CN110565047B

CN110565047B - Titanium alloy surface nitriding process

Info

Publication number: CN110565047B
Application number: CN201910983906.2A
Authority: CN
Inventors: 冯志浩; 孙信阳; 秦雪明; 孟致安; 胡希磊; 韩鹏彪
Original assignee: Hebei University of Science and Technology
Current assignee: Hebei University of Science and Technology
Priority date: 2019-10-16
Filing date: 2019-10-16
Publication date: 2021-11-16
Anticipated expiration: 2039-10-16
Also published as: CN110565047A

Abstract

The invention specifically discloses a titanium alloy surface nitriding process. The method comprises the following steps: putting the titanium alloy matrix with the surface subjected to pretreatment into a vacuum nitriding device, vacuumizing, and heating the vacuum nitriding device to a nitriding temperature; the nitriding temperature is 400-1300 ℃; nitriding the vacuumIntroducing nitriding gas into the titanium alloy, nitriding for 0.1-15h, introducing inert gas, and cooling to room temperature to form a titanium nitride layer on the surface of the titanium alloy; the nitriding gas is nitrogen and argon in a volume ratio of 1: 0-5. The surface nitriding process of the titanium alloy can prepare modified titanium alloys with different properties at low temperature and high temperature according to the specific requirements of the service of the titanium alloy, wherein the thickness of the titanium nitride layer can reach 20-200 mu m, and the highest hardness can reach 2100HV_0.5The average abrasion weight loss is 1.67-3.14g, and compared with the matrix hardness and the abrasion resistance, the matrix hardness and the abrasion resistance are obviously improved.

Description

Titanium alloy surface nitriding process

Technical Field

The invention relates to the technical field of titanium alloy surface modification, in particular to a titanium alloy surface nitriding process.

Background

Compared with other metal materials, the titanium alloy has the advantages of low density, high specific strength, corrosion resistance, low temperature resistance and high temperature resistance, and is widely applied to the fields of military industry, nuclear industry, chemical industry, automobile industry and the like. However, the titanium alloy has a low surface hardness, a poor wear resistance, and a high sensitivity to adhesive wear and fretting wear, and thus its range of use is greatly limited. How to improve the hardness and wear resistance of titanium alloys has become one of the hot spots in the field of titanium alloys.

The titanium nitride has the advantages of high melting point, high hardness, excellent wear resistance, excellent high-temperature stability and the like. The preparation of the titanium nitride modified layer on the surface of the titanium alloy is an effective measure for improving the surface hardness, improving the wear resistance, prolonging the service life and expanding the application range of the titanium alloy. At present, a titanium nitride modified layer is prepared on the surface of a titanium alloy by mainly adopting magnetron sputtering, ion nitriding, laser gas nitriding and gas nitriding methods. An obvious interface exists between the film layer and the substrate treated by the magnetron sputtering method, the bonding strength is poor, and the coating is thin; ion nitriding cannot process parts with complex shapes, and the cost is high; laser nitriding is prone to structural defects (e.g., porosity, etc.) and cracks. The gas nitriding is simple and easy to implement, has low cost, can form nitride hard phase on the surface of the titanium alloy, and obviously improves the wear resistance and the corrosion resistance, thereby being widely applied. However, the existing gas nitriding is generally carried out at low temperature, and has the defects of slow nitriding speed, thin permeated layer, brittle permeated layer, overlong treatment time and the like, the prepared modified titanium alloy can not meet the requirement of long-term service of industrial production,

disclosure of Invention

Aiming at the problems that nitriding is generally carried out at low temperature in the prior art, and the defects of low nitriding speed, thin diffusion layer, brittle diffusion layer, overlong processing time and the like exist, the invention provides a titanium alloy surface nitriding process.

In order to solve the technical problems, the technical scheme provided by the invention is as follows:

a surface nitriding process for titanium alloy specifically comprises the following steps:

step a, putting a titanium alloy matrix with a pretreated surface into a vacuum nitriding device, vacuumizing, and heating the vacuum nitriding device to a nitriding temperature; the nitriding temperature is 400-1300 ℃;

and b, introducing nitriding gas into the vacuum nitriding device, nitriding for 0.1-15h, introducing inert gas, and cooling to room temperature to form a titanium nitride layer on the surface of the titanium alloy.

Compared with the prior art, the modified titanium alloy with different properties can be prepared at low temperature and high temperature according to the specific requirements of the service of the titanium alloy, and the problems of rough surface, cracks, uneven tissue and the like of the modified titanium alloy prepared by the existing preparation process are solved. The thickness of the titanium nitride layer of the modified titanium alloy prepared by the nitriding process provided by the invention can reach 20-200 mu m, and the highest hardness can reach 2100HV_0.5The average abrasion weight loss is 1.67-3.14g, the friction coefficient is 0.15-0.36, compared with the matrix hardness and the abrasion resistance, the prepared titanium nitride layer has the advantages of fine and uniform crystal grains, flat and crack-free surface, simple process, strong controllability and wide application prospect.

Preferably, in the step a, the nitriding temperature is 400-900 ℃.

Preferably, when the nitriding temperature is 400-900 ℃, in the step b, the nitriding gas is a mixed gas of nitrogen and argon in a volume ratio of 1: 0-1.

Preferably, when the nitriding temperature is 400-900 ℃, the nitriding time in the step b is 3-15 h.

Nitriding is carried out at the low temperature of 400-900 ℃, and mixed gas of nitrogen and argon with the volume ratio of 1:0-1 is adopted, so that nitrogen atoms can be uniformly diffused in matrix tissues, the flatness of a nitriding layer can be improved, the nitriding layer has no defects of pits, bulges and the like, and the wear resistance and hardness of the nitrogen carbide layer are improved.

Preferably, in the step a, the nitriding temperature is 900-1300 ℃.

Preferably, in the step b, the nitriding gas is a mixed gas of nitrogen and argon in a volume ratio of 1: 1-5.

The nitriding temperature is an important factor influencing the diffusion rate of nitrogen atoms, and the higher the temperature is, the higher the diffusion speed is, and the larger the thickness of a diffusion layer is. However, at higher temperatures (above 950 ℃), the nitride layer increases in thickness, and due to the difference in the coefficients of expansion between the nitride layer and the substrate, the nitride layer can generate a large stress on the substrate during the nitriding process of the titanium alloy, so that the film layer cracks, and the cracked and exposed substrate continues to react with nitrogen, so that the surface layer structure becomes loose. Thus, the prior art generally selects nitriding at temperatures up to 850-.

The nitrogen-argon ratio is controlled within the range of 1:1-5, the content of nitrogen in the mixed gas is controlled, so that the nitrogen absorption speed and the diffusion rate of nitrogen atoms in a matrix are controlled, the nitriding is smoothly realized at the temperature of above a phase transition point of 900 plus 1300 ℃, and the problems of coarse grains, uneven structure, cracks and the like of a nitrided layer, which are easily caused in the high-temperature nitriding, are avoided. When the nitrogen-argon ratio exceeds 1:1, high-density nitride is concentratedly distributed on the surface layer, so that the elastic modulus of titanium nitride prepared on the surface layer is increased, the brittleness is increased, and defects such as cracks are generated.

Preferably, in step b, the nitriding time is 0.1-3 h.

The longer the nitriding time, the larger the thickness of the carburized layer. But when the nitriding time reaches a certain value, the speed of increasing the thickness of the infiltrated layer is reduced, but the tissue defects are increased, and the optimal high-temperature nitriding time can effectively reduce the tissue defects on the premise of ensuring the thickness of the infiltrated layer.

Preferably, in the step a, the temperature is raised to the nitriding temperature by adopting a temperature programming mode, and the temperature raising rate is 1-10 ℃/min.

The preferred rate of temperature increase may result in a more uniform titanium alloy structure.

Preferably, in the step b, the temperature is reduced to room temperature in a programmed cooling mode, and the cooling rate is 0.5-3 ℃/min.

The preferable cooling rate can avoid the problem that the nitride layer and the matrix structure are too stressed to fall off.

Preferably, in step b, the introduction rate of the nitriding gas is 1-10L/min.

The preferred introduction rate of the nitriding gas is beneficial to increasing the nitrogen content of each thickness part in the nitrided layer, thereby improving the strength of the whole carburized surface.

Alternatively, the inert gas in step b may be an inert gas conventional in the art, such as argon, helium, and the like. The inert gas is continuously introduced from the end of nitriding until the nitrided sample is cooled to room temperature

Preferably, in step a, vacuum is applied until the vacuum degree is less than 0.1 Pa.

Before nitriding, the device is vacuumized until the vacuum degree is lower than 0.1Pa, so that other gases and impurities adsorbed on the surface of the titanium alloy substrate can be desorbed and discharged out of the nitriding device, the surface of the titanium alloy can be further purified, the absorption and diffusion of nitrogen atoms in the nitriding process can be promoted, and the nitriding efficiency can be improved; meanwhile, the harmful atmosphere in the nitriding device can be reduced, other gases are effectively prevented from forming an adsorption layer on the surface of the titanium alloy substrate, and oxidation, hydrogen embrittlement and black tissues are prevented from being generated.

Preferably, in the step a, the surface of the titanium alloy substrate is pretreated by the following method: and (3) sequentially grinding the titanium alloy substrate by using 100-mesh, 200-mesh and 500-mesh water-grinding abrasive paper, then ultrasonically cleaning for 20-30min under the condition of power of 30-50Hz, and drying by cold air.

The optimal pretreatment method for the titanium alloy surface can effectively remove impurities adsorbed on the surface and reduce the generation of the defects of the nitriding layer.

The nitrogen and the argon are high-purity gases with the purity of more than or equal to 99.99 percent. The titanium alloy substrate may be a titanium alloy conventional in the art, such as a Ti-6Al-4V, TB6, TC4, or TA15 titanium alloy, and the like. The nitriding apparatus may be a vacuum furnace or an atmosphere tube furnace.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to these drawings without creative efforts.

FIG. 1 is a cross-sectional view of a titanium nitride layer on the surface of a titanium alloy prepared in example 1 of the present invention;

FIG. 2 is a cross-sectional view of a titanium nitride layer on the surface of a titanium alloy prepared in example 2 of the present invention;

FIG. 3 is a cross-sectional view of a titanium nitride layer on the surface of a titanium alloy prepared in example 3 of the present invention;

fig. 4 is a graph showing hardness distribution of the titanium nitride layer on the surface of the titanium alloy in examples 1 to 3 of the present invention.

Fig. 5 is a bar graph of the wear loss of the titanium nitride layer and the titanium alloy matrix prepared in examples 1 to 3 of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

In order to better illustrate the invention, the following examples are given by way of further illustration.

Example 1

A surface nitriding process for titanium alloy comprises the following steps:

step one, pretreatment of the surface of a titanium alloy substrate: cutting Ti-6Al-4V titanium alloy material into 15mm multiplied by 80mm multiplied by 10mm, successively grinding by 100-mesh, 200-mesh and 500-mesh water grinding abrasive paper, then cleaning for 25min under the condition of 40Hz frequency by ultrasonic waves, and drying by a blower in a cold air mode;

secondly, putting the titanium alloy matrix into an SK-1600 ℃ series vacuum/atmosphere tubular electric furnace nitriding device, sealing the device, and vacuumizing the nitriding device to below 0.1Pa by a vacuum pump;

setting the temperature rise rate of the nitriding device to be 5 ℃/min, raising the temperature to the nitriding temperature of 600 ℃, and maintaining the vacuum environment in the furnace;

and step four, introducing high-purity nitrogen into the nitriding device at the flow rate of 3L/min, nitriding for 10 hours, and cooling to room temperature at the speed of 3 ℃/min.

Example 2

A surface nitriding process for titanium alloy comprises the following steps:

step one, pretreatment of the surface of a titanium alloy substrate: cutting a TB6 titanium alloy material into 15mm multiplied by 80mm multiplied by 10mm, sequentially polishing the titanium alloy material by 100-mesh, 200-mesh and 500-mesh water-grinding abrasive paper, then cleaning the titanium alloy material for 30min by using ultrasonic waves under the condition of 35Hz, and drying the titanium alloy material by using a blower in a cold air mode;

setting the heating rate of the nitriding device to be 8 ℃/min, raising the temperature to the nitriding temperature of 950 ℃, and maintaining the vacuum environment in the furnace;

and step four, introducing mixed gas of nitrogen and argon in a volume ratio of 1:2 into the nitriding device at a flow rate of 5L/min, nitriding for 1h, and cooling to room temperature at a speed of 1 ℃/min.

Example 3

A surface nitriding process for titanium alloy comprises the following steps:

step one, pretreatment of the surface of a titanium alloy substrate: cutting a TA15 titanium alloy material into 15mm multiplied by 80mm multiplied by 10mm, sequentially polishing the material by 100-mesh, 200-mesh and 500-mesh water-grinding abrasive paper, then cleaning the material for 20min by using ultrasonic waves under the condition of 45Hz frequency, and drying the material by using a blower in a cold air mode;

setting the heating rate of the nitriding device to be 10 ℃/min, raising the temperature to the nitriding temperature of 1200 ℃, and maintaining the vacuum environment in the furnace;

and step four, introducing mixed gas of nitrogen and argon in a volume ratio of 1:5 into the nitriding device at a flow rate of 8L/min, nitriding for 0.5h, and cooling to room temperature at a speed of 0.5 ℃/min.

The TiN layers prepared in examples 1 to 3 were subjected to structural characterization, and the surface and cross-sectional morphology of the coating was observed by using a Scanning Electron Microscope (SEM), as shown in fig. 1 to 3, it can be seen from the figure that the thickness of the TiN layer prepared in example 1 was about 20 μm, the thickness of the TiN layer prepared in example 2 was about 50 μm, the thickness of the TiN layer prepared in example 3 was about 200 μm, and the TiN layer had uniform texture transition, exhibited good metallurgical bonding, compact texture structure, and no structural defects such as pores and cracks were found.

The hardness distribution of the TiN layers prepared in examples 1 to 3 was measured by a micro Vickers hardness tester, and the results are shown in FIG. 4, from which it can be seen that the hardness of the titanium nitride coating in the nitrided region after nitriding treatment can reach up to 2100HV_0.5(hardness of the base alloy is about 280 HV)_0.5). The titanium nitride coating prepared by surface nitriding can obviously improve the hardness of the surface layer.

The frictional properties of the TiN coating layers and the substrate prepared in examples 1-3 were measured respectively using an MMW-1 type microcomputer controlled universal frictional wear tester, and the results are shown in FIG. 5. A quenched 45-grade steel small pin disc friction pair with the diameter of 32mm is used as a counterpart, the effective wear diameter of the friction pair is 25mm, and all samples rotate for 10min under the condition that the load is 20N and the rotating speed is 5 r/min.

The wear rate is the amount of wear loss per unit wear area. The formula for calculating the wear rate can be represented by the following formula:

in the formula: m is the wear loss before and after wear, d is the rotation diameter, and n is the rotation speed of rotation.

As can be seen from the figure, the wear resistance of examples 1, 2 and 3 is respectively improved by 8, 10 and 15 times relative to the matrix structure, which shows that the titanium nitride layer has good friction and wear resistance and friction reduction effect.

The wear loss and coefficient of friction results for examples 1-3 are summarized in Table 1.

TABLE 1

Numbering	Loss of wear (g)	Coefficient of friction
			Example 1	3.14	0.36
Example 2	2.51	0.28
			Example 3	1.67	0.15
Ti-6Al-4V titanium alloy matrix	25.12	0.58

Example 4

A surface nitriding process for titanium alloy comprises the following steps:

setting the heating rate of the nitriding device to be 1 ℃/min, raising the temperature to the nitriding temperature of 400 ℃, and maintaining the vacuum environment in the furnace;

and step four, introducing high-purity nitrogen into the nitriding device at the flow rate of 10L/min, nitriding for 15h, and cooling to room temperature at the speed of 0.5 ℃/min.

The TiN layer prepared in this example had a thickness of about 90 μm and a hardness of 1820HV_0.5The abrasion loss was 2.1 g.

Example 5

A surface nitriding process for titanium alloy comprises the following steps:

step one, pretreatment of the surface of a titanium alloy substrate: cutting a TA15 titanium alloy material into 15mm multiplied by 80mm multiplied by 10mm, sequentially polishing the material by 100-mesh, 200-mesh and 500-mesh water-grinding abrasive paper, then cleaning the material for 25min by using ultrasonic waves under the condition of 40Hz, and drying the material by using a blower in a cold air mode;

setting the heating rate of the nitriding device to be 10 ℃/min, raising the temperature to the nitriding temperature of 1300 ℃, and maintaining the vacuum environment in the furnace;

and step four, introducing mixed gas of nitrogen and argon in a volume ratio of 1:5 into the nitriding device at a flow rate of 1L/min, nitriding for 0.1h, and cooling to room temperature at a speed of 3 ℃/min.

The SK-1600 ℃ series vacuum/atmosphere tubular electric furnace in examples 1 to 5 has a rated power of 4KW and a maximum hearth temperature of 1700 ℃. The vacuum pump adopts an F100/110 molecular pump as a main working pump, is connected through standard accessories, is completely made of stainless steel materials, is subjected to electro-polishing treatment, has a pre-pumping function, and has a limiting vacuum degree superior to 1.0 multiplied by 10^-3Pa。

The TiN layer prepared in this example had a thickness of about 150 μm and a hardness of 1900HV_0.5The abrasion loss was 1.93 g.

Comparative example 1

A method for preparing TiN coating on the surface of titanium alloy comprises the following steps:

step one, selecting a test sample as TC4 titanium alloy, cutting TC4 titanium alloy matrix warp into the size of 30mm x 22mm x 6mm, successively polishing 100-mesh, 200-mesh and 500-mesh water-mill sandpaper, then cleaning for 20min by using ultrasonic waves under the condition that the frequency is 45Hz, and drying by using a blower in a cold air mode;

secondly, placing the pretreated TC4 titanium alloy matrix into a gas supply controller, and laterally conveying high-purity nitrogen through a gas supply nozzle, wherein the purity of the nitrogen is more than 99.999%; the air supply parameters are set as follows: the distance between a nozzle opening and a light spot is 5-15 mm, and the nitrogen flow rate is controlled to be 20L/min by a nitrogen flow rate meter;

thirdly, after the TC4 titanium alloy matrix is subjected to air supply treatment for 5-10 s, opening an optical fiber coupling all-solid-state laser to perform laser nitriding treatment on the surface of the TC4 titanium alloy matrix; the laser parameters are set as follows: the focal length of the laser is 20mm, and the diameter of a light spot is 3 mm; the center distance of the laser focusing lens to the sample processing surface is 200mm, the laser power is 800w, the spot speed is 300mm/min, and the lap joint rate is 20%; the laser nitriding treatment time is 3min, and the TiN coating is prepared.

The hardness of the TiN layers prepared in examples 1-5 and comparative examples 1-2 was measured using a micro Vickers hardness tester, and the frictional properties of the TiN coating and the substrate were measured respectively using a MMW-1 type microcomputer controlled universal frictional wear tester, and the increase in hardness and wear resistance compared to the substrate material Ti-6Al-4V titanium alloy substrate was calculated, and the results are summarized in Table 2.

TABLE 2

In summary, the titanium nitride layer prepared by the present invention has a compact texture structure and fine grains (about 50nm), and the hardness of the titanium nitride layer is significantly improved compared to the conventional process (laser gas nitridation). The hardness value from the base material to the surface of the titanium nitride layer is gradually increased, and the maximum value can reach 3-7 times of the hardness of the base. The friction and wear test shows that: after nitriding, the friction coefficient, the abrasion loss and the abrasion mark depth of the titanium nitride layer are all smaller than those of a base body and a titanium nitride coating prepared by the traditional process, and the abrasion resistance is improved by 8-15 times. The prepared titanium nitride layer has the advantages of uniform tissue distribution, no oxide inclusion, good high-temperature stability, and high hardness and wear resistance.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents or improvements made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A surface nitriding process for titanium alloy is characterized by comprising the following steps:

step a, putting a titanium alloy matrix with a pretreated surface into a vacuum nitriding device, vacuumizing, and heating the vacuum nitriding device to a nitriding temperature; the nitriding temperature is 1200-1300 ℃, the temperature is raised to the nitriding temperature by adopting a programmed heating mode, and the heating rate is 1-10 ℃/min;

b, introducing nitriding gas into the vacuum nitriding device, nitriding for 0.1-3h, introducing inert gas, and cooling to room temperature to form a titanium nitride layer on the surface of the titanium alloy; wherein the nitriding gas is a mixed gas of nitrogen and argon in a volume ratio of 1:2-5, the introduction rate of the nitriding gas is 1-10L/min, and the nitriding gas is cooled to room temperature by adopting a programmed cooling mode, wherein the cooling rate is 0.5-3 ℃/min.

2. The process for surface nitriding of titanium alloys according to claim 1, wherein in step a, vacuum is applied to a vacuum degree of 0.1Pa or less; and/or

In the step a, the surface pretreatment method of the titanium alloy substrate comprises the following steps: and (3) sequentially grinding the titanium alloy substrate by using 100-mesh, 200-mesh and 500-mesh water-grinding abrasive paper, then ultrasonically cleaning for 20-30min under the condition of power of 30-50Hz, and drying by cold air.