RU2365671C1 - Method of hardening titanium alloys in gas environment - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to the methods of nitride hardening of metals and can be used for manufacturing the parts from titanium alloys working at cyclic loads. High-temperature nitride hardening is performed at 700-750°C for 30-10 min. This is followed by recovery annealing in argon at the temperature exceeding the nitride hardening temperature by 100-150°C, for as long as the time τ_annealing, selected from the condition:

, where K_{nitride hardening}, K_p is empirical coefficients accordingly allowing the rate of formation and the rate of dissolving of the nitride gas-saturated layer, mcm²/sec; E_{nitride hardening} is the process activation energy controlling increase in the nitrogen concentration in the brittle nitride hardened layer J/mole; E_p - is the process activation energy controlling decrease in the nitrogen concentration in the brittle nitride hardened layer J/mole; R is gas constant, J/Kmole; T_{nitride hardening} is nitride hardening temperature, K; T _annealing is recovery annealing temperature, K; τ_{nitride hardening} is nitride hardening time, sec.

EFFECT: reduced power inputs and enhanced cyclic endurance and durability of the titanium alloy structures.

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Description

The invention relates to metallurgy, and in particular to methods of hardening metals by nitriding, and can be used in the manufacture of parts from titanium alloys operating under cyclic loads.

A known method of surface hardening of products from titanium and titanium alloys, including annealing in a saturating atmosphere and subsequent partial removal of the gas-saturated layer by etching, the value of which is judged by the optimal value of the microhardness drop of the etched surface relative to the core, calculated by the formula (description of patent RU 2205890, IPC ⁷ C22F 1/18, 2001/09/12).

The disadvantages of this method are the complexity and high complexity of the used method of destructive testing to detect changes in the microhardness of the treated surface at different etching depths.

A known method of surface hardening of titanium and titanium alloys, which includes stages of high-temperature nitriding and subsequent removal by etching of the gas-saturated layer corresponding to twice the depth of its zone h, which has increased fragility (description of application No. 2006124054).

The disadvantage of this method is the loss of expensive metal during etching and high complexity.

A known method of high-temperature nitriding of chromium-nickel alloys alloyed with titanium, providing for sequential stages of heat treatment with exposure in an active gas medium containing nitrogen for 15 hours and in a neutral gas medium of argon for 1.5-2.5 hours at a temperature of 1200 ° C (description of patent RU 2148675, IPC 7 C23C 8/24, C23F 17/00, 06/26/1998).

The known method improves the heat resistance and heat resistance.

A known method of hardening titanium alloys in a gaseous medium, including high-temperature nitriding and reduction annealing in argon (Minkevich AN Chemical-thermal treatment of metals and alloys. Moscow, Mechanical Engineering, 1965, p.334).

The objective of the invention is to reduce energy consumption and increase cyclic endurance and durability of structures made of titanium alloys.

EFFECT: optimization of high-temperature nitriding modes for surface hardening of titanium alloys.

The technical result is achieved by the fact that in the method of hardening titanium alloys in a gaseous medium, including high-temperature nitriding and reductive annealing in argon, nitriding is carried out at a temperature of 700-750 ° C for 10-30 minutes, and reductive annealing at a temperature exceeding the nitriding temperature at 100-150 ° C, during the time τ _ot , selected from the condition:

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 / s;

E _nitrogen is the activation energy of the process that controls the increase in nitrogen concentration in the nitrided embrittled 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, s.

The essence of the technical solution lies in the fact that during the reductive annealing in the surface layer of titanium, the nitrogen concentration decreases as a result of the development of its diffusion into the metal base. It was experimentally established that when conducting reductive annealing for a time selected from condition (1), the nitrogen concentration in the surface layers is redistributed and reduced to a certain optimal value, which ensures the restoration of plasticity and, as a result, an increase in the endurance and durability of nitrated titanium.

The nitriding time for τ _nitrogen ≤0.5 hours at a temperature of T _nitrogen ≥700 ° С and the temperature of reductive annealing Т _anne ≈ Т _nitrogen + (100-150) ° С are optimal for carrying out chemical-thermal processes with minimal energy consumption. The value of the selected temperature regime of nitriding is a prerequisite for nitriding titanium alloys in an active nitrogen gas environment.

The values of the selected temperature regime for conducting reductive annealing are necessary conditions for the possibility of implementing the method. So, after nitriding at 700 ° C for 0.5 hours in the case of reductive annealing at T _anne <800 ° C, its duration increases by an order of magnitude. This leads to unjustified additional energy costs. At T _anne > 850 ° C, the duration of reductive annealing, on the contrary, is reduced by an order of magnitude or more, which makes it impossible to control the process and the implementation of the method.

Example

The method was tested on samples of 2 × 5 × 10 mm in size made of VT6 sheet alloy, which, after polishing and degreasing, were nitrided at a temperature of T _nitrogen = 700 ° C = 973.15 K for τ _nitrogen = 0.5 hours = 30 min = 1800 c in a gas mixture of nitrogen (40%) and argon (60%) at atmospheric pressure. As a characteristic of the surface condition of the titanium alloy, the depth of the embrittled layer and the surface microhardness measured at a load on the indenter of 0.2 N were used.

As a result of nitriding, a brittle layer with a depth of ≈ 1 μm was formed on the surface of the samples. The microhardness was 9-10 GPa.

For the selected nitriding regime of the VT6 alloy experimentally (according to the experimental technique for determining the numerical values of the rate constants and activation energy of the processes of growth and dissolution of oxide films on the surface of titanium alloys, given in the work of Bondar A.V., Peshkov V.V., Kireev L. S., Shurupov VV Diffusion welding of titanium and its alloys. - Voronezh: Publishing house of the Voronezh State University, 1998 p.29-50) numerical values of the coefficients are taken into account, taking into account the formation rate, respectively dissolution rate of the nitride layer of gas-saturated, _nitrogen-K = 3.5 × 10 ⁷ m ² / s and K _p = 6.3 × 10 ⁶ mm ² / s, and the process value of the activation energy, controlling increase of nitrogen concentration in the embrittled layer, E _nitrogen = 203000 J / mol and the activation energy of the process controlling the decrease in nitrogen concentration in the embrittled layer, E _p = 215000 J / mol.

Then the samples were annealed at a temperature T _anne = 800 ° С in the medium of argon of the highest grade for a time τ _anne = 45 minutes.

The duration of the reductive annealing was calculated according to the dependence (1):

When conducting reductive annealing at a temperature of T _anne = 850 ° C, the holding time according to dependence (1) is chosen equal to, respectively, τ _anne ≅15 min (882 s).

Measurement of the microhardness of the samples after reductive annealing showed that it is (2.9-3.1) GPa, i.e. significantly less than the microhardness of the initial nitrided surface (9-10) GPa, and is comparable with the microhardness of the VT6 alloy in the delivery state (2.95 GPa).

The samples were tested for re-static tension, and the test results showed that the number of N cycles before failure compared with the samples in the delivery state increased by 12-17%.

Claims (1)

A method of hardening titanium alloys in a gaseous medium, including high-temperature nitriding and reductive annealing in argon, characterized in that nitriding is carried out at a temperature of 700-750 ° C for 30-10 minutes, and reductive annealing at a temperature exceeding the nitriding temperature by 100- 150 ° C, during the time τ _ot , selected from the condition:

,
where K _nitrogen , K _p - empirical coefficients, respectively taking into account the rate of formation and dissolution rate of the nitride gas-saturated layer, μm ² / s;
E _nitrogen is the activation energy of the process that controls the increase in nitrogen concentration in the nitrided embrittled 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 / Kmol;
T _nitrogen - nitriding temperature, K;
T _anne — temperature of reductive annealing, K;
τ _nitrogen - nitriding time, s.