JPS589821B2 - Manufacturing method of cemented carbide containing molybdenum - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a cemented carbide, particularly an alloy containing a composite compound whose purpose is to replace WC in the alloy with MoC.

Conventionally, the raw material for cemented carbide is mainly composed of WC powder, to which high melting point metal carbides or carbonitrides such as Ti, Ta, Nb, Mo, Hf, V, Cr, etc. are added depending on the required properties of the alloy. Iron group metals are mainly used as bonding metals.

However, tungsten is a relatively expensive metal;
Since it can only be found in very small quantities on earth,
It is considered a so-called "strategic" material, and its degree of use can be said to have political value.

Therefore, if the demand for cemented carbide whose main component is WC increases, this resource problem will naturally arise.

If WC could be replaced with other high melting point metal carbides, it would have a significant impact on the industry.

Molybdenum monocarbide is the most promising candidate.

Only this carbide is a simple hexagonal type having the same crystal structure as WC, and its mechanical properties are thought to be close to WC.

However, the existence of molybdenum monocarbide alone has been questioned to this day, and attempts have been made to stabilize MoC by forming a solid solution with tungsten carbide.

This method was introduced in 1950 by W. Although it was first discovered by David Dawihl, at that time no industrial value was found in this solid solution, and not much study was conducted.

Recently, with the rise in the price of W, research using solid hemorrhoids of (MOXWY)C (X+Y=1) has become active again.

However, it is very interesting to know why little research has been done on it, and no active attempts have been made to use it.

The solid solution manufacturing method of MoC-WC that has been reported so far is to separate cobalt into WC, Mo and C powder, or W, Mo and C powder, make a mixed powder, and then fill it into a carbon container at 1600 to 2000 °C. A method in which the reaction is carried out at a temperature of

(W. Dawihl, Z. ancg, Chem262(
1950) 212) At this time, the role of cobalt is said to be to promote the formation of carbide and the dissolution of Mo into tungsten carbide.

Indeed, it seems that a solid solution of (MoW)C cannot be obtained without the presence of Co.

However, even if the (MoW)C powder obtained by this method is used as a raw material for a (MoW)C-Co alloy as a substitute for WC, MoC is not stable in the alloy and Mo2C often precipitates.

If even a small amount of Mo2C precipitates in the form of needles in the alloy, the strength of the alloy will deteriorate.

For these reasons, active consideration as a substitute for WC has not been made until now.

The present inventors conducted a detailed study on why Mo2C precipitates when a solid solution of MoC and WC is used as a raw material for cemented carbide.

As a result, the sintering mechanism of the (MoW) C-Co alloy was considered as follows.

Normally, in WC-Co cemented carbide, W-C-
A liquid phase of Co is generated, and a large amount of WC is dissolved in the Co.

Complete densification occurs simultaneously with the generation of this liquid phase.

Particle size can also be controlled by appropriately selecting the heating temperature and holding temperature.

In the cooling process, WC is precipitated from the Co at 1320° C. or below, but if the amount of carbon is insufficient at this time, the cobalt is cooled with the W dissolved in the cobalt.

Therefore, unless the amount of carbon is abnormally insufficient, two phases, WC and two phases (a phase with W in Co), are observed in the structure of the cemented carbide, and the alloy properties are relatively stable.

On the other hand, in (Mo/W)C-Co alloy, W in Co
Mo and C dissolve to produce a liquid phase, and densification progresses in the same way.

However, in the cooling process, elements dissolved in Co do not precipitate at the same temperature.

When only WC precipitates at the liquidus line of We-Co at 1320°C, only Mo and C remain in the Co binder phase.

MoC is stable in the form of Mo2C+C up to 1180°C, but if it is cooled as it is, it will not become MoC.

Therefore, even if the amount of carbon is close to the stoichiometric amount, it is cooled in the form of coexistence of Mo2C and carbon.

There is also a method in which the amount of free carbon that is precipitated is reduced in advance at the blending stage.

In this case as well, WC precipitates first, and in response to the lack of carbon, precipitates as MO2C, but since this Mo2C precipitates last, it becomes aggregated particles.

This Mo2C aggregate is undesirable because it deteriorates the alloy strength.

As mentioned above, in WC-Co alloys, W and C dissolved in Co combine to form WC crystals and precipitate, and the lack of carbon is adjusted by the amount of W dissolved in Co. As a result, the carbon content forms an IE normal range of about 0.1%.

However, in cemented carbide containing MoC, there are only two methods for adjusting the carbon content: (1) precipitation as free carbon, and (2) precipitation as Mo2C aggregates.

Therefore, in such alloys, the free carbon and Mo that precipitate
In order to prevent the agglomeration of 2C, there is no choice but to disperse the precipitates by rapid cooling treatment.

Although such a method can provide a sufficient cooling effect for small-sized alloys because they have a small heat capacity, it is difficult to apply to large-sized cemented carbides that have a large heat capacity.

Unless these essential sintering problems are solved (M
O-W)C solid solution cannot be used as a substitute for WC powder.As a result of repeated various sintering experiments, the present inventors found that
We have found the most suitable sintering conditions for sintering this alloy.

The present invention relates to a method for sintering a cemented carbide, which is a composite carbide of tungsten and molybdenum or a compound of molybdenum, in which a hard phase with a simple hexagonal crystal structure is bonded mainly with iron group metals, and one part of the sintering process. Part or all of the furnace is placed in a CO gas atmosphere, and in particular, CO gas is introduced into the furnace once at a temperature of 1180° C. or higher, which is the unstable temperature range of MoC.

The following two effects are considered to be caused by this CO gas.

One is that alloys containing MoC have a very narrow good carbon region, so variations in the amount of carbon in the alloy must be strictly controlled.

This method includes (1) removing oxygen in a hydrogen stream under reduced pressure to prevent decarburization fluctuations;

(2) To suppress decarburization after the appearance of a liquid phase and to bring the carbon content of the alloy closer to the theoretical carbon content.

For this purpose, it is preferable to use CO gas.

That is, the first effect is that by controlling the CO gas partial pressure in the furnace, the carbon potential in the furnace is determined, so it is possible to converge the alloy carbon content to a target value.

The second effect is to bring the amount of alloy carbon into a stable region where Mo2C does not precipitate, and furthermore, in the cooling process, MoC is reduced to 1% by Mo2C.
Decomposition can be prevented by CO gas.

This is because when these alloys are sintered in CO gas, CO
This is because the gas dissolves in cobalt, prevents the precipitation of free carbon dissolved in the atomic state of C and 0, and serves as a buffer for supplying C to Mo2C.

This is based on the following idea. The partial pressure of CO in the outside air is kept in equilibrium with respect to C and O dissolved in the co phase of the alloy as shown below.

ΔG0 = -58600 - 116.2 TJ/mol As a function of pressure, the amount of CO gas dissolved in the bath is (%O) x (%C) - PCO/Q, assuming that the activity is equal to the amount of C and O hemolyzed. (-3069/T-2
・34) (3) At 1350°C, equation (3) becomes (%O)×(%C)2Pco・6×10−5(4).

That is, when sintering is performed in vacuum (Pco = 10-4 atm) and in a CO atmosphere (Pco = 10-1 to 10-2 atm).
When baking with Co, about 100 times more oxygen and carbon can enter the Co binder.

As the amount of carbon increases, oxygen is supplied and ultimately released as CO, allowing excess free carbon to be removed.

Furthermore, when carbon is insufficient in the binder phase, carbon can be supplied and stabilized as Mo2C+C→MoC upon cooling.

From the above, it has been found that sintering in a CO gas flow has a significant effect on expanding the normal range of alloys containing MoC.

In the present invention, the most effective temperature is 1180°C or higher.

Since it easily decomposes into MoCMo2C+C at a temperature of 1180°C, it is possible to increase the carbon concentration in the Co binder by introducing CO gas.

If the temperature is below 1180°C, it is difficult for CO gas to enter the alloy and no effect is produced.

Further, even if CO gas is introduced at a temperature below 1180° C., no effect will be produced because the already decomposed Mo2C will not become MoC.

However, the temperature at which the CO gas is introduced may be at any stage above 1180°C.

This is because once the carbon concentration in Co is adjusted, the effect can be achieved.

However, in order to reliably prevent the decomposition of MoC, a cooling process is preferable.

On the other hand, it is desirable that the amount of CO gas input and the gas pressure in the furnace be under reduced pressure.

This is because as the CO gas concentration increases, reactions such as carburization and decarburization occur in a complicated manner.

Although only alloys whose main components are composite carbides of molybdenum and tungsten have been described above, the present sintering method is also effective with alloys in which a portion of carbon is replaced with nitrogen or oxygen.

It is also generally expressed as M(C,N,0)z (M=Ti,
(Zr, Hf, V, Nb, Ta, 0.5≦Z≦1.0) The effect is not lost even if it coexists with a so-called B1 type solid-state solid material.

Example 1 A monocarbide solid solution (Mo7W3)C having a Mo to W ratio of 7:3 (at%) was obtained.

86% by weight of this (Mo7W3)C and 14% by weight of Co powder were blended, and wet mixing was performed in an organic solvent using a ball mill.

After drying, the mixed powder was embossed and sintered at 1380° C. in vacuum.

The carbon content of the obtained alloy was as follows.

The alloy sintered under vacuum (10-2 Torr), which is the conventional sintering method, precipitates free carbon and has a bound carbon content of 98.7
It contained only %.

When looking at the structure of the alloy, free carbon was clearly seen and the properties of the alloy were poor.

On the other hand, after holding the stamped body in vacuum at 1380°C for 1 hour, CO gas was introduced into the furnace during the cooling process to reduce the CO in the furnace.
The partial pressure was set at 30 Torr.

The properties of the obtained alloy were as shown in Table 2.

When CO gas was introduced into the furnace, the amount of carbon increased by about 0.05%.

Further, the bonded carbon content reached the theoretical value of 99.6, and the alloy properties were also sufficiently satisfactory.

This proves that the carbon concentration in cobalt increased by increasing Pco, and that the decomposition reaction of MOC→MO2C+C was prevented by increasing the carbon concentration in cobalt. 70 g of Co was blended with 550 g of a composite carbide of tungsten and molybdenum consisting of Mo7w3)C and 380 g of a composite carbide consisting of (Ti5, Ta2W3)C, and mixed in a ball mill for 100 hours in an organic bath medium to obtain a mixed powder.

After stamping this, use a vacuum furnace to heat it to 1200℃ for 10 minutes.
Heated under a vacuum of -2 Torr.

For the temperature range from 1200° C. to 1400° C., CO gas was introduced into the furnace at a rate of 0.3 l/min, and the furnace pressure was set at 100 Torr by adjusting the furnace exhaust.

After raising the temperature to 1400°C under reduced pressure CO atmosphere, it was heated again to 10°C.
It was maintained under a vacuum of -2 Torr for 1 hour.

After heating is completed, turn off the power, stop heating, and at the same time introduce CO gas into the furnace to raise the internal pressure of the sintering furnace to 100 Torr.
A CO2 vacuum was applied.

In order to compare with the method of the present invention, the entire sintering process was carried out at 10-2
Samples made under a vacuum of Torr were also obtained and their properties were compared.

Table 3 summarizes the properties of alloys obtained by the method of the present invention and the conventional method.

Since a normal alloy can be obtained by the method of the present invention, the properties can be fully satisfied.

On the other hand, in the conventional sintering method, free carbon precipitates, resulting in low hardness and insufficient transverse rupture strength.

Claims (1)

[Claims] 1 A composite compound of tungsten and molybdenum or a compound of molybdenum, containing only one or more hard phases having a simple hexagonal crystal structure, or a Bl-type hard compound containing these hard phases and groups IVa, Va, and ■a. A cemented carbide whose phases are mainly bonded by iron group metals containing molybdenum, characterized in that the sintering process is partially or completely sintered in an atmosphere containing CO gas in a temperature range of 1180°C or higher. Manufacturing method of cemented carbide.