Tensile performance is one of the main performance indicators of metal materials, among which yield and tensile strength are the most representative mechanical performance indicators of metal materials, and are also important bases for stress calculation in engineering design and mechanical design.

For metal materials used in high-temperature environments such as aircraft engines, pressure vessels, nuclear power equipment and thermal pipelines, high-temperature tensile performance data is the most basic and indispensable assessment data.

In 1984, my country issued the first edition of the national standard for high-temperature tensile test of metal materials. So far, it has undergone three revisions in 1995, 2006 and 2015. The latest version of the standard has been renamed GB/T 228.2-2015 “Metallic Materials Tensile Test Part 2: High-temperature Test Method”, replacing GB/T 4334-2006 “Metallic Materials High-temperature Tensile Test Method”.

The standard specimen used in the tensile test of metal materials is machined from the sample blank, which usually includes turning, milling, planing, grinding and other processes. The feed speed and cooling speed should be controlled in each machining process to prevent the material properties from being affected by heat or cold working hardening.

For the processed standard specimens, the specimen size and roughness must first be guaranteed to meet the standard requirements, which is the basis for ensuring the accuracy of the test results. Then determine and mark the specimen gauge length according to the specimen size.

1 Specimen clamping

Before clamping the specimen, check whether the equipment and fixture are in normal condition. The most important thing when clamping the specimen is the centering of the specimen, which directly affects the test results.

Once there is a large deviation between the loading force axis and the center of the specimen, the specimen will be subjected to a certain degree of additional bending stress. At the same time, it may also cause the specimen to slip or break abnormally during the test.

2 Thermocouple, extensometer, high temperature furnace installation

For high temperature tensile tests, the temperature sensor is a key component of the temperature monitoring system, and its control accuracy will directly affect the test results.

High temperature tensile tests usually use mechanical ceramic rod high temperature extensometers. The extensometer should be installed in the middle of the specimen, the blade must be perpendicular to the specimen surface, the two rods of the extensometer should be parallel to the specimen and on the same line, and finally adjust the gauge length of the extensometer to ensure the accuracy of the gauge length of the extensometer.

Most high-temperature furnaces used for high-temperature tensile tests of metal materials are vertical split structures. In order to ensure the uniform temperature in the vertical direction of the furnace, three-zone control is often adopted, that is, the upper, middle and lower heating wires of the furnace body are controlled separately, so that a longer soaking zone can be obtained in the furnace.

3 Temperature control

The so-called soaking zone refers to the temperature fluctuation in a certain area in the furnace that does not exceed the specified range after the furnace temperature reaches the test set temperature and stabilizes, that is, the soaking zone is a relatively stable temperature area. However, due to the “hot flue gas upward” effect, the soaking zone is not in the geometric center area of ​​the furnace body, but the upper end of the furnace body is hotter than the lower end, that is, the soaking zone is in the area slightly above the center of the furnace body. Therefore, it is necessary to measure the soaking zone of the high-temperature furnace used before the test to determine the specific area of ​​the soaking zone and make position marks. Once the soaking zone is determined, the upper and lower positions of the sample can be adjusted to ensure that the working section of the sample is in the soaking zone of the high-temperature furnace.

The heating process of the high-temperature furnace usually has thermal inertia, and its temperature control has a certain lag. The reason is that the temperature change rate of heating elements such as electric heating wires is far behind the temperature measurement speed of thermocouples, causing the actual temperature of the high-temperature furnace to always lag behind the set temperature, usually with a deviation of several degrees to more than ten degrees.

4 Tensile process

The temperature is the most difficult link to control during the sample loading process. From the beginning of the stress to the yielding of the sample, the sample itself is basically not heated because it is in the elastic deformation stage, and the temperature is easy to control at this stage.

However, from the yield deformation stage, the sample undergoes plastic deformation, causing the sample itself to generate heat. This phenomenon may cause the sample temperature to rise by dozens of degrees Celsius (such as austenitic stainless steel). The increase in the temperature of the sample itself makes the test temperature difficult to control or even exceeds the specified test temperature.

Therefore, in the initial loading stage of tension, the test temperature needs to be controlled within the negative deviation of the specified temperature range to prevent the temperature from exceeding the standard requirements in the later stage; at the same time, during the loading process, attention should be paid to temperature changes at all times, and necessary manual adjustments should be made to the heating controller in a timely manner.

Cherry
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#High-temperature tensile test