CN111028898B - Evaluation method for damage failure life of aluminum electrolysis cathode material - Google Patents

Evaluation method for damage failure life of aluminum electrolysis cathode material Download PDF

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CN111028898B
CN111028898B CN201911395919.4A CN201911395919A CN111028898B CN 111028898 B CN111028898 B CN 111028898B CN 201911395919 A CN201911395919 A CN 201911395919A CN 111028898 B CN111028898 B CN 111028898B
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creep
cathode
cathode material
life
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CN111028898A (en
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薛济来
张城
李海鹏
刘轩
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University of Science and Technology Beijing USTB
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Abstract

An evaluation method for damage failure life of an aluminum electrolysis cathode material belongs to the technical field of materials. The method comprises the steps of adopting a material testing machine to externally apply controllable constant pressure, adopting an improved Rapoport-Samoilenko device to online measure an expansion-creep curve of a cathode material under a selected service condition in real time, and obtaining parameters A and B in a linear equation epsilon=A+Bt of strain epsilon along with time t of the cathode material in a steady creep stage under the condition by fitting through a mathematical method; in addition, when the cathode material enters the steady creep initial stage under the same electrolysis and pressure conditions, the compressive strength test is carried out to obtain the ultimate strain epsilon of the material max And taking the creep failure life end as the creep failure life end, calculating and evaluating the residual life time of the cathode material in the service process
Figure DDA0002346305920000011
The invention can evaluate the residual service life of the cathode sample under the selected service condition by accurately measuring the creep damage data of the cathode sample on line in a certain period of time, and can provide a short-time, simple and feasible evaluation technical method for screening various commercial carbon cathode materials, analyzing and improving the quality and service life of cathode products.

Description

Evaluation method for damage failure life of aluminum electrolysis cathode material
Technical Field
The invention relates to an evaluation method for damage failure life of an aluminum electrolysis cathode material, and belongs to the technical field of materials.
Background
Modern metallic aluminum is produced by adopting an electrolysis method, and an aluminum electrolysis carbon cathode material is subjected to expansion phenomenon caused by infiltration of Na and electrolyte components in the electrolysis process. But this expansion is typically constrained by the cell steel shell and thus creates cumulative stresses in the cathode material of the cell. Under the stress, the cathode material also undergoes creep damage. The cathode material is continuously accumulated in the creep deformation process, so that the cathode carbon block is deformed, cracked and failed, and the overall service life of the electrolytic tank is seriously influenced.
The carbon cathode material, which is a typical elastoplastic material, exhibits a sodium expansion phase at constant pressure in the initial stage of electrolysis. When sodium and electrolyte permeation reaches saturation, rapid creep of the cathode structure further occurs under the associated stresses. When electrolyte permeation, internal stress and material structure reach an equilibrium state, creep enters a steady creep stage, the process mainly accompanies slip dislocation of a cathode structure, creep deformation is slow, strain changes in a linear rule along with time in a long time, and destructive damage occurs until the material reaches a strain limit. Therefore, the steady-state creep law provides a possible basis for estimating the estimated end of creep life of the cathode material.
At present, the method for evaluating and predicting the residual life of high-temperature creep damage of materials (most of metal materials) at home and abroad is generally based on a method related to a creep process by extrapolation of mechanical properties, and a large number of experiments show that parameters such as stress, temperature, steady-state creep rate, breaking time and the like of the materials in the creep process are consistent with Norton, arrhenius, monkman-Grant and other epitaxial formulas, and the influence of different temperature and stress conditions on the creep damage life of the materials is evaluated and presumed by a Larson-Miller method and a theta project method based on classical extrapolation methods generated by the relations. The method for testing creep life of the high-temperature component is based on the prediction of creep service life of the externally-propelled high-temperature component by Larson-Miller method. The related method is widely researched and applied in the field of metal materials, however, the carbon cathode material and the metal material have different creep damage rules, and the plastic deformation mode in the creep process is also essentially different in the microstructure change process. However, to date, no literature report or patent publication has been made on a method for evaluating and predicting the creep damage life of a carbon cathode material. Therefore, the invention and implementation of the assessment method for the creep damage life of the carbon cathode material have important significance for selecting and purchasing commercial carbon cathode products, judging the quality of the cathode products, prolonging the service life of the cathode service, saving the production cost of aluminum electrolysis and improving the economic benefit.
Disclosure of Invention
Aiming at the problems existing in the background technology, the invention aims to provide an evaluation method for the damage failure life of an aluminum electrolysis cathode material, which provides a judgment method for the creep life end point of the cathode material in the electrolytic creep process and provides reliable and convenient technical evaluation for the damage failure life of the cathode material according to relevant experimental data.
The specific implementation steps of the technical scheme of the invention are as follows:
step 1, measuring the electrolytic expansion-creep curve of more than one cathode material sample in the whole course under the steady state condition of selected parameters such as cathode current density, temperature, electrolyte composition, constant pressure, time and the like by using an improved Rapoport-Samoilenko device.
And 2, performing mathematical binomial fitting on the steady-state creep line segments in the curve to obtain a steady-state creep equation, namely a linear equation epsilon=a+bt of creep strain epsilon with time t, wherein A, B is a fitting constant.
Step 3, under the same experimental conditions as described above, determining the limit strain value of the same cathode material, wherein the specific method is to switch the constant-pressure mode of the material testing machine in the creep process into a constant-speed rate pressurizing mode when the creep curve of the cathode material just reaches a steady-state creep stage, select proper pressurizing rate and sample breaking conditions, and determine the stress strain curve of the sample material on line; then according to the stress-strain curve of the pressurizing process, determining that the strain value corresponding to the compressive strength is the limit strain epsilon max 。
Step 4, limiting the strain value epsilon max Substituting parameters A and B into the steady state creep equation in step 2, from which the calculation can be made
Obtaining residual creep life
Figure GDA0002381145660000021
Further, the creep data in the range of 20-150min in the steady-state creep stage is adopted to fit a complete steady-state creep linear equation epsilon=A+Bt, and meanwhile, the parameters A and B can adopt the average value of multiple groups of experimental data to increase the reliability of a prediction result.
Further, the compressive strength test result of the sample in the steady creep stage is adopted to correspond to the limit strain epsilon max As an end of creep life for the cathode material.
Further, the testing method of the creep life end is as follows: carrying out on-line compressive strength test on a sample in a steady creep stage by adopting a material testing machine, wherein the pressurizing rate is set to be 5-50N/s during the test, and the sample fracture condition is set to be 50-300N; under the specified pressurizing rate, until the sample reaches the set fracture condition; the slower the pressurization rate, the closer the limit strain amount is to the true creep rupture strain.
Further, the cathode material is a semi-graphite cathode, a graphitized cathode or a graphite composite cathode.
Further, the electrolysis conditions are characterized by: the temperature is 700-980 ℃, and the cathode current density is 0.01-2A/cm 2 The constant pressure of the liquid electrolyte system is 0.5-30MPa.
The invention has the main advantages that: ultimate strain ε obtained by testing compressive strength at steady state creep stage max As the end of the creep life of the cathode material, the relationship between the steady-state creep rate and the material limit deformation in the creep process of the cathode material can be reflected more reliably; meanwhile, the residual creep life of the material can be simply estimated according to a steady creep strain-time equation, and further quantitative evaluation, analysis and comparison are carried out on the damage failure life of different cathode materials or under the parameter change conditions of different electrolysis, pressure and the like.
Drawings
Figure 1 is a schematic diagram of a modified Rapoport-samailenko device,
1-a material testing machine pressurizing head; 2-a laser range finder probe; 3-a pressure conducting frame; 4-corundum guide rod; 5-an air outlet hole; 6-cathode conductive bars; 7-graphite crucible; 8-cathode sample; 9-melting the electrolyte; 10-corundum insulating gaskets; 11-heating the resistance wire; 12-an air inlet hole; 13-an anode conductive rod; 14-pressurizing a base of the testing machine; 15-a main body frame.
Fig. 2: graphitized cathode electrolysis creep curve;
fig. 3: compressive strength curve of graphitized cathode in steady state creep stage;
fig. 4: a semi-graphitic cathodic electrolysis creep curve;
fig. 5: compressive strength curve of semi-graphite cathode in steady state creep stage;
fig. 6: a graphite-titanium boride composite cathode electrolytic creep curve;
fig. 7: compression strength curve of graphite-titanium boride composite cathode in steady state creep phase.
Detailed Description
Example-1:
the expansion and creep of the aluminum electrolytic graphite cathode material are measured by a modified Rapoport-samailenko device, as shown in fig. 1, a graphite crucible 7 is used for containing molten electrolyte 9, a corundum insulating gasket 10 is added between a cathode sample and the bottom of the crucible for electric insulation, electrolytic current flows into the cathode sample from a bottom anode conductive rod 13 and then flows out of a cathode conductive rod 6, high temperature required by experiments is provided by a heating resistance wire 11 in a furnace, and protective atmosphere flows into the furnace from an air inlet hole 12 so as to prevent the cathode sample, the graphite crucible and the like from being oxidized to influence the test precision. The test system is vertically placed on the pressurization base 14 of the testing machine, and the main body frame 15 is made of steel with rigidity and strength so as to ensure that the whole equipment system has good mechanical stability.
It can also be used for on-line compressive strength test of sample under high temperature and electrolysis conditions. Wherein: a material testing machine (CMT 4304, SUST, pressure value error < +/-0.5%) is adopted to provide controllable pressure, a pressurizing head 1 of the material testing machine is connected with a pressure conducting frame 3, and the pressure is conducted to directly act on a cathode sample 8 through a cathode conducting rod 6; under the action of the pressure, a displacement change signal of the height of the cathode sample is reflected by the corundum guide rod 4 which is in direct contact with the signal and passes through a through hole in the cathode conductive rod, and the signal is measured in a non-contact mode by the laser range finder probe 2 (OptNCDT, micro-Epsilon, precision is 1 mu m) in real time and is input into a computer for recording; in experimental tests, a graphite crucible 7 is used for containing molten electrolyte 9, a corundum insulating gasket 10 is added between a cathode sample and the bottom of the crucible for electric insulation, electrolytic current flows into the cathode sample from a bottom anode conductive rod 13 and then flows out of a cathode conductive rod 6, high temperature required by experiments is provided by a heating resistance wire 11 in a furnace, and protective atmosphere flows into the furnace from an air inlet hole 12 so as to prevent the cathode sample, the graphite crucible and the like from being oxidized to influence the test precision. The test system is vertically placed on the pressurization base 14 of the testing machine, and the main body frame 15 is made of steel with rigidity and strength so as to ensure that the whole equipment system has good mechanical stability.
Graphitized cathode samples with the size phi of 25mm multiplied by 50mm are adopted, the cathode current density is 0.5A/cm < -2 >, the electrolyte system comprises 87wt.% of Na3AlF6-8wt.% of Al2O3-5wt.% of CaF2, the experiment is ended when the steady-state creep lasts for about 60min under the constant pressure condition of 940 ℃ and 4MPa, and the electrolytic expansion-creep curve of the graphite cathode in the obtained electrolytic process is shown in figure 2.
Mathematical binomial fitting is performed on the steady-state creep line segments (within 20-150 min) in the curve to obtain a steady-state creep equation, that is, a linear equation of creep strain epsilon over time t, which is: epsilon= -0.60987-6.28X10 -4 t, see fig. 2, where the fitting yields a=0.60987, b=6.28x10-4.
Under the same electrolysis and pressure conditions, the electrolysis creep test was again performed, and when the steady state creep was started, the stress strain curve of the graphitized cathode was measured at a constant pressurization rate of 10N/s, see fig. 3. Measuring maximum limit strain epsilon from stress-strain curve max Substituting the graphitized cathode steady-state creep linear equation, calculating to obtain the residual creep life, and the related results are shown in table 1.
Example-2:
the experimental test rig system was the same as in example 1.
The semi-graphite cathode material is adopted, the sample size and the test condition are the same as in example 1, a steady-state creep curve of the semi-graphite cathode material is obtained, and fitting is carried out to obtain a steady-state creep equation, as shown in fig. 4.
Then, in the same manner as in example 1, a stress strain curve and a maximum limit strain ε are measured max As shown in fig. 5, the residual creep life of the semi-graphitic cathode material was calculated by substituting the steady state creep equation, as shown in table 1.
Example-3:
the experimental test rig system was the same as in example 1. The graphite-titanium boride composite cathode material is adopted, the sample size and the test condition are the same as those of example 1, and the steady-state creep curve is obtained and simulatedThe steady state creep equation is summed as shown in fig. 6. Then, in the same manner as in example 1, a stress strain curve and a maximum limit strain ε are measured max As shown in fig. 7, the residual creep life of the graphite-titanium boride composite cathode material was calculated by substituting the steady-state creep linear equation, as shown in table 1.
The measurement, calculation and evaluation results of the residual service life of the electrolytic creep of the three different cathode materials of graphitization, semi-graphitization and graphite-titanium boride show that the residual service life of the cathode is mainly related to the ultimate deformation capacity and steady creep rate of the cathode. The graphite-titanium boride cathode has better ultimate strain capacity and smaller steady state creep rate, and longer residual creep life than graphitized cathodes and semi-graphitized cathodes. This is consistent with the results of prior studies and shows the reliability of evaluating the damage failure life of cathode materials by this method.
TABLE 1
Figure GDA0002381145660000041
Related reference patent literature:
[1]200710039899.8 method and system for predicting creep life of high-temperature part of steam turbine
[2]200710308160.2 method for predicting creep life of heat-resistant material of power station boiler
[3] Method for testing creep life of 03134314.7 high-temperature component
[4] Creep life prediction method of 200910198409.8 high-temperature material
[5]201310237622.1 method for judging creep failure life of polymer material and prediction method.

Claims (6)

1. The method for evaluating the damage failure life of the aluminum electrolysis cathode material is characterized by comprising the following steps of:
(1) Adopting an improved Rapoport-Samoilenko device to determine the electrolytic expansion-creep curve of the cathode material in the whole process under the conditions of stable electrolysis and constant pressure;
(2) Fitting a linear equation epsilon=A+Bt of the strain epsilon in the steady-state creep stage along with the change of time t by a mathematical method, and obtaining parameters A and B by fitting;
(3) Under the same electrolysis and pressure conditions, determining the limit strain epsilon of the cathode material through an online compressive strength test max The method comprises the steps of carrying out a first treatment on the surface of the At the initial stage of entering a steady creep stage, changing from a constant pressure mode to a constant rate pressurizing mode until the material strength fails, and determining the limit strain epsilon according to a stress strain curve max ；
(4) Will limit strain epsilon max As the end of the creep residual life, the creep residual life of the cathode material can be estimated and evaluated by taking a linear equation epsilon=A+Bt of the change of the strain epsilon with the time t in the steady-state creep stage in the step (2)
Figure FDA0004159824100000011
2. The method for evaluating the damage failure life of a cathode material according to claim 1, wherein: and fitting a complete steady-state creep linear equation epsilon=A+Bt by adopting creep data in a range of 20-150min in a steady-state creep stage, and simultaneously, increasing the reliability of a prediction result by adopting an average value of a plurality of groups of experimental data by adopting parameters A and B.
3. The method for evaluating the damage failure life of a cathode material according to claim 1, wherein: compressive strength test results using steady state creep stage samples correspond to limit strain ε max As an end of creep life for the cathode material.
4. A method of evaluating the damage life to a cathode material according to claim 3, wherein: the testing method of the creep life end is as follows: carrying out on-line compressive strength test on a sample in a steady creep stage by adopting a material testing machine, wherein the pressurizing rate is set to be 5-50N/s during the test, and the sample fracture condition is set to be 50-300N; under the specified pressurizing rate, until the sample reaches the set fracture condition; the slower the pressurization rate, the closer the limit strain amount is to the true creep rupture strain.
5. The method for evaluating the damage failure life of a cathode material according to claim 1, characterized in that: the cathode material is a semi-graphite cathode, a graphitized cathode and a graphite composite cathode.
6. The method for evaluating the damage failure life of a cathode material according to claim 1, wherein the electrolysis condition is characterized in that: the temperature is 700-980 ℃, and the cathode current density is 0.01-2A/cm 2 The constant pressure of the liquid electrolyte system is 0.5-30MPa.
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