Preparation method of stress corrosion resistant Ti-6Al-4V alloy component

Abstract

The invention belongs to the technical field of titanium alloy materials, and particularly relates to a preparation method of a stress corrosion resistant Ti-6Al-4V alloy component. The method comprises the following steps: pretreating a Ti-6Al-4V alloy workpiece, and then performing air cooling to obtain a pretreated workpiece; the pretreated workpiece is subjected to solution treatment, a segmented cooling mode is adopted, and a segmented cooling workpiece is obtained; performing two-stage aging on the segmented cooling workpiece to form an alpha colony and / or lamellar alpha structure in the Ti-6Al-4V alloy, so as to obtain the stress corrosion resistant Ti-6Al-4V alloy component; the average size of the alpha colony is greater than or equal to 50 microns; the average thickness of the lamellar alpha phase structure is 1-5 [mu] m, and the crystallographic orientation difference between adjacent lamellar alpha phases is greater than or equal to 5 degrees. Through a composite heat treatment process comprising pretreatment, controllable cooling and graded aging, the alpha-phase size, orientation and distribution of a sheet layer are accurately regulated and controlled, so that the stress corrosion resistance of the alloy in a severe corrosion environment is systematically improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of titanium alloy materials, and particularly relates to a preparation method of a stress corrosion resistant Ti-6Al-4V alloy component. BACKGROUND

[0002] Ti-6Al-4V alloy is a typical (alpha + beta) two-phase titanium alloy, and is widely used in the fields of aerospace, biomedical treatment, marine engineering and petroleum chemical industry due to its low density, high specific strength, excellent toughness and good corrosion resistance. In recent years, with the extension of oil and gas resource exploration and development to deep wells, super deep wells and harsh corrosion environments such as media containing high Cl - , CO2 and H2S, Ti-6Al-4V alloy is considered as an ideal material for manufacturing key components such as drill pipes, oil pipes and valve bodies due to its excellent corrosion resistance.

[0003] However, titanium alloy components will occur stress corrosion cracking (SCC) under the combined action of tensile stress and specific corrosion environment. SCC is a potential and disastrous failure mode, and the crack propagation rate is much higher than that under pure corrosion or stress, which often leads to brittle fracture of the component without warning at a stress much lower than the yield strength of the material, thereby seriously threatening the safety production.

[0004] In the prior art, a process for improving the strength of Ti-6Al-4V alloy by double solid solution treatment is disclosed. However, the process ignores the systematic regulation of lamellar alpha phase orientation and size, and the optimization of stress corrosion environment. In addition, the traditional heat treatment process mainly focuses on obtaining equiaxed or bimodal structure to balance the strength and toughness, and cannot effectively solve the problem of high SCC sensitivity in oilfield environments containing Cl - , CO2 and H2S. SUMMARY

[0005] To solve the above technical problems, the application provides a preparation method of a stress corrosion resistant Ti-6Al-4V alloy component.

[0006] The application accurately regulates the size, orientation and distribution of lamellar alpha phase by a composite heat treatment process including pretreatment, controllable cooling and staged aging, so as to systematically improve the stress corrosion resistance of the alloy in harsh corrosion environments.

[0007] The method of the application combines pretreatment, multi-stage solid solution, controllable cooling and staged aging to optimize the size, orientation and distribution of lamellar alpha phase in Ti-6Al-4V alloy, thereby significantly improving the stress corrosion resistance of the alloy in halide-containing corrosion media.

[0008] To achieve the above purpose, the technical scheme of the application is as follows.

[0009] The first aspect of the present application provides a method for preparing a stress corrosion resistant Ti-6Al-4V alloy component, comprising the following steps: The Ti-6Al-4V alloy workpiece is pretreated at 800-850℃, and then air-cooled to obtain a pretreated workpiece; the pretreated workpiece is heated to 10-80℃ above the beta phase transition point for solid solution treatment, then cooled to 800℃ at a rate of ≥15℃ / min by using a step cooling method, and then cooled to room temperature at a rate of ≤5℃ / min to obtain a step-cooled workpiece; the step-cooled workpiece is subjected to two-stage aging at 500-650℃ to form alpha colonies and / or lamellar alpha phase structure in the Ti-6Al-4V alloy, thereby obtaining a stress corrosion resistant Ti-6Al-4V alloy component.

[0010] Preferably, the average size of the alpha colonies is ≥50μm; the average thickness of the lamellar alpha phase structure is 1-5μm, and the crystallographic orientation difference between adjacent lamellar alpha phases is ≥5°.

[0011] The heat treatment method of the present application can improve the stress corrosion resistance of Ti-6Al-4V alloy in a halide ion-containing corrosion medium, thereby obtaining a Ti-6Al-4V alloy component with excellent stress corrosion resistance.

[0012] The pretreatment step of the present application aims to refine the original beta grains, providing a uniform nucleation basis for the subsequent growth of alpha colonies; the solid solution treatment step can fully solid-solute the alloying elements to obtain a supersaturated solid solution; the step cooling step first cools to 800℃ at a faster rate of ≥15℃ / min to inhibit excessive grain growth; then cools to room temperature at a slow rate of ≤5℃ / min to promote the precipitation of thick lamellar alpha phase and form different crystallographic orientations; the two-stage aging step precisely matches the temperature and time to stabilize the beta transformed structure, adjust the alpha / beta phase interface state, and further improve the organizational stability and crack propagation resistance.

[0013] The preparation method of the present application forms alpha colonies with an average size of ≥50μm and / or lamellar alpha phase structure with an average thickness of 1-5μm and a crystallographic orientation difference between adjacent lamellar alpha phases of ≥5° in the alloy.

[0014] Preferably, the pretreatment time is 1-3h.

[0015] Preferably, the solid solution treatment time is 0.5-3h.

[0016] Preferably, the two-stage aging includes first-stage aging and second-stage aging; the holding time of the first-stage aging is 2-4h; the holding time of the second-stage aging is 3-6h.

[0017] Preferably, the step cooling is carried out under the protection of inert gas; the inert gas is argon. The inert gas (e.g. argon) is introduced during the step cooling to prevent the surface of the alloy from being oxidized at high temperature.

[0018] Preferably, the pretreatment further comprises: rolling or forging at 20-100℃ below the beta transus point, with a deformation of 10-30%, to obtain the Ti-6Al-4V alloy workpiece. The present application can further refine the grain and introduce dislocations through the thermal mechanical treatment, to provide more nucleation sites for the formation of more complex multi-oriented lamellar alpha structure in the subsequent heat treatment.

[0019] The second aspect of the present application provides a stress corrosion resistant Ti-6Al-4V alloy component, which is prepared by the method for preparing a stress corrosion resistant Ti-6Al-4V alloy component according to the first aspect.

[0020] Preferably, the Ti-6Al-4V alloy component has an alpha colony and / or a multi-oriented lamellar alpha phase composite structure.

[0021] The third aspect of the present application provides the use of the stress corrosion resistant Ti-6Al-4V alloy component according to the second aspect in a halide ion-containing corrosive medium, wherein the halide ion-containing corrosive medium is a corrosive medium containing Cl - ; the concentration of Cl - is ≥10g / L.

[0022] The Ti-6Al-4V alloy component is a drill pipe, a tubing, a casing or a valve body for oil and gas field, and is used in a corrosive medium containing Cl - ≥10g / L. The Ti-6Al-4V alloy component is used in a corrosive medium containing Cl - ≥10g / L, so that the average deflection angle of the crack during propagation is ≥10.5°, and the crack propagation path is discontinuous zigzag.

[0023] Preferably, the concentration of Cl - is 10-25g / L.

[0024] Preferably, the stress corrosion sensitivity index of the Ti-6Al-4V alloy component in the halide ion-containing corrosive medium is controlled to be ≤18%; preferably ≤11%.

[0025] The beneficial effects of the present application are as follows: 1、The present application systematically combines pre-treatment, solution treatment, step cooling and two-stage aging to form a complete and synergistic heat treatment system. The present application precisely controls the size, orientation and distribution of lamellar alpha phase through a composite heat treatment process comprising pre-treatment, solution treatment, step cooling and two-stage aging, thereby systematically improving the stress corrosion resistance of Ti-6Al-4V alloy in harsh corrosive environments.

[0026] 2、Through the above-mentioned composite process, a unique microstructure of large-size alpha colony and / or multi-oriented lamellar alpha phase composite structure is obtained.

[0027] 3、The stress corrosion sensitivity index of the Ti-6Al-4V alloy component treated by the preparation method of the present application can be stably controlled below 18%. More importantly, the preparation method of the present application induces frequent deflection of cracks with an average of ≥10.5° and forms a discontinuous zigzag propagation path, which significantly improves the crack propagation resistance from a mechanism and exhibits excellent stress corrosion resistance. The application environment of the Ti-6Al-4V alloy component obtained by the preparation method of the present application is limited to corrosive media containing Cl - ≥10g / L, which has good industrial applicability. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is a flowchart of the preparation method of the stress corrosion resistant Ti-6Al-4V alloy component in the embodiments of the present application.

[0029] Figure 2 It is a typical microstructure metallographic phase diagram of the Ti-6Al-4V alloy component treated by the method of Example 1 of the present application.

[0030] Figure 3 It is a Widmanstatten structure diagram of the Ti-6Al-4V alloy component after traditional solution and aging in Comparative Example 1.

[0031] Figure 4 It is a bimodal structure diagram of the Ti-6Al-4V alloy component after traditional solution and aging in Comparative Example 2. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present application more clear and explicit, the following embodiments are used to further illustrate the present application. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0033] Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0034] Figure 1 Flow chart of the method for preparing the stress corrosion resistant Ti-6Al-4V alloy component in the embodiments of the present application.

[0035] The present application provides a method for preparing a stress corrosion resistant Ti-6Al-4V alloy component, comprising the following steps: Pre-treatment: heat the alloy workpiece at 800-850℃ for 1-3 hours, and then air cool; this step aims to refine the original β grains, and provide a uniform nucleation basis for the subsequent α colony growth.

[0036] Solution treatment: heat the pre-treated workpiece to 10-80℃ above the β phase transition point (T β ) for 0.5-3 hours; fully dissolve the alloying elements to obtain a supersaturated solid solution.

[0037] Controllable cooling: adopt a stepwise cooling method, first cool at a faster rate of ≥15℃ / min to 800℃ to inhibit excessive grain growth; then cool at a slow rate of ≤5℃ / min to room temperature to promote the precipitation of thick lamellar α phase and form different crystallographic orientations.

[0038] Staged aging: perform two-stage aging in the range of 500-650℃, the first stage for 2-4 hours, and the second stage for 3-6 hours; through the precise combination of temperature and time, stabilize the β transformed structure, adjust the α / β phase interface state, and further improve the organizational stability and crack propagation resistance.

[0039] The method forms α colonies with an average size of ≥50μm and / or lamellar α structures with an average thickness of 1-5μm and a crystallographic orientation difference of ≥5° between adjacent lamellar α phases in the alloy.

[0040] In the present application, the method constructs a mixed microstructure with large size α colonies (average size ≥50μm) and / or multi-orientation interlaced distribution of thick lamellar α phase (average thickness 1-5μm) in the alloy by sequentially performing pre-treatment, multi-stage solution treatment, controllable cooling and staged aging on the alloy workpiece. This structure can effectively induce frequent deflection of stress corrosion cracks during the propagation process, significantly improve the crack propagation resistance, and thus control the stress corrosion sensitivity index of the alloy in a halide-containing corrosion medium to be below 18%, preferably ≤11%. The method has stable process and good repeatability, and is particularly suitable for the heat treatment of titanium alloy components such as drill pipes, oil pipes and valve bodies used in oil and gas field exploration and development, and can significantly improve the service safety and life of the components in harsh corrosion environments.

[0041] The technical solutions of the present application are further described below through specific examples. In each of the following examples, the methods are conventional methods unless otherwise specified; and the reagents and materials are commercially available unless otherwise specified.

[0042] Example 1 A method for preparing a Ti-6Al-4V alloy component resistant to stress corrosion, comprising the following steps: Material: Commercially available hot-rolled Ti-6Al-4V alloy plate with a thickness of 10 mm, and a beta phase transition point (T β ) of about 990℃.

[0043] The heat treatment steps are as follows: Thermal mechanical treatment: hot rolling at 950℃ (T β -40℃) with a deformation of 20%.

[0044] Pretreatment: holding at 830℃ for 1.5 hours and air cooling.

[0045] Solution treatment: holding at 1010℃ (T β +20℃) for 2 hours.

[0046] Controlled cooling: first cooling to 800℃ at ≥15℃ / min, and then furnace cooling to room temperature at ≤5℃ / min.

[0047] Step aging: first stage: holding at 580℃ for 2.5 hours; second stage: holding at 620℃ for 3.5 hours and air cooling.

[0048] Microstructure and performance: the microstructure is more fine and uniform, but a multi-oriented lamellar alpha tri-state structure of the alloy component is successfully constructed. Figure 2 A typical microstructure metallographic image of the Ti-6Al-4V alloy component after being treated by the method of Example 1 of the present application.

[0049] The Ti-6Al-4V alloy component of Example 1 is applied to SSRT test in simulated oilfield brine. The corrosion medium of the simulated oilfield brine contains Cl - , and the concentration of Cl - is 10g / L-25g / L; the stress corrosion test is carried out according to the national standard GB / T1804-m, the sample used is a sheet sample with a thickness of 3mm, the rate is 1×10 -6 S -1 , and the stress sensitivity coefficient (I SSRT ) obtained by the test is 12%, and the strength of the sample is 920MPa. The crack propagation path observation of the SSRT test shows that the Ti-6Al-4V alloy component in the Cl -≥10g / L of corrosion medium, the average deflection angle of the crack during propagation is ≥10.5°, and the crack propagation path is discontinuous zigzag.

[0050] Comparative Example 1 A method for preparing a stress corrosion resistant Ti-6Al-4V alloy component, comprising the following steps: Material: Commercially available hot-rolled Ti-6Al-4V alloy plate with a thickness of 10 mm, and a β phase transition point (Tβ) of about 990℃. β

[0051] The heat treatment steps are as follows: Solution treatment: 1020℃ (T β +30℃) for 1.5 hours, and water cooling.

[0052] Aging: 550℃ for 2 hours, and then air cooling.

[0053] Microstructure and performance: a Widmanatatten microstructure alloy component containing large size α bundles is obtained. Figure 3 A Widmanatatten microstructure diagram of the Ti-6Al-4V alloy component after traditional solution and aging in Comparative Example 1.

[0054] The Ti-6Al-4V alloy component of Comparative Example 1 is applied to the SSRT test in the simulated oilfield brine, and the same test conditions as in Example 1 are adopted. The stress sensitivity coefficient (I SSRT ) obtained by the test is 11%, and the strength of the sample is 910MPa. The crack propagation path observation of the SSRT test shows that the crack is obviously discontinuous zigzag, and is straight in one grain, and the single grain has weak crack propagation resistance.

[0055] Comparative Example 2 A method for preparing a stress corrosion resistant Ti-6Al-4V alloy component, comprising the following steps: Material: Commercially available hot-rolled Ti-6Al-4V alloy plate with a thickness of 10 mm, and a β phase transition point (Tβ) of about 990℃.

[0056] The heat treatment steps are as follows: Thermal mechanical treatment: hot rolling at 950℃ (T β -40℃) with a deformation of 20%.

[0057] Pretreatment: 830℃ for 1.5 hours, and air cooling.

[0058] Controlled cooling: first cooling to 800℃ at ≥15℃ / min, and then furnace cooling to room temperature at ≤5℃ / min.

[0059] ​Staging aging: first stage: 580℃ for 2.5 hours; second stage: 620℃ for 3.5 hours, air cooling.

[0060] Tissue and performance: the microstructure is more fine and uniform, but the alloy component of multi-oriented lamellar alpha duplex structure is still successfully constructed. Figure 4 The duplex structure diagram of the Ti-6Al-4V alloy component in the comparative example 2 after traditional solid solution aging.

[0061] The Ti-6Al-4V alloy component in the comparative example 2 is applied to the SSRT test in the simulated oilfield brine, and the same test conditions as in the example 1 are adopted, the stress sensitivity coefficient (I SSRT ) is 20%, and the strength of the sample is 915 MPa.

[0062] Industrial applicability verification: The heat treatment method in the example 1 of the application is completed in the pilot verification of a certain titanium alloy pipe production line, the service life of the Ti-6Al-4V oil pipe after treatment is increased by about 30% or more in the 25g / L NaCl solution, and the process is stable, which is suitable for the existing titanium alloy component heat treatment production line.

[0063] The above is only a preferred embodiment of the application, and is not used to limit the application, any modification, equivalent replacement and improvement made within the spirit and principle of the application shall be included in the protection scope of the application.

Claims

1. A method of preparing a Ti-6Al-4V alloy component resistant to stress corrosion, characterized in that, The method comprises the following steps: pre-treating a Ti-6Al-4V alloy workpiece at 800-850 DEG C, and then air cooling to obtain a pre-treated workpiece; heating the pre-treated workpiece to 10-80 DEG C above the beta transus point for solid solution treatment, then cooling at a rate of ≥15 DEG C / min to 800 DEG C, and then cooling at a rate of ≤5 DEG C / min to room temperature to obtain a workpiece cooled in stages; aging the workpiece cooled in stages at 500-650 DEG C in two stages to form alpha colonies and / or lamellar alpha phase structure in the Ti-6Al-4V alloy to obtain a stress corrosion resistant Ti-6Al-4V alloy component.

2. The method of claim 1, wherein the Ti-6AI-4V alloy component is subjected to a solution treatment and a subsequent aging treatment. The average size of the alpha colonies is ≥50 μm; the average thickness of the lamellar alpha phase structure is 1-5 μm, and the crystallographic misorientation between adjacent lamellar alpha phases is ≥5 DEG.

3. The method for preparing the stress corrosion resistant Ti-6Al-4V alloy component according to claim 1, characterized in that, The pre-treatment time is 1-3 h.

4. The method of claim 1, wherein the Ti-6Al-4V alloy component is subjected to a solution treatment and a subsequent aging treatment. The solid solution treatment time is 0.5-3 h.

5. The method for preparing the stress corrosion resistant Ti-6Al-4V alloy component according to claim 1, characterized in that, The two-stage aging comprises first stage aging and second stage aging; the holding time of the first stage aging is 2-4 h; and the holding time of the second stage aging is 3-6 h.

6. The method for preparing the stress corrosion resistant Ti-6Al-4V alloy component according to claim 1, characterized in that, The cooling in stages is carried out under inert gas protection; the inert gas is argon.

7. The method of claim 1, wherein the Ti-6Al-4V alloy component is subjected to a solution treatment and a subsequent aging treatment. Before the pre-treatment, the method further comprises: rolling or forging at 20-100 DEG C below the beta transus point with a deformation of 10-30% to obtain the Ti-6Al-4V alloy workpiece.

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