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Application Notes
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Application

Testing the integrity of metallic braze joints in metal tubing and other parts.

Background

Brazing is a common process for joining sections of small-diameter metal tubing and certain other small metal parts, such as electrical contacts. It bonds two pieces of metal by means of a metallic filler that is melted and then flowed into the space between them by capillary attraction. The process is similar to soldering. The filler metal, which is commonly an alloy of copper, silver, tin, and/or zinc with a relatively low melting point, must fill the space between the two parts being joined, wet the surfaces, and fuse them when it cools and solidifies. If the braze metal does not flow properly or does not fuse, the integrity of the joint is affected. This problem can usually be identified with an ultrasonic flaw detector.

transducer testing a braze joint

Equipment

Most braze joints can be tested with an Olympus EPOCH® 6LT, EPOCH 650, or EPOCH 1000 ultrasonic flaw detector. Given the typical geometry of braze joints, small-diameter contact or delay line transducers are usually used. The transducer should be selected with respect to the geometry of the specific joint being tested, but for typical small-diameter tubing, the V208-RM (20 MHz) and V260 Sonopen® (15 MHz) transducers are most commonly recommended. Spring-loaded holders are available with these transducers as an alignment aid.

Procedure

Braze joints are typically tested using a straight beam test that monitors the amplitude of the echo from the braze location at the inner surface of the outer metal part. In the case of a good braze joint at the test point, the metal-to-metal bond will return a relatively small reflection, while an air gap at the test point will return a much larger reflection. These levels should be established in each specific case by setting up on reference samples representing known brazed and un-brazed conditions.

The example below represents typical echoes from a braze joint between 9.5 mm (0.4 in.) and 12.5 mm (0.5 in.) copper tubing that was tested with a V208-RM delay line  transducer and a flaw detector. The first waveform shows a reference echo from an un-brazed area of the outer tube, which has been marked with a gate and set to nearly full-screen height. The second waveform shows the response from a properly brazed joint, in which the echo in the gated region drops to a very low level. It is followed by a larger peak representing the echo from the inside surface of the inner tube. By observing the amplitude of the echo within the gate, the operator can determine the joint condition at each test point by comparing the echo amplitude to these reference levels.

A partial braze within the area of the sound beam, due to conditions such as porosity, will typically be represented by a mid-level echo. Note: it is important to remember that very small discontinuities, such as pinholes that are a small fraction of the beam diameter, will not be detected through this procedure.

Typical echo response from a brazed joint, with a minimal echo in the gate.

unbrazed joint screen

Typical echo response from an unbrazed joint, with a large echo in the gate.

brazed joint screen

Olympus IMS

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EPOCH 6LT 휴대용 초음파 결함 탐상기는 한 손 작업에 최적화되어 있으며 로프 접근 및 고소 작업이 필요한 고휴대성 응용 분야에서 탁월한 성능을 제공합니다. 가볍고 인체 공학적 디자인으로 사용자의 손에 단단히 고정시키나 로프 접근이 필요한 응용 분야에서는 다리에 묶을 수 있습니다.
EPOCH 650은 다양한 응용 분야에 대한 우수한 검사 성능과 유용성을 갖춘 재래식 초음파 결함 검출기입니다. 이 직관적이고 견고한 장비는 인기 있는 EPOCH 600 결함 탐상기 후속작이며 추가 기능을 갖추고 있습니다.
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