Ultrasonic inspection normally involves the use of a piezo electric transducer which is both a transmitter and receiver of ultrasound. The transducer must be coupled to the item to be inspected, as ultrasound at the frequency used is not transmitted in air. Coupling of the transducer to the item under inspection may be achieved in two main ways: In the case of the contact inspection method a coupling medium (oil or wall paper paste, or similar) is used. The other common process used is immersion testing where the component or material under inspection is immersed in a water bath and ultrasound is transmitted through the water and into the component. (Corrosion inhibitors are normally added to the water bath where immersion testing is involved).
A proportion of the ultrasound is reflected back at interfaces such as the coupling medium / material interface, but the majority of the ultrasound should ideally be transmitted into the material
/ steel under inspection. Ultrasound is reflected from the back surface (back wall) of the item under inspection and off any planar discontinuities that may be present in the steel. (Note: to be able to detect any the discontinuities present in the steel matrix the acoustic properties of the discontinuity must be different to those of the defect free steel).
The transducer (as described above) comprises of a piezo electric crystal. Initially, the piezo crystal converts electrical energy to ultrasound which is transmitted into the item under inspection. Ultrasound reflected back to the crystal is then converted to electrical energy and this signal is amplified, and in turn the signal is monitored on a cathode ray tube or digital display screen. The time base can be calibrated providing information on the location of defects in relation to their position between the front and back wall of the item under inspection. Also using relative reflectivity (at known gain setting and beam path lengths) it is possible to determine the size of discontinuities present. An experienced ultrasonic technician can accurately map the location, size and shape of discontinuities that may be present. Based on the location, size, shape / geometry of a discontinuity it may then be categorised as a defect
An analogy is sometimes used to help describe the ultrasonic test method. Imagine being in a darkened room and a torch light producing a diverging beam of light is allowed to shine on a sheet of A4 paper. Where the sheet of paper is presented at right angles to the diverging beam of light more light energy is reflected backwards. If on the other hand the sheet of paper is placed in the light beam and placed parallel to the diverging beam of light (i.e. edge on) then very little light is reflected backwards. Similar principles apply to ultrasonic testing in that a diverging beam of ultrasound is used and discontinuities that present themselves at right angles to the ultrasonic beam may be detected (assuming a large enough reflectivity applies), while those that present themselves with their plane parallel to the ultrasonic beam are very difficult or impossible to detect.
Therefore, scanning methods may involve the use of normal and / or shear wave probes. A normal “0°” probe essentially directs ultrasound into the component under inspection normal to the surface (i.e. at right angles to the surface). A shear wave probe on the other hand directs ultrasound into the component at an angle. Common angles might be in the range 30° through to around 60°. Ultrasound may be diffracted and bent as it moves between material having differing acoustic attenuations, (this is similar to light through a prism). Also ultrasound changes velocity depending on the material through which it is travelling. The formula V=fl, where V=velocity, f = frequency and l = wavelength may be used by an ultrasonic technician. Ultrasonic probes are normally identified with the following basic information: Frequency (MHz), Diameter and Angle. Please note that the information provided above is not meant to be exhaustive, rather it is an introduction to the some of the major principles of the ultrasonic test method.
All rolled and forged steel products contain microscopic discontinuities, such as non-metallic inclusions. Volumetric discontinuities such as voids or cavities may also be present, originating from shrinkage or gas porosity. In addition, surface discontinuities or defects such as cracks or seams may be present, remnant from hot processing of billets or created during heat treatment.
The issue is: when does a discontinuity become a defect? Discontinuity size and shape, frequency of occurrence, distribution and location all need to be considered. Rejection or acceptance of discontinuities also depends on the application of the component. The loss of effective cross-section due to the presence of any discontinuity, loss of strength due to fatigue, and implications of sudden impact are all relevant considerations.
Ultrasonic inspection is a non-destructive testing method used to locate volumetric discontinuities and to determine their size, geometry and frequency of occurrence. 85
Products benefiting from inspection: Atlas 4140, Atlas 4340, Atlas 6582, Atlas 6580, Atlas 6587, Atlas 6657 and Atlas 8620H.
5.2.1 WHAT DOES IT MEAN?
• All alloy steel bar products that are sold from Atlas stock are ultrasonic inspected.
• Ultrasonic inspection has been performed in accordance with a defined standard.
• The inspection standard is explicit and transparent: AS 1065 to Level 2.
• AS1065 is the Australian Standard for “Non-Destructive Testing of Carbon and Low Alloy Steel Forgings”.
• Level 2 refers to the precise inspection criteria.
5.2.2 WHY AS1065 LEVEL 2?
AS1065 defines the method of ultrasonic inspection. There are three inspection levels. The higher the level (i.e. Level 1 is higher than Level 2 which in turn is higher than Level 3), the lower the frequency and the smaller the size of the discontinuities allowed to be found by the ultrasonic inspection.
If a product meets AS1065 Level 2 it will be suitable for most general engineering applications. A small number of applications will call for an even more stringent specification regarding ultrasonic inspection; this can be carried out if requested.
5.2.3 WHAT ARE THE BENEFITS TO THE CUSTOMER?
• End user Quality of the Product is Guaranteed.
• Mill certificate based on test results is available, which can be submitted to the end user. The mill certificate will state the Ultrasonic Test Method and Acceptance Criteria used.
• Reduction in the likelihood of incurring costs and loss of time where defects are found to be present at the time of machining.
• Reduction in the likelihood of production time losses and the associated costs if a component fails in service.
Contact Person: Ms. Florence Tang
Fax: 86-731-89853933
