Titanium alloy sheet and exhaust system component

Abstract

The present invention relates to metallurgy, and more particularly to sheets made of titanium alloys which are resistant to high heat and oxidation and exhibit structural stability when exposed to temperatures in the range of up to 800 DEG C over long-term operation, and can be used to manufacture components of vehicle exhaust systems. The titanium alloy sheet for manufacturing a part contains 1.5 to 3.0 wt% of aluminum, 0.1 to 0.5 wt% of molybdenum, 0.1 to 0.6 wt% of silicon, not more than 0.2 wt% of iron, not more than 0.15 wt% of oxygen, not more than 0.1 wt% of carbon, not more than 0.03 wt% of nitrogen, not more than 0.015 wt% of hydrogen and the balance of titanium. The sheet has high creep and oxidation resistance values, and has a stable structure when exposed to temperatures in the range of up to 800 DEG C for long term operation. The material is suitable for cold forming.

Description

[0001] The present invention relates to non-ferrous metallurgy, and more particularly to the manufacture of sheets from low-alloy heat-resistant and oxidation-resistant titanium alloys, which have a stable structure during long-term operation in a temperature range up to 800° C. and can be used to manufacture products that operate for a long time at high temperatures, especially components of exhaust systems of vehicle engines.

[0002] Titanium-based alloys are used for the manufacture of components in different commercial applications, such as internal combustion engines and exhaust systems. Such components include intake and exhaust valves, housings, turbine blades, pipes and tanks. The operating temperature of such engine and exhaust system components made of low-alloyed titanium-based alloys is about 500-800°C. Therefore, the operating characteristics of the material, such as heat resistance and oxidation resistance, are a priority. In addition, the material used should have sufficient process plasticity, because the components are mainly produced by cold forming from rolled sheets and by bending welded pipes. In order to obtain high plastic properties, it is important to produce a spherical morphology of α-phase grains in the material, because the spherical microstructure provides better forming properties than the needle-like structure.

[0003] As internal combustion engine designers improve engine efficiency, properties such as boost pressure, compression ratio and operating temperature are correspondingly increased. The increased levels of these properties have led to the need for materials that can resist strain (creep) at higher operating temperatures and pressures within the combustion chamber and exhaust system than are currently achievable with conventional low alloyed titanium alloys. Creep is the tendency of a solid material to slowly shift or retain strain under load, and occurs when a metal is subjected to a constant tensile load at high temperatures. High creep resistance allows the material to be used for long periods of time without changes in shape and size, while maintaining the material's original performance levels is important.

[0004] Therefore, the material is required to have the greatest possible combination of high mechanical properties and operating properties in addition to its low price.

[0005] Flat rolled products and exhaust system components made of oxidation resistant, high strength titanium alloys are known, the titanium alloys consisting of (weight %): 0.06 to 0.5 iron, 0.02 to 0.12 oxygen, 0.15 to 0.46 silicon and the balance titanium and incidental impurities. However, the average grain size of the titanium alloy is 15.9 μm or less. (U.S. Patent No. US8349096, published on January 8, 2013, IPC C22C14 / 00).

[0006] Rolled products have high ductility but reduced resistance to high temperature oxidation.

[0007] A material for exhaust systems made of a low alloyed titanium alloy is known, which has excellent resistance to high temperature oxidation and corrosion. It contains (in wt%) Al: 0.30-1.50%, Si: 0.10-1.0%, and additionally Nb: 0.1-0.5 (U.S. Pat. No. US7166367, published on Jan. 23, 2007, IPC B32B15 / 01; C22C14 / 00, F01N7 / 16). This material is prior art.

[0008] Materials made from the above alloys have high strength and ductility at room and elevated temperatures, but have an insufficient level of resistance to high temperature creep.

[0009] An object of the present invention is to develop a low alloyed titanium alloy sheet having a spheroidal microstructure, thereby allowing the manufacture of a wide range of products, including those used in engine components and exhaust systems for vehicles.

[0010] The technical result achieved by implementing the present invention is the production of titanium alloy sheets having a combination of high mechanical properties and operating properties, including improved levels of creep resistance and oxidation resistance, as well as structural stability under long-term operating conditions in a temperature range of up to 800°C, and with the possibility of cold forming.

[0011] This technical result is achieved in the following way: the titanium alloy sheet for manufacturing components operating at high temperatures for a long time according to the present invention contains the following elements, in % by weight:

[0012] Aluminum 1.5-3.0,

[0013] Molybdenum 0.1-0.5,

[0014] Silicon 0.1-0.6,

[0015] Iron up to 0.2,

[0016] Oxygen up to 0.15,

[0017] Carbon up to 0.1,

[0018] Nitrogen up to 0.03,

[0019] Hydrogen up to 0.015,

[0020] The remainder is Ti.

[0021] Provided that the ratio of Mo to Si (weight %) is equal to 0.4-3, the sheet contains at least 90% (volume %) of alpha phase. The total content of intermetallic compound particles of beta phase and titanium silicide is 0.5-5% by volume. The average grain size of the alpha phase is 5 to 100 μm. In addition, the sheet is made in the form of a rolled sheet with a thickness of up to 6 mm. Technical results are also achieved when a component of a vehicle exhaust system that operates for a long time at high temperatures and is made of a titanium alloy sheet is proposed.

[0022] Alloying elements from various stabilizer groups are added to the titanium alloy material: alpha stabilizers: aluminum, oxygen, carbon, nitrogen; beta stabilizers: molybdenum, silicon.

[0023] Aluminum improves heat resistance and creep resistance and reduces the formation of oxide scale at high temperatures. The aluminum content in the alloy is 1.5-3.0% by weight. In order to maintain optimal process plasticity, the maximum aluminum content in the alloy is limited to 3.0% by weight.

[0024] Oxygen, nitrogen and carbon contents within specified limits increase the allotropic transformation temperature of titanium with increasing strength and allow high levels of strength and ductility to be maintained. Higher concentrations of oxygen, carbon and nitrogen reduce process ductility and affect alloy strength.

[0025] Group of β stabilizers (Mo, Si).

[0026] The alloy is alloyed with molybdenum in an amount of 0.1-0.5 wt.% to improve strength due to solid solution strengthening and the appearance of a β phase interlayer in the structure, which is an interface boundary that slows down strain dislocation movement during deformation and also prevents the aggregation growth of α grains at high temperatures during heat treatment and operation. A molybdenum content exceeding 0.5 wt.% reduces heat resistance because the β transformation temperature of the alloy is reduced, thereby increasing the proportion of the β phase in the structure.

[0027] The presence of silicon (contained in the titanium solid solution) in the alloy improves creep resistance. The silicon content in the alloy is set in the range of 0.1-0.6% by weight. Within this range, silicon forms an intermetallic compound with titanium, a silicide (TixSiy) of complex stoichiometric composition. The generation of the required amount of silicide in the alloy can improve heat resistance, creep resistance and prevent the growth of alpha grains at high temperatures. In addition, if the silicon content does not exceed 0.8% by weight, silicon significantly improves the oxidation resistance of the alloy. Greater concentrations reduce process plasticity / formability due to the generation of coarse-grained silicides. The absence of Zr and Sn in the alloy reduces the eutectoid transformation temperature of silicide formation, maximizes the Si content in the solid solution, and thus maximizes heat resistance.

[0028] The maximum hydrogen content in the alloy is limited to 0.015 wt. % to avoid embrittlement of the alloy due to possible formation of titanium hydrides.

[0029] The iron content in the alloy is limited to 0.2 wt. % since higher contents have a negative impact on creep resistance and short-term heat resistance.

[0030] The main factor for structural stability during long-term operation at elevated temperatures is the presence of particles that inhibit grain growth. These are the β-phase particles in the alloy as well as the silicide particles. The presence of these two types of particles in the alloy is very important and is achieved by the close content of Mo and Si. The preferred ratio Mo / Si (in weight percentage) of β-isostructural molybdenum and β-eutectoid silicon is in the range of 0.4-3. This ratio allows for increased oxidation resistance and creep resistance, as well as structural stability during long-term operation.

[0031] The composition of elements added to the alloy in the amounts claimed and each characterized by having a favorable effect on the oxidation resistance of titanium makes it possible to achieve an additional effect in terms of obtaining high creep resistance values ​​while providing a combination of strength, plasticity and oxidation resistance relative to known low-alloyed titanium alloys.

[0032] By adjusting the organization that affects the cold forming properties, an additional improvement in material properties is achieved. The spherical organization of α-phase grains has higher plasticity and formability values ​​than the needle-shaped organization. Therefore, in order to improve the formability of the sheet, a uniform spherical microstructure with an average grain size of 5 to 100 μm is preferred. Obtaining a microstructure with an α-phase average grain size of less than 5 μm requires a large number of process operations, so the cost is high. In a microstructure with an α-phase average grain size greater than 100 μm, the boundaries of large grains become the starting point of cracking. The average diameter of α-phase grains in titanium billet organization is measured according to the international standard ASTM E112 practice. The proportion of β-phase and silicide particles is calculated using a scanning electron probe microscope (SEM) in backscattered electron mode, and the resulting image is processed using software to quantitatively analyze the microstructure by elemental contrast.

[0033] To ensure the stability of the alpha grain structure during operation, the preferred alpha content in the material should be at least 95 volume %. The total content of beta phase and titanium silicide intermetallic compound particles in the material in the range of 0.5-5 volume % can improve creep resistance at high temperatures.

[0034] The industrial applicability of the present invention is demonstrated by concretely implemented examples.

[0035] In order to investigate the properties of the proposed material, ingots weighing 2100 kg were melted using industrial technology by vacuum arc remelting. The chemical composition of the alloy is shown in Table 1.

[0036] Table 1

[0037]

[0038] The ingot was forged and further rolled to obtain a 0.9 mm thick coil, the final stage of rolling being carried out at a β-transformation temperature of 945°C, which is necessary to produce a spherical α grain structure. Samples for evaluation of the mechanical properties of the alloy were obtained under delivery conditions. Tensile tests were carried out at temperatures of 20°C, 500°C, and 700°C to evaluate the mechanical properties, and deep drawing tests were carried out according to Eriksen to evaluate the formability benchmark of the material. The tensile mechanical properties of the material in the delivery state (annealed state) are given in Table 2, Figure 1 A comparison diagram is shown in FIG.

[0039] Table 2

[0040]

[0041] In order to simulate the handling of the material in the product, the samples were isothermally annealed in static laboratory air at 560°C, 625°C, with a holding time of 1000 hours, and also at 800°C with a holding time of 200 hours. The weight gain of the samples (in mg / cm 2 to study the antioxidant activity. Figure 2 , 3 The results of the oxidation resistance evaluation compared with the prior art alloys are shown in the graphs in 4 representing the dependence of the alloy weight increase on the square root of the oxidation time at temperatures of 560°C, 625°C and 800°C, respectively.

[0042] Furthermore, the creep resistance of the samples in the as-delivered state was determined at a temperature of 500°C and a duration of 100 hours; it was expressed as a function of the relative strain of the specimen at a stress of 30 MPa. The creep resistance results of the material to be protected compared with the prior art are presented in Figure 5 The curve diagram given in shows.

[0043] The average grain size of the α phase in the blank structure in the longitudinal section measured according to the international standard ASTM E112 is 15 μm. The proportion of the α phase is 98% by volume, and the proportion of the β phase and titanium silicide particles is 2% by volume. The proportion of the β phase and titanium silicide particles is measured using a scanning electron probe microscope (SEM) in backscattered electron mode and calculated using an image analysis program.

[0044] The grain structure of the material with titanium silicide particles and β phase intermediate layer after annealing at 625℃ for 1000 hours did not change compared with the initial structure ( Figure 6 ), which indicates the stability of the organization.

[0045] Analysis of test results and research data showed that the proposed titanium alloy sheet has a combination of high mechanical properties and operating properties, including resistance to high temperature creep, compared to known low-alloyed alloys. The results of oxidation resistance evaluation of the specimens after long-term isothermal annealing demonstrated the durability of the material.

Claims

1. Titanium alloy sheet for manufacturing parts that operate at high temperature for a long time, characterized in that: In terms of weight %, the element contents in the titanium alloy are as follows: Aluminum 1.5-3.0, Molybdenum 0.1-0.5, Silicon 0.1-0.6, Iron up to 0.2, Oxygen up to 0.15, Carbon up to 0.1, Nitrogen up to 0.03, Hydrogen up to 0.015, The remainder is Ti.

2. The sheet material according to claim 1, characterized in that The ratio (weight %) of Mo to Si in the alloy is comprised between 0.4 and 3.

3. The sheet material according to claim 1, characterized in that The average grain size of the α phase is 5 to 100 μm.

4. The sheet material according to claim 1, characterized in that It contains at least 95% by volume of the alpha phase.

5. The sheet material according to claim 1, characterized in that The total content of the β phase and the titanium silicide intermetallic compound particles comprises 0.5-5% by volume.

6. The sheet material according to claim 1, characterized in that The sheets are produced in the form of flat rolled products having a thickness of at most 6 mm.

7. A vehicle exhaust system component operated at high temperature for a long time and made of titanium alloy sheet, characterized in that: The component is made from a sheet material as claimed in any one of the preceding claims.

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