ISO 14420:2020 pdf download Carbonaceous products for the production of aluminium — Baked anodes and shaped carbon products — Determination of the coefficient of linear thermal expansion
4 Principle The average coefficient of linear thermal expansion is determined by means of a push-rod dilatometer. The test specimen is contained in a sample holder made from low-expansivity material (such as flint glass). It is heated in a furnace and the length change is transmitted to a mechanical, optical or electronic measuring system outside the furnace by a push-rod.
The average coefficient of linear thermal expansion is calculated from the measured length change, the original length and the temperature change of the test specimen, taking the expansion of the sample holder and the push-rod into account. Unless otherwise stated, the determination is performed between a lower limit for the temperature interval of 20 °C (i.e. room temperature) and an upper limit for the temperature interval of 300 °C.
5 Apparatus
5.1 Dilatometer, with sample holder and push-rod, for example, made from flint glass, as well as a mechanical, optical or electronic length-measurement device (error limits ± 0,5 µm), for temperatures above 300 °C in a vacuum or in a protective gas atmosphere.
5.2 Furnace, capable of holding the temperature constant to within ± 0,5 % over the length of the test specimen.
5.3 Temperature-measuring device, for example, a thermocouple with indicating instrument, accurate to within ± 0,5 %, to determine the average test-specimen temperature.
5.4 Instrument for measuring lengths, with error limit of ± 0,2 %, for example a vernier calliper according to ISO 13385-1.
5.5 Calibration samples, made from materials with known thermal expansion in the range of the material to be measured and made with similar geometry. The thermal expansion of calibration samples shall have been predetermined by the producer of the measuring equipment or by a recognized calibration authority. 6 Specimens Prepare a test specimen of cylindrical or prismatic geometry. The cylinder diameter or the prism transverse edge length shall be at least twice the diameter of the largest structural constituent (for example maximum grain size) of the material to be examined, and in no case smaller than 4 mm (typically 30 mm to 50 mm).
The length of the test specimens shall be at least 25 mm, but preferably should be 50 mm to 120 mm. The test specimens shall be machined on all surfaces by turning or grinding, so that the surfaces in contact with the push-rod do not deviate from plane parallelism by more than 0,2 mm. Remove existing stresses in a test specimen by annealing at 1 000 °C in a non-oxidizing atmosphere.
7 Procedure
7.1 Calibration Calibrate the dilatometer according to 7.2 using calibrated samples.
7.2 Measurement Measure the sample length l 1 of the test specimen at temperature ϑ 1 . Insert the test specimen into the dilatometer, taking care that the specimen ends are firmly in contact with the push-rod. Measure the original length l 1 of the test specimen at the lower limit of the temperature interval ϑ 1 .
If the push-rod planes contacting the specimen end surfaces are not spherically or conically shaped, use connecting pieces to make a point of contact with the specimen end planes.
At the beginning of the measurement, set the measuring system to zero by either adjusting the zero point of the apparatus, or marking on the recording chart or the photosensitive paper. When using double dilatometers, with the two dilatometer motions recorded orthogonally, the assignment of the recording axes to the dilatometers shall also be determined and recorded. Position the furnace (which may be preheated) around the sample holder. Allow the test specimen to attain the upper limit of the temperature interval ϑ 2 . Then measure and record the length of the test specimen l 2 .
If the upper limit of the temperature interval ϑ 2 is above 300 °C, avoid oxidation of the test specimen by applying a suitable protection gas or vacuum.ISO 14420:2020 pdf download