WO2002076659A1

WO2002076659A1 - A high-strength, sintered binder alloy for powder metallurgy

Info

Publication number: WO2002076659A1
Application number: PCT/KR2002/000494
Authority: WO
Inventors: Kang-Hyung Kim
Original assignee: Yeonwoo Industry Co., Ltd
Priority date: 2001-03-24
Filing date: 2002-03-22
Publication date: 2002-10-03
Also published as: KR100415315B1; KR20010067783A

Abstract

Discolsed is a high-strength, sintered binder alloy binder alloy for powder metallurgy, based on a composition comprising 15.45 atomic % of an early transition metal element, 30-65 atomic % of a late transition metal element, 10-30 atomic % of a deposit-reinforcing and stabilizing element selected from among elements of Ib, IIb, IIIb and IVb groups and mixtures thereof, and inevitable impurities. Also, a high-strength sintered binder alloy with improved durability and wear resistance is provided, which is prepared by sintering a powder previously alloyed from the composition, along with hard particles.

Description

A HIGH-STRENGTH, SINTERED BINDER ALLOY FOR POWDER

METALLURGY

TECHNICAL FIELD

The present invention is directed to sintered binder alloys for powder metallurgy, which are advantageous in terms of excellent wear resistance and durability, by using a high strength, binder alloy powder having low melting point when various carbides, nitrides, borides, oxides, including tungsten carbide, titanium carbide, tantalum carbide, alumina and boron nitride, are sintered along with hard ceramic particles, such as diamond. The present invention provides an alloy having excellent corrosion resistance and strength as well as restraining solution of carbides and deformation of carbide particles during sintering, which are problems of conventional carbide materials containing cobalt and nickel as a binder alloy, and a method for preparing the same.

PRIOR ART

Typically, hard particle powders are mixed with cobalt powders, and sintered in a liquid sintering manner by heating at a high temperature of about 1400-

1550 °C for 1-2 hours based on particle sizes and a composition of the powders.

Such reaction of the liquid binder metal and hard particles is effectively used to decrease porosity and improve density, but suffers from the disadvantages of deformation of hard particles from a spherical shape to a square shape and increase of the particle size, due to heating at a high temperature for a long period of time.

A widely used cobalt element has a tensile strength of 250 - 700 Mpa, a broader range than other metal elements, while being excellent in wettability with hard particles, such as carbides or nitrides. Cobalt of HRA 90 - 92 is added in the amount of 6 - 12 wt% for sintering, depending upon a necessary hardness and toughness.

The cobalt element is advantageous in terms of high hardness but is disadvantageous in light of low toughness. As for header dies or stamping dies for a cold work requiring high toughness, such element is added in the amount of 18- 27 % for increasing toughness. As such, there are problems, such as lowering of wear resistance with decrease of hardness to HRA 84-87. In addition, since a sintering temperature of 1450 °C or higher and a long period of time are required to obtain a sufficient sintered density, the size of hard particles is increased and the particles of spherical shape are deformed to a brick shape, thereby decreasing wear resistance. Moreover, when used in chemical fields or upon contact with combustible gases, cobalt is low in corrosion resistance, generating problems related to durability.

There is thus an effort to solve such problems by adding nickel, chromium, aluminum, niobium, vanadium, molybdenum and so on. In this regard, U.S. Pat. No. 4,198,234 discloses a binder alloy by alloying chromium, iron, silicon, boron and carbon added to nickel or by adding 20-50 wt% of copper, to sinter metal powders at a relatively low temperature of 1050-1100 °C. But this patent has disadvantages of low wettability for carbides, nitrides and oxides, and insufficient sintered strength, so being unsuitable for use in a binder alloy for hard particles.

U.S. Pat. No. 4,466,829 refers to a tungsten carbide-based hard alloy incorporating Ni₃Al formed from aluminum and nickel, which is advantageous in light of improved heat resistance, and increased corrosion resistance by containing chromium. But this patent suffers from low wettability and decreased density. In U.S. Pat. No. 4,497,660, there is provided a sintered hard metal alloy, in which nickel having excellent durability while being similar in strength to cobalt, is used as a binder metal. Fine cobalt powders used as binders, which are also expensive, are limited in their production amount, thus their supply and demand being out of balance. Meanwhile, as for nickel, its supply and demand are smoothly synchronized, and nickel is one third as expensive as cobalt. But nickel has drawbacks, such as inferior wettability, increased porosity, and a sintering temperature higher than cobalt, thus hard particles being extremely deformed.

In U.S. Pat. No. 5,238,481, there is disclosed a method for sintering a tungsten-boron compound by a strong bonding of tungsten and boron, in the presence of cobalt used as a binder. However, this method is disadvantageous in that since tungsten, cobalt and boron are mixed as elements and sintered, a resultant structure becomes non-uniform and a strength is not constant, due to the strong oxidation of boron, thus reducing reliability and a quality of products. U.S. Pat. No. 5,309,874 describes a power-train component with adherent amorphous or nano-crystalline ceramic coating system, in which amorphous Si₃N is coated by use of a thin film deposition method such as PVD, CVD and PECVD to provide wear resistance, corrosion resistance and heat resistance. But such patent is disadvantageous in that a coated layer is thin film of 0.5-0.8 μm, and thus under the conditions of high load and impact load, a coated film is easily broken due to low durability, while increase of a thickness of the film results in weak strength of the film by attachment of droplets.

A basic solution for low wettability, sintering strength, corrosion resistance, and thermal resistance uses an alloy composition having excellent corrosion resistance and high strength. However, materials having high strength, while being superior in wettability to cobalt, are rare, expensive and inferior in wettability.

Therefore, there are required process techniques, such as a solid phase sintering process, a pressurization sintering process, and a spark discharging sintering process. Through such sintering process, hard particles are not deformed and their spherical shape can be maintained by restraining a grain growth and controlling a shape of hard particles during a shortened sintering time period, instead of increasing a pressure and a temperature which affect energy of the system between a grain boundary and porosity. Through the pressurization sintering process, including pressing, rolling, extrusion and centrifugal pressurization, powders are formed to a predetermined shape under a predetermined density using equipment having a capacity of 20-30 ton, after which iron is sintered at 1095 °C, stainless steel at 1180 °C, copper at 870 °C, tungsten carbide at 1480 °C, for 20-40 minutes. This process has advantages, such as lower sintering temperature and shorter time period, compared to a general sintering process.

But this process requires an expensive pressurization device or a pressure container, and has a difficulty in its practical use owing to limited preparation conditions. In addition, a forming and sintering process is performed at low temperature by use of high-energy sources, such as plasma, laser, and microwave. However, there are problems such as a double heating process, expensive equipment and high production cost.

DISCLOSURE OF THE INVENTION

Therefore, it is an object of the present invention to alleviate the problems as described above and to provide a sintered binder alloy having excellent ductility and corrosion resistance, using conventional equipment.

It is another object of the present invention to provide a method for preparing the sintered binder alloy.

To achieve the object, the present invention provides a sintering binder alloy powder mainly comprising cheap iron, instead of using cobalt, which is advantageous in light of excellent wettability, high strength and high ductility, and low price.

BEST MODES FORCARRYINGOUT THEINVENTION

The present invention pertains to a binder alloy comprising 15-45 atomic % of an early transition metal (hereinafter, referred to ETM), 30-65 atomic % of a late transition metal (hereinafter, referred to LTM), 10-30 atomic % of a deposit-reinforcing and stabilizing element selected from among elements of the Groups lb, lib, ITIb, IVb and mixtures thereof, with inevitable impurities. Preferably, carbon, silicon, boron and aluminum of the Groups IUb and IVb, which are used alone or in combinations thereof, are added in the amount of 10-25 % by atom, in order to obtain a suitable amount of hard deposit.

In the present invention, the ETM elements belong to the Rows Ilia, IVa, Va and Via in the periodic table. In particular, chrominum of the Group VIb mainly used in the present invention is advantageous in terms of high solid solubility for iron, cobalt, and nickel among LTM elements of the Groups Vila and Vm, and low price.

The chromium solid solution forms borides and carbides, together with boron and carbon, and thus is desirable in terms of processibility and strength. The ETM elements, exclusive of chromium, are mainly used for enhancing the effects of chromium. However, they have lower solid solubility, compared to chromium, and are added only in the amount of up to 5 % by atom. Of them, molybdenum is formed to a complete solid solution, along with chromium, to reinforce a matrix structure and to stabilize borides and carbides. Meanwhile, a deposit of the present invention is intended to have a structure of face-centered cubic (FCC) or tetragonal form compounds with superior ductility and heat resistance, rather than orthorhombic or hexagonal form compounds with high brittleness. Other ETM elements, such as titanium, vanadium, zirconium, niobium, hafnium, tantalum, tungsten, lanthanide and actinide, are responsible for reinforcing the matrix and formation of stable borides and carbides, and thus are highly resistant for fatigue wear, including spalling, pitting, chipping and heat checking. Further, such elements can restrain oxidation of the elements of LTM and the Groups lb, lib, Ufa and IVa during a sintering process, and thus the amount of impurities and the extent of porosity are decreased. But if the total amount of ETM elements is less than 15 % by atom, sufficient strength and ductility are not ensured and the alloy with low melting point or wettability cannot be obtained. Meanwhile, if the amount exceeds 45 % by atom, such elements form excessive deposits, together with the elements of the Hla and IVa groups, thus reducing ductility.

The LTM elements play a role in strengthening the binder alloy and belong to the Groups VHb and VHIb. In the present invention, inexpensive and abundant iron is mainly used, and the other LTM elements, such as nickel, cobalt, etc., may be added to improve corrosion resistance and thermal resistance. The element nickel is cheaper than cobalt used for a conventional binder, and activates metals of high melting points, such as tungsten and molybdenum, thereby a sintering process being promoted. The element cobalt is excellent in wettability for steel or ceramic, and improves heat resistance. So, if necessary, a small amount of cobalt is used. Additionally, other LTM elements may be added as matrix elements. But such elements are expensive or low in solid solubility for chromium, and thus used in small amounts. In the present invention, when the amount of the LTM elements is less than 30 % by atom, hard compounds are excessively generated and ductility is decreased. On the other hand, when the amount exceeds 65 % by atom, matrix strength becomes poor and wear resistance is reduced.

The Groups lb and lib are responsible for solid-solution hardening of the matrix of the binder alloy, and the Groups πia and TVa are responsible for forming and reinforcing the hard deposit. As elements of the Groups Hla and IVa, boron and carbon are mainly used, and silicon and aluminum bear the responsibility of stabilizing compounds related to boron and carbon. With the total amount of these elements less than 10 % by atom, matrix hardening effect is poor. Meanwhile, with the total amount of more than 30 % by atom, the hard deposit becomes coarse and brittleness increases. Preferably, in order to obtain the hard deposit being suitably resistant to wear, carbon and boron of the Groups Ilia and IVa, which are used alone or in combinations thereof, are added in the amount of 10-25 % by atom. A mixing ratio in the binder alloy is determined depending on a required hardness, in which hard particles are added to occupy about 45-90 % by area in the alloy, considering a specific gravity, so as to improve internal compression of a hard layer in an engine valve train system presented in the following Example. In the present invention, on the basis of the mixing ratio and the sintering temperature, the hard particles comprise about 57-90 % by area, calculated by a Point counting method.

The inventive binder alloy having high surface energy is excellent in wettability with metals or ceramics, and suitable for a binder of hard ceramic particles. Using such property, the present binder alloy can be substituted for cobalt widely used as a conventional binder alloy.

Thus, the sintered products by use of the present binder alloy have superior internal compression, heat resistance and corrosion resistance, and are applicable for wear resistant parts for engines, die punch, drawing dice, guides, bearings, processing tools and sintered binder materials for cutters.

In the Example as described below, as for use of the binder alloy in a powder form, particle size, particle size distribution, shape, purity and surface state affect quality of products. Therefore, with a view to obtaining fine particles with very uniform particle size, the binder alloy powders having the composition ratios presented in the present invention are prepared by a gas atomization method.

Then, considering packing with hard particles, the powders for sintering should have a particle size of 15 μm or smaller. When the binder alloy powder which has smaller particle size than hard particles is used, density, strength and young's modulus of the formed products are increased. If the particle size is excessively increased, density of sintered products becomes low. Hence, the particle size should be in the range of up to 15 μm. In the present binder alloy powder, alloy elements are previously mixed and then alloyed, or a pre-alloy powder comprising at least one element of ETM or LTM, the Group Hla element and the Group IVa element, is mixed with other alloy elements, to yield a desired alloy composition for a final sintering.

Hard particles, such as carbides, nitrides and oxides, which are mixed with the binder alloy, are limited in size to 25.0 μm or less, and preferably, to the size of 2.0 μm or less, when subjected to continuous strong impact fatigue using a tappet. In the case that the hard particles having an average particle size larger than about 25.0 μm are present, a crystal plane of a Miller index (001) turns into a cleavage plane and thus cracks parallel to the crystal plane are easily formed.

Moreover, in order to increase formability and density, hard particles are previously mixed with the binder alloy particles to make up to 15 wt%, and formed. As such, the mixed particles are made to a size of 45-125 μm. The hard particle materials comprise tungsten carbide, titanium carbide, zirconium carbide, tantalum carbide, silicon carbide, chromium carbide, boron nitride, zirconium nitride, titanium nitride, silicon nitride, hafnium boride, titanium boride, zirconium boride, chromium boride, aluminum boride, cobalt boride, iron boride, aluminum oxide, zirconium oxide and combinations thereof. Other hard ceramic particles or diamond particles may also be used.

A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

EXAMPLES 1-9

A binder alloy having an amount (the remainder: hard particles) and a chemical composition shown in the following Table 1 was subjected to gas atomization, to prepare a binder alloy powder. The powder for sintering was pulverized to 15 μm or less, considering packing with hard particles. A composition and an average particle size of hard particles were adjusted as shown in

Table 1, after which such materials and an organic binder were uniformly mixed by use of a kneader. The finer the powder, the higher the density and the higher the transverse rupture strength. So, a plasma atomization method might be adopted. As for the organic binder, polymeric elements such as paraffin, polyethylene wax or EBS wax, and a liquid binder, such as stearic acid, glycol, polyvinyl alcohol, were mixed based upon a size and a shape of a final product. In the present invention, paraffin was added in the amount of 0.5 wt% as the organic binder. Other than the organic binder, in order to improve compressibility of the powder and aid a flow of the particles by decreasing friction with a mold, lubricants, such as graphite, resins, soaps are additionally added in the amount of 0.1 wt%. In the present invention, such lubricants were not used. Thusly mixed materials were formed to a coin shape having a thickness of

1.0-1.5 mm and a diameter of 28 mm, which was then pre-sintered at 450-550 °C, attached onto S45C carbon steel disk block using a nickel based alloy paste, heated to a sintering temperature shown in the table 1, and simultaneously subjected to sintering and brazing. Thereafter, a hard layer of the sintered block was crown- processed to a curvature of 1,500 mm radius and finished to a final illumination of

Rmax 1.2.

A test piece of the block having a hardness of HV 850 or more, which was prepared as mentioned above, was tested for a pitting resistance with a single- acting tappet-cam tester. A test condition was under a cam rotation rate 1000 rpm, a spring static load 175 kgf, a test rotational frequency 1 x 10⁷ cycle, and an oil temperature 75-85 °C. As the other party in the test, use was made of the cam obtained from a camshaft having a hardness of HRC 55 or more by hardening SCM440 steel at high frequency.

As the results, it was found that the block test piece sintered by a high strength alloy of the present invention and the cam are resistant to wear.

COMPARATIVE EXAMPLES 1-5

As shown in the following table 1, conventional binder alloy powders were mixed with hard particles and sintered. The binder alloy powder for sintering had an average particle size of 1.5-25 μm. The hard particles of the table 1 were uniformly mixed with 0.5 wt% of paraffin by a kneader.

Thusly mixed materials were formed to the same form as in the above examples, which were then pre-sintered in the same manner as in the above examples, heated to the sintering temperature stated in Table 1 and sintered. Thusly prepared blocks were tested for wear resistance. As the test results, the blocks were poor in wear resistance, thus defects by wear and pitting occurring.

TABLE 1

Note: 0 : good, Δ : shallow wear defect , : pitting

INDUSTRIAL APPLICABILITY

The sintered products using the binder materials of the present invention are excellent in wettability as well as durability because of being highly resistant to fatigue, pitting and toughness in the surface layer by maintaining the spherical shape of the hard particles even after a liquid sintering process.

The sintered binder alloy of the present invention can be applied in the fields of wear resistant parts for engines, die punch, drawing dice, guides, bearings, processing tools and sintered binder materials for cutters.

The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

1. A sintered binder alloy for powder metallurgy, comprising 15-45 atomic% of an early transition metal, 30-65 atomic% of a late transition metal, and 10-30 atomic% of a deposit hardening and stabilizing element selected from among elements of the Groups lb, πb, LUb, IVb and combinations thereof, with incidental impurities.

2. The sintered binder alloy as defined in claim 1, wherein a pre-alloy powder comprising at least one element of the early transition metal and the late transition metal, the Group Hlb element and the Group IVb element, is mixed with other alloy elements to yield an alloy composition desired in a final sintered composition.

3. The sintered binder alloy as defined in claim 1, wherein the early transition metal is selected from the group consisting of chromium, molybdenum, titanium, vanadium, zirconium, niobium, hafnium, tantalum, tungsten, lanthanide, actinide, and combinations thereof.

4. The sintered binder alloy as defined in claim 3, wherein the total amount of the early transition metal contained in the binder alloy powder, exclusive of chromium, is in the range of up to 5 % by atom.

5. The sintered binder alloy as defined in claim 1, wherein the late transition metal comprises the element of Groups Vila and VLU, selected from among manganese, iron, cobalt, nickel and combinations thereof.

6. The sintered binder alloy as defined in claim 1, wherein the element of the Groups lb and lib is selected from among copper, silver and combinations thereof, and the element of the Groups LUb and IVb is selected from among boron, carbon, silicon, aluminum and combinations thereof.

7. The sintered binder alloy as defined in claim 6, wherein the alloy comprising 10-25 % by atom of the element of the Groups mb and IVb selected from among carbon, boron and combinations thereof, is used to form a hard structure.

8. The sintered binder alloy as defined in claim 1, wherein the binder alloy is used in a powder form having a particle size of 15 μm or less, considering packing with hard particles, to form a hard structure.

9. The sintered binder alloy as defined in claim 1, wherein hard particles are mixed with the binder alloy, said hard particles being selected from the group consisting of tungsten carbide, titanium carbide, zirconium carbide, tantalum carbide, silicon carbide, chromium carbide, boron nitride, zirconium nitride, titanium nitride, silicon nitride, hafnium boride, titanium boride, zirconium boride, chromium boride, aluminum boride, cobalt boride, iron boride, aluminum oxide, zirconium oxide and combinations thereof, or other hard ceramic particles, diamond particles and combinations thereof.

10. The sintered binder alloy as defined in claim 1, wherein hard particles are mixed with the binder alloy, and the hard particles having an average particle size of 25 μm or less are used to improve impact resistance and fatigue resistance of a hard structure.

11. The sintered binder alloy as defined in claim 10, wherein, as the hard particles, carbides having a particle size of 2 μm or less are used to improve impact resistance and fatigue resistance of the hard structure.

12. The sintered binder alloy as defined in claim 1, wherein hard particles are mixed with the binder alloy, and the hard particles have 45-90 % by area, to improve internal compression of a hard structure.