JPS58722B2 - Reduction sintering method for high speed steel powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for reducing and sintering high speed steel powder.

Recently, methods have been used to manufacture tools by densifying sprayed powder of high-speed steel by hot isostatic pressing, hot powder forging, or the like.

This is because by using the quenched spray method, it is possible to produce tools with extremely fine and uniform textures that cannot be obtained with conventional ingot processes.

When producing raw material powder by a spraying method, it is necessary to take measures to prevent oxidation of the surface layer as much as possible.

If oxides remain, mechanical strength such as impact value will decrease, and the quality of the powder product will deteriorate.

Therefore, it is necessary to use raw material powder with a low oxygen content.In the water spray method, an atmosphere of inert gas such as nitrogen or argon is used, but in the gas spray method, nitrogen gas is used, and the oxygen content is further reduced. For this purpose, argon spray is used for cooling in water, and further cooling in argon is used.

However, the argon gas atomization method is undesirable because it increases the cost of the raw material powder, and the water atomization method is the cheapest to produce than the gas atomization method.

The components of high-speed steel powder include W, Ni, which is relatively easy to reduce.
It contains Mo, and Mn, Cr, and V, which are difficult to reduce elements.

The latter element has a strong affinity for oxygen, and a small increase in oxygen content during dissolution, high temperature during the spraying process, and an increase in surface layer oxide due to an increase in specific surface area are unavoidable in this production method.

Next, when this type of powder is used in powder bed metallurgy, it must undergo a hot heating process to achieve sintering.
The surface of a sintered body in a reducing atmosphere is highly activated and easily oxidized.

Generally, a reducing atmosphere of ammonia decomposition gas is used.
The dew point must also be controlled low.

The present invention was made in view of the above circumstances, and its purpose is to use high-speed steel powder obtained by an inexpensive water spray method to effectively reduce surface layer oxides at the same time as sintering. Our goal is to provide the following.

The present invention will be explained below.

The surface layer of high-speed steel particles obtained by the water spraying method has a large amount of oxides on the surface layer.
A resin-coated powder (R, C, P) is obtained by uniformly coating the powder with a phenolic resin (weight ratio) in close contact with the powder.

After compacting this into the required shape, it is inserted into a reduction furnace heated by a heater in a nitrogen atmosphere.

As a result, the resin thermally decomposes, and in the low temperature range CH4, H2
This waste comes out of the discharge tower and is burned outside the furnace.

When carrying high-speed steel powder compacts into the furnace in this way,
If you suddenly enter the high temperature range of 1000 to 1300°C, the main sintering range, the rapid decomposition of the resin will generate a large amount of soot, which will accelerate the deterioration of the furnace material and heating element.
It is preferable to provide a low temperature region of 600° C. and allow the material to pass through this range for 5 to 10 minutes, during which time it is decomposed and discharged as a combustible gas.

After that, reduction at 1000-1300℃, 30-30℃ in the main sintering area.
Hold for 120 minutes.

The reduction of the sintered steel powder obtained by the above production method is determined by temperature and time, and the reduction rate is predominantly influenced by temperature.

The oxygen content of the sintered steel powder needs to be 11,000 pp or less in order to ensure the mechanical strength of the sintered body.

On the other hand, if the reduction temperature is below 1000°C, the rate of decomposition and reduction of the copper powder oxide is slow, and if it is above 1300°C, there is a problem in the durability of the heating furnace, and higher temperatures are not desirable industrially.

And the amount of oxygen is 1.000 ppm at the reduction temperature in the above range.
When performing the following reduction, sufficient reduction will not be achieved in a time shorter than 30 minutes, and there is no need to exceed 2 hours, sufficient reduction can be achieved in 2 hours, and it is uneconomical to take longer than 30 minutes. be.

Here, the carbon remaining on the surface of the metal powder after resin decomposition reacts strongly with the oxide on the surface of the metal powder and is reduced.

The atmosphere inside the furnace is nitrogen instead of an atmosphere containing hydrogen, which tends to cause decarburization.

The reaction at this time is:

The reaction residue generated in this reduction reaction is discharged together with the nitrogen stream, and the reduction reaction proceeds.

The coated phenolic resin has a reducing effect and at the same time has the effect of carburization that progresses in the late stages of reduction, and the residual carbon meter after completion of reduction sintering can freely adjust the target carbon amount depending on the amount of phenolic resin coating, reduction time, and temperature. can be adjusted to

This phenolic resin also has the effect of lubricating between metal particles and the mold during powder compaction, and has the effect of acting as a binder between metal particles.

By coating metal particles with phenolic resin, the above-mentioned reduction effect is improved.

In addition, in conventional mixtures with graphite powder, etc., separation due to the difference in specific gravity (
Lamination) had the disadvantage of poor workability due to scattering and inhibiting the uniformity of the molded product, but coating with resin eliminates the above disadvantages, improves workability, and further reduces the reduction reaction. The reduction reaction proceeds evenly and in close contact with the oxide, making it more effective.

That is, in the water spray method, the molten metal is pulverized by a high-pressure, high-speed water stream, and at the same time is rapidly cooled, so that the spray shape has a complicated shape with many irregularities.

For this reason, the surface area of the oxide film on the surface layer is large, and in order to reduce the oxides in the deep parts, it is necessary to sufficiently penetrate the resin into the inside and make it adhere.

For this reason, coating is the most effective method for dissolving the resin and making it adhere.

As a method for producing resin-coated powders (R, C, P), there is a cold method which is carried out at a low temperature, and this is a method in which a phenol resin is dissolved in methanol, mixed and coated, and then the methanol is vaporized.

However, this method has disadvantages in that it generates irritating gas, which is not a pollution hazard, and that it takes time to vaporize methanol, and that it tends to form blocks.

If this is done using the dry hot method, approximately 15% of the copper powder is
Heat it to 0℃, mix it with the necessary amount of flaky or needle-shaped phenol resin in a mixer, and make a diameter of about 1 min.
The powder is discharged at about 00°C and dispersed while being cooled by blowing air on a conveyor in order to forcefully disperse the powder so as not to form a compound.

This method is cheaper than the cold method, has no pollution problems, and has better storage stability.

As for the type of phenolic resin, novolac type (thermoplastic resin) is used.

This is because it is difficult to reduce the amount of residual carbon after combustion with resol type (thermosetting resin), and the curing speed is fast when coating copper powder with weak bonding strength between molded particles. This is because it has some flaws.

In the novolak type, the amount of residual carbon can be changed up to 20% by changing the degree of polymerization, and the curing speed is slow (
Easy to coat.

In this case, hexamethylenetetramine as a curing agent is not used because it generates ammonia gas, increases the amount of residual carbon, and slows down the curing speed.

Another major feature of using the novolak mold is that if the Rattler value, which determines the strength of the powder compact (before sintering), is high, the compact can be baked at a low temperature of 200 to 300°C for about 10 minutes. There is also.

The degree of polymerization n in the above molecular formula is 10 or less, and by changing the degree of polymerization, the amount of residual carbon can be changed to 20.3040% and the amount added can be adjusted.

If the amount of phenolic resin to be coated on the copper powder is less than 1%, the strength of the compacted product will be weak, causing cracks such as missing parts during baking, and if it is more than 7%, it will be difficult to form during baking. The decomposition gas of the phenol resin causes the molded article to collapse, and as a result, the appropriate amount of the phenol resin is 1 to 7%, preferably 2.5 to 3.5%.

Next, the components of copper powder must be as low as possible, including difficult-to-reducible aluminum oxide, magnesium oxide, calcium oxide, silica, etc., which may be mixed in from deoxidizers, furnace materials, etc. during the melting of raw materials. Needless to say.

Example High-speed steel powder (SKH9, 5KH57) with the components shown in Table 1 obtained by the water spray method was coated with 2.5% by weight phenolic resin with a residual carbon content of 20% by the hot method. It was set as R, C, and P.

After compacting to a sample size of 12.25 φ x 2.7 mm at a compacting pressure of 5 t/cm 2 , the sample was circularly sintered at 1200° C. for 60 minutes and 120 minutes in a nitrogen flow of 51/min.

Table 2 shows the amount of oxygen before and after reduction sintering, and the initial amount of oxygen was reduced to about 1/8 after 2 hours.

The reduction results are obtained when the copper powder contains a considerable amount of Si and V, which are difficult to reduce at 1200°C.

Table 3 also shows the nitrogen content before and after reduction sintering under the above conditions, and a high nitrogen content of 0.6 to 0.8% was obtained after 120 minutes.

After cooling to room temperature in a reduction sintering furnace at 1200℃ for 1 hour,
After reheating at 40°C for 10 minutes in a N2 stream, upsetting forging was performed.

The forged product was held in alumina for 5 minutes at 1230°C, quenched by cooling in oil, held at 550°C for 95 minutes, and then air-cooled and tempered.

Table 4 shows the amount of 01N and Hv hardness after forging heat treatment.

Since the present invention is as described above, the surface layer oxides can be effectively reduced at the same time as sintering by using high-speed steel powder obtained by an inexpensive water spray method.

In addition, by performing the above sintering reduction in a nitrogen gas stream, the nitrogen content of the sintered body is increased, the performance after hot forging is improved, and the grain size is reduced and the tempering hardness is increased. be able to.

Furthermore, the toler value is improved, and the strength of the compacted colored body can be obtained.

Claims (1)

[Claims] 1. High speed steel powder obtained by a water spray method is blended with a phenol resin to uniformly coat the surface of the steel powder in a range of 1 to 7% (weight ratio) to form a resin-coated powder. After that, this was compacted and then heated to 1000 to 130
By heating to a reduction temperature of 0°C for 0.5 to 2 hours, the surface layer oxides are reduced by the carbon that remains in close contact with the surface layer after the thermal decomposition of the phenolic resin, and the amount of residual carbon can also be freely adjusted. A method for reducing and sintering high-speed steel powder, characterized by increasing the nitrogen content. The high-speed steel powder obtained by the two-water spray method is heated to about 150°C and kneaded with the required amount of phenol resin in a kneader, and then dispersed on a conveyor while being cooled with air to form a resin. A method for reducing and sintering high-speed steel powder according to claim 1, characterized in that the powder is a coated powder. 3. The method for reducing and sintering high speed steel powder according to claim 1, wherein the phenolic resin coated on the surface of the high speed steel powder is a novolac type which can improve the Rattler value by low temperature baking. 4. The high-speed steel powder according to claim 1, wherein the powder compact is baked in air at a heating temperature of 200 to 300°C for about 10 minutes in order to improve the Rattler value that determines the strength of the compact. reduction sintering method.