SU1731842A1 - Cromel-alumel thermocouple electrodes treatment method - Google Patents

Abstract

The invention relates to metallurgy, in particular, to a method for treating thermoelectrode alloys, and can be used in instrument engineering for the manufacture of highly stable thermocouples designed to measure temperatures in oxidizing environments up to 1250 ° C for a long time (over 100 hours) and 1350 ° C briefly (less than 1 hour) in industrial furnaces, heating devices, power equipment, etc. The aim of the invention is to increase the thermocouple thermopower stability while maintaining heat resistance. The method includes the preliminary annealing of a chromelide electrode at 700–750 ° C and an alumel electrode at 900–1000 ° C. After pre-annealing, the surfaces of both electrodes are coated by thermal evaporation in vacuum and additional annealing is performed at a temperature of 0.4-0.45 Tpl of the coating material, and the time for coating and additional annealing is determined by the formula. The method allows to increase the thermopower stability of chromel-alumel thermocouples. 1 tab. cl

Description

The invention relates to metallurgy, in particular, to a method for treating thermoelectrode alloys, and can be used in instrument making for the manufacture of highly stable thermocouples, which are designed to measure temperatures in oxidizing environments up to 1250 ° C for a long time (more than 100 hours) and 1350 ° C for a short time (less than 1 h) in industrial furnaces, heating devices, power equipment, etc.

During the operation of thermoelectrodes from chromel-alumel alloys, their thermoelectric power is changed. This leads to the fact that during long-term operation of chromel-alumel thermocouples, the temperature readings are too high.

Therefore, it is necessary to increase the thermoelectric stability of the thermocouple. It has been established that the thermoelectric stability of electrodes substantially depends on heat treatment.

A known method of heat treatment of chromel wire, including annealing at 700-750 ° C and additional annealing for 10-50 hours at 350-500 ° C. This heat treatment method is currently used in the manufacture of chromel-alumel thermoelectrodes. And although the thermoelectric stability of X-A thermocouples processed by this method, you VJ

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four

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more than when processed by other methods, however, it is not sufficient for using this thermocouple in modern devices of high accuracy.

The known method of heat treatment of electrodes of a chromel-alumel thermocouple, according to which chromel is subjected to annealing at 700–750 ° C, and aluminum - at 900–1000 ° C, depending on the diameter of the wire. Thermocouples whose electrodes are treated according to the described mode do not provide the necessary thermoelectric stability.

The purpose of the invention is to increase the thermocouple stability of thermocouples while maintaining heat resistance.

The goal is achieved by the method of treating electrodes with a chromel-alumel thermocouple, which includes pre-annealing of the chromel electrode at 700-750 ° C and alumel electrode at 900-1000 ° C, in which, after preliminary annealing on the surface of both electrodes, by thermal evaporation in vacuum the coating is applied, additional annealing is performed at a temperature of 0.4-0.45 Tpl. the coating material, and the time of additional annealing and coating is determined by the formula

04 10 exp (™ °) + 10 ° mp - ° - 6t 10) + (3 - 4) 4 V 2

R, po ... (jv)

8.12 10 "p (- Рв- °) + V66 -29V1 °) + (V-4) 4V.

Ttm „. , (

x5O

where t is the time of additional annealing and coating, min;

V - speed, micron / min;

T is the temperature of additional annealing and coating, ° C, and chromel is used as the coating material, and chromel is used as the coating material.

It is known that the main cause of instability of thermoelectrode alloys is their chemical interaction with the environment (oxidation), as well as the evaporation of components of thermoelectrode alloys (Cr, Mn, AI). These processes lead to a change in the chemical composition, mainly to the depletion of the alloys by any one or several components and, consequently, to a change in the thermoEMF.

The proposed method involves coating a surface of thermoelectrodes with a coating of the main alloying components of these alloys, which prevents them from selective oxidation and, therefore, changes in thermoelectric power.

The choice of coating temperature combined with annealing is determined by the following conditions. Processing of thermoelectrodes at a temperature lower than 0.4 Tpl. the coating material leads to the formation of cracks and porosity on their surface, and also leads to a decrease in the adhesion of the applied coating to the thermoelectrode material.

Processing at temperatures higher than recommended (0.45 Tpl.) Promotes the growth of condensed chromium grains, the appearance of pores at the grain boundaries and drip condensation of aluminum atoms. The latter does not allow to obtain a continuous condensate, uniform in thickness, And diffusion is also enhanced, due to which an intermediate layer is formed deep into the electrode, distorting the initial thermoelectric characteristic. All this reduces the stability of thermopower and heat resistance of thermoelectrodes.

The method is implemented as follows.

Chromel and alumel thermoelectrodes were subjected to separate processing. Chromel was annealed at 725 ° C, then degreased in carbon tetrachloride, placed in a vacuum chamber, outgassed at 720 ° C for 1 min in vacuum with a pressure of 5-6 mm Hg. Then the substrate was heated to 780 ° C and the chromium of the ERH brand was evaporated. Evaporation time 63 s. The thickness of the chromium condensate was 2.5 µm. The condensation rate of the chromium coating is 2.5 µm / min.

In a similar way, the coating was applied on alumel, after its preliminary annealing at 900–1000 ° С for 2 h and outgassed at 500 ° С for 3 min in vacuum with a pressure of 4–6 × 10 mm Hg. Coated with aluminum grade A995D.

The temperature of the alumel heating during condensation of aluminum was kept constant and equal to 300 ° C. Aluminum evaporation time is 56 s, condensate thickness 3.0 µm, condensation rate 3.5 µm / min.

The table shows the experimental data on the thermoelectric stability and heat resistance of electrodes of a chromel-alumel thermocouple, processed in a known manner and proposed.

As can be seen from the table, the proposed method increases the stability of the thermoelectric electrode made of chromel by 2-8 times, and alumel by 2 times while maintaining the level of heat resistance.

Claims (1)

Invention Formula The method of processing electrodes chromel-alumel thermocouple, including pre-annealing chromel electrode at 700-750 ° C and alumel electrode at 900-1000 ° C, characterized in that, in order to increase the thermopower thermopower stability while maintaining heat resistance, after pre-annealing, the surfaces of both electrodes are coated by thermal evaporation in vacuum and additional annealing is performed at a temperature of 0 4-0.45 TPl of the coating material, and the time of coating and additional annealing is determined by the formulas TxpOMI / IR -) + (S -4) 4V  f .84 1B "., p () + Y1B Mi (- “i 4) + v6n.io..p -; ° ":. 1 ° i) + (a-4) 4v (IsvI where t is the time of additional annealing and coating, min; V is the velocity μm / min; T-temperature of additional annealing and coating, ° C, and chromel, alumel-aluminum is used as a coating material for chromel. )  Thermoelectric stability is given after preliminary oxidation at C 1 h. Preliminary annealing of chromel T 700- 75 C C 2 h; alumel T 900-1000 ° C 2 h. The condensation rate of chromium is 2.5 μm / min, aluminum - μm / min.