GOST 3248-81

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  ГОСТ 3248-81

GOST 3248−81 Metals. Test method for creep (Change No. 1)

GOST 3248−81

Group B09

STATE STANDARD OF THE USSR
METALS
Test method for creep

Metals.
Creep test method

Date of introduction 1982−07−01

APPROVED AND put INTO EFFECT by Decision of the USSR State Committee on standards of 2 September 1981 No. 4137 the introduction of set with 01.07.82

PROVEN in 1986 by Decree of the state standard from 05.12.86 No. 3691 validity extended to 01.07.92*
_______________
* Expiration removed according to the Protocol of the Interstate Council for standardization, Metrology and certification (I & C No. 2, 1993). — Note the CODE.

REPRINT (March 1988) with amendment No. 1, approved in December 1986 (IUS 2−87).

REPLACE GOST 3248−60

This standard specifies the test method for creep of ferrous and nonferrous metals and alloys at temperatures up to 1200 °C

The essence of the method is that the sample is subjected to a constant tensile load and constant temperature while fixing the specimen in time.

The test determines the creep strength of the material, i.e. the voltage at which the speed or creep deformation for a certain period of time does not exceed a predetermined value.

Standard corresponds to ISO/R-203, ISO/R-204.

1. THE SHAPE AND DIMENSIONS OF SAMPLES

1.1. Set basic samples:

cylindrical — diameter 10 mm, with an estimated length of 100, 150 and 200 mm;

flat — of a width of 15 mm, with an estimated length of 100 mm.

Sample thickness is determined by the thickness of the rental.

Allowed if there is technical justification to apply a proportional samples of other sizes and shapes, with an initial estimated length or mm, where is the initial cross — sectional area of the working part of the specimen in mm. The diameter of the cylindrical specimen should not be less than 5 mm.

1.2. Permissible deviations on dimensions and surface roughness parameter RA of the processed surfaces of the samples shall conform to the requirements specified in table. 1.

Table 1

1.2.1. The allowable deviation from the specified design length of the sample, the length of the section of the working part of the specimen on which the elongation is measured, should not exceed ±1%.

1.2.2. The runout of the cylindrical sample at check in centers must not exceed 0,02 mm.

1.2.3. Allowable deviation of the largest cross-sectional area should not exceed ±0,5%.

1.3. Sample form and sizes of heads of the sample and the transitional part from the head to its working length is determined by the adopted method of mounting the measuring elongation on the sample and method of fixing the specimen in the grips of the testing machine. The mate of the head of the sample with its working parts must be smooth.

Samples can be applied in two types: surface-treated or preservation of the surface layer (for example, the samples of rolled steel plates or samples fabricated by precision casting method) in accordance with the terms and conditions of supply, use and technology of manufacture of semi-products and products.

(Changed edition, Rev. No. 1).

1.4. Samples with warpage, mechanical damage, surface defects such as foreign inclusions, delaminations, pores, cavities, cracks (caused by mechanical or thermal treatment), the test is not exposed. Straightening or straightening workpieces or test specimens is not allowed.

1.5. If the metal to be tested in heat-treated, then heat treatment of the workpiece exposed to the samples. If after heat treatment the metal has poor cutting, the workpiece must first be brought to the dimensions, including allowance for final processing and possible warping. Requirements for metal and size of workpiece are established by standards or specifications for steel.

In the manufacture of samples should not occur changes in the structure and properties of the tested metal (for example, due to heating or cold working).

1.6. Accuracy requirements for measuring samples prior to testing shall meet the requirements of GOST 9651−84, if not imposed more stringent requirements.

2. USED EQUIPMENT

Technical requirements to machines for testing metals for creep shall comply with the GOST 15533−80.

3. TESTING

3.1. The correct installation of the specimen in the grips of the testing machine (no eccentricity) is the measurement at normal temperature elastic elongation of the specimen at two diametrically opposite sides and must be no more than 10% of the average deformation of the specimen.

(Changed edition, Rev. No. 1).

3.2. The sample mounted in the grips of the testing machine and placed in an oven and heated to a predetermined temperature (the heating time should be no more than 8 hours) and kept at this temperature for at least 1 h. If necessary, the duration of exposure is regulated in the standards or technical conditions of steel products.

Notes:

1. In special cases, if the test material has a stable structure and designed for long service life, the heating time can be more than 8 hours, and a material having an unstable structure and is intended for small deployments, pre-exposure less than 1 h.

2. The heating device can be used with or other protective atmosphere, if required by the test conditions.

3.3. For temperature measurement on samples with an estimated length of up to 100 mm inclusive shall be installed at least two thermocouples, and samples with an estimated length of over 100 mm of at least three thermocouples distributed evenly across the estimated length. Thermocouples are installed so that the hot junctions are tightly in contact with the sample surface: the hot end should be protected from exposure to the hot walls of the furnace.

3.4. Thermocouples must be tested out according to GOST 8.338−78. Time to calibrate thermocouples from base metal shall be as specified in table. 2.

Table 2

If the test duration exceeds specified time to calibrate thermocouples, the latter must be tested out before and after the test. In case of change of the readings of the thermocouples used are different thermocouples, preserves stability until the end of the test.

The cold junction of thermocouple in the test process must have a constant temperature.

3.5. Deviations from the specified test temperature at any time during the duration of the test and at any point the calculated length of the sample must not exceed:

heating temperature

Periodically, not less than 2 h, to measure the test temperature. It is recommended that automatic recording of the temperature throughout the test.

3.6. Test temperature is chosen a multiple of 25, if a study does not require a special temperature.

3.7. After heating of the sample and exposure at a given temperature to the sample gradually applied pre-load equal to approximately 10% of the given total load, but preload should not cause the sample voltage is more than 10 MPa.

If the temperature of the sample and the measured value of elongation remain unchanged for 5 min, then carry out a smooth loading of the specimen to a predetermined load.

3.8. Simultaneously with the application of the loads should be check the elongation of the sample, starting with the pre-load and at each stage of loading, if the latter is carried out by steps.

Check the elongation of the specimen shall conduct or continuously, or one at a time, to fully establish the character of change of elongation.

Devices emerging from the furnace, must be designed and protected so that short-term temperature changes do not affect the readings.

Note. Allowed the use of devices with precision reference deformation to 0.02 mm, if the test requirements such accuracy is sufficient.

If the design of the testing machine creep does not allow for the loading steps, the plastic deformation under loading can be obtained by subtracting the elastic deformation from the total. Elastic deformation is calculated from the magnitude of the modulus of the material at the test temperature.

Breaks during the test of short duration are not allowed. Long-term tests after the interruption can be continued. Samples when a break test is not unloaded. Make sure that no additional stresses in the sample during its cooling.

3.9. The test duration and a tolerance for deformation under given stress and temperature (i.e. load, referred to the initial cross sectional area of the sample) are set for each material depending on its purpose.

Definition of creep limit is recommended during the tolerances for the elongation of 0.1 to 1% if the duration of the trial 50, 100, 300, 500, 1000, 3000, 5000, 10000 h if you do not want a different duration or a different tolerance for deformation.

In the case of determining the creep strength at creep speed the test duration shall be not less than 2000−3000 hours, provided the duration of the linear part of creep curve not less than 500 hours, this specifies the determination method.

3.10. The room temperature during the test must be constant. Fluctuations in room temperature during the measurement of elongation should not exceed ±3 °C.

3.11. After finishing the test, the creep sample is unloaded to the amount of pre-load and determine the absolute value of the residual elongation.

4. TEST RESULTS

4.1. The results of the tests (except acceptance) represent graphically in the form of primary creep curves in the coordinates «elongation (i.e. creep deformation) — time», which then determine the voltage (i.e., load, referred to the initial cross sectional area of the sample), the corresponding conditional limit creep of the material.

4.2. Conventional creep strength — the voltage that causes during the set test time at a given temperature, a given elongation of the sample (total or residual) or a given rate of creep at the straight portion of the curve of the creep.

The total or residual elongation in percent (see drawing) is calculated according to the formulas:

where is the initial gauge length, measured at room temperature before the test sample with an accuracy to 0.05 mm;

 — absolute elongation recorded at the next measurement of the deformation of the sample during the test.

Note. You can define the total or residual elongation without considering the plastic part of the elongation during loading. In the presentation of results is required. The initial gauge length shall be measured at room temperature before the test sample.

4.3. The definition of conditional creep strength is carried out as follows:

experiencing a series of samples at a given temperature and three or four levels of stress. The number of samples in a series should provide the necessary determination accuracy of the conditional limit of creep;

based on the obtained primary creep curves define the average value , and a creep speed at the straight portion of each stress level and build the chart of dependence between stress and elongation, or between voltage and an average uniform speed of creep on the straight portion in a logarithmic coordinate system (allow for the processing of test data in other coordinate systems). On these charts, find the voltage that is recorded with an accuracy of 5 MPa;

 — elongation at loading;  — full (elastic + residual elongation on curved section);  — the total (elastic + residual) elongation during the test; elastic elongation;  — the residual elongation

after the desired voltage is found no less than three different temperature tests can be built diagram of dependence between the conditional limit of creep and temperature.

Example of designation of the magnitude of the deformation of creep limit when the tolerance for deformation of 0.2% over 100 h of testing at a temperature of 700 °C:

.

It should further specify the total or the residual deformation, which was determined by the limit of creep.

Example of designation of the value of creep strength at creep speed 1·10%/h at a temperature of 600 °C:

.

It should further specify the time of the test, which was reached set speed creep.

(Changed edition, Rev. No. 1).

4.4. The results of tests on creep for each sample is recorded on the form shown in the recommended application.

APPLICATION (recommended). FORM OF RECORD OF RESULTS OF TESTING METALS FOR CREEP

APP
Recommended

1. Material

1. Name brand (to the appropriate standard).

2. Chemical composition.

3. Smelting (furnace type) and size of ingot, billet.

4. Form (bars, sheets, pipe, casting, etc.).

5. The types and modes of heat treatment.

6. Mechanical properties (tensile strength, yield strength, elongation, necking, fracture toughness, hardness).

7. The microstructure and grain size.

2. Samples

1. The place from which sample taken (axis distance of the sample from the axis of the ingot of the workpiece or product; the longitudinal, transverse).

2. The estimated size of the sample (length and diameter of the cross section in mm, before and after the test, measured in the room where the test is performed).

3. Equipment

1. Installation type for the test.

2. The accuracy of temperature measurement.

3. The accuracy of strain measurement (absolute).

4. The conditions and test results

The text of the document is verified by:
the official publication of the
M.: Publishing house of standards, 1988