A method of heat treating a k-split armored thermocouple

Abstract

The application provides a heat treatment method of K-graduated armored thermocouple, belongs to the technical field of thermocouple processing, and is used for processing K-graduated armored thermocouples by using armored materials. The method comprises the following steps: S1: cutting a sample in the armored material; S2: simultaneously processing the sample and the armored material; S3: performing heat treatment on the sample and the armored material after S2; S4: performing sampling inspection; S5: cutting a test sample, decomposing the nickel-chromium wire in the test sample, and performing sample scanning electron microscope analysis; and S6: if the sampling inspection result and the chromium content meet the process requirements of the K-graduated armored thermocouple, the K-graduated armored thermocouple to be detected is a K-graduated armored thermocouple meeting the process requirements. According to the heat treatment method, the K-graduated armored thermocouple precision qualified rate can reach more than 95% (meeting the I-grade precision), and the precision stability of the armored couple after heat treatment is good after long-time use.

Description

Technical Field

[0001] This application belongs to the field of thermocouple processing technology, specifically relating to a heat treatment method for a K-type armored thermocouple. Background Technology

[0002] K-type armored thermocouples use nickel-chromium and nickel-silicon wires as their internal sensing materials. While the nickel-silicon wire has a stable structure, the chromium in the nickel-chromium wire is highly susceptible to environmental influences. Stress-relief annealing after processing can cause cracking or peeling of the oxide film on the surface of the nickel-chromium wire, thus promoting the oxidation of internal elements. Due to the selective oxidation of chromium, the chromium beneath the oxide layer is depleted. Reducing the chromium content lowers the thermoelectric potential of the nickel-chromium alloy. However, since the chromium content in nickel-chromium alloys is generally slightly higher than the chromium content equivalent to the maximum thermoelectric potential, although some thermocouples that pass through conventional heat treatment methods may reach their maximum accuracy in a very short time, their accuracy will deteriorate negatively during use, resulting in significant deviations from tolerances.

[0003] Furthermore, during forging and bending processes, the thermocouple wires may experience stress distortion or uneven deformation, potentially leading to thermocouple degradation or characteristic drift. While inappropriate heat treatment methods may eliminate stress distortion or uneven deformation, they can also reduce the chromium content within the nickel-chromium wire. This not only fails to restore the accuracy of the armored thermocouple but can also cause a permanent loss of its accuracy performance.

[0004] Therefore, a new heat treatment process is needed to reduce or avoid selective oxidation of chromium in the nickel-chromium wire during heat treatment, thereby removing processing stress without causing irreversible damage to the accuracy of the armored thermocouple. Summary of the Invention

[0005] The purpose of this application is to provide a heat treatment method for K-type armored thermocouples, so as to reduce or avoid selective oxidation of chromium in the nickel-chromium wire during the heat treatment process, and remove processing stress without causing irreversible damage to the accuracy of the armored thermocouple.

[0006] To achieve the above objectives, this application provides the following technical solution: a heat treatment method for K-type armored thermocouples, used for processing K-type armored thermocouples using armor material, the method comprising the following steps:

[0007] S1: Take a sample from the armor material;

[0008] S2: Process the sample and the armor material simultaneously in one operation;

[0009] S3: Heat-treat the sample and armor material after processing in S2 to obtain the sample thermocouple and the K-type armored thermocouple to be tested.

[0010] S4: Randomly inspect the K-type armored thermocouples to be tested;

[0011] S5: Cut a test sample from the thermocouple of the sample, decompose the nickel-chromium wire in the test sample, and perform scanning electron microscopy analysis on the sample to detect the chromium content at each location;

[0012] S6: If the sampling results in S4 and the chromium content in S5 meet the process requirements of the K-type armored thermocouple, then the K-type armored thermocouple to be tested is a K-type armored thermocouple that meets the process requirements.

[0013] The heat treatment method for K-type armored thermocouples provided in this application also has the feature that the sample length in S1 is 100 mm.

[0014] The heat treatment method for K-type armored thermocouples provided in this application also has the feature that the processing in S2 includes a processing method selected from rotary forging and bending.

[0015] The heat treatment method for K-type armored thermocouples provided in this application also has the following feature: the heat treatment step in S3 is as follows:

[0016] S3.1: After cleaning the sample and armor, place them into the nitrogen furnace;

[0017] S3.2: Evacuate the nitrogen furnace to 0.01 Pa, then introduce nitrogen gas to 50 kPa;

[0018] S3.3: Heat the furnace to 1140℃~1160℃ and hold for about 1 hour;

[0019] S3.4: After the sample and armor are cooled to below 300°C in the furnace, the heat treatment process is complete.

[0020] The heat treatment method for K-type armored thermocouples provided in this application also has the following features: in step S4, the sampling rate is not less than 10%, the number of samples is not less than 5, and during the sampling process, the accuracy error values ​​of the K-type armored thermocouples to be tested at 600℃ and 900℃ are measured and the measurement results are recorded.

[0021] The heat treatment method for K-type armored thermocouples provided in this application also has the feature that the length of the test sample in S5 is 15 mm.

[0022] The heat treatment method for K-type armored thermocouples provided in this application also has the following feature: in step S5, before performing scanning electron microscopy analysis of the sample, the nickel-chromium wire decomposed from the test sample is polished into a semi-cylindrical shape.

[0023] Beneficial effects

[0024] The heat treatment method for K-type armored thermocouples provided in this application effectively removes stress distortion and uneven deformation of the thermocouple wires, and also avoids the precipitation of chromium during the heat treatment process. While ensuring a significantly improved pass rate for Class I accuracy after processing, it also avoids irreversible damage to the accuracy of the K-type armored thermocouples. In addition to providing effective heat treatment parameters, the relevant performance of the K-type armored thermocouples is tested through product accuracy sampling and scanning electron microscopy analysis of the samples. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this disclosure. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a flowchart of the heat treatment method provided in the embodiments of this application;

[0027] Figure 2 This is a structural diagram of an armored thermocouple;

[0028] Figure 3 This is a scanning electron microscopy grading line scan. Detailed Implementation

[0029] The present application will be further described in detail below with reference to the accompanying drawings and embodiments. However, it should be noted that these embodiments are not intended to limit the present application. Equivalent transformations or substitutions in function, method, or structure made by those skilled in the art based on these embodiments are all within the protection scope of the present application.

[0030] In the description of the embodiments of this application, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. They are only for the convenience of describing the creation of this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the creation of this application.

[0031] Furthermore, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0032] The terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0033] like Figure 2 The diagram shows the structure of a K-type indexed armored thermocouple, which includes a metal protective sleeve, end caps, a positive thermoelectric electrode and a negative thermoelectric electrode placed inside the metal protective sleeve, and magnesium oxide powder for filling. During processing, the laser welding point and the transition range of the rotary forging cross-section are located near the end cap. In the processing of K-type indexed armored thermocouples, the silicon and nickel elements in the nickel-silicon alloy wire of both qualified and unqualified thermocouples are relatively uniformly distributed from the core to the surface, with no significant difference in silicon and nickel content. However, in the nickel-chromium alloy wire, the chromium and nickel elements are unevenly distributed from the core to the surface, with chromium segregating on the outer surface. Furthermore, as the accuracy error increases, the chromium content on the outer surface gradually increases.

[0034] Thermocouples with a continuous and dense oxide layer on the surface of nichrome alloy wire provide excellent protection for the internal alloy. Even after multiple annealing processes, the oxidation process is slow, and the surface remains relatively continuous and dense. The chromium content distribution within the nichrome alloy wire remains relatively uniform, and the thermocouple accuracy shows no significant fluctuations within the acceptable range. In contrast, thermocouples with a discontinuous and dense oxide layer on the surface of nichrome alloy wire oxidize rapidly after multiple annealing processes. Furthermore, during heating and cooling, the oxide film cracks or peels off, further promoting oxidation. Due to the selective oxidation of chromium, the chromium beneath the oxide layer is depleted, reducing the chromium content. This lowers the thermoelectric potential of the nichrome alloy. However, since the chromium content in nichrome alloys is generally slightly higher than the chromium content equivalent to the maximum thermoelectric potential, the thermocouple accuracy passes its peak point and changes negatively in a very short time, resulting in significant deviations from tolerance.

[0035] For the reasons mentioned above, such as Figure 1 As shown, this embodiment provides a heat treatment method for K-type sheathed thermocouples, used for processing K-type sheathed thermocouples using sheathing material. The method includes the following steps:

[0036] S1: Take a sample from the armor material;

[0037] S2: Process the sample and the armor material simultaneously in one operation;

[0038] S3: Heat-treat the sample and armor material after processing in S2 to obtain the sample thermocouple and the K-type armored thermocouple to be tested.

[0039] S4: Randomly inspect the K-type armored thermocouples to be tested;

[0040] S5: Cut a test sample from the thermocouple of the sample, decompose the nickel-chromium wire in the test sample, and perform scanning electron microscopy analysis on the sample to detect the chromium content at each location;

[0041] S6: If the sampling results in S4 and the chromium content in S5 meet the process requirements of the K-type armored thermocouple, then the K-type armored thermocouple to be tested is a K-type armored thermocouple that meets the process requirements.

[0042] In the above embodiments, the electron microscopy analysis results in S5 are as follows: Figure 3 As shown, ① is the Ni content curve, ② is the O content curve, ③ is the Cr content curve, and ④ is the Mg content curve.

[0043] In some embodiments, the sample length in S1 is 100 mm.

[0044] In some embodiments, the processing in S2 includes a processing method selected from rotary forging and bending.

[0045] In some embodiments, the heat treatment step in S3 is as follows:

[0046] S3.1: After cleaning the sample and armor, place them into the nitrogen furnace;

[0047] S3.2: Evacuate the nitrogen furnace to 0.01 Pa, then introduce nitrogen gas to 50 kPa;

[0048] S3.3: Heat the furnace to 1140℃~1160℃ and hold for about 1 hour;

[0049] S3.4: After the sample and armor are cooled to below 300°C in the furnace, the heat treatment process is complete.

[0050] In some embodiments, in step S4, the sampling rate is not less than 10%, the number of samples is not less than 5, and during the sampling process, the accuracy error values ​​of the K-type armored thermocouple to be tested at 600°C and 900°C are measured and the measurement results are recorded.

[0051] In some embodiments, the length of the test sample in S5 is 15 mm.

[0052] In some embodiments, in step S5, before performing scanning electron microscopy analysis on the sample, the nickel-chromium wire decomposed from the test sample is polished into a semi-cylindrical shape.

[0053] Comparative tests were conducted using armored thermocouples from the same batch. The experimental group underwent heat treatment using the method provided in the above embodiments, while the control group underwent conventional heat treatment. Conventional heat treatment refers to localized annealing in a box furnace with a furnace temperature uniformity of ±10℃ and an effective heating zone of 250*150*100mm, followed by furnace heating to 850℃, holding for 2 hours, and then furnace cooling. The obtained thermocouple accuracy values ​​and the chemical composition of the nickel-chromium alloy wire are as follows:

[0054]

[0055]

[0056] As shown in the table above, in the experimental group using the heat treatment method provided in the embodiments of this application, there is less chromium segregation on the surface of the nickel-chromium alloy wire, and the microscopic distribution of chromium and nickel elements from the core to the surface is relatively uniform. The thermometric results show less accuracy deviation. In contrast, in the control group, there is more chromium segregation on the surface of the nickel-chromium alloy wire, and the microscopic distribution of chromium and nickel elements from the core to the surface is uneven. The thermometric results show a larger accuracy deviation, greater than ±2.4℃.

[0057] In summary, the heat treatment method for K-type sheathed thermocouples provided in this application can effectively remove stress distortion and uneven deformation of the thermocouple wires, and also avoid the precipitation of chromium during the heat treatment process. While ensuring a significant increase in the pass rate of K-type sheathed thermocouples meeting Class I accuracy after processing, it also avoids irreversible damage to their accuracy. In addition to providing effective heat treatment parameters, the relevant performance of the K-type sheathed thermocouples is tested through product accuracy sampling and scanning electron microscopy analysis of the samples.

[0058] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application. The above description is merely a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the protection scope of this application.

Claims

1. A heat treatment method for K-type sheathed thermocouples, used for processing K-type sheathed thermocouples using sheathing materials, characterized in that, The method includes the following steps: S1: Take a sample from the armor material; S2: Process the sample and the armor material simultaneously in one operation; S3: Heat-treat the sample and armor material after processing in S2 to obtain the sample thermocouple and the K-type armored thermocouple to be tested. S4: Randomly inspect the K-type armored thermocouples to be tested; S5: Cut a test sample from the thermocouple of the sample, decompose the nickel-chromium wire in the test sample, and perform scanning electron microscopy analysis on the sample to detect the chromium content at each location; S6: If the sampling results in S4 and the chromium content in S5 meet the process requirements for K-type sheathed thermocouples, then the K-type sheathed thermocouple to be tested is a K-type sheathed thermocouple that meets the process requirements. The heat treatment steps in S3 are as follows: S3.1: After cleaning the sample and armor, place them into the nitrogen furnace; S3.2: Evacuate the nitrogen furnace to a vacuum level of 0.01 Pa, then introduce nitrogen gas to a pressure of 50 kPa; S3.3: Heat the furnace to 1140℃~1160℃ and hold for 1 hour; S3.4: After furnace cooling to below 300℃, remove the sample and armor. The heat treatment process is now complete. In step S4, the sampling rate is not less than 10%, the number of samples is not less than 5, and during the sampling process, the accuracy error values ​​of the K-type armored thermocouple to be tested at 600℃ and 900℃ are measured and the measurement results are recorded.

2. The heat treatment method for K-type armored thermocouples according to claim 1, characterized in that, The sample length in S1 is 100 mm.

3. The heat treatment method for K-type armored thermocouples according to claim 1, characterized in that, The processing in S2 includes one of the processing methods selected from rotary forging and bending.

4. The heat treatment method for K-type armored thermocouples according to claim 1, characterized in that, The length of the test sample in S5 is 15 mm.

5. The heat treatment method for a K-type armored thermocouple according to claim 1, characterized in that, In step S5, before performing scanning electron microscopy analysis on the sample, the nickel-chromium wire decomposed from the test sample is polished into a semi-cylindrical shape.

Images