected in the low-temperature deposition process (900-1150 ℃) before a liquid phase point appears₄The carburizing atmosphere is CO in the high-temperature carburizing process₂CO is filled in the temperature range of 1150-1500 DEG C₂Carbon atoms with higher activity can be generated by reaction at high temperature, which is beneficial to the proceeding of the carburization process and the increase of the thickness of the gradient layer. The functionally gradient structure hard alloy prepared by the two-step carburizing method does not contain eta phase and graphite phase, has good comprehensive performance, the hardness of the surface layer with high hardness under the load of 30 kilograms of force is higher than the hardness of the core part by 60-300 Vickers hardness, the thickness of the surface layer is 2-6 millimeters, the service life of the alloy is prolonged by more than 30%, the use requirements of the hard alloy, particularly the gradient structure hard alloy for a mining rock drilling tool, are met, and the functionally gradient structure hard alloy has good market value and application prospect.

Drawings

The invention will be further explained with reference to the drawings.

FIG. 1 is a graph showing a Vickers hardness distribution of a functionally graded cemented carbide obtained in example 1 of the present invention.

Fig. 2 is a cobalt content distribution diagram of the functionally graded cemented carbide obtained in example 1 of the present invention.

Detailed Description

In order that the present invention may be more readily understood, the following detailed description will proceed with reference being made to examples, which are intended to be illustrative only and are not intended to limit the scope of the invention. The starting materials or components used in the present invention may be commercially or conventionally prepared unless otherwise specified.

Example 1: preparation of functionally graded hard alloy spherical teeth with 6% cobalt content in alloy matrix integral material

The method comprises the steps of taking commercially available tungsten carbide (Fsss particle size is about 4.0 mu m) and cobalt powder as raw materials, doping 2 wt% of forming agent PEG, carrying out alcohol ball milling (the adding amount of alcohol is 250ml/kg, the ball-material ratio is 3:1, the rotating speed is 80 r/min; wet milling time is 20h), and drying to obtain uniform tungsten carbide-cobalt mixed powder (the cobalt content is 6 wt%, the balance is tungsten carbide, and the substoichiometric carbon content is 5.75%). Then pressing and sintering at low pressure (the sintering temperature is 1450 ℃) to prepare the hard alloy with a uniform structure, wherein free graphite and eta phase do not exist in the alloy.

And carrying out carburizing heat treatment on the cemented carbide with the uniform structure in a sintering furnace with a carburizing atmosphere, wherein the sintering step and the carburizing heat treatment step in the preparation process of the cemented carbide with the uniform structure are carried out in two separate furnaces. The first step of the carburizing heat treatment is to introduce a carburizing atmosphere of methane (CH)₄) Heat treatment is carried out at 900 deg.C for 100 min at gas partial pressure of 600mba, argon (Ar) is introduced for gas replacement at the end of the first step, and carbon dioxide (CO) is introduced as carburizing atmosphere after evacuation₂) And heat treated at 1200 ℃ for 90 minutes at a gas partial pressure of 300 mbar. The hard alloy after the two-step carburization heat treatment has a cobalt-poor surfaceThe cobalt content of the surface of the gradient layer is lower than 85% of the average cobalt content of the nominal composition of the hard alloy matrix, the hard alloy with the functional gradient structure after final heat treatment does not contain free graphite and eta phase, the Vickers hardness value of the surface layer with high hardness under the load of 30 kilograms of force is higher than the core hardness of the material by 80 Vickers hardness values, and the thickness of the gradient layer of the surface layer is about 2.2 millimeters. The vickers hardness profile of the finally obtained functionally graded cemented carbide is shown in fig. 1, and the cobalt content profile is shown in fig. 2. The properties of the alloy before and after the carburizing heat treatment are shown in table 1. The cemented carbide ball tooth with the functional gradient structure is mainly suitable for high-wind-pressure drill bits for rock drilling.

Table 1: properties of alloy before and after carburizing heat treatment

Example 2: preparation of functionally gradient structure hard alloy spherical tooth with 11.5% cobalt content in alloy matrix integral material

The method comprises the steps of taking commercially available tungsten carbide (Fsss particle size is about 4.0 mu m) and cobalt powder as raw materials, doping 2 wt% of forming agent paraffin, carrying out alcohol ball milling (the adding amount of alcohol is 300ml/kg, the ball-material ratio is 3:1, the rotating speed is 80 r/min; wet milling time is 22h), and drying to obtain uniform tungsten carbide-cobalt mixed powder (the cobalt content is 11.5%, the balance is tungsten carbide, and the substoichiometric carbon content is 5.42%). Then pressing and sintering at low pressure (the sintering temperature is 1450 ℃) to prepare the hard alloy with a uniform structure, wherein free graphite and eta phase do not exist in the alloy.

And carrying out carburizing heat treatment on the cemented carbide with the uniform structure in a sintering furnace with a carburizing atmosphere, wherein the sintering step and the carburizing heat treatment step in the preparation process of the cemented carbide with the uniform structure are carried out in a heat cycle of the same furnace. The carburizing atmosphere introduced in the first step of the carburizing heat treatment is methane (CH)₄) Heat treatment is carried out at 1000 ℃ for 120 minutes with a gas partial pressure of 800mba, in the second step argon (Ar) is introduced at the end of the first step for gas replacement, and the carburizing atmosphere introduced after evacuation is twoCarbon Oxide (CO)₂) And heat-treated at 1300 ℃ for 100 minutes at a gas partial pressure of 400 mbar. The hard alloy after the two-step carburization heat treatment has a gradient layer with a poor cobalt surface layer, the cobalt content of the surface of the gradient layer is lower than 85% of the average cobalt content of the nominal composition of the hard alloy matrix, free graphite and eta phase are not existed in the finally heat-treated functionally gradient structure hard alloy, the Vickers hardness value of the surface layer with high hardness under the load of 30 kilograms of force is higher than the core hardness of the material by 180 Vickers hardness values, and the thickness of the gradient layer of the surface layer is 3.1 millimeters. The properties of the alloy before and after the carburizing heat treatment are shown in table 2. The hard alloy ball tooth with the functional gradient structure is mainly suitable for roller bit used in mines and oil fields.

Table 2: properties of alloy before and after carburizing heat treatment

Example 3: preparation of gradient structure hard alloy spherical tooth with 13% cobalt content in alloy matrix integral material

The method comprises the steps of taking commercially available tungsten carbide (Fsss particle size is about 2.0 mu m) and cobalt powder as raw materials, doping 2 wt% of forming agent PEG, carrying out alcohol ball milling (the adding amount of alcohol is 300ml/kg, the ball-material ratio is 3:1, the rotating speed is 80 r/min; wet milling time is 26h), and drying to obtain uniform tungsten carbide-cobalt mixed powder (the cobalt content is 13%, the balance is tungsten carbide, and the substoichiometric carbon content is 5.34%). Then pressing and sintering at low pressure (the sintering temperature is 1450 ℃) to prepare the hard alloy with a uniform structure, wherein free graphite and eta phase do not exist in the alloy.

And carrying out carburizing heat treatment on the cemented carbide with the uniform structure in a sintering furnace with a carburizing atmosphere, wherein the sintering step and the carburizing heat treatment step in the preparation process of the cemented carbide with the uniform structure are carried out in two separate furnaces. The carburizing atmosphere introduced in the first step of the carburizing heat treatment is methane (CH)₄) Heat treatment is carried out at 1100 deg.C for 160 min under a gas partial pressure of 1000mba, and in the second step, argon (Ar) is introduced at the end of the first step for gas replacement, followed by pumpingCarburizing atmosphere carbon dioxide (CO) introduced after vacuum₂) And heat treated at 1390 ℃ for 150 minutes at a gas partial pressure of 500 mbar. The hard alloy after the two-step carburization heat treatment has a gradient layer with a poor cobalt surface layer, the cobalt content of the surface of the gradient layer is lower than 85% of the nominal cobalt content of the hard alloy, the final heat-treated functionally gradient structure hard alloy does not contain free graphite or eta phase, the Vickers hardness value of the surface layer with high hardness under the load of 30 kilograms of force is higher than the core hardness of the material by 220 Vickers hardness value, and the thickness of the gradient layer of the surface layer is 4.3 millimeters. The properties of the alloy before and after the carburizing heat treatment are shown in table 3. The hard alloy ball tooth with the functional gradient structure is mainly suitable for a roller bit for oil fields.

Table 3: properties of alloy before and after carburizing heat treatment

Example 4: preparation of gradient structure hard alloy spherical tooth with 8% cobalt and nickel content in alloy matrix integral material

The method comprises the steps of taking commercially available tungsten carbide (with the Fsss granularity of about 1.5 mu m), tantalum carbide, niobium carbide (with the Fsss granularity of about 2.0 mu m) and cobalt powder and nickel powder as raw materials, adding 3 wt% of forming agent PEG, carrying out alcohol ball milling (with the alcohol addition of 280ml/kg, the ball-material ratio of 4:1, the rotating speed of 90 r/min and the wet milling time of 30h), and drying to obtain uniform tungsten carbide-tantalum carbide-niobium carbide-cobalt-nickel mixed powder (with the cobalt content of 6%, the nickel content of 2%, the tantalum carbide content of 0.5%, the niobium carbide content of 0.3%, the balance of tungsten carbide and the substoichiometric carbon content of 5.64%). Then pressing and sintering at low pressure (the sintering temperature is 1410 ℃) to prepare the hard alloy with a uniform structure, wherein free graphite and eta phase do not exist in the alloy.

And carrying out carburizing heat treatment on the cemented carbide with the uniform structure in a sintering furnace with a carburizing atmosphere, wherein the sintering step and the carburizing heat treatment step in the preparation process of the cemented carbide with the uniform structure are carried out in a heat cycle of the same furnace. The carburizing atmosphere introduced in the first step of the carburizing heat treatment isMethane (CH)₄) Heat treatment is carried out at 1000 deg.C for 70 min at a gas partial pressure of 500mbar, and in the second step, argon (Ar) is introduced at the end of the first step for gas replacement, and carbon dioxide (CO) is introduced in the carburizing atmosphere after evacuation₂) And heat treated at 1350 deg.C for 70 min at a gas partial pressure of 200 mbar. The hard alloy after the two-step carburization heat treatment has a gradient layer with a poor cobalt surface layer, the cobalt content of the surface of the gradient layer is lower than 85% of the nominal cobalt content of the hard alloy, the final heat-treated functionally gradient structure hard alloy does not contain free graphite or eta phase, the Vickers hardness value of the surface layer with high hardness under the load of 30 kilograms of force is higher than the core hardness of the material by 150 Vickers hardness values, and the thickness of the gradient layer of the surface layer is 2.8 millimeters. The properties of the alloy before and after the carburizing heat treatment are shown in table 4. The cemented carbide spherical tooth with the functional gradient structure is mainly suitable for medium and low air pressure drill bits for rock drilling.

Table 4: properties of alloy before and after carburizing heat treatment

Example 5: preparation of gradient structure hard alloy spherical tooth with 20% cobalt and iron content in alloy matrix integral material

The method comprises the steps of taking commercially available tungsten carbide (with the Fsss granularity of about 12 mu m), titanium carbide (with the Fsss granularity of about 3.0 mu m), cobalt powder and iron powder as raw materials, doping 2.3 wt% of forming agent paraffin, performing alcohol ball milling (with the alcohol addition of 350ml/kg, the ball-material ratio of 3.5:1, the rotating speed of 36 r/min, wet milling time of 24h), and drying to obtain uniform tungsten carbide-titanium carbide-cobalt-iron mixed powder (with the cobalt content of 19%, the iron content of 1%, the titanium carbide content of 2%, the balance of tungsten carbide, and the substoichiometric carbon content of 5.10%). Then pressing and sintering at low pressure (the sintering temperature is 1480 ℃) to prepare the hard alloy with a uniform structure, wherein free graphite and eta phase do not exist in the alloy.

Carrying out carburizing heat treatment on the cemented carbide with the uniform structure in a sintering furnace with carburizing atmosphere, and preparing the cemented carbide with the uniform structureThe sintering step and the carburizing heat treatment step in the process are implemented in the heat cycle of the same furnace. The carburizing atmosphere introduced in the first step of the carburizing heat treatment is methane (CH)₄) Heat treatment at 1150 deg.C for 180 min under 1000mbar partial pressure, second step introducing argon (Ar) for gas replacement at the end of the first step, and introducing carbon dioxide (CO) in carburizing atmosphere after vacuumizing₂) And heat treated at 1450 ℃ for 180 minutes with a gas partial pressure of 500 mbar. The hard alloy after the two-step carburization heat treatment has a gradient layer with a poor cobalt surface layer, the cobalt content of the surface of the gradient layer is lower than 85% of the nominal cobalt content of the hard alloy, the final heat-treated functionally gradient structure hard alloy does not contain free graphite or eta phase, the Vickers hardness value of the surface layer with high hardness under the load of 30 kilograms of force is higher than the core hardness of the material, namely 260 Vickers hardness value, and the thickness of the gradient layer of the surface layer is 5.8 millimeters. The properties of the alloy before and after the carburizing heat treatment are shown in table 4. The hard alloy spherical tooth with the functional gradient structure is mainly suitable for a cutter for a tunneling shield machine.

Table 5: properties of alloy before and after carburizing heat treatment

Comparative example 1

The preparation process is substantially the same as that of example 1 except that the carburizing heat treatment is conducted in the first step by introducing a carburizing atmosphere of methane (CH4) at 1200 ℃ for 100 minutes at a gas partial pressure of 600 mba.

The functionally gradient structure hard alloy after the two-step carburizing heat treatment does not contain free graphite or eta phase, the Vickers hardness value of the surface layer of the functionally gradient structure hard alloy under the load of 30 kilograms of force is only higher than the core hardness of the material by 30 Vickers hardness values, and the thickness of the gradient layer of the surface layer is about 2.3 millimeters. The properties of the alloy before and after the carburizing heat treatment are shown in table 6.

Table 6: properties of alloy before and after carburizing heat treatment

Comparative example 2

The preparation process is substantially the same as that of example 1, except that in the second step of the carburizing heat treatment, argon (Ar) gas replacement is introduced at the end of the first step, and the carburizing atmosphere introduced after evacuation is carbon dioxide (CO)₂) And heat treated at 900 ℃ for 90 minutes with a gas partial pressure of 300 mbar.

The hard alloy after the two-step carburizing heat treatment does not contain free graphite or eta phase; the Vickers hardness value of the alloy surface layer under the load of 30 kilograms of force is lower than the core hardness of the material by 20 Vickers hardness values, and no obvious gradient layer appears in the alloy. The properties of the alloy before and after the carburizing heat treatment are shown in table 7.

Table 7: properties of alloy before and after carburizing heat treatment

It should be noted that the above-mentioned embodiments are only for explaining the present invention, and do not constitute any limitation to the present invention. The present invention has been described with reference to exemplary embodiments, but the words which have been used herein are words of description and illustration, rather than words of limitation. The invention can be modified, as prescribed, within the scope of the claims and without departing from the scope and spirit of the invention. Although the invention has been described herein with reference to particular means, materials and embodiments, the invention is not intended to be limited to the particulars disclosed herein, but rather extends to all other methods and applications having the same functionality.