JPH07166288A - High strength sintered hard alloy having superfine-grained surface - Google Patents

Abstract

PURPOSE:To inhibit the growth and propagation of fine cracking liable to occur at the surface of a tool material at the time of use and to prolong service life by constituting a tool material of a high strength sintered hard alloy where the part in the vicinity of surface is composed of fine-grain WC and the inner part is composed of coarse-grain WC. CONSTITUTION:The part in the vicinity of surface, between the surface and the part at a depth of 50-100mum from the surface, is constituted of WC of <=1mum average particle diameter and the inner part is constituted of WC sintered hard alloy of >=3mum average particle diameter. Moreover, the average content (weight ratio) of V is regulated to 0.1-2.0% in a fine-particle WC part in the surface layer and to <=0.05% in a coarse-particle WC part in the inner part, or the average content of Cr is regulated to 0.3-5% in the fine-particle WC part and to <=0.1% in the coarse-particle part. V and Cr are WC particle growth inhibitors, and a green compact before sintering is immersed in a solution in which respective compounds of V and Cr are dispersed, or the solution is applied or sprayed to or onto the green compact. Moreover, as another means, the part in the vicinity of surface is constituted of a green compact consisting of fine-particle WC and growth inhibitor as an additive and the inner part is constituted of a green compact composed of coarse-particle WC, and these are formed into sandwich-like state and sintered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength cemented carbide used for plastic working, mining tools and the like.

[0002]

2. Description of the Related Art Conventionally, cemented carbide has been widely used for dies used for rolling, extrusion, plastic working, etc., and bits used for rock drilling, excavation bits and the like. However, when the existing cemented carbide is used for such an application, an accident often occurs in which the cemented carbide die is damaged. As a result of diligent investigations into the cause of the damage accident, the present inventors have found that the current cemented carbide satisfies both strength (hardness, compressive strength) and toughness (breaking strength, high fracture toughness value K _1C ) at the same time. It was judged that there was no.

[0003]

That is, most of the causes of breakage accidents are caused by (1) failure of the cemented carbide die under load stress (low bending strength) or (2) repeated load stress. It was confirmed that the growth rate of microcracks was high (the fracture toughness value was low) and the cracks rapidly became large, resulting in damage.

[0004]

SUMMARY OF THE INVENTION However, as a property of the cemented carbide, when the strength is high, the toughness is low, and when the toughness is high, the strength is low. The present invention intends to create a cemented carbide having both toughness and strength at the same time, which has never been obtained before, and to use it for a plastic working mold material, a mining tool, or the like.

[0005]

[Means for Solving the Problems] As a result of the investigation and examination of the above-mentioned damage accident case, the inventors have obtained a more interesting conclusion.
That is, hardness and compressive strength, which have been used as indicators of strength in the past,
Strength and fracture toughness index (K _1C ), which was used as an index of strength and toughness
Are not synonymous with each other and show the following tendency. Strength index Toughness index Hardness Compressive strength Fracture strength Fracture toughness value (K _1c ) Fine grain alloy High High High Low Coarse grain alloy Low Low Low High Exactly opposite values as above, and fine grain alloy and coarse grain alloy Then, it was concluded that each index value is just reversed. Therefore, the mold using the coarse grain alloy has a high probability of being damaged by load stress (low bending strength), and the mold using the fine grain alloy has a high growth rate of microcracks generated by repeated load stress ( The fracture toughness value is low.) It was found that the probability of a large crack rapidly becoming a failure accident increases.

In either case, there is a high probability of damage under high load load and high load frequency. However, when observing the process leading to the damage in more detail, first, minute cracks are generated on the surface of the stress concentration portion or in the vicinity of the surface, and then these cracks grow toward the inside while connecting with each other. Then, at the final stage, if the size of the crack becomes large, it will be damaged. From the above examination results, it is supposed that the vicinity of the surface will be atomized so that it can sufficiently resist the load stress (large bending strength) to suppress the occurrence of cracks, and the inside will suppress the growth of cracks (larger fracture toughness value). We have created an unprecedented alloy with coarse grains. This makes it possible to provide a cemented carbide mold having high fracture resistance, wear resistance, and fatigue strength.

[0007]

The operation will be described in more detail below according to the claims. First, it is desired that the WC constituting the cemented carbide near the surface has an average particle size of 1 μm or less. If it exceeds 1 μm, the effect of suppressing the generation of cracks due to high load decreases. (The transverse rupture strength is reduced.) The average grain size is 3 μm or more inside the alloy. Fracture toughness value (K _1C ) when less than 3 μm
The crack propagation rate is high and the time to failure is short.

At least 50 μm in the vicinity of the surface (the portion which is atomized to increase the transverse rupture strength and hardness) from the surface to the inside.
The effect of the present invention is further enhanced when m, at most 5000 μm. If the thickness is less than 50 μm, the crack resistance is low and the effect of the present invention is not remarkable. 5000 μm
When it exceeds, the progress rate of cracks generated in the stress concentration portion near the surface is fast and the effect of the present invention for extending the time to failure is slightly reduced. It is difficult to make the grain size of the WC near the surface smaller and to make the inside coarser by conventional techniques. As a result of diligent studies, the inventors dip a so-called green before sintering into a solution in which a compound such as V or Cr, which is a conventionally known grain growth inhibitor, is dispersed, or apply or spray the solution. Or try various methods such as sintering a green body in the vicinity of the surface of which is a green body to which fine WC and a grain growth inhibitor are added and whose inside is a green composed of coarse WC, and which is sintered. In either case, the gist of the present invention could be realized by optimizing the conditions.

Further, it has been proved that the effect of the present invention is remarkable in any case when the following conditions are satisfied. When V is used as the grain growth inhibitor, the average content of V in the vicinity of the surface is preferably 0.1% or more and 2% or less. If it is less than 0.1%, the grain growth effect is not clear, and the grain size difference between the WC near the surface and the inside is unclear, and the effect of the present invention is weakened. If it exceeds 2%, the transverse rupture strength decreases and cracks are likely to occur. The average internal content is preferably 0.05 or less. If it exceeds that value, the internal fracture toughness value decreases, and the gist of the present invention cannot be fully realized. Similarly, when Cr is used as a grain growth inhibitor, C near the surface is used.
It is desirable that the average content of r is 0.3% or more and 5% or less. If it is less than 0.3%, the grain growth effect is not clear, and the grain size difference between the WC in the vicinity of the surface and the inside is unclear, and the effect of the present invention is weakened. If it exceeds 5%, the transverse rupture strength decreases and cracks are likely to occur. The average internal content is preferably 0.1 or less. If it exceeds that value, the internal fracture toughness value decreases, and the gist of the present invention cannot be fully realized.

[0010]

Example: Raw material powders such as WC, Co and Cr were weighed to a predetermined ratio and then alcohol was used as a dispersant in a ball mill.
Wet mixed for 2 hours. After completion of the mixing, dry granulation was performed using spray drying to prepare two types of mixed powders of the fine particle alloy mixed powder (A) and the coarse particle alloy powder (B). Next, using a special press machine, a press-molded body is formed on the surface so as to have a sandwich structure of the (A) system mixed powder and the (B) system mixed powder inside, and the processing temperature is appropriately selected. Vacuum sintering was performed. After sintering, it is processed into a predetermined shape and hardness, Young's modulus, transverse rupture strength, fracture toughness (K _1C ), fatigue fracture strength (1
Cyclic compressive stress that causes fatigue failure at 0 ⁶ times, kg / mm ² )
Was measured. Further, press type and forming rolls were prototyped and the effect was investigated. The results are shown in Table 1, Table 2 and Table 3.

[0011]

As described above, by forming a fine grained cemented carbide in the vicinity of the surface and a coarse grained cemented carbide in the interior, it is possible to create an alloy with an increased resistance to crack generation and resistance to progress. It is possible to provide a mold that withstands a high load and has a long life, which is unprecedented.

[Table 1]

[Table 2]

[Table 3]

Claims (6)

[Claims] 1. A WC-based cemented carbide composed of WC having an average particle size of 1 μm or less in the vicinity of the surface, and a WC-based cemented carbide composed sufficiently of WC having an average particle size of 3 μm or more. A high-strength cemented carbide characterized by that. 2. The high-strength cemented carbide according to claim 1, wherein the vicinity of the surface is 50 μm to 5 in the depth direction from the surface of the material.
High-strength cemented carbide characterized by having a thickness of 000 μm. 3. The high-strength cemented carbide according to claim 1, wherein the average V content in the vicinity of the surface is 0.1% by weight or more and 2%.
And the average content of V in the interior is 0.0
High-strength cemented carbide characterized by being 5% or less. 4. The high-strength cemented carbide according to claim 3, wherein the vicinity of the surface is 50 μm to 5 in the depth direction from the surface of the material.
High-strength cemented carbide characterized by having a thickness of 000 μm. 5. The high-strength cemented carbide according to claim 1, wherein the average Cr content in the vicinity of the surface is 0.3% by weight or more and 5
%, And the average content of Cr in the interior is 0.1% by weight or less, a high-strength cemented carbide. 6. The high-strength cemented carbide according to claim 5, wherein the vicinity of the surface is 50 μm to 5 μm in the depth direction from the surface of the material.
High-strength cemented carbide characterized by having a thickness of 000 μm.