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A brief analysis of the application of the ultrasonic flaw detection technique o

Update time:2015-02-03 17:26:06Hit:3452
Forgings and castings is an important blank pieces of various machinery and equipment and boiler and pressure vessel.
A cast of ultrasonic flaw detection.
Due to the casting coarse grain, acoustical permeability difference, low SNR, so testing the difficulty is great, it is the use of having a high frequency acoustic energy acoustic beam in the casting internal communication, touch the internal surface or defects generated when the reflection and find defects. Reflected energy is the size of the inner surface or defect directivity and the nature and function of acoustic impedance of the reflector, and therefore can be applied to a variety of defects or internal surface reflected sound energy to detect the presence of position, wall thickness or surface defects under the depth of defects. Ultrasonic testing nondestructive testing as a means of more extensive application, its main advantage lies in: high sensitivity, can detect tiny crack; has high penetrating ability, can detect heavy section castings. Its main limitation is that: for contour size and point to the complex reflection waveform bad disconnected defects interpretation difficulties; the internal structure of the undesirable, such as grain size, organization structure, porous, inclusion content of dispersed precipitates or small, also hinder the waveform interpretation; in addition, the detection need to refer to standard test block.
Two the forging of ultrasonic flaw detection.
(a) forging processing and common defects
Forgings are made of hot steel ingot forging deformation and. Forging process includes heating, deformation and cooling. Forging defects can be divided into the defects in casting, forging and heat treatment defect defect. Casting defects are: shrinking hole remnant, osteoporosis, inclusions, cracks etc.. Forging defects mainly include: folding, white spots, cracks etc.. Heat treatment is the main crack defect.
Shrinking hole remnant is shrinkage porosity in the ingot during forging head cutting quantity shortage remained, found in the end of forgings.
Osteoporosis is a steel ingot solidification shrinkage formation in the dense and cavities, due to lack of forging forging ratio and not fully dissolve, mainly exists in the ingot center and head.
Inclusion has internal impurities, foreign non metallic inclusions and metallic inclusion. Internal inclusions are mainly concentrated in the ingot center and head.
A casting, forging crack crack crack crack and heat treatment etc.. Austenitic steel axial intergranular crack is caused by the crack of casting. Forging and improper heat treatment, forging cracks will occur in the surface or center.
White spots are forging hydrogen content is high, the cooling after forging steel too fast, too late to escape the dissolved hydrogen, cause stress cracking caused by the. White spots are mainly concentrated in the forging of large section center. White always appear in groups in the steel.
(two) testing method overview
According to the classification of the flaw detection time, forging can be divided into the inspection of raw materials and the manufacturing process inspection, product inspection and in service inspection.
Inspection of raw materials inspection and the manufacturing process of the objective is to find defects early, in order to take timely measures to avoid defects caused by enlarged scrapped development. Product testing to ensure product quality. In service inspection purpose is the defect of supervision after the operation may produce or development, is mainly fatigue crack.
Detection of 1 axial forgings
The forging process of shaft forgings is the main pull long, so most of the parallel axis orientation and defect, detect such defects in longitudinal transducer radially from the best detection results. Considering the defects will have distribution and orientation of the other, so the axial forgings, inspection, also should be supplemented by direct probe axial and oblique probe detection of circumferential and axial detection detection.
Detection of 2 cakes, bowl of forgings
The forging process of cake class and bowl of forgings mainly upsetting primarily, the distribution of defects is mainly parallel to the end, so use straight probe in face detection is the best method for defect detection.
Detection of 3 cylinder forgings
The forging process of tube category forging is upsetting, after punching, rolling. Therefore, the defect orientation is more complex than the orientation of defects of shaft forgings and forging in the. But due to the removal of the ingot in the worst quality center part has been punching, so the quality is generally good cylinder forgings. The main orientation of the defects is still with the outer circular surface parallel, so the detection tube category forging still straight probe outer circular face detection, but for the thick wall cylinder forgings, must be added for the detection of angle probe.
(three) the detection condition selection
1 probe selection
Forgings ultrasonic flaw detection, the main use of longitudinal transducer, chip size is Phi 14 mm to 28mm, the commonly used 20mm diameter. For small forgings, consider the near field and coupling loss reasons, usually with small chip probe. Sometimes in order to detect the defect and the detection surface into a certain angle, oblique probe can also use the K value detection. For close defects, due to blind straight probe and the near field effect, often using dual crystal straight probe.
Forging grains generally smaller, inspection frequency so as to select high, commonly used 2.5 ~ 5.0MHz. For a few materials of coarse grain severe attenuation of forgings, in order to avoid the emergence of "forest like echo", to improve the signal-to-noise ratio, should choose a lower frequency, usually 1 ~ 2.5MHz.
(Author:dong)
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