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Non-Destructive Testing (NDT) in Welding Inspection (Part I): General

By: Eng. Jesus Eduardo Contreras

 

When talking about this topic, we must consider at least 3 important aspects:

i) Welding, which is already an extensive topic, but here we can define very briefly that it is a process that leads to the union of two or more materials together, in such a way that they remain as a single piece. For the purposes of this article, we are talking about metallic materials.

There are many welding processes (At SOLDOZA we are experts in everything related to welding, see references), in other articles we will describe the welding processes in detail.

ii) The defects present in the weld,

as in the previous point, we will detail these defects in other related articles (see references). And the…

iii) Non-Destructive Testing including:

…techniques both surface detection and volumetric inspection. Each technique has its own advantages and limitations, but in most cases, they can be complementary and combined to provide operators with the most comprehensive data. Thanks to the continued advances in NDT technology enables improved detection of defects and sizing.

Let us start with the most traditional methods to detect volumetric discontinuities:

Technique Description Advantages Limitations
Radiography (RT) Is the general term given to material inspection methods that are based on the differential absorption of penetrating radiation–either electromagnetic radiation of short wavelength or particulate radiation–by the part or test-piece (object) being inspected.    
 

Conventional:

X-Ray



Gamma-Ray    


It detects volumetrics discontinuities such as porosity, inclusions, and even cracks if the crack opening runs parallel to the radiation beam.

Can be used on almost any material  

Well established standards and codes of practice

The radiogram or film provides a ‘visual’ indication of flaws      

X-Rays and gamma rays are hazardous radiations  

Access to both sides of the test object is necessary to produce a radiograph

Discontinuities such as cracks, laminations, lack of fusion, etc., must be aligned with or parallel to the radiation beam to be detected clearly  

Choice of radiation energy for a particular thickness of weld is a critical factor  

Location of defect in test object’s cross section is difficult to determine

 

 

Digital & Computerized 

 

 

                       

On site immediate results Instant image transmission, no developing, no scanning. Results appear on laptop/tablet screen in seconds of shooting  

Minimal surrounding safety distance  

No more film consumables (Films paper, Chemicals, etc.) More inspections a day are made possible

No more dark rooms  

Very portable systems    

Less photosensitive than traditional films    
Ultrasound (UT)                

ULTRASONIC INSPECTION is a nondestructive method in which beams of high-frequency sound waves are introduced into materials for the detection of surface and subsurface flaws in the material.

The sound waves travel through the material with some attendant loss of energy (attenuation) and are reflected at interfaces.

The reflected beam is displayed and then analyzed to define the presence and location of flaws or discontinuities.

   
       

 

 

 

Conventional

 

 

 

 

 

 

 

 

 

 

 

Capable of detecting planar defects not detectable by radiography.  

High sensitivity, permitting detection of minute defects.  

Great penetrating power, allowing examination of extremely thick sections, e.g., up to 10 inches of steel.

Accuracy in the measurement of flaw position and estimation of flaw size.

Fast response, permitting rapid and automated inspection.

May be performed with access to only one surface of the object.

Test conditions which may limit the application of ultrasonic methods usually relate to one of the following factors:  

Unfavorable geometry of test object; for example, size, contour, complexity and defect orientation.  

Undesirable material structure; for example, grain size, structure porosity, or inclusion content.
Examples of materials difficult to test by ultrasonics are austenitic steel and welds involving nickel base alloys.
Penetration of sound into these materials is limited and interpretation of results may be difficult. (It should be noted that austenitic materials are now widely used for fabrication of chemical tankers and -installations as well as nuclear reactors.)  

Coupling and scanning problems, surface conditions etc.  

When using normal probes, defects located less than 4-5 mm below the test object’s surface is difficult to detect. (This is due to the equipment dead zone, the width of the pulse, and the probes near zone where interference will affect the measurements).  

Due to high sensitivity false or irrelevant indications may occur.  

TOFD

 

 

Called Time-of-Flight Diffraction (TOFD) can be used for a variety of applications, its primary use is rapid weld testing of circumferential and axial weld seams

 

 

 

 

 

 

Based on diffraction, so relatively indifferent to weld bevel angles and flaw orientation Uses time of arrival of signals received from crack tips for accurate defect positioning and sizing  

Precise sizing capability makes it an ideal flaw monitoring method  

Quick to set up and perform an inspection, as a single beam offers a large area of coverage

Rapid scanning with imaging and full data recording  

Can also be used for corrosion inspections Required equipment is more economical than phased array, due to conventional nature (single pulser and receiver) and use of conventional probes

Extremely sensitive to all weld flaw types

Requirement to operator’s qualifications
Phased Array (UTPA)

Phased array is an advanced pulse-echo technique utilizing multiple miniaturized transducers and time-delays to shape the ultrasonic sound beam in a desired angle and focus.

Therefore, the technique has a wide range of applications Phased array ultrasonic testing (PAUT) probes are composed of several piezoelectric crystals that can transmit/receive independently at different times.

To focus the ultrasonic beam, time delays are applied to the elements to create constructive interference of the wavefronts, allowing the energy to be focused on any depth in the test specimen undergoing inspection.    

                 

PA can reduce inspection times by eliminating or reducing the need for mechanical scanning and by taking advantage of the ability to perform electronic scanning  

A single phased-array probe allows the user to change the shape and focal point of the ultrasonic beam to optimize each inspection.  

Digital storage of all data, location, and system settings  

Visualization of indications in weld and/or base material, using B-, C-, D- and S-scans (with all A-scans included)  

Compared to measurements with conventional single-element probes, the detection and sizing of defects is much easier and more robust  

Possibility to facilitate inspections of complex geometries

Requirement to operator’s qualifications  

Materials as SS alloys sometimes need special transducers to increase the PoD.

Expensive accessories        

Total Focus Method (TFM)  

 

The Total Focusing Method (TFM) is an ultrasonic array technique which is used to synthetically focus on every point of a region of interest  

 

 

               

Optimal focusing and spatial resolution everywhere

Direct imaging of a large area for one probe position  

All reachable angles with the array simultaneously  

Defect characterization  
Comprehensive imaging of defect  

3D imaging

In some application files will be very heavy  

The data processing speed could be slow in some cases

Next time, we will be talking about NDT techniques for evaluating surface and sub-surface defects.

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