A method for predicting the change trend of elongation after fracture of 625 alloy in a non-destructive and in-situ detection manner

Abstract

The present invention discloses a method for predicting the change trend of the elongation after fracture of 625 alloy in a non-destructive and in-situ detectable manner. Based on the principle that carbide and other precipitation phases belong to the second phase in corrosion-resistant alloys, and there are significant differences in their structures and compositions from the matrix, so corrosion usually occurs preferentially in this region and the adjacent regions thereof. By using electrochemical impedance technology, the change trend of the elongation after fracture of 625 alloy after long-term aging treatment is predicted. The impedance fitting parameter charge transfer resistance R t is used as a prediction index to detect the second phase precipitated in 625 alloy, evaluate the change of the elongation after fracture of the material, and further guide the safe progress of corresponding production.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochemical testing of Alloy 625, and more specifically, to a method for predicting the change trend of the elongation after fracture of Alloy 625 in tension. Background Art

[0002] Alloy 625 has high temperature oxidation resistance, corrosion resistance, and fatigue resistance, and is widely used in oil and gas pipelines, marine, nuclear industry and other fields. Due to performance or environmental requirements, Alloy 625 is usually subjected to heat treatment or long-term aging treatment. High temperature provides the required diffusion activation energy for element diffusion. Solute atoms diffuse and segregate at grain boundaries or dislocations to form and grow precipitation phases. Since there are significant differences in the composition and crystal structure of the precipitation phases and the matrix, on the one hand, the precipitation phases can increase the strength and hardness of the alloy and reduce its plasticity and toughness by pinning dislocations. On the other hand, the corrosion resistance of the region adjacent to the grain boundary is reduced due to the diffusion of solute atoms, especially Cr elements, which increases the intergranular corrosion sensitivity of the alloy. The research on precipitation phases usually uses methods such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The advantage of these methods is that they can accurately analyze the composition, type and morphology of precipitation phases microscopically, but they are limited to micro qualitative or semi-quantitative analysis and difficult to macroscopically and quantitatively analyze precipitation phases. In addition, these methods generally have high requirements for equipment, samples and experimental personnel and can usually only be carried out in scientific laboratories. Summary of the Invention

[0003] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a non-destructive and in-situ detectable method for predicting the change trend of the elongation after fracture of Alloy 625 (especially after heat treatment or long-term aging treatment) in tension. Since the precipitation phases in Alloy 625 belong to the second phase, there are significant differences in their structure and composition from the matrix, so corrosion usually occurs preferentially in this region and its adjacent regions. In addition, the coarsening precipitation phases precipitated in large quantities will also have an adverse effect on the plasticity and toughness of the alloy. Therefore, based on this principle and electrochemical impedance technology, this patent proposes a new index for predicting the change trend of the elongation after fracture of Alloy 625 in tension.

[0004] A non-destructive and in-situ detectable method for predicting the change trend of the elongation after fracture of Alloy 625 in tension, which performs electrochemical impedance testing on an Alloy 625 specimen and uses electrochemical impedance analysis software to establish an equivalent circuit to fit the impedance spectrum to obtain the charge transfer resistance R t As an index for detecting precipitation phases in Alloy 625, the lower the value of the charge transfer resistance R t the more serious the precipitation of the second phase in the alloy, and the greater the tendency for the elongation after fracture of the specimen to decrease.

[0005] In the above method, when performing electrochemical impedance testing, the 625 alloy specimen was subjected to electrochemical impedance testing in an aqueous solution of 5 wt% H2SO4 + 0.02 wt.% KSCN.

[0006] In the above method, the SiC sandpaper on the surface of the 625 alloy specimen was polished from coarse to fine to 2000 mesh and mechanically polished, and then the sample was washed and dried with deionized water and absolute ethanol.

[0007] In the above method, the electrochemical impedance testing was carried out at a stable open-circuit potential, and the test frequency was 10 5 -10 - 2 Hz, and the amplitude was 10 mV.

[0008] In the above method, an equivalent circuit was established using the electrochemical impedance analysis software ZSimpWin to fit the impedance spectrum obtained from the electrochemical test.

[0009] In the technical solution of the present invention, the corrosion of the corrosion-resistant alloy in the medium always occurs in the second phase with higher energy and its adjacent regions. Therefore, the lower the charge transfer resistance R t value indicates that the precipitation of the second phase in the alloy is more serious, and the tendency of the elongation after fracture of the specimen to decrease is greater. When the elongation after fracture decreases to a certain extent, safety protection or replacement of the material is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is the Nyquist diagram (1) of the 625 alloy samples at different aging temperatures in the embodiments of the present invention.

[0011] Figure 2 is the Nyquist diagram (2) of the 625 alloy samples at different aging temperatures in the embodiments of the present invention.

[0012] Figure 3 is the Bode diagram of the 625 alloy samples at different aging temperatures in the embodiments of the present invention.

[0013] Figure 4 is the fitting circuit diagram of the N1-N4 and N7 specimens in the embodiments of the present invention.

[0014] Figure 5 is the fitting circuit diagram of the N5 and N6 specimens in the embodiments of the present invention.

[0015] Figure 6 is the relationship diagram between the charge transfer resistance and the elongation after fracture of the 625 alloy samples at different aging temperatures in the embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0017] The electrochemical impedance test was carried out on 625 alloy specimens in an aqueous solution of 5 wt% H2SO4 + 0.02 wt.% KSCN. The SCN in the applied electrolyte solution - is a typical depolarizer, which can preferentially damage the areas with weak passivation film structure on the specimen surface. Usually, this weak area is in the second adjacent area, and the charge transfer resistance R obtained by fitting the electrochemical impedance t is the result of the parallel connection of the charge transfer resistance of the weak passivation film area and the charge transfer resistance of the complete area. Therefore, R t is mainly controlled by the weak area of the passivation film. Therefore, based on this principle, the present invention uses the charge transfer resistance R obtained from the electrochemical impedance test t as a new index to detect the precipitated phases in 625 alloy, and then evaluate the changes in the plasticity and toughness of the material, especially the elongation after tensile fracture. This method can predict the changes in the elongation after tensile fracture of 625 alloy non-destructively, quickly and quantitatively, and guide the safe production. The electrochemical impedance test was carried out at a stable open-circuit potential, and the test frequency was 10 5 -10 -2 Hz, and the amplitude was 10 mV. The obtained impedance spectrum was fitted by fitting software to obtain various parameters with physical meanings.

[0018] The specific steps are as follows:

[0019] (1) Prepare an electrolyte solution of 5 wt.% H2SO4 + 0.02 wt.% KSCN as the medium for the electrochemical reactivation test.

[0020] (2) Connect the back of the test surface of the specimen to be tested to a wire and seal it in epoxy resin, exposing only the test surface (ensuring that the exposed areas of the specimens to be tested are equal).

[0021] (3) Grind the SiC sandpaper on the sample surface from coarse to fine to 2000 mesh and mechanically polish it. Then wash and dry the sample with deionized water and absolute ethanol.

[0022] (4) Immerse the prepared specimen in the prepared solution and measure the open-circuit potential. After the open-circuit potential is stable, perform the electrochemical impedance test, and the test frequency is 10 5 -10 -2 Hz, and the amplitude is 10 mV.

[0023] (8) Use the electrochemical impedance analysis software ZSimpWin to establish an equivalent circuit to fit the impedance spectrum to obtain various parameters, and use the obtained parameter R t to analyze in combination with the test conditions.

[0024] The corrosion of corrosion-resistant alloys in a medium always occurs in the second phase with higher energy and its adjacent regions. Therefore, the charge transfer resistance R t The lower the value, the more serious the precipitation of the second phase in the alloy, and the greater the tendency for the elongation after fracture of the specimen to decrease. When the elongation after fracture decreases to a certain extent, safety protection or replacement of the material is required.

[0025] Taking the change trend of the elongation after tensile fracture of 625 alloy at different aging temperatures as an example:

[0026] Table 1: Heat treatment system of 625 alloy (7 hours)

[0027] Specimen number N1 N2 N3 N4 N5 N6 N7 Heat treatment regime (K) 873 893 913 933 953 973 993 Elongation after fracture (%) 48.0 48.8 47.6 46.8 46.0 42.4 45.2

[0028] Table 2 Fitting results of electrochemical impedance spectroscopy of 625 alloy after aging treatment

[0029]

[0030] In this experiment, Figure 1 is the Nyquist diagram of each sample, Figure 2 is Figure 1 a partial enlarged view of the lower left part of, Figure 3 is the Bode diagram.

[0031] From Figure 1 and Figure 2 it can be seen that the impedance arcs of specimens N1 to N4 and N7 are relatively large, while the arc radii of specimens N5 and N6 are much smaller than those of other specimens. Combining Figure 3 , two time constants appear in N5. Therefore, for specimens N1 to N4 and N7 with larger impedance arcs, a circuit with a complete passivation film is selected for fitting, such as Figure 4 (Impedance spectrum fitting circuit 1), while for specimens N5 and N6 with smaller impedance arcs, a circuit with a damaged local passivation film is selected for fitting, such as Figure 5 (Impedance spectrum fitting circuit 2).

[0032] The R t value is obtained by fitting the measured electrochemical impedance spectrum through the software ZSimpWin to establish equivalent circuits such as Figure 4 and Figure 5 . In the equivalent circuit, R s represents the solution resistance between the counter electrode and the working electrode, Q1 is a constant phase angle element related to the double layer, the dispersion coefficient n characterizes the degree to which the constant phase angle element approaches a pure capacitor. The closer n is to 1, the closer it is to a pure capacitor. Q2 is a constant phase angle element related to the passivation film capacitance, R f is the passivation film resistance, and R t is the charge transfer resistance of the entire test surface. From Figure 4 and Figure 5It can be seen that the charge transfer resistance R obtained by fitting t is the result of parallel testing of the weak area and the intact area of the passivation film on the specimen surface. Therefore, the value of R t mainly depends on the weak area of the passivation film, that is: the weaker the passivation film, the smaller R t . Therefore, the change rule of R t can indicate that with the increase of the aging temperature, the precipitation of the second phase leads to local corrosion and the deterioration of the uniformity of the passivation film.

[0033] From Table 2, Figure 6 it can be seen that after the electrochemical impedance spectrum is fitted, when the charge transfer resistance R t is relatively high (> 2.68×10 4 (Ω·cm 2 ²)), the elongation after fracture of the alloy has a relatively high plateau. As the value of R t decreases, the elongation after fracture decreases sharply. Under the conditions of this experiment, when the value of R t is lower than 2.68×10 4 (Ω·cm 2 ²), the elongation after fracture of the 625 alloy decreases sharply. Therefore, this value can be defined as the warning value of the elongation after fracture of the 625 alloy. When the value of R t is lower than this value, the ductility of the material will be significantly reduced and there is a risk of brittle fracture. Corresponding measures such as protection or replacement of parts need to be taken.

[0034] The above makes an exemplary description of the present invention. It should be noted that without departing from the core of the present invention, any simple deformation, modification or equivalent replacement that can be made by those skilled in the art without creative labor falls within the protection scope of the present invention.

Claims

1. A method for predicting the change trend of elongation after fracture of 625 alloy in a non-destructive and in-situ detection manner, characterized in that, The electrochemical impedance spectroscopy (EIS) test was performed on the 625 alloy sample, and the equivalent circuit was established using the EIS software to fit the impedance spectrum to obtain the charge transfer resistance R. t As an indicator for detecting the precipitation phase in 625 alloy, the charge transfer resistance R t The lower the value, the more serious the precipitation of the second phase in the alloy is, and the greater the tendency of the elongation after fracture of the sample to decrease.

2. The method for predicting the change trend of the elongation after fracture of 625 alloy by non-destructive and in-situ detection according to claim 1, characterized in that, When performing electrochemical impedance testing, the 625 alloy specimen was subjected to electrochemical impedance testing in an aqueous solution of 5 wt% H2SO4 + 0.02 wt.% KSCN.

3. A method for predicting the change trend of the elongation after fracture of 625 alloy by non-destructive and in-situ detection according to claim 1, characterized in that, The surface of the 625 alloy specimen was polished from coarse to fine with SiC sandpaper up to 2000 mesh and then mechanically polished, and then the sample was washed and dried with deionized water and absolute ethanol.

4. A method for predicting the change trend of the elongation after fracture of 625 alloy in a non-destructive and in-situ detection manner according to claim 1, characterized in that, The electrochemical impedance test was carried out at a stable open-circuit potential, and the test frequency was 10 5 -10 - 2 Hz, and the amplitude was 10 mV.

5. A method for predicting the change trend of the elongation after fracture of 625 alloy in a non-destructive and in-situ detection manner according to claim 1, characterized in that, The equivalent circuit was established using the electrochemical impedance analysis software ZSimpWin to fit the impedance spectrum obtained from the electrochemical test.

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