Heat treatment method for improving mechanical property of long-time service K412 high-temperature alloy

Abstract

A heat treatment method for improving the mechanical property of a long-time service K412 high-temperature alloy comprises the steps that the K412 high-temperature alloy to be treated is subjected to segmented heating to the solid solution temperature T for solid solution, then heat preservation is conducted, and air cooling is conducted to the room temperature after heat preservation; the K412 high-temperature alloy cooled to the room temperature is put into a furnace to be subjected to aging treatment; the segmented temperature rising comprises a first temperature rising stage, a second temperature rising stage and a third temperature rising stage which are carried out in sequence; in the first heating stage, the temperature is raised to 65-70% solid solution temperature T for solid solution; in the second heating stage, the temperature is raised to 80%-85% solid solution temperature T for solid solution; and in the third heating stage, the temperature is raised to the solid solution temperature T solid solution.

Description

Technical Field

[0001] The present invention relates to the technical field of K412 superalloy, and particularly relates to a heat treatment method for improving the mechanical properties of K412 superalloy during long-term service. Background Art

[0002] Nickel-based superalloys have high high-temperature strength, creep strength, and good fatigue resistance. They are mainly used to manufacture hot-end components in the aerospace industry. K412 is a precipitation-hardening superalloy, which has good high-temperature mechanical properties below 1000°C and is widely used to manufacture aerospace structural components, such as guide vane blades, outer rings of guide vanes, etc.

[0003] The excellent high-temperature properties of nickel-based superalloys are closely related to their duplex structure, which is mainly composed of γ matrix phase and γ' phase. The γ' phase is a precipitation strengthening phase Ni3(Al,Ti) that precipitates and is formed by the combination of aluminum atoms that easily enrich around dislocations at the interface with nickel and titanium atoms in the matrix. The hindrance of the γ' phase to the movement of dislocations strengthens the strength of the alloy.

[0004] Due to the long-term action of high-temperature gas and thermal alternating stress during service, the degradation of mechanical properties and the evolution of microstructure will greatly affect the service life of parts. Since the composition of K412 material is complex and the production process is cumbersome, the cost of replacing new parts is relatively high. Therefore, there is an urgent need for a suitable method to solve the problem of the decrease in mechanical properties of nickel-based superalloys after service.

[0005] The above information disclosed in the background art section is only used to enhance the understanding of the background of the present invention, and thus may include information that does not constitute the prior art known to those of ordinary skill in the art. Summary of the Invention

[0006] The present invention provides a heat treatment method for improving the mechanical properties of K412 superalloy during long-term service, which can restore the mechanical properties of nickel-based alloys whose structures have degraded during long-term service, and thus effectively extend the service life.

[0007] A heat treatment method for improving the mechanical properties of K412 superalloy during long-term service includes:

[0008] The K412 superalloy to be treated is heated in stages to the solution temperature T 固溶 , then held for a certain time, and after holding, it is air-cooled to room temperature; and the K412 superalloy cooled to room temperature is put into the furnace for aging treatment;

[0009] The staged heating includes a first heating stage, a second heating stage, and a third heating stage that are carried out in sequence;

[0010] In the first heating stage, it is heated to 65%-70% of the solution temperature T固溶 ;

[0011] In the second heating stage, heat up to 80% - 85% of the solution treatment temperature T 固溶 ;

[0012] In the third heating stage, heat up to the solution treatment temperature T 固溶 .

[0013] In the heat treatment method for improving the mechanical properties of K412 superalloy during long-term service, the K412 superalloy is air-cooled to room temperature after holding at the solution treatment temperature T 固溶 ±15°C.

[0014] In the heat treatment method for improving the mechanical properties of K412 superalloy during long-term service, the K412 superalloy after being cooled to room temperature is aged at 75% of the solution treatment temperature T 固溶 for 2 - 2.5 times the solution treatment time, and then air-cooled to room temperature.

[0015] In the heat treatment method for improving the mechanical properties of K412 superalloy during long-term service, the K412 superalloy is a K412 nickel-based superalloy that has been in service for 1000h to 4000h.

[0016] Compared with the prior art, the present invention has the following advantages: The present invention can increase the volume fraction of the γ' phase, reduce its size, and make the γ' phase uniformly distributed in the γ matrix, restoring the strength and hardness of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] By reading the detailed description of the preferred specific embodiments below, various other advantages and benefits of the present invention will become clear to those of ordinary skill in the art. The accompanying drawings of the specification are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present invention. Obviously, the drawings described below are only some embodiments of the present invention, and those of ordinary skill in the art can obtain other drawings based on these drawings without creative efforts. Moreover, throughout the drawings, the same reference numerals are used to represent the same components.

[0018] In the drawings:

[0019] Figure 1 is a schematic diagram of the microstructure of the K412 superalloy that has been in service for 1500h without heat treatment;

[0020] Figure 2 is a schematic diagram of the microstructure of the K412 superalloy that has been in service for 1500h after heat treatment;

[0021] Figure 3Schematic diagram of the microstructure of K412 superalloy that has not been heat-treated after 3500 h of service;

[0022] Figure 4 Schematic diagram of the microstructure of K412 superalloy after heat treatment after 3500 h of service;

[0023] Figure 5 Schematic diagram of the critical resolved shear stress in different service states before and after heat treatment.

[0024] The present invention will be further explained below in conjunction with the accompanying drawings and embodiments. Detailed implementation manners

[0025] The specific embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although specific embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present invention can be more thoroughly understood and the scope of the present invention can be completely conveyed to those skilled in the art.

[0026] It should be noted that certain terms are used in the description and claims to refer to specific components. Those skilled in the art should understand that technicians may use different terms to refer to the same component. The description and claims of this specification do not use the difference in terms as a way to distinguish components, but use the difference in the functions of components as the criterion for distinction. As mentioned throughout the description and claims, the term "comprising" or "including" is an open-ended term and should be interpreted as "including but not limited to". The subsequent description of the specification is for the purpose of implementing the preferred embodiments of the present invention, but the description is for the general purpose of the specification and is not intended to limit the scope of the present invention. The scope of protection of the present invention shall be determined by the terms defined in the appended claims.

[0027] To facilitate the understanding of the embodiments of the present invention, the following will further explain with specific embodiments as examples in conjunction with the accompanying drawings, and each accompanying drawing does not constitute a limitation to the embodiments of the present invention.

[0028] As Figures 1 to 5 shown, the heat treatment method for improving the mechanical properties of K412 superalloy during long-term service includes the following steps:

[0029] The K412 superalloy to be treated is heated in stages to the solution temperature T 固溶 , then insulated, and after insulation, air-cooled to room temperature; and the K412 superalloy cooled to room temperature is put into the furnace for aging treatment;

[0030] The staged heating includes a first heating stage, a second heating stage, and a third heating stage that are carried out in sequence;

[0031] The first heating stage heats up to 65% - 70% of the solution treatment temperature T 固溶 ;

[0032] The second heating stage heats up to 80% - 85% of the solution treatment temperature T 固溶 ;

[0033] The third heating stage heats up to the solution treatment temperature T 固溶 .

[0034] In the preferred embodiment of the heat treatment method for improving the mechanical properties of the K412 superalloy during long-term service, the K412 superalloy is air-cooled to room temperature after holding at the solution treatment temperature T 固溶 ±15°C

[0035] In the preferred embodiment of the heat treatment method for improving the mechanical properties of the K412 superalloy during long-term service, the K412 superalloy after cooling to room temperature is aged for 2 - 2.5 times the solution time at 75% of the solution treatment temperature T 固溶 and then air-cooled to room temperature

[0036] In the preferred embodiment of the heat treatment method for improving the mechanical properties of the K412 superalloy during long-term service, the K412 superalloy is a K412 nickel-based superalloy that has been in service for 1000h to 4000h

[0037] Example 1:

[0038] In this example, the heat treatment method for restoring the mechanical properties of the K412 superalloy that has been in service for 1500h is as follows:

[0039] 1. Solution treatment: Select the K412 superalloy that has been in service for 1500h. Place the specimen in a KSL-1700X-A1 box furnace for stepped heating, heat up to T 固溶 , hold for a period of time and then air-cool to room temperature

[0040] In the first heating stage, heat from room temperature to 800°C at a heating rate of 26°C / min

[0041] In the second heating stage, heat from 800°C to 1000°C at a heating rate of 2°C / min

[0042] In the third heating stage, heat from 1000°C to 1150°C at a heating rate of 1°C / min

[0043] After heating up to 1150°C, hold for 2h and then air-cool to room temperature

[0044] 2. Aging treatment: Place the sample cooled to room temperature after solution treatment in a KSL-1700X-A1 box furnace at 855°C for aging for 17h, and then air-cool to room temperature

[0045] Example 2:

[0046] In this example, the heat treatment method for restoring the mechanical properties of the superalloy K412 after 3500 h of service is as follows:

[0047] 1. Solution treatment: Select the superalloy K412 after 3500 h of service and place the specimen in a KSL-1700X-A1 box furnace for stepwise heating. Heat up to T 固溶 +15 °C, hold for a period of time and then air cool to room temperature;

[0048] In the first heating stage, heat from room temperature to 800 °C at a heating rate of 26 °C / min;

[0049] In the second heating stage, heat from 800 °C to 1000 °C at a heating rate of 2 °C / min;

[0050] In the third heating stage, heat from 1000 °C to 1170 °C at a heating rate of 1 °C / min.

[0051] After heating up to 1170 °C, hold for 2 h and then air cool to room temperature;

[0052] 2. Aging treatment: The specimen after solution treatment is aged at 75% T 固溶 for 2 - 2.5 times the solution time, and then air cool to room temperature.

[0053] Table 1 shows the statistical results of the specimens before and after heat treatment in Example 1 and Example 2

[0054]

[0055] As can be seen from Table 1, for the specimen heat treated in Example 1, after detection, the γ' size decreased by 60.7% and the volume fraction increased by 98.2%.

[0056] As can be seen from Table 1, for the specimen heat treated in Example 2, after detection, the γ' size decreased by 41.2% and the volume fraction increased by 55.8%.

[0057] When the γ′ size is relatively small, dislocations pass through the γ′ phase by the cutting mechanism. The γ' phase has an LI2 long-range ordered structure. When dislocations shear the ordered γ' phase, the ordered arrangement of atoms above and below the slip plane is disrupted, generating an elastic stress field that hinders the movement of dislocations, thus leading to an increase in the strength of the nickel-based superalloy.

[0058] The shear mechanisms in nickel-based superalloys can be divided into two types: the weak coupling model and the strong coupling model. When the size of the γ' phase is small, dislocation pairs may not exist in a single γ' phase, and the shear mechanism at this time is the weak coupling model. When the size of the γ' phase is large, dislocation pairs can exist in a single γ' phase simultaneously, and the shear mechanism is the strong coupling model. As the size of the γ' phase further increases, the hindering effect of the γ' phase on dislocations decreases. When the shear stress required for dislocations to bypass the γ' phase is less than the shear stress of the strong coupling shear mechanism, the strengthening mechanism changes from shear to the Orowan bypass mechanism.

[0059] The critical resolved shear stress (CRSS) refers to the minimum resolved shear stress required to activate the slip system. When the external stress exceeds the critical resolved shear stress of the material, the material will undergo plastic deformation. The critical resolved shear stress of the Orowan model is

[0060] (1)

[0061] where d is the diameter of γ′; f is the volume fraction of γ′; the shear modulus G = E / 2(1 + υ); υ is the Poisson's ratio; E is the elastic modulus; b is the Burgers vector.

[0062] The critical resolved shear stresses of the weak coupling model and the strong coupling model are respectively

[0063] (2)

[0064] (3)

[0065] where γ APB is the APB energy, which is related to alloy composition, γ / γ′ misfit degree, etc.; k is an empirical constant, and for screw dislocations k = 1.

[0066] The diameter (d) of γ′ is characterized by scanning electron microscopy (SEM) and measured by nano measure software. The volume fraction (f) of γ′ is statistically analyzed by image pro plus software. The Poisson's ratio υ is 0.33, the elastic modulus is 210 GPa, and the Burgers vector (b) takes the common a / 2<011> in FCC. Since the APB energy is unknown, the thermodynamic software Jmatpro is used for simulation and estimation, and γ APB = 0.231 J / m 2 .

[0067] In formulas (1), (2), and (3), d and f are regarded as independent variables, and the critical resolved shear stress (CRSS) is the dependent variable. The range of the independent variable d is 80 - 350 nm, and the iteration step size is 10 nm. The range of the independent variable f is 5% - 50%, and the iteration step size is 5%. Using Matlab software, two independent variables in the three formulas are continuously enumerated to obtain three sets of data, and the minimum values of the three sets of data are taken to plot the contour lines, that is, the contour lines are the theoretical critical resolved shear stress. Toshio Osada et al. have studied and proved that the yield strength is often proportional to the Vickers hardness. Generally speaking, the higher the yield strength of the material, the corresponding increase in the critical resolved shear stress. The critical resolved shear stress is proportional to the increase in microhardness value due to precipitation strengthening. After heat treatment, the critical resolved shear stress increases significantly as Figure 5 .

[0068] Compare the test results of the specimens before and after heat treatment: After heat treatment, the volume fraction of γ' phase increases, the size decreases, and the Vickers hardness of the specimens has been significantly improved. Heat treatment can effectively improve the mechanical properties and extend the service life.

[0069]

[0070] Although the embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments and application fields. The above specific embodiments are merely illustrative and guiding, rather than restrictive. Those of ordinary skill in the art can also make many forms under the inspiration of this specification and without departing from the scope protected by the claims of the present invention, and these all belong to the scope of protection of the present invention.

Claims

1. A heat treatment method for improving the mechanical properties of K412 high-temperature alloy in long-term service, characterized in that: The steps include: The K412 high temperature alloy to be treated is heated up in stages to the solution temperature T 固溶 , then keep warm, and then air cool to room temperature; and put the K412 high temperature alloy after cooling to room temperature into the furnace for aging treatment; The staged heating includes a first heating stage, a second heating stage, and a third heating stage which are performed sequentially; The first heating stage is to heat up to 65%-70% of the solid solution temperature T 固溶 ; The second heating stage is to heat up to 80%-85% of the solid solution temperature T 固溶 ; The third heating stage is to heat up to the solid solution temperature T 固溶 .

2. A heat treatment method for improving the mechanical properties of K412 high temperature alloy in long-term service according to claim 1, characterized in that: Preferably, the K412 high temperature alloy has a solid solution temperature T 固溶 Keep warm at ±15℃ and then air cool to room temperature.

3. A heat treatment method for improving the mechanical properties of K412 high temperature alloy in long-term service according to claim 1, characterized in that: After cooling to room temperature, the K412 high temperature alloy is at 75% solution temperature T 固溶 Aged at temperature for 2-2.5 times the solution time, then air cooled to room temperature.

4. A heat treatment method for improving the mechanical properties of long-term service K412 high-temperature alloy according to claim 1, characterized in that: K412 high temperature alloy is a K412 nickel-based high temperature alloy with a service time of 1000h to 4000h.

Images