JPH0461058B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to sintered alloys such as cemented carbide and cermets, and particularly to high-strength sintered alloys made of fine crystal grains. [Prior Art] Generally, sintered alloys such as WC cemented carbide and TiC cermet are used for cutting tools and wear-resistant tools because of their high hardness and excellent heat resistance. When these sintered alloys are used as tools, the damage to the tools can be roughly divided into two types. One type of damage is a gap-like damage in which a small amount of the tool surface is removed due to contact between the tool and the workpiece, and the other type is brittle damage in which a part of the tool breaks due to chipping or chipping. This is devastating damage. In the former case, where the damage increases until the end of the tool life, the tool life can be roughly predicted depending on the type of tool material and tool usage conditions, whereas in the case of the latter, brittle fracture damage. There is a problem in that it is difficult to predict when this will occur during use of the tool, and therefore the tool life cannot be predicted, reducing the reliability of the tool. Fields of use that are susceptible to brittle fracture damage that occurs in tools include tools that are subject to large impact such as large side cutters and hobs, as well as cutting tools that are subject to impact at low speeds such as gun drills, gun reamers, end mills, and various drills. There are also wear-resistant tools such as slitters, dot pins, etc. In each of these fields, work conditions have recently become more efficient and automated, and there is also a demand for improved tool life, especially stability or reliability of tool life. It is increasing. [Problems to be Solved by the Invention] The present invention solves the above problems, and specifically improves wear resistance by increasing toughness without reducing hardness, which is a measure of wear resistance. The purpose of the present invention is to provide a sintered alloy with improved brittle fracture damage. [Means for Solving the Problem] The inventors of the present invention investigated the origin of fracture as a cause of brittle fracture in sintered alloys, particularly sintered alloys mainly composed of WC, by observing fracture surfaces using a microscope and using As a result of tissue analysis using a line microanalyzer, the present invention was completed by confirming that Ca and S, or a foreign substance containing Ca, was the origin of the fracture. The sintered alloy of the present invention is mainly composed of tungsten carbide with an average grain size of 1 μm or less (tungsten carbide alone, or carbides of metals of groups IVa, Va, and VIa of the periodic table excluding tungsten carbide, and nitrides). substances, carbonitrides, carbonates, nitrides,
70 to 97% by weight of a hard phase (consisting of at least one of carbonitoxides and mutual solid solutions thereof);
At least one of Fe, Ni, Co, Cr, Mo, W
Among the inevitable impurities of the sintered alloy consisting of the seed binder phase and inevitable impurities, Ca is 0.01% by weight or less and S is
It is characterized by having high strength by limiting the content to 0.005% by weight or less. The hard phase in the sintered alloy of the present invention has an average grain size
If it is made of tungsten carbide with a thickness exceeding 1 μm,
The effect of limiting unavoidable impurities becomes weaker, abnormally grown tungsten carbide appears, and the strength decreases significantly. The method for producing a sintered alloy of the present invention includes
It is necessary to strictly limit the content of Ca or Ca and S. For example, it is necessary to devise a refinement to the conventional refining process of starting materials so as to produce starting materials with even lower Ca and S contents.
After blending a predetermined amount of starting materials with low Ca and S content obtained by devising the refining process, paraffin, stearic acid, etc., which are used to make the mixed powder easier to mold, are added. It is necessary to pay attention to impurities consisting of compounds of Ca and S contained in the lubricant, or impurities contained in carbon or graphite used in the preliminary sintering and sintering steps. In addition to suppressing the amount of impurities mixed in, the density of the starting material is, for example,
CaS2.80 for WC15.7g/cc, Co8.9g/cc
g/cc and CaO3.37g/cc, which is extremely small, so CaS and CaO tend to segregate as impurities even in small amounts during the drying process of the mixed powder. Alternatively, it is necessary to devise measures to significantly reduce the segregation of CaS and CaO by rapid cooling treatment during the sintering process. The sintered alloy of the present invention, which is obtained by suppressing the amount and segregation of these impurities, can be further treated by hot isostatic pressing (HIP) after sintering to have the effect of further increasing the anti-soldering strength. Highly desirable. [Function] The sintered alloy of the present invention can increase the anti-soldering strength without reducing the hardness in the relationship between hardness and anti-soldering strength, which are typical characteristics of sintered alloys.
Specifically, sintered alloys made of cemented carbide or cermets contain Ca and S as unavoidable impurities in carbides, nitrides, etc. of metals in groups a, Va, and a of the periodic table used as starting materials. Ca and S are also included as unavoidable impurities in the iron group metal as a binder phase.
For example, in the case of W, it contains about 100 ppm of Ca, and the carbon used to carbonize this W contains S, so when it is turned into WC, it contains about 90 ppm of Ca.
It contains about 500 ppm of S. Additionally, Co and Ni, which are iron group metals, contain approximately 100 ppm of Ca and S.
Contains approximately 30ppm. As these starting materials become finer particles, the content of Ca and S tends to increase. In addition, the content of Ca and S in the sintered alloy is
As mentioned above, in addition to those due to the starting materials,
It is also caused by the sintering atmosphere, that is, the materials used as the heating element, heat insulating material, and product support plate used during sintering, such as Ca and S contained in graphite. Therefore, the sintering tool must be fired in advance at a temperature higher than the sintering temperature of the product, preferably at a temperature of 1773K or higher, to sufficiently volatilize Ca and S. For this reason, conventional sintered alloys generally contain about 0.02% Ca and about 0.01% S, although this varies depending on the component composition and the size of crystal grains.
If Ca is contained in the starting material in this way, CaO aggregates are generated during the sintering process, which becomes a source of fracture after sintering and reduces toughness. In addition, if Ca and S are contained in the starting materials, aggregates of CaS and CaO will be generated during the sintering process, or a crystallized phase of sulfides will be formed. After the phase is sintered, it becomes a source of fracture and reduces toughness. In particular, the average particle size as the hard phase
When a small amount of vanadium carbide, chromium carbide, etc. is added to a material mainly consisting of tungsten carbide with a diameter of 1 μm or less, the strength will be significantly reduced due to the formation of aggregates of CaS and CaO. These CaS, CaO
The size of the aggregates is approximately 10 to 20 μm in the sintered example.
, and this large aggregate becomes the defect as the origin of the fracture. In order to reduce the size of such aggregates of CaS and CaO to 10 μm or less, preferably 5 μm or less, the unavoidable impurities in the sintered alloy must be
It is necessary to reduce Ca to 0.01% by weight or less and S to 0.005% by weight or less. In particular, reducing Ca to 0.007% by weight or less and S to 0.003% by weight or less in the sintered alloy is
This is preferable for improving strength. By limiting the amount of Ca and S in this way, defects are eliminated in the sintered alloy made of cemented carbide or cermet, and therefore the transverse rupture strength, which is a measure of toughness, is reduced without decreasing the hardness. This is a marked improvement. [Example] As starting raw material powders, WC powder with an average particle size of 0.50 μm (Ca: 0.001% or less, S: 0.0015% or less) and Co powder with an average particle size of 1.4 μm (Ca: 0.002% or less, S:
0.0015 or less) is used for the sintered alloy of the present invention,
(However, the W used to make the WC is
WO ₃ W is obtained by treating with pure water passed through an ion exchange resin in all the steps of refining and refining from wolframite to W. Conventionally commercially available WC powder with an average particle size of 0.50μm, average particle size
Co powder of 1.5 μm was used for comparison, and other starting materials with an average particle size of 1.0 to 1.5 μm were used in common, and the prescribed amounts were mixed with the composition shown in Table 1, and the mixture was heated in a ball mill for 72 hours. After drying and heating while uniformly mixing and heating to prevent the formation of aggregates, the material was vacuum sintered (approximately 7 Pa) at a temperature of 1643 to 1673 K for a holding time of 3.6 Ks. At this time, 1 of the present invention,
For 2, 4, 6 and comparative product 8, only the graphite product support plate was charged into the furnace prior to sintering,
I air-fired it at 1873K and 22Kg. Then 0.1GPa
Samples Nos. 1 to 6 of the present invention and comparative samples Nos. 7 to 10 were obtained by HIPing in an argon stream at a temperature of 1623K for a holding time of 3.6Ks. For each sample thus obtained, the amount of Ca was measured by atomic absorption spectrometry, and the amount of S was measured by infrared analysis using a carbon-sulfur-in-metal analyzer. After measuring the hardness and transverse rupture strength of each sample, the fracture surface on which the transverse rupture strength was measured was examined using a metallurgical microscope, a scanning microscope, and an X-ray microanalyzer to determine the type and size of the main defects in each sample. We measured the These results are shown in Table 2. As is clear from the results in Table 2, the Ca and S contents were adjusted through the selection of starting materials and the dry firing of the sintering furnace and equipment such as product support plates charged into the furnace during sintering. It was confirmed that the product of the present invention had a higher transverse rupture strength and higher strength than the comparative product.

【table】

〔Effect of the invention〕

From the above explanation and results, the sintered alloy of the present invention has fewer defects caused by impurities that cause fracture, and therefore has high transverse rupture strength while maintaining the same hardness as conventional sintered alloys. It is something. Therefore, tools that apply impact or compressive stress, such as shearing tools such as punches and dies, cutting tools such as slitters and cutting blades, guide bushes, etc.
Machine parts jigs and valves for rolls and gauges,
Wear-resistant tools such as mechanical seals, cutting tools including not only turning tools but also milling tools that are subject to large impact forces and drilling tools such as end mills, reamers, drills, etc., especially micron drills using fine grain WC, It is an industrially useful sintered alloy that is ideal for dot pins and may also be used as a base material for various coated sintered alloys.

Claims (1)

[Claims] 1 70 to 97% by weight of a hard phase mainly composed of tungsten carbide with an average particle size of 1 μm or less, and the remainder Fe, Ni,
In a sintered alloy consisting of at least one binder phase among Co, Cr, Mo, and W and unavoidable impurities, Ca (calcium) among the unavoidable impurities is 0.01% by weight.
A sintered alloy characterized by having high strength by containing the following and S (sulfur) at 0.005% by weight or less.