GB/T 38683-2020Method of ultrasonic testing for large inclusions in bearing steel (English PDF)

4.2 Testing sample state

4.2.1 Sample requirements

The selection of testing samples can be divided into two categories: process testing (C) and final testing (L). The sample shall be cylinder and cuboid. The interior and surface of the sample shall not have defects that affect the testing, and the surface roughness shall meet the testing requirements. The sampling location and requirements shall comply with the product standards or agreements; if not specified, it shall comply with 4.2.2 and 4.2.3.

4.2.2 Process testing (C)

4.2.2.1 The sample can be taken from the head, middle or tail of continuous casting billets. If it is a steel ingot, it can be taken from the steel ingot head, steel ingot middle or steel ingot tail. The sample need to be deformed by rolling or forging, and the compression ratio shall be greater than 4.

4.2.2.2 In order to meet the testing sensitivity, the sample needs to be heat-treated before surface processing to make its grain size reach level 5 or finer as specified in GB/T 6394.

4.2.2.3 The surface of the sample shall be clean and smooth. The opposite faces of the cuboid sample shall be parallel and all faces shall be rectangular. The sample processing shall meet the requirements of Table 1.

4.2.2.4 During the preparation process, the sample is marked to ensure traceability; the mark can neither be marked on the testing surface nor the opposite surface. The compression ratio and size of the sample shall be included in the testing record.

The instrument shall be able to meet the requirements of GB/T 27664.1, and shall have a deep compensation function.

5.4 Ultrasound probe

The liquid immersion probe shall meet the requirements of GB/T 27664.2. The diameter of the straight probe element shall be (6.3 ± 0.3) mm; the nominal frequency of the probe is 10 MHz, and its 6dB bandwidth shall be (6 ± 1) MHz.

NOTE: If the probe parameters change, it shall be agreed upon between the supply and demand parties in the agreement. However, the purpose of this Standard is to ensure the consistency of the testing results. When the equipment parameters change, the consistency of the testing results cannot be guaranteed.

5.5 Coupling agent

Water is generally used as the coupling agent, and necessary additives can be added to prevent rusting of the sample. The water temperature shall be between 15 °C ~ 30 °C. The coupling agent used for calibration and testing shall be the same, and it shall be ensured that no physical or chemical changes that affect testing sensitivity occur in the sink. When necessary, the sink shall be equipped with a circulating filter device to ensure that the suspended particles that affect the testing can be removed. The temperature difference between the calibration of the comparison sample and the testing of the sample to be tested shall not exceed 5 °C.

6.1 Comparison sample

6.1.1 Sample requirements

6.1.1.1 The equipment shall be verified with comparison samples. The comparison sample used to adjust and set the testing sensitivity shall have the same or similar acoustic performance and the same surface condition as the sample to be tested (see 4.2.2 and 4.2.3).

6.1.1.2 For cylindrical samples, a comparison sample with the same or similar diameter shall be used. The diameter of the sample to be tested shall be within ±20 % of the diameter of the comparison sample.

6.1.1.3 For cylindrical samples with a diameter greater than 100 mm, a cuboid comparison sample can be used instead. For cuboid samples, the side length of the sample shall at least correspond to the thickness (sound path) of the respective measured area (see 7.1).

6.2 Water layer thickness

During adjustment and testing, the thickness of the water layer between the probe and the sample to be tested shall be 40 mm ~ 45 mm.

6.3 Sensitivity adjustment

6.3.1 The reference sensitivity is adjusted by a comparison sample with flat-bottomed holes of which the diameter is 1.0 mm. The reflected wave height of the flat-bottomed hole with the highest echo is adjusted to 80 % of the full screen height (FSH) of the display, at which the sensitivity is the reference sensitivity. For the flat-bottomed hole of other depths, the echo amplitude is adjusted through depth compensation, that is, add the gain VJ on the basis of the reference sensitivity, so that the wave is as high as 80 % of the full screen height (FSH) of the display.

6.3.2 According to the required sensitivity level (see 6.4), different gain factors Vk need to be added to the reference sensitivity; in this way, the depth compensation is included, and the actual amplification gain VR is equal to the sum of the depth compensation gain VJ and the gain factor VK, as shown in formula (1).

6.4 Sensitivity level

The sensitivity level is determined by the product standards or agreements.

Specific values are given in Table 2. If it is not specified, the sensitivity is implemented as level 1. It is not allowed to convert the size and depth of the flat-bottomed hole according to the law of the AVG curve. It shall be measured based on the sample and the 1.0 mm flat-bottomed hole. The signal-to-noise ratio in the automatic ultrasonic testing shall be not less than 6 dB, preferably 10 dB. If necessary, perform special heat treatment before testing (see 4.2.2 and 4.2.3).

6.5 Testing volume

The minimum testing volume is shown in Table 3. The specific testing volume shall be implemented according to the requirements of product standards or agreements. If there is no specific requirement, it usually implements category

7.2 Scanning speed, pulse repetition frequency and pulse spacing

7.2.1 According to the pulse repetition frequency and scanning speed, the distance between the two pulses (pulse spacing) can be obtained. Considering the statistical interference suppression (pseudo echo suppression), the pulse spacing cannot be greater than the grid distance.

Example: If the scanning speed is 150 mm/s and the pulse frequency is 1500 Hz, the pulse spacing is (150 mm/s)/1500 s-1 = 100 µm.

7.2.2 Too high pulse repetition frequency will cause phantom echo, so it shall be avoided.

7.3 Testing method

7.3.1 An ultrasonic pulse can produce reflected echoes at different distances, and it shall select the largest echo within the evaluation window.

7.3.2 If a rectangular sample is tested from several faces, the defects will be repeatedly evaluated (test twice at 90° perpendicular to each other). The same defect will produce 2 echoes on different testing surfaces, and only one defect is displayed, which can only be counted once.

7.3.3 During the testing of cylindrical workpieces, repeated scanning of defects may also occur due to the setting of the gate covering. After being identified as the same defect, only one evaluation will be made.