Non Destructive Test (NDT)

Non Destructive Test (NDT)

NDT Technology | SEED Group

TECHNICAL PAPER ON

NDT TECHNIQUES IN WELDING INDUSTRY

Website : 1.Introduction:Non-destructive Testing, the testing carried out without impairing the final usefulness of the product is becoming more alive in todays world than ever before. New and newer techniques are being developed. The latest machines, carefully developed procedures and qualified manpower are always striving for the reliability of product through NDT. NDT is not only encompassing the industrial field like material testing, service inspection, corrosion monitoring, laser technology, process control, instrument development, nuclear power but it also covers equally the health physics and medicine. X-rays and Ultrasound are very common methods used in medical field.

When we talk about welding industry, the NDT plays a vital role in the evaluation of weldments. Many techniques are developed to find out the soundness of the weld at different stages of manufacturing cycle. NDT is also used in raw materials as well as service inspection.

This paper takes you for a tour of different Non-destructive testing techniques applicable for welding industry alongwith their principle, application and selection criteria. It also covers the very important factors for a meaningful NDT which are qualified manpower ,calibrated machines and correct procedures. At the end, it poses the areas of concern in todays context.

2. NDT techniques used in Welding Industry :

Considering the definition of NDT, needless to say that Visual Testing becomes first technique of NDT. Some of the commonly used NDT techniques are :

Visual Testing (VT)

Liquid Penetrant Testing (PT)

Magnetic Particle Testing(MT)

Ultrasonic Testing (UT)

Radiographic Testing (RT)

Eddy Current Testing (ET)

Leak Testing (LT)

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The basic principle, equipment required, applications and advantages / limitations are as given in Annexure A.

It is important to mention here that NDT cannot eliminate the need for Destructive Testing. The different NDT techniques also do not substitute each other. However, they can compliment each other.

Further, the techniques like Acoustic Emission, Neutron Radiography, Thermal Emission and Vibration Analysis are also used for the evaluation of material and weldments.

3.Importance of Training :

The success of NDT always depends on the knowledge and skill level of the operator. NDT results are operator dependant and cannot be cross checked by the simple alternatives. The moment a probe is removed from the surface there are no indications and one has to go by the operators report. This operator dependability has forced the technical societies to formulate the norms for training, qualification and certification of NDT personnel. The worldwide used standard is ASNTs

SNT- C T-1A: The international standard ISO 9712 is also used. Indian Standard applicable is IS13805. These standards give the requirements for Training and Certification of NDT personnel. It divides the personnel into 3 categories for each technique namely, level I (operator level) level II (supervisory level) and level III (managerial level). One has to ensure that the candidate meets the requirements of Education,Training, Eye test and minimum grade in Examination (Theory + Practical).

4.Latest Developments :

The developments in NDT are very fast. Flexible boroscopes, remote visual inspection, video cameras for visual inspection are the latest things in Visual Testing In Magnetic Particle Testing and in Penetrant Testing fluorescent inspection methods are catching over visible methods. In Japan fluorescent penetrant / magnetic particle testing has become very common. In ultrasonic testing the recording systems are developed and one can transmit the data through internet and view it from a distant place without loss of time. In addition to the automatic on-line Ultrasonic tube testing machines, new ultrasonic inspection machines for remote inspection for tube wall thinning are very common now. In radiography the filmless radiography and development in detecting media is taking over conventional radiography. In eddy current testing new techniques are coming up in service inspection. In Helium leak testing, leak detection of the order of 10-9 std. cc/sec. is possible.

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5.Selection of a particular NDT technique :

(Annexure B is tabulating NDT technique selection w.r.t. welding discontinuity and Annexure C gives the generally applicable NDT w.r.t weld joint types.

Advantages / Limitations of the NDT Method :

The advantages and limitations of the NDT method can be used to determine which method will provide the best results for a particular test. For example radiography can detect cracks whose major planes are aligned parallel with the radiation beam; such cracks are usually normal to the plate surfaces. Radiography, however, usually cannot detect laminations in plate or cracks oriented parallel to the plate surface. On the other hand, Ultrasonic can more readily detect cracks oriented in either direction provided the proper scanning technique is used.

Acceptance standards :

The statement the weld shall be of radiographic quality has no meaning unless acceptance standards are stated. Acceptance standards define different types of characteristics of discontinuities and whether particular types of discontinuities are permissible. If a particular type of discontinuity is permissible, then the acceptance standards must specify the maximum size at which that discontinuity is acceptable. Acceptance standards are an integral part of most codes and specifications and are commonly used as reference in purchase specifications.

Cost :

Different inspection methods have different costs in any particular situation. Two basic cost factors, which should be considered in the selection of a nondestructive method, are the initial equipment availability cost and the cost of performing the inspection. Visual inspection is almost always the least expensive, but it is also limited to the detection of surface discontinuities. In general, costs of Radiographic, Ultrasonic, Eddy current and Helium Leak testing inspections are greater than those of Visual, Magnetic and Liquid Penetrant inspections. X-ray films are generally imported which constitutes the major consumable cost in Radiography.

Selection of the proper NDT method can be quite complex. It involves manufacturing technology, possible discontinuities, orientation of discontinuities, accessibility for testing, physical condition of the job and so many other factors. To meet the intended purposes and minimize cost, it is suggested that help be obtained from a qualified nondestructive testing engineer or technician.

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6.Case Studies :

While going through the tour of the different NDT techniques it is worthwhile to spend some time to have a look on some of the case studies which will emphasize on the basic advantage / limitations of NDT techniques and it will convey the subject in right sense. The case studies are the events happened in last few years and they are to be considered as representative types

6.1 New porosities were revealed in the space science motor shell radiography after increasing the density of radiograph from 2.5 to 3.5. This was due to increased contrast at higher densities. This shows that, for a specific purpose the outer limits of parameters given by specification are not sufficient. One has to design new limits to suit the purpose.

6.2 In Aluminium weld radiography a black band was observed in the weld zone. This was due to weld material composition different than parent metal composition. Investigation proved that there was no discontinuity. One can easily get misled with such situations unless proper study of materials and consumables is taken up.

6.3 New defects were revealed due to change of shooting direction in high thickness Pressure Vessel Radiography . This emphasizes operators to follow procedures religiously and also demands a good record keeping.

6.4 Comparison of UT & RT results to 3, 4 and 5 thickness pressure vessels. This data was collected to compare RT and UT results for the welds. The data indicates that RT & UT results are not interchangeable and they cannot replace each other.

6.5 The results of Ultrasonic Testing of welds with 2 MHz frequency probe and 4 MHz frequency probe are not matching. This demands the procedure be to be followed religiously.

6.6 Cracks after back chipping seen by PT and not seen by RT in groove spot. This shows PT can be superior to RT when it comes for surface defects.

6.7 Narrow gap welding techniques demand the ultrasonic testing procedure to be totally changed with tandem technique. This shows study of configuration is a must before selecting a technique and one has to change the testing procedure if weld configuration is changed.

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7.Areas of concern :

Availability of trained / qualified manpower :

NDT is an operator dependant technique and demands highly skilled and trained personnel. This is a concern of the day . The improperly trained manpower cannot give justice to the NDT techniques and can lead to misleading results.

Adequate Machinery :

This is an important factor contributing to the success of the NDT. If the machine is not capable of producing results the test will not be meaningful. Calibration of the machines is an important issue.

Adequacy of Procedures :

Even though the trained manpower and adequate machinery is available, one cannot get fruitful NDT results unless the correct procedures are adopted. The correctness of procedures will be verified by cross checking, demonstration of techniques and ensuring that the procedure is reviewed and approved by a person having authority in the subject.

While going through the above concerns one has to realize that Cheapest is not the Best Bye.

The careful selection of NDT subcontractor (the subcontracting of NDT is very common) is very important and monitoring the subcontractors work is also recommended. The subcontractor cannot put properly qualified manpower and adequate machines and correct procedures unless it is paid for. So the important thing always remains in the following three aspects.

Is the NDT requirement clear?

Is subcontractor capable of fulfilling the requirement?

Is subcontractor actually fulfilling the requirement ?

Unless this monitoring is taken up seriously the reliable results cannot be guaranteed.

Non-destructive testing is a serious business. The health and lives of people and money is at stake. Even experienced technicians can also encounter problems. Hence technicians starting a career in NDT shall get the best possible training and never take routine test results granted. New technicians should always be teamed up with experienced personnel and the doubtful results shall be concluded with the guidance of expects.

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ANNEXURE A

NDT Method Selection Table

Equipment NeedsApplicationAdvantagesLimitations

Visual

Principle : Use of eye and instruments

Magnifiers, color enhancement, projectors, other measurement equipment, i.e. rulers, micrometers, optical comparators, light sourceWelds which have discontinuities on the surface.Economical, expedient, requires relatively little training and relatively little equipment for many applications.Limited to external or surface conditions only. Limited to the visual acuity of the inspector.

Liquid Penetrant

Principle : Capillary Action - detects surface discontinuities only.

Fluorescent or visible dye penetrant developers cleaners (solvents, emulsifiers, etc) Suitable cleaning agent, Ultraviolet light source if fluorescent dye is used.Weld discontinuities open to surface, i.e. cracks, porosity.Portable, relatively inexpensive equipment,. Expedient inspection results. Results are easily interpreted requires no electrical energy except for light sources.Surface films such as coatings, scale smeared metal mask or hide rejectable defects. Seepage from weld porosity at the surface can also mask indications. Parts must be cleaned before and after inspection.

Magnetic Particle

Principle : Magnetic flux leakage detects discontinuities

Prods, yokes, coils suitable for inducing magnetism into the weld. Power source (electrical). Magnetic powders. Some application require special facilities and ultraviolet lights. Most weld discontinuities open to the surface some large voids slightly subsurface. Most suitable for cracks.Relatively economical and expedient, Inspection equipment is considered portable. Unlike dye penetrants, magnetic Particle can detect some discontinuities slightly below the surface. Can be used for higher temperatures. Applicable only to ferromagnetic material. Parts must be cleaned before and after inspection. Thick coatings may mask rejectable discontinuities. Some applications require parts to be demagnetized after inspection. Magnetic particle inspection requires use of electrical energy for most applications.

Radiography (Gamma)

Principle: Differential absorption of radiation by the object gives density variation in the film

Gamma ray sources, gamma ray camera projectors, film holders, film, lead screens, film processing equipment, film viewers, exposure facilities, radiation monitoring equipmentWelds which have voluminous discontinuities such as porosity, incomplete joint penetration, corrosion, etc, Lamellar type discontinuities such as cracks and incomplete fusion can be detected with a lesser degree of reliability. May also be used in certain applications to evaluate dimensional requirements such as fit-up, root conditions, and wall thickness.Generally not restricted by type of material or grain structure. Surface and subsurface inspection capability. Radiographic images aid in characterizing discontinuities. Provides a permanent record for future review.Planar discontinuities must be favorably aligned with radiation beam to be reliably detected, radiation poses a potential hazard to personnel. Cost radiographic equipment, facilities, safety programs, and related licensing is relatively high. A relatively long time between exposure process and availability of results. Accessibility to both sides of the weld required.

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Radiography (X-rays)

Principle : Differential absorption of radiation by the object gives density variation in the

Film

X-ray sources (machines), electrical power source, same general equipment as used with gamma sources (above).Same application as above.Adjustable energy levels. Generally produces higher quality radiographs than gamma sources. Also same advantages as above.High initial cost of X-ray equipment. Not generally considered portable. Also, same limitations as above.

Ultrasonic

Principle : Reflection of Sound beam by a flaw gives signal on the oscilloscope.

Pulse-echo instrument capable of exciting a piezoelectric material and generating ultrasonic energy within a weld, and a suitable cathode ray tube scope capable of displaying the magnitudes of received sound energy. Calibration standards, liquid couplant.Most weld discontinuities including cracks, slag and incomplete fusion. Can also be used to verify weld thickness.Most sensitive to planar type discontinuities. Test results known immediately. Portable. Most ultrasonic flaw detectors do not require an electrical power outlet. High penetration capability.Surface condition must be suitable for coupling of transducer. Couplant (liquid) required. Small thin welds may be difficult to inspect. Reference standards are required. Requires a relatively skilled operator or inspector.

Eddy Current

Principle : Electromagnetic Induction

An instrument capable of inducing electromagnetic fields within a weld and sensing the resulting electrical currents (eddy) so induced with a suitable probe or detector. Calibration standards. Weld discontinuities open to the surface (i.e cracks, porosity, incomplete fusion) as well as some subsurface discontinuities. Alloy content, heat treatment variations. Equipment used with surface probes is generally light weight and portable. Painted or coated welds can be inspected. Can be partially or completely automated for high speed relatively inexpensive test.Relatively shallow depth of inspection. Many material and test variables can affect the test signal.

Leak Testing

Principle : Leakage of fluid detected by different detectors.

Bubble test

Pressure change measurement test

Halogen Diode leak detection

Mass Spectrometer Leak detection.Leakage of storage tanks, leak tightness of systems, very fine leak testing by, Helium leak detector for electronic components and nuclear components.Very sensitive, Used for checking vacuum tightness.Limited to thro and thro leak only. Cleaning and drying is mandatory. Expensive in case of MSLDT

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ANNEXURE B

Applicable NDT methods / Welding Discontinuities

Discontinuities Applicable NDT Methods

RTUTPTMTVTET

Porosity(*(a*b(a*

Slag Inclusion((NA*bNA*

Incomplete fusion*(NA*bNA*

Incomplete Penetration((NA*NA*

Undercut(***(*

OverlapNA*((**

Cracks*((a(b(a(

LaminationsNA((a,c(b,c(a,cNA

Note 1 :

a. Surface

b. Surface and slightly subsurface

c. Weld preparation or edge of base metal

(Applicable method

*Marginal applicability depending on other factors such as material thickness,

Discontinuity size, orientation and location

Note 2 :

Leak testing can reveal only through and through leaks.

ANNEXURE C

Applicable NDT methods for Weld joint types

Joints NDT Methods

RTUTPTMTVTET

Butt ((((((

Corner*((((*

T*((((*

Lap**(((*

(Applicable method

*Marginal applicability depending on other factors such as material thickness,

Discontinuity size, orientation and location

Note 2 :

Leak testing can reveal only through and through leaks.

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