Ultrasonic testing and ultrasonic thickness measurement
Services
- Ultrasonic thickness measurement
- Ultrasonic testing of metal structures
- Ultrasonic pipe inspection
- The cost of ultrasonic testing
Advantages of ultrasonic inspection method in comparison with X-ray
- equipment is light, mobile, as a result, there are no problems with accessibility
- the NDT specialist is ready to announce the results of the control on the spot
- ability to control large thicknesses
- ultrasonic control is cheaper than RK
- Ultrasound detects cracks well
Preparation of the object during the inspection of welds:
The customer provides access for laboratory specialists directly to the places of control. If work is carried out at a height, then he organizes scaffolding, a lift. The near-weld zone on both sides of the seam must be cleaned from splashes of metal, paint, rust.
Surface roughness of products and welded joints should not exceed:
Ra 6.3 (Rz 40) – with ultrasonic testing.
What is a defect?
The insidious role of defects has been noticed since time immemorial. Human handicrafts are not perfect. Defects devalue things, sometimes making it impossible to use them for their intended purpose.
A defect in a welded joint is a discontinuity or broken continuity of the weld material.
A defect in the continuity of the weld metal drastically reduces its strength or load-bearing capacity.
In the echo method, the defect reflects an ultrasonic probing pulse, transforming into an echo signal of the opposite direction. In this case, the echo signal is enriched with information content about the parameters of the defect.
“Defect not found” – does not mean at all that it is not in the metal. Its detection depends on a number of reasons, of which the primary ones are: the angular orientation relative to the transducer, the size, the state of the boundary, and the small difference in acoustic impedances.
In this case, we have a stealth defect – a defect that completely or partially hides its presence in the weld metal and its heat affected zone.
Examples of stealth defects during ultrasonic testing of welded joints:
- a defect with a loose surface scale of its boundary due to its high ability to absorb and scatter an ultrasonic pulse;
- highly compressed lack of fusion – is not an obstacle to the ultrasonic pulse, therefore, does not give an echo signal;
- a planar crack-like defect, the orientation vector of which does not coincide with the transducer acoustic axis vector by more than half the angle of its directivity diagram.
In this case, the echo signal from the defect does not return to the transducer and, as a result, is not displayed on the screen of the flaw detector.
Examples of real defects in welded joints: crack, lack of fusion at the root of the weld, non-fusion along the edge of the groove of elements for welding, gas pore, slag inclusion, undercut, burn-through, and others.
What is a dangerous defect?
Roughly, this is a discontinuity of the metal of such a critical size, the excess of which leads to the destruction of the seam.
More precisely: the product of the maximum defect size and its gradient.
It is generally accepted that the danger of a defect increases in the series: “volumetric – volumetric-planar – planar”. A quantitative characteristic of the degree of danger of a defect is the geometric coefficient of stress concentration – the ratio of two mutually perpendicular dimensions of the defect. It can be taken as a defect gradient.
The most dangerous defects are cracks. Their gradient is maximal among all other defects.
All regulatory documents of all industries recognize cracks in welded joints as unacceptable – neither in terms of their opening, nor in their size, nor in their orientation with respect to the main acting stresses of the OK weld under load. The presence of a main crack is considered to be a fact of the fracture of the seam.
Smaller cracks during operation of the test object under stress only grow with time. And the faster, the more intensively the seam works under a higher level of operating voltages.
How and what is the guarantee of reliable and confident detection of dangerous defects in structures using welded seams?
To do this, at a minimum, you need to have:
1) An operable and adequately tuned system for testing: “flaw detector – transducer – tuning sample with a measured defect according to an industry standard document”.
2) Setting up a flaw detector with a transducer to organize relative measurements of defect parameters in comparison with similar parameters of an artificial measuring defect-reflector of a tuning sample prescribed by the relevant regulatory document.
3) Relevant experience, skill and qualifications for the successful conduct of ultrasonic testing of the SS of this OK, with an understanding of the meaning of indications, their clear and unambiguous division into real and false (phantom). Their misinterpretation can lead to disastrous consequences, with all that it implies. This imposes certain requirements on the personnel allowed to conduct ultrasonic testing of hazardous production facilities.
In order to sound the seam control zone with ultrasound, it is necessary to create a reliable and stable acoustic contact. The ultrasound input surface must comply with the standard values of its roughness and waviness.
Creating a reliable acoustic contact for introducing ultrasound into the weld metal involves the use of special fluids. These are gels/oils of appropriate consistency and wettability at the ambient temperature of the input surface.
An essential condition for successful ultrasonic testing of a welded joint of the OK is its testability: each point of the OK control zone must be able to and be sounded by ultrasound at angles from significantly different directions.
If this condition cannot be met, then the OK welded joint is considered to be limited testable or not testable according to ultrasonic testing. According to various documents, the term “testability” is changed to “controllability”, but the meaning of the task does not change with such a replacement.