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Flaw detection condition flaw detector: CTS-33 type (on-site), VSIP-12 type (laboratory) probe: B2S-N, B4S-N (á24) Frequency: 2MHz, 4MHz Method: P-wave flaw detection sensitivity: á2 flat bottom hole quantitative method : Large flat bottom method scanning direction and scope: 360° radial scanning couplant: engine oil, experimental material experimental material is 20Cr2Ni4 bar material, the chemical composition of the mass fraction () is: C0.2, Si0.22, Mn0. 5, Cr1.5, Ni3.4. The blanks of furnace number 61311 and spindle number 1-2, 1-5 were subjected to non-destructive testing.
The results of the ultrasonic analysis of the defect analysis are as follows: defect distribution: no defects at both ends of the round bar blank. The defects are distributed at 1/5 of the axis. The probes were separately probed with 2 MHz and 4 MHz, and there was no significant change. Reflection of the defect signal: The defect signal of the spindle number 1-5 is strongly reflected, the local bottom wave is lowered, and the defect signal is higher than the bottom wave. The ingot section number 1-2 defect reflection signal is not particularly strong, has an angular relationship, and is difficult to detect. The coarse adjustment knob plate is in the bottom position, and the moving probe is observed separately. The defect signal changes rapidly, or disappears, or appears, there is no connection between the signals, and the correspondence of the defects is good. Defects have a certain depth range and are intensive defects. This defect has a waveform characteristic of a "white point" defect.
Take a sample from the section number 1-5 as a fracture, and make two fractures. From the test piece, the heart has different radial white points. The elliptical long axis is 6 to 10 mm. Sampling from section number 1-2 is done at low magnification and the result is white. Causes of white spots and improvement of hydrogen diffusion process 4.1 Causes of white spots White spots are internal defects that are not allowed in forgings, and their shapes are micro-cracks of different lengths and parallel flow lines. The main cause of white spots is that the interior of the steel contains excess hydrogen, and the other reason is the presence of stress. The reason why hydrogen enters the steel is that it is brought in by the furnace material or is not dried enough by the ladle or the steel ingot mold, and then the material is not subjected to vacuum smelting. Since the inside of the steel ingot is loose, even if the steel ingot contains more hydrogen (0.0005 to 0.0007), white spots do not occur. However, if forging is not carried out in time after forging, white spots will appear in the forgings.
Before the process improvement is improved, we use the process as shown; the improvement is followed by the process as shown. The improved process mainly has the following features: reduced low temperature holding temperature. Due to the high Ni content in 20Cr2Ni4 steel, Ni can enlarge the austenite region, and there are many residual austenite inside the forging. In addition, the solubility of hydrogen in austenite is high, so austenite must be decomposed before hydrogen diffusion. Therefore, the low temperature holding temperature is adjusted to 250 to 270 °C. Appropriately increase the hydrogen retention time. That is, 1h/10mm becomes 1.5h/10mm. 20Cr2Ni4 steel is a steel with high white point sensitivity, which is necessary to increase the hydrogen diffusion time. Reduce the cooling rate. That is, the air cooling from 650 to 400 ° C becomes ≤ 30 ° C / h cooling, reducing the forging stress. By improving the process, the test pass rate was increased from 91 to 98.