RU2562185C1 - Modification method of surface of items from titanium alloys in vacuum - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: modification method of surface of items from titanium alloys is performed by nitriding in glow discharge in vacuum chamber in gas mixture 15 wt % of nitrogen and 85 wt % of argon at temperature 650-700°C by vacuum heating in plasma with increased density with effect of hollow cathode. Plasma of increased density is formed between the part and screen made with holes and out of titanium alloy.

EFFECT: intensification of the nitriding and increased hardness and contact wear resistance of strengthened alloy of titanium alloys at lesser pressure during nitriding and lesser holding time.

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Description

The invention relates to the metallurgical industry, and in particular to chemical-thermal surface treatment of products made of titanium alloys, and can be used in the manufacture of engine parts operating in conditions of wear, in medicine and other industries.

A known method of chemical-thermal treatment of parts made of titanium alloys (RF patent No. 2460826, C23C 8/54. 05/18/2011), which includes saturating the surface of the parts with nitrogen and carbon in a crucible or electrode bath with molten salts, heated to a temperature of 800 ° C, this uses a molten salt of the following composition, wt.%: NaCN 10, NaCl 40, BaCl ₂ 50.

The disadvantages of this method are:

- environmentally harmful production;

- a decrease in the efficiency of nitrogen diffusion deep into titanium alloys, since the process proceeds in an open atmosphere, which leads to the formation of an oxide film on the surface.

A known method of surface hardening of products from titanium and titanium alloys (RF patent No. 2318077, C23C 8/06. 07/04/2006), which is carried out using heat treatment. Heat treatment is carried out in an active gas environment. Then carry out a partial removal of the gas-saturated layer having increased fragility, etching. The depth of the zone with increased fragility is determined by the formula, and the depth can also be determined by the average distance between the cracks formed in the gas-saturated layer upon fracture of the sample by bending.

The disadvantages of this method are:

- high complexity;

- reduction of the resource of work in conditions of intensive wear, since during processing by this method the etchant can remove part of the diffusion zone from the surface of the parts.

A known method of nitriding steel products in a glow discharge (RF patent No. 2276201, C23C 8/36. November 9, 2004), which is carried out by vacuum heating the products in high-density nitrogen plasma formed between the part and the screen due to the hollow cathode effect. The nitriding process is carried out at a temperature of 700-750 ° C. After nitriding, surface quenching is carried out by cooling in an argon stream at a rate exceeding the critical rate of steel quenching.

The disadvantages of this method are:

- the impossibility of nitriding titanium alloys in high-density plasma, since the use of steel screens can lead to the penetration of atomized iron particles on the treated surface and blocking the diffusion of nitrogen deep into the treated surface;

- a decrease in the efficiency of nitrogen diffusion deep into titanium alloys, since nitriding is carried out in a nitrogen medium, which leads to the formation of a continuous nitride film on the surface.

A known method of hardening titanium alloys in a gaseous medium (RF patent No. 2365671, C23C 8/80. 06.12.2007), which carry out high-temperature nitriding at temperatures of 700-750 ° C for 10-30 minutes Then conduct reductive annealing in argon at a temperature exceeding the nitriding temperature by 100-150 ° C, the annealing time is calculated by the formula:

τ _anne = 0.75 · (K _nitrogen / K _r ) · exp (E _p / RT _T anne -Ε _nitrogen / RΤ _nitrogen ) · τ _nitrogen ,

where K _nitrogen , K _p - empirical coefficients, taking into account, respectively, the rate of formation and dissolution rate of the nitride gas-saturated layer, μm ² / sec;

E _nitrogen is the activation energy of the process that controls the increase in nitrogen concentration in the embrittled nitriding layer, J / mol;

E _p is the activation energy of the process controlling the decrease in the nitrogen concentration in the nitrided embrittled layer, J / mol;

R is the gas constant, J / (K · mol);

T _nitrogen - nitriding temperature, K;

T _anne — temperature of reductive annealing, K;

τ _nitrogen - nitriding time, sec.

The disadvantages of this method are:

- a decrease in the efficiency of the saturation process during nitriding, since high-temperature treatment in an open atmosphere leads to the formation of oxide films on the treated surface;

- short duration of nitriding, as a result of which a small thickness of the diffusion zone is formed, or the diffusion zone is not formed.

A known method of hardening the surface of products made of titanium alloys (RF patent No. 2427666, C23C 8/36, 12/21/2009), which is carried out by heating the surface of the product in a nitrogen environment, while heating is carried out by a concentrated heat source with a power density of 10 ³ -10 ⁴ W / cm ² , a current of 80-150 A and a speed of movement of the source relative to the product of 0.005-0.01 m / s.

The disadvantages of this method are:

- uneven hardening of the surface due to the lack of timely calibration of the plasma head of the installation during processing;

- the complexity of the process associated with the installation and alignment of the product in the device;

- reducing the diffusion of nitrogen deep into the titanium alloys, since the process is carried out in a nitrogen environment, which leads to the formation of a continuous nitride film on the surface;

- high nitrogen consumption.

The closest in technical essence and the achieved effect to the claimed is a method of surface modification of titanium alloy products (RF patent No. 2346080, C23C 8/02, 01/25/2007), which is carried out using electrospark alloying of the surface layer with subsequent oxidation or nitriding. Spark alloying is carried out with nitride-forming elements or alloys based on them. Then carry out thermal oxidation in an oxidizing air at a temperature of 600-800 ° C for 2-16 hours or diffusion nitriding, carried out in catalytically prepared gas ammonia environments at a temperature of 500-680 ° C for 15-40 hours.

The disadvantages of the prototype are:

- high complexity;

- long duration of the process;

- nitriding of the titanium alloy is carried out in an ammonia medium, which can lead to embrittlement of the surface due to the formation of titanium hydrides.

The problem to which the invention is directed, is to increase the productivity of the nitriding process of titanium alloys.

The technical result is the intensification of the nitriding process and increasing the hardness and contact wear resistance of the hardened layer of titanium alloys, with less pressure on the working process and shorter holding time.

The problem is solved and the technical result is achieved through a method of modifying the surface of titanium alloy products, including surface nitriding, according to which, according to the invention, nitriding is carried out in a vacuum chamber in a gas mixture of 15 wt.% Nitrogen and 85 wt.% Argon at a temperature of 650- 700 ° C by vacuum heating in a plasma of increased density with the effect of a hollow cathode, and plasma of increased density is formed between the part and the screen made with holes and made of titanium alloy.

High density plasma is provided due to the hollow cathode effect.

The hollow cathode effect is manifested in a significant increase in current density, an increase in the degree of ionization, while reducing the discharge burning voltage.

Nitriding is carried out at a temperature of 650-700 ° C, which is lower than the polymorphic α → β transformation, as a result of which there is no growth of structural components.

The screen is made of titanium alloy in the form of a plate with holes.

The invention is illustrated by drawings.

In FIG. 1 shows the structure of the initial state of the VT6 titanium alloy. In FIG. Figure 2 shows the screen parameters for creating the hollow cathode effect, where a is the diameter of the hole, b is the distance between the centers of the holes. In FIG. 3 depicts a titanium alloy shield to create a hollow cathode effect. In FIG. Figure 4 shows the microstructure of a VT6 titanium alloy sample after nitriding in a glow discharge (I — nitrided layer, II — base). In FIG. 5 shows the microstructure of a VT6 titanium alloy sample after nitriding in a glow discharge with the effect of a hollow cathode (I — nitrided layer, II — base). In FIG. 6 shows a diagram of the implementation of the method of ion nitriding of a titanium alloy in a glow discharge with a hollow cathode effect.

The circuit contains a power source 1, anode 2, cathode 3, cathode-part 4, high-density plasma 5, a screen 6 made of a titanium alloy in the form of a plate with holes mounted at a certain distance from the cathode-part 4, the casing is made of metal in a vacuum chamber 7.

An example of a specific implementation of the method.

The method is as follows: in a vacuum chamber (Fig. 6) at a certain distance from the surface to be machined, a screen (Fig. 3) is made of a titanium alloy with the calculated parameters a and b (b = 2 a ) (Fig. 2), detail and the screen is connected to the negative electrode, the chamber is sealed and air is pumped out to a pressure of 133 Pa. After evacuation of air, the chamber is purged with working gas for 5-15 minutes at a pressure of ~ 1330 Pa, then the chamber is pumped out to a pressure of 20-30 Pa, voltage is applied to the electrodes and a glow discharge is excited. At a voltage of 900-1100 V, cathodic sputtering is carried out at this stage. After a 5-20 minute surface treatment according to the cathodic sputtering mode, the voltage is reduced to the working one, and the pressure is increased to 90 Pa. The working mixture has a gas composition of 15 wt.% Nitrogen and 85 wt.% Argon.

Using the hollow cathode (EPC) effect that occurs in the cavity between the screen and the surface to be treated, the component is heated and nitrided in a high-density plasma, which increases the surface hardness of the titanium alloy.

The study of the microhardness of the surface of the samples showed that after nitriding in a glow discharge with EPA, the microhardness increased by 4.1 times, and without EPA by 2.6 times compared to the initial state (see table of microhardness from the surface of nitrided samples).

The increase in microhardness is due to the formation of titanium nitrides on the surface of the samples. It was found that the microhardness of the sample after nitriding under the conditions of EPA manifestation is 1.6 times higher than nitriding without EPA, which is associated with a decrease in pressure to 90 Pa and an increase in the concentration of nitrogen ions in the cathode cavity under the action of oscillating electrons.

In FIG. 5 shows the microstructure after nitriding with the hollow cathode effect of the VT6 titanium alloy, and FIG. 4. shows the microstructure after ion nitriding without the effect of a hollow cathode. An analysis of microstructures showed that nitriding under the conditions of the manifestation of EPA compared with nitriding without EPA is more than twice as advantageous in the thickness of the resulting nitrided layer as compared to ion nitriding. A distinct separation is observed in zones, while the transition from the nitrided layer to the underlying layers is smooth.

As a result, an increase in the surface hardness of VT6 titanium alloy was obtained, thereby providing an increase in contact wear resistance with a decrease in processing time and a decrease in energy characteristics (U, I).

Using EPA, the surface of the part is uniformly heated to the required temperature, while the working pressure in the chamber is reduced, which reduces the possibility of ignition of the arc, and the surface saturation rate also increases.

Claims (1)

A method of modifying the surface of titanium alloy products by nitriding in a glow discharge, characterized in that nitriding is carried out in a vacuum chamber in a gas mixture of 15 wt.% Nitrogen and 85 wt.% Argon at a temperature of 650-700 ° C by vacuum heating in high density plasma with the effect of a hollow cathode, and a plasma of increased density is formed between the part and the screen made with holes and made of a titanium alloy.

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