Ultrasonic testing (UT) of steel is a non destructive testing process that normally uses very short ultrasonic pulse (high frequency sound energy) waves that have frequencies ( of between 0.1 and 15 MHz, even though they occasionally get up to 50 MHz. Frequency range for common metals is used 1 to 4 MHz. These pulse waves are transmitted into materials that will actually detect the internal flaws found in various metals. They can also be used to characterize different materials.
If you’re curious about a common example of this process, here it is: the sound energy is introduced and propagates through the object/materials in the form of waves. When there is a discontinuity (such as flaw, crack, impedance difference) in the wave path, part of the energy will be reflected back from the flaw surface. From the reflected signal information about reflector(flaw) location, size, orientation other features can be gained. With this one of the NDT techniques internal soundness of the materials, coating thickness etc. is often measured so that the object/material can be tested/characterised. This is often done to monitor pipework corrosion, for example. UT is also used on steel, including alloy steel, and numerous other metals, although it can also be used on wood, composites, concrete, and many other materials.
More About the Process
The UT process involves several functional units such as pulser/receiver, transducer and display devices. transducer being connected to a diagnostic machine and then ultrasonic waves passed over the object that is being inspected. Usually, this transducer is separated from the object being tested by either a couplant (e.g., oil) or water, the latter being used mostly in immersion testing. If UT is conducted with an EMAT — electromagnetic acoustic transducer — oil, or any other couplant, is not required.
Common Misconceptions –
- It is used for testing slag, that is incorrect, it only helps to detect cracks and the shape of the crack.
- UT 100% ok, there is always an acceptance level which has to be agreed based on which the material is 100% ok. There are various standards like ASTM A388, EN 10228, EN 10308 which specify the acceptance criteria and levels.
- Due to different chemical compositions, the UT is different for different lots, this is incorrect as Ultrasonic testing is not dependent on the chemical composition, the testing technique might change.
Naturally, there are both pros and cons of ultrasonic NDT testing of steel. Some of those pros and cons are listed below.
Pros:
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It is highly sensitive and will detect even the smallest flaws and offers high penetrating power and can therefore detect flaws deep in the part.
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It has a lot of accuracy compared to other nondestructive methods when it comes to determining the depth of internal flaws
Cons:
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Some parts are difficult to inspect, including very thin or small parts or parts that are rough, irregular in shape, or not homogeneous.
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Surface defects cannot be detected using UT.
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For the inspection procedures to take place, the operator has to have extensive technical knowledge. (As said “Little knowledge can be dangerous”)
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You have to prepare the surfaces by cleaning them and removing any loose paint, scales, etc and use couplants in order to provide efficient transfer of ultrasonic wave energy between the parts that are being inspected and the transducers unless you use a non-contact technique.
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Cast iron and other coarse grain materials are difficult to inspect due to low sound transmission and high signal noise.
At KISCO we have a complete in-house team of 8-12 Level 1 and Level 2 certified UT personnel and also conduct regular in house courses and training programs by Level 3 to keep our team of engineers and operators up to date. 100% of the material is tested during processing and before dispatch to adhere to the feasibility reports between our customer and Kisco.
