KR100594998B1 - Method for nitriding of Ti and Ti alloy - Google Patents

Abstract

The present invention relates to a method of nitriding a surface of a titanium-based metal such as titanium or a titanium alloy.

The method of the present invention comprises an initial purge step (I) of charging a nitride with a nitride furnace and purging the interior of the nitride furnace with either nitrogen gas or an inert gas; A first temperature rising and plasma sputtering step (II) for performing plasma sputtering at 300 ± 50 ° C .; A secondary temperature rising and plasma sputtering step (III) for performing plasma sputtering at 600 ± 50 ° C .; A third elevated temperature and nitriding step (IV) carried out at 700 to 800 ° C. under a mixed gas atmosphere of nitrogen and ammonia; It consists of a cooling step (V) to cool down to room temperature, and it is a technical feature of the present invention to volatilize the contaminants remaining on the surface of the nitride using plasma, and to activate the surface and to perform gas nitriding at low pressure. .

In the method of nitriding the titanium-based metal of the present invention, the nitriding treatment productivity is improved by surface activation using plasma, and it is possible to treat the nitride having a complicated shape, which is difficult to plasma nitrate treatment, and the amount of change in dimension before and after nitriding treatment is small. have.

Plasma, Gas Nitriding, Titanium, Purge

Description

Nitriding Method of Titanium Metals {Method for nitriding of Ti and Ti alloy}             

1 is a heat treatment cycle of the method of one embodiment of the present invention.

Figure 2 is a graph of the correlation between the nitride layer and the hardened layer thickness according to the nitriding temperature.

Figure 3 is a graph of the correlation between the thickness of the nitride layer and the cured layer and the dimensional change according to the nitriding temperature.

            ((Explanation of symbols for main part of drawing))

I. Initial Purge Phase II. First Temperature and Plasma Sputtering Steps

III. Second temperature rising and plasma sputtering step IV. Third temperature rising and nitriding step

Ⅴ.Cooling stage

The present invention relates to a method for nitriding titanium-based metals such as titanium or titanium alloy, and more particularly, to improve the surface strength and abrasion resistance of various products or parts made of titanium-based metals by using plasma. The present invention relates to a method of forming a nitride layer by performing gas nitriding on a surface of a titanium-based metal activated by plasma after activation under low pressure.

In general, nitriding treatment, which is one of the representative heat treatment methods performed on metal surfaces in order to improve wear resistance of various metals, is a surface treatment method applied to the surface of mechanical parts requiring high levels of wear resistance, fatigue resistance, and corrosion resistance. According to the nitriding method, the gas nitriding method, the salt bath nitriding method, the ion nitriding method, and the like, the gas nitriding method is a surface treatment method which infiltrates and diffuses the generator nitrogen generated by the following chemical reaction formula into steel.

NH ₃ → 3/2 H ₂ + [N]

In addition, the salt bath nitriding method is a method of nitriding at about 570 ° C. using a molten salt bath containing KCN + KCNO + Na ₂ CO ₃ as a main component, which makes it difficult to treat wastewater and control processes due to the use of toxic chemicals. The gasification method is used for the glow discharge between two electrodes generated by applying a DC voltage of several hundred to several thousand volts while putting two electrodes of a cathode and an anode in a closed vacuum vessel and reducing the pressure in the vessel to several millibars (mbar). It is also called plasma nitridation method as a method using the plasma produced | generated.

The conventional nitriding method as described above has been mainly performed on steel materials, but recently, various products or parts made of titanium-based metals have been developed, which are known as metals that are harmless to the human body, and have been developed on the surface of titanium-based metals to improve their wear resistance. Surface hardening treatment should be performed, look at the surface hardening treatment applied to titanium-based metals are as follows.

Titanium is a silver-white metal, similar in strength to carbon steel, but its weight-to-strength ratio is about twice that of iron and about six times that of aluminum. In addition to the material, it is also used as a corrosion resistant container material for the chemical industry.

However, since the titanium-based metal has a disadvantage of low surface hardness, the surface-hardening treatment should be performed to improve wear resistance or surface hardness, and the surface-hardening treatment method includes coating a hard film on the titanium-based metal surface. As a method of hardening the titanium-based metal surface itself, the method of coating the hard film has a limitation in adhesion between the titanium-based metal and the hard film, which causes a problem that the hard film is peeled off when used for a long time.

Therefore, it is necessary to perform self-curing treatment without the problem of peeling as described above, and nitriding treatment is mainly used as the method.

However, in the case of titanium-based metals, the titanium oxide layer formed on the surface interferes with the internal diffusion of generator nitrogen during the nitriding treatment. In the case of nitriding the titanium-based metals, vacuum exhaust is performed at a temperature of 580 ° C. or higher to remove titanium oxide. After that, plasma and gas nitriding, which are carried out at a temperature range of 700 to 800 ° C, are mainly performed.

The plasma nitriding is carried out by nitrogen gas being ionized near the cathode where the plasma is formed, and the ionized nitrogen atoms are accelerated at high speed to impinge on the surface of the nitride, thereby heating the nitride and allowing nitrogen to penetrate into the inside of the nitride. Plasma nitriding is advantageous in that the nitriding rate is high due to the high potential of nitrogen.

However, there is a disadvantage in that the deformation is large due to local overheating of the nitride due to the plasma heat source, and in the fine gap, the nitriding treatment is severely limited in the case of the precise product due to the unevenness of the glow discharge.

Moreover, when the shape of the nitride becomes complicated, such as small through holes or minute holes penetrating the nitride, plasma is not completely formed up to the inner surface thereof, so that the inner surface is properly nitrided. There is a problem in that the plasma processing apparatus is expensive, and thus the nitriding treatment cost is higher than that of gas nitriding.

In order to solve the problem of incomplete plasma nitridation treatment for complex shape nitrides as described above, gas nitriding was carried out at a temperature of 800 ° C. or higher under normal pressure, but the dimensional change due to high temperature heating and the decrease of physical properties due to grain coarsening were caused. Could not be avoided.

In order to solve the problems of the high temperature gas nitriding, a method of forming an oxide layer together with a nitride layer is disclosed in Korean Patent Publication No. 10-0301677 and Japanese Patent Application Laid-Open No. 2000-0067920.

Since the method forms a nitride layer and an oxide layer at a temperature of 700 to 800 ° C. by supplying a mixed gas of nitrogen and oxygen, the dimensional deformation is reduced compared to high temperature nitriding, and the surface hardness is further increased because the oxide layer is formed together with the nitride layer. There is an advantage that the scratch resistance is excellent, but there is a disadvantage that brittleness increases due to the effect of oxygen penetrated into the base material.

That is, although the scratch resistance is excellent in the case of an ornament or the like due to an increase in surface hardness, there is a problem that brittle fracture is caused in the case of mechanical parts that are subjected to shock or vibration due to increased brittleness.

The present invention is to form a titanium nitrogen hardened layer having high corrosion resistance and abrasion resistance of several tens to hundreds of micrometers thick on the surface of the titanium-based metal such as titanium or titanium alloy, it is possible to minimize the dimensional deformation and grain coarsening due to nitriding It is an object of the present invention to provide a complex nitriding method.

The object of the present invention is achieved by surface activation by plasma and low pressure gas nitridation atmosphere conditions maintained at 700 to 800 ° C. under a low pressure of 0.1 to 1000 mbar.

In the method of surface nitriding a titanium-based metal of the present invention, after activating the surface of the nitride using plasma, the nitriding treatment is a composite nitriding in which gas nitriding is performed instead of nitriding by plasma. In order to eliminate the influence of dissolved oxygen inside, the low-pressure condition as described above is formed, and ammonia gas is mixed with nitrogen gas, which is an atmospheric gas, as a processing gas at a predetermined ratio, thereby generating hydrogen generated by decomposition of ammonia gas. There is a technical feature of the invention in that dissolved oxygen is removed by.

At this time, the nitrogen gas and ammonia gas is preferably used in a mixture of 50 to 90 vol%: 50 to 10 vol% ratio, when the nitrogen gas is less than 50 vol%, embrittlement by hydrogen gas generated by decomposition reaction of ammonia gas This phenomenon occurs because if the nitrogen gas exceeds 90 vol%, the rate of nitriding reaction may be drastically lowered and productivity may be reduced.

In addition, since the nitriding in the method of the present invention is carried out at 700 to 800 ° C., when the nitriding temperature is less than 700 ° C., the grain coarsening phenomenon of the nitride caused by high temperature nitriding is effectively suppressed, but the time for forming the nitride hardened layer is long. The productivity may drop excessively, and if it exceeds 800 ° C, the nitride grains are coarsened and mechanical properties are degraded, so that nitriding is performed in the above temperature range. The suppression of nitriding and grain coarsening caused by high temperature gas nitriding can be minimized.

As described above, the nitriding method of the present invention in which nitride is nitrided using a mixed gas of nitrogen and ammonia at 700 to 800 ° C., as shown in FIG. 1, has an initial purge that starts after charging the nitride into a nitride furnace. Of step (I), first temperature rising and plasma sputtering step (II), second temperature rising and plasma sputtering step (III), third temperature rising and nitriding step (IV), and cooling step (V) It consists of a series of continuous processes in order, and each step is described in detail as follows.

The initial purge step performed after charging the nitride to the nitriding furnace involves evacuating the inside of the nitriding furnace to 0.01 mbar or less, and then injecting either nitrogen gas or an inert gas into the nitriding furnace so that the pressure inside the nitriding furnace is 0.1 to 1000 mbar. It is a process.

In addition, the first temperature raising and plasma sputtering step is a cleaning process using plasma, which is heated to 300 ± 50 ° C. in an initial purge state, and then purged in the same manner as in the initial purge step. Nitrogen gas and hydrogen gas were mixed at a ratio of 80 to 20 vol%: 20 to 80 vol%, and the internal pressure of the nitriding furnace was maintained at 0.1 to 10 mbar while maintaining a constant temperature and a voltage of 350 to 700 V and 2.5 to The plasma is generated for 0.5 to 2 hours by applying power at a current density of 5 mA / cm 2 to volatilize contaminants remaining in the surface of the nitride and the inside of the nitride by the plasma.

At this time, if the internal temperature of the nitriding furnace does not reach 250 ° C, volatilization of contaminants is insufficient, which adversely affects nitriding. When the temperature exceeds 350 ° C, the metastable ω phase starts to be generated during the aging treatment of α-β titanium. As it is embrittled, the temperature near 400 deg. C should be especially avoided, and the holding time may be added or subtracted at this time depending on the capacity of the nitriding furnace and the loading amount of the nitride.

When the temperature is reached, the purge is carried out in the same manner as the purge method performed in the initial purge step to remove the contaminants volatilized in the temperature increase step, and then enters the constant temperature process, by the plasma generated in the constant temperature process. Since the contaminants are volatilized, the plasma may be sputtered by generating plasma even at an elevated temperature in order to volatize the contaminants more effectively.

The second temperature raising and plasma sputtering step is a process of activating the surface of the nitride, and immediately after the first temperature rising and plasma sputtering step, a mixture of nitrogen gas and argon gas in a ratio of 80 to 20 vol%: 20 to 80 vol% is used. Purge in the same manner as in the first elevated temperature and plasma sputtering step to completely remove the remaining contaminants volatilized in the first elevated temperature and plasma sputtering step and to form a new in-furnace atmosphere. After heating to ℃, the plasma generated by applying the power at a voltage of 350 ~ 700V and a current density of 2.5 ~ 5mA / ㎠ while maintaining a constant temperature to activate the surface of the nitride for 0.5 to 2 hours to induce the initial nitriding This step may also generate plasma at elevated temperatures as in the first elevated temperature and plasma sputtering steps. have.

As in the first temperature rising and the plasma sputtering step, the second temperature rising and the sputtering step may further purge when the temperature rises, which minimizes the amount of contaminants and oxygen remaining in the furnace atmosphere. For this purpose, it may be performed only once after completion of the incubation process depending on the state of the nitride and the nitriding furnace.

In the second temperature raising and sputtering step, the constant temperature is within the above range because the temperature for maximizing the embrittlement of the hydrogen gas remaining inside the furnace by the dehydrogenation of the titanium-based metal nitride is 600 ± 50 ° C. In addition, the constant temperature holding time may be added to or subtracted from the time according to the capacity of the nitriding furnace and the loading amount of the nitride, as in the first temperature raising and constant temperature steps.

In the third temperature raising and nitriding step, after the second temperature rising and plasma sputtering step, the mixture is purged so that the pressure inside the nitriding furnace is 0.1 to 1000 mbar using a mixed gas of nitrogen and ammonia, and heated to 700 to 800 ° C., and then 5 to 21 As a core process of the present invention for forming a nitride layer on the surface of the nitride while maintaining a constant temperature for a time, purge may be additionally performed immediately after the elevated temperature as in the first and second elevated temperatures and the constant temperature step.

At this time, when the nitriding temperature exceeds 800 ℃, the grain is coarse to decrease the mechanical properties, if it does not reach 700 ℃ grain coarsening phenomenon can be suppressed, but the formation time of the nitride hardened layer increases the productivity sharply decreases As a result, the nitriding temperature range of 700 ° C to 800 ° C minimizes the inhibition of nitriding by titanium oxide caused by general gas nitriding and the grain coarsening caused by hot gas nitriding.

In addition, since the N ₂ + NH ₃ mixed gas is used for the purge nitriding performed in the third temperature raising and constant temperature step, the nitriding treatment in the method of the present invention is performed under an N ₂ + NH ₃ mixed gas atmosphere.

As described above, mixing and using ammonia gas with nitrogen gas as the nitriding gas, at least 70% of the ammonia gas is decomposed into nitrogen and hydrogen at the nitriding temperature, and the resulting nitrogen is formed by forming nitrogen compounds on the surface of the nitride material. In order to promote diffusion and to prevent hydrogen from reacting with the nitride and oxygen by remaining in the nitriding furnace, the mixing ratio of N ₂ : NH ₃ is 50 to 90 vol%: 50. It is preferable to set it as -10 vol%.

The final cooling step after nitriding is a process of purging with nitrogen gas or an inert gas immediately after the third elevated temperature and nitriding and then cooling to room temperature.

Summarizing the titanium-based metal nitride method of the present invention made as described above is shown in Table 1.

  division   Temperature (℃)    Hours   Vacuum degree (mbar)   Mood with boss  Plasma (on / off) Initial purge stage   Room temperature     -  0.1-1000 N ₂ or inert gas    off    1st temperature rise 300 ± 50 ℃   0.5 to 2   0.1 to 10 N ₂ + H ₂ on or off  Primary plasma     on    2nd temperature rise 600 ± 50 ℃   0.5 to 2   0.1 to 10 N ₂ + Ar on or off  Secondary plasma     on    3rd temperature rise 750 ± 50 ℃    5 to 21  0.1-1000 N ₂ + NH ₃    off    3rd constant temperature (nitridation stage)   Cooling stage   Rowing      -       - N ₂ or inert gas    off

2 and 3 show graphs of thickness changes of the cured layer and the nitride layer according to the nitriding time and the nitriding time according to the present invention.

It can be seen from FIG. 2 that as the nitride temperature is increased, the thickness of the nitride layer is increased, and the thickness of the cured layer is almost constant from around 760 ° C or the degree of increase is decreased.

In addition, it can be seen that the nitride object contracts at a temperature below the 760 ° C and expands at a temperature below that of 750 ° C. It is preferable to adjust the nitriding temperature accordingly.

In the above, the cured layer is a layer having a hardness of 50 points or more higher than the base material on the basis of the micro Vickers hardness, and the nitride layer is a surface layer that is clearly distinguished from the general cured layer as the surface layer of the cured layer.

As described above, the titanium-based metal nitriding method of the present invention is a complex nitriding process that utilizes only the advantages of overcoming the disadvantages of plasma nitriding and atmospheric gas nitriding. Because of the low pressure gas nitriding, nitriding is possible regardless of the shape conditions of the nitride, and the amount of change in dimension before and after nitriding is expected to be particularly effective for precision parts.

Claims (7)

In the method of nitriding a titanium-based metal part, an initial pressure in which the internal pressure becomes 0.1 to 1000 mbar after vacuum evacuating the inside of the nitriding furnace to 0.01 mbar or less with either nitrogen gas or an inert gas while the nitride is charged into the nitride furnace. Purge step (I); After heating up to 300 ± 50 ℃ in the initial purge state, evacuating the inside of the nitriding furnace with a mixed gas of nitrogen gas and hydrogen gas to 0.01 mbar or less, and then allowing the pressure inside to be 0.1 to 1000 mbar and plasma sputtering at constant temperature. Performing a first elevated temperature and plasma sputtering step (II); Immediately after the first temperature rising and plasma sputtering step, the inside of the nitriding furnace was evacuated to 0.01 ± mbar by a mixed gas of nitrogen gas and argon gas, and then heated to 600 ± 50 ° C. in a state such that the internal pressure was 0.1 to 1000 mbar. A secondary temperature rising and plasma sputtering step (III) for carrying out plasma sputtering while maintaining; Immediately after the second temperature rising and plasma sputtering step, the interior of the nitriding furnace was evacuated to 0.01 mbar or less using a mixed gas of nitrogen and ammonia, and then heated to 700 to 800 ° C. in a state such that the internal pressure became 0.1 to 1000 mbar. A third elevated temperature and nitriding step (IV) to maintain constant temperature for ˜21 hours; Cooling step (V) after evacuating the interior of the nitriding furnace to 0.01 mbar or less by using any one of nitrogen gas or inert gas immediately after the third temperature increase and nitriding, and then internalizing the pressure to 0.1 to 1000 mbar. ; Nitriding method of the titanium-based metal, characterized in that comprises a. delete The method of claim 1, wherein the mixed gas of the primary temperature rising and plasma sputtering step (II) is a gas in which nitrogen gas and hydrogen gas are mixed at a ratio of 80 to 20 vol%: 20 to 80 vol%. Nitriding method. The titanium-based metal nitride according to claim 1, wherein the mixed gas of the secondary temperature rising and plasma sputtering step (III) is a gas in which nitrogen gas and argon gas are mixed at a ratio of 80 to 20 vol%: 20 to 80 vol%. Way. The method of claim 1, wherein the mixed gas of the third temperature raising and nitriding step (IV) is a gas in which nitrogen gas and ammonia gas are mixed at a ratio of 50 to 90 vol%: 50 to 10 vol%. . delete delete

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