RU2800089C1 - Method for heat treatment of intermetallic titanium ortho-alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: intermetallic titanium alloys based on the Ti₂AlNb compound, which can be used for the manufacture of gas turbine engine parts operating at elevated temperatures. Method for vacuum heat treatment of billets made of intermetallic titanium ortho-alloys includes loading billets into a charge, heating the charge with billets to the heat treatment temperature, holding and cooling. Heating of the charge with billets is carried out stepwise in a vacuum with a vacuum value of not more than 0.0093 Pa: first, heating is carried out to a temperature of 500°C at a speed of 10 to 12°C⋅ min ^-1 with an exposure of at least 3 hours, then heating is carried out to temperatures from 900 to 940°C at a speed of 5 to 8°C⋅ min^-1 with an exposure of at least 3 hours, after which the charge with billets is cooled in an oven in a vacuum of 0.0093 Pa to a temperature of 250°C and further in air up to a temperature of 20°C.

EFFECT: efficient removal of hydrogen, improvement of the mechanical properties of the billets.

3 cl, 2 tbl

Description

The invention relates to the field of non-ferrous metallurgy, namely to intermetallic titanium alloys based on the compound Ti ₂ AlNb (Ortho-alloys), which are intended for the manufacture of parts of a gas turbine engine (blades, disks, housings, etc.) operating at elevated temperatures.

It is known that cast intermetallic titanium alloys based on the Ti ₂ AlNb compound have low plastic properties at a temperature of 20°C - ultimate strength σ _В ²⁰ ~ 700 MPa and relative elongation δ ²⁰ ~ 1%. This level of mechanical properties of ortho-alloys makes it difficult to produce deformed semi-finished products - forgings, stampings, bars, sheets, etc.

To obtain semi-finished products from intermetallic Ortho-alloys, technological schemes are used, including multiple precipitation - drawing - precipitation of cast billets at polymorphic transformation temperatures (Tpp) equal to Tpp + (20-30) ° C, followed by a decrease in temperature to Tpp - (50-150) ° C. The use of such technological schemes leads to increased oxidation and additional operations - etching to remove gas-saturated surface layers and machining - turning, gouging, and milling, which leads to a decrease in the metal utilization factor (CMM) and an increase in the cost of finished semi-finished products.

To facilitate the processes of plastic deformation of intermetallic titanium Ortho-alloys, it is proposed to introduce hydrogen into Ortho-alloys [Khadzhieva O.G., Grib S.V., Malevich Yu.A., Illarionov A.G. Influence of hydrogen on transformations in an alloy based on Ti ₂ AlNb intermetallide // Collection of abstracts of the VIII All-Russian school-conference of young scientists "KoMU-2010" - Izhevsk: FTI UrO RAS, UdGU, IzhGTU, 2010. p. 108-109] which, being a temporary alloying element, increases the amount of plastic β-phase, facilitates plastic deformation processes at temperatures Тpp - (100-150)°С and reduces gas saturation of the surface of semi-finished products. In the presented source, heat treatment modes are not used, since the introduction of hydrogen is used to increase technological plasticity during hot forming.

Study [Zhang LT, Ito K., Vasudevan VK, Yamaguchi M. Beneficial effects of O-phase on the hydrogen absorption of Ti-Al-Nb // Journal of Intermetallics, 2001, v. 9, pp. 1045-1052] showed that the Ti ₂ AlNb intermetallic compound can contain at a temperature of 50°C residual hydrogen from 1 to 1.2 (wt.%), which is present in the structure in the form of solid solutions and hydrides with the formulas (TiNb) _X H _Y where X=2 or 1, and Y=1. The presence of hydrogen in such alloys leads to the fact that, under mechanical action on solid solutions, hydrides are released at the interphase boundaries, which lead to low plasticity or brittle fracture. To remove hydrogen from solid solutions and hydrides, it is necessary to use vacuum thermal treatment (VacTO).

There is a method [SU 1506916 A1, Method of heat treatment of α-titanium alloys (MPK C22F 1/18, publ. - 03/27/1995)], according to which hydrogenated α-alloys (VT5L and VT5-1 grades) are heated to temperatures Tpp - (20-50) ° C, held for 1 hour, cooled at a rate of 0.03 up to 10 C⋅s ^-1 to a temperature of 200 to 400°C above the end of the eutectoid transformation and aging at this temperature for 1 to 10 hours. Then vacuum annealing is carried out at temperatures from 600 to 650°C and exposure required for complete degassing of the product. This method has significant disadvantages:

- the method is used for α-alloys (i.e. alloys with α-stabilizing elements - Al and Sn) and cannot be used for highly alloyed alloys of the Ti-Al-Nb system;

- the method does not indicate the initial and final hydrogen content after heat treatment;

- vacuum annealing is carried out at temperatures from 600 to 650°C, at which there is no effective removal of hydrogen dissolved in alloys.

Известен способ [RU 2081201 О, Способ обработки изделий из титановых деформированных сплавов (МПК C22F 1/18, опубл. - 06.10.1997)], в соответствие с которым изделия из технического титана (сплавы марок ВТ1-00, ВТ 1-0) и двухфазные титановые сплавы (сплавы марок ВТ6, ВТ6С) подвергаются чистовой механической обработке и последующему вакуумному отжигу при температурах (0,8-0,9) Тпл (от 1336 до 1530°С) в вакууме от 0,0133 до 0,00133 Па (от 10 ^-4 до 10 ^-5 мм. рт.ст.) в течение от 15 до 30 мин с регламентированными скоростями нагрева от 350 до 800°С⋅ч ^-1 (от 5,8 до 13,3°С мин ^-1 ) и охлаждения от 10 до 200°С мин ^-1 . The given VakTO method has significant disadvantages:

- VakTO at such high temperatures (from 1336 to 1530°C) is designed to relieve residual stresses in titanium parts and leads to the production of a material with a coarse-grained structure, which provides low strength and increased ductility and is used as a structural material in the jewelry industry;

- VakTO at the indicated temperatures leads to vacuum etching (change in surface roughness) and its activation, which is necessary for applying various coatings with the required performance properties.

The closest analogue, taken as a prototype, is the VacTO method proposed in [Borisova E.A., Shashenkova I.I., Glebova R.D., on vacuum annealing of titanium alloys // Metal Science and Heat Treatment of Metals, 1972, No. 5, p. 10-13]. According to this method, VacTO was carried out on serial (α + β) titanium alloys of the VT6S and VT20 grades at temperatures from 650 to 800°C, a vacuum of 0.0133 Pa (10 ^-4 mm Hg) and a holding time of 1 to 2 hours. holding for 1 hour. These VacTO regimes led to the removal of hydrogen to values not exceeding 0.005 (wt.%). The studied VacTO regimes had little effect on the strength mechanical properties (σ _B , δ _0.2 , δ, ψ), but reduced the cyclic and low-cycle strength from 10 to 15% at a temperature of 20°C, which was caused by a deterioration in the quality of the surface of heat-treated samples due to surface etching and the formation of stress concentrators.

The use of the method specified in the prototype (Borisova E.A., Shashenkova I.I., Glebova R.D., On vacuum annealing of titanium alloys // Metal Science and Heat Treatment of Metals, 1972, No. 5, pp. 10-13) for Ortho-alloys is not possible, since the solubility of hydrogen in these alloys is from 1 to 1.2 (wt.%) (at a temperature of 50 ° C) and the resulting hydrides have greater resistance to heating. To destroy titanium and niobium hydrides in ortho-alloys and remove the resulting hydrogen, it is necessary to use temperatures above 800°C and a vacuum better than 0.0133 Pa.

The technical task and technical result of the claimed invention is the development of VacTO regimes for a hydrogen-containing Ortho-alloy, which ensures effective removal of hydrogen with a content of 0.1 to 0.2 (wt.%) up to 0.004 (wt.%), an increase in the level of mechanical properties such as tensile strength by 38%, yield strength by 33% and relative elongation and narrowing by 3.2 times.

To achieve the stated technical result, a method is proposed for vacuum heat treatment of workpieces made of intermetallic titanium ortho-alloys, including loading workpieces into a charge, heating the charge with workpieces to the heat treatment temperature, holding and cooling, characterized in that the charge with workpieces is heated stepwise in a vacuum with a vacuum value of not more than 0.0093 Pa: first, heating is carried out to a temperature of 500 ° C at a speed of 10 to 12 °C min" ¹ with a holding time of at least 3 hours, then heating is carried out to temperatures from 900 to 940°C at a rate of 5 to 8°C min' ¹ with a holding time of at least 3 hours, after which the charge with blanks is cooled with a furnace in a vacuum of 0.0093 Pa to a temperature of 250°C and then in air to a temperature of 20°C.

Preferably, the heat treatment is carried out in a vacuum heat treatment furnace having a leakage of not more than 130 Pa in a time of at least 15 minutes.

Preferably, the workpiece charge is cooled with high purity inert gases - argon and/or helium with a dew point of at least minus 60°C.

Experiments have shown that VacTO at a temperature of 800°C, a vacuum of 0.0133 Pa, and a time of 2 h provide partial removal of hydrogen from a content of 0.2 (wt.%) to values of 0.12 (wt.%) (i.e., 1.8 times) and such a hydrogen content cannot ensure reliable operation of Ortho-alloys. VakTO alloy samples at a temperature of 880°C reduced the hydrogen content to 0.012 (wt.%) (ie 18 times), which is also not enough for the structural use of Ortho-alloys. VakTO at temperatures from 900 to 920°C reduces the hydrogen content to values of 0.005 (wt.%), and a further increase in the VakTO temperature to 940°C and above leads to a decrease in the hydrogen content in the alloy to a level of 0.004 (wt.%). The use of VakTO at a temperature of 960°C does not lead to a decrease in the hydrogen content, but causes noticeable changes in the surface roughness (a so-called "vacuum stain" is formed), which requires additional mechanical surface treatment. It should be noted that high VakTO temperatures increase the duration of the heat treatment cycle, which leads to a decrease in equipment productivity by 25% due to a longer blank cooling time.

Reducing the cooling time of the samples after VakTO is achieved by accelerated cooling with inert gases - argon and/or helium, which provide increased productivity of VakTO.

VacTO of workpieces from intermetallic titanium Ortho-alloy in the temperature range from 900 to 940°C, vacuum not worse than 0.0093 Pa (7⋅10 ^-5 mm Hg) and time not less than 3 h showed that workpieces containing 0.2 (wt.%) hydrogen and processed according to the above regime had a low hydrogen content of 0.004 (wt.%), which exceeded hydrogen content in serial titanium alloys.

The reduced hydrogen content in the blanks of the intermetallic titanium Ortho-alloy led to the stabilization of the phase composition and the formation of dispersed precipitates of hardening phases, which led to an increase in the strength and plastic characteristics of the material.

Implementation examples

An ingot of an intermetallic titanium alloy with the composition Ti-21Al-23Nb-Zr-Mo-Ta-W-Si(at. %) was produced using the production technology of serial titanium alloys. The technology included the preparation of charge materials, the manufacture of a consumable electrode by continuous pressing and the smelting of ingots by repeated vacuum-arc remelting.

The machined cast billet was subjected to all-round forging, forging to an intermediate size, mechanical cleaning of the billet surface, and hydrogenation. Hydrogenated workpieces were subjected to rolling at heating temperatures (Tpp - 80)°C and a total degree of deformation of ~ 90%.

The rods were cut into blanks for samples to determine the initial hydrogen content and test mechanical properties at a temperature of 20°C.

Sample blanks after hot deformation had high internal stresses, which, upon subsequent rapid heating, caused the formation of microcracks and led to the destruction of samples during mechanical tests. To eliminate these phenomena, stepwise heating of sample blanks was used to a temperature of 500°C at a rate of 10 to 12°C⋅min ^-1 and then to temperatures from 900 to 940°C at a rate of 5 to 8°C⋅min ^-1 . These heating regimes led to the removal of deformation and thermal stresses and stabilization of the phase composition of the alloy.

The workpieces were subjected to VakTO and used to determine the content of residual hydrogen and test mechanical properties at a temperature of 20°C.

VakTO was carried out at temperatures of 880, 900, 920, 940 and 960°C, a vacuum of 0.0093 Pa (7 10 ^-5 mm Hg) for at least 3 hours, cooling with an oven to a temperature of 200°C and then in air to a temperature of 20°C. 0133 Pa (10 ^-4 mm Hg), cooling with an oven up to 200°C and further in air. The results of determining the hydrogen content in the samples after such treatments are presented in Table 1.

The test results of the VakTO variants showed that heating according to the prototype and heating at a temperature of 880°C (experiment 2) does not provide effective hydrogen removal, which decreases only 1.8 times (for the prototype) and by the proposed method from 40 to 50 times.

The efficiency of the HTO according to the conducted modes (samples 3-5) from 900 to 940°C and a vacuum pressure of 0.0093 Pa (7⋅10 ^-5 mm Hg) for at least 3 hours led to a decrease in the hydrogen content to a level of 0.004 (wt.%). Increasing the temperature to 960°C does not provide a further reduction in the hydrogen content in the workpieces.

VakTO according to the proposed method allowed to improve the mechanical properties of the workpieces. The results of mechanical tests of bar blanks in the initial state and after VakTO according to the proposed mode at a temperature of 20°C are presented in Table 2.

Thus, the proposed VacTO mode makes it possible to remove residual hydrogen in the workpieces to a level of 0.004 (wt.%), to increase the mechanical properties of the workpieces - the tensile strength of the alloy by 38%, the yield strength by 33%, the relative elongation and narrowing by ~ 3.2 times.

The use of the proposed VakTO method for deformed semi-finished products in the form of bars made of intermetallic titanium Ortho-alloy provides:

- effective destruction of hydrides and removal of residual hydrogen;

- obtaining a low hydrogen content in the alloy blanks and an increased level of mechanical properties.

The proposed method of VacTO semi-finished products in the form of bar stocks from intermetallic titanium ortho-alloys can be used for the manufacture of gas turbine engine parts, which will ensure reliable operation of the structure and the required technical life of the products.

Claims (3)

1. The method of vacuum heat treatment of workpieces from intermetallic titanium ortho-alloys, including loading workpieces into a charge, heating the charge with workpieces to the heat treatment temperature, holding and cooling, characterized in that the charge with workpieces is heated stepwise in a vacuum with a vacuum value of not more than 0.0093 Pa: first, heating is carried out to a temperature of 500°C at a speed of 10 to 12°C ⋅min ^-1 with a holding time of at least 3 h, then heating is carried out to temperatures from 900 to 940°C at a rate of 5 to 8°C⋅min ^-1 with a holding time of at least 3 h, after which the cage with the blanks is cooled with a furnace in a vacuum of 0.0093 Pa to a temperature of 250°C and then in air to a temperature of 20°C. 2. The method according to p. 1, characterized in that the heat treatment is carried out in a vacuum thermal furnace having leakage of not more than 130 Pa for a time of at least 15 minutes. 3. The method according to p. 1, characterized in that the cage with blanks is cooled with inert gases of high purity - argon and/or helium with a dew point of at least minus 60°C.