Visual Inspection, Part 1

This is a new series of articles put together by the BINDT Practitioner Committee to describe the principles of different NDT methods and techniques as an introduction to practitioners...

Introduction^[1,2]
The simplest and most basic method of non-destructive testing of a component or part is to look at it either using the naked eye or with some aid to vision, such as a magnifying glass. A simple visual inspection can reveal gross surface defects, leading to an immediate rejection of the component and consequently saving much time and money that would otherwise be spent on further manufacturing processes followed by additional, more complicated non-destructive and mechanical testing.

However, it is often necessary to examine a component surface for the presence of finer/smaller defects and, for this purpose, NDT methods have been developed to enhance the contrast between the defect and the background surface, therefore making the defect more easily visible. These methods include both magnetic particle inspection and penetrant inspection. In many cases, especially for the inspection of small components and where it is undesirable to contaminate the surface, the use of optical techniques may be more suitable.

Eddy current inspection is another NDT method that has also been developed to detect surface and slightly subsurface defects and, in some cases, eddy currents can detect surface defects that may not be detected visually.

Purpose^[2]
Often overlooked when compared to other NDT methods, visual inspection is one of the most common and useful means of non-destructive testing.

Visual testing (VT) requires adequate illumination of the test surface and satisfactory eyesight of the inspector. Eyesight tests may include the near-vision requirements of Jaeger 1^[6,7], colour differentiation and, in some cases, a shades of grey assessment^[8] to meet specific technique requirements. To be most effective, visual testing requires training (a knowledge of products and processes, anticipated service conditions, acceptance criteria, reporting requirements, etc).

It is also a fact that all surface defects found by other NDT methods ultimately must be substantiated by visual testing. Visual testing can be classified as direct visual testing or remote visual testing^[6].

Direct visual testing is usually used for local visual testing when access is sufficient to place the eye within 600 mm of the surface to be tested at an angle not less than 30°. When direct visual testing cannot be utilised, it may have to be substituted using remote visual testing.

Remote visual testing uses visual aids such as endoscopes and fibre optics coupled to cameras or other suitable instruments.

For visual inspection, often the equipment needed is simple, including a portable light, a mirror on a stem, a 2× or 4× hand lens and one illuminated magnifier with a magnification of 5× or 10×.

For internal inspection, the equipment is more complex; light lens systems such as borescopes allow remote surfaces to be examined. More sophisticated devices of this nature using fibre optics permit the introduction of the device into very small access holes and channels. Most of these systems provide for the attachment of a camera (remote visual testing) to permit permanent recording.

The applications of visual testing include:

Inspection of the surface finish of the component;
Inspection of the alignment of mating surfaces;
Inspection of the shape and dimensions of the component;
Inspection for evidence of leaking;
Inspection for surface defects;
Warping;
Corrosion;
Wear; and
Physical damage.

Some of the advantages of visual testing are as follows:

Testing is simple;
Testing speed is high;
It is versatile;
Testing is possible while the test object is being used;
It is applicable to the testing of complex or irregular shapes;
Mobility;
Permanent records are available when cameras are used; and
Mirrors may allow direct visual testing of more inaccessible outer surfaces.

Some of the limitations of visual testing are as follows:

It can detect only surface discontinuities;
Adequate lighting conditions are required;
Eye fatigue;
Inadequate training and/or product knowledge;
Remote visual equipment may be required to access interior surfaces;
Remote visual equipment providing adequate illumination, resolution, sensitivity and viewing angle can be expensive.

Visual testing is widely used on many different components to detect surface-breaking discontinuities associated with a variety of failure mechanisms.

As with any NDT method, visual testing requires personnel to be adequately trained and certified to the specific contract requirements.

The difference between the visual inspection of welds and the role of a Weld Inspector
Within industry, there has been some confusion between the differing roles of a Visual Inspector of welds compared to a Weld Inspector.

A Visual Inspector of welds is trained and certified to inspect a finished weld to ensure it meets the relevant acceptance criteria.

The role of a Weld Inspector is much more involved and includes responsibilities before, during and after welding. Both roles are described in more detail below.

The role and duties of a Visual Inspector of welds^[5]
The visual inspection of the finished weld is carried out to determine whether it meets the requirements of the application or product standard or other agreed acceptance criteria.

The requirements described below for the visual inspection of a finished weld are taken directly from BS EN ISO 17637:2016, ‘Non-destructive testing of welds –Visual testing of fusion-welded joints’.

Finished welds shall at least be examined in accordance with the following requirements:
Cleaning and dressing
The weld shall be examined to check the following:

All slag has been removed by manual or mechanical means to avoid imperfections being obscured.
There are no tool impressions or blow marks.
When weld dressing is required, overheating of the joint due to grinding is avoided and grinding marks and an uneven finish are also avoided.
For fillet welds and butt welds to be dressed flush, the joint merges smoothly with the parent metal without under flushing. If imperfections (caused by dressing or otherwise) are observed, they shall be reported so that remedial action can be taken.

Profile and dimensions
The weld shall be examined to check the following:

The profile of the weld face and the height of any excess weld metal meet the requirements of the acceptance criteria.
The surface of the weld is regular: the pattern and the pitch of weave marks present an even and satisfactory visual appearance; the distance between the last layer and the parent metal or the position of runs has been measured where required by the Welding Procedure Specification (WPS).
The weld width is consistent over the whole of the joint and it meets the requirements given in the weld drawing or acceptance criteria; in the case of butt welds, the weld preparation shall be checked to ensure it has been completely filled and meets the requirements of drawing or acceptance criteria.

Weld root and surfaces
The visually accessible parts of the weld, ie the weld root for a single-sided butt weld, and the weld surfaces shall be examined for deviations from the acceptance criteria.
The weld shall be examined to check the following:

In the case of single-sided butt welds, the penetration, root concavity and any burn-through or shrinkage grooves are within the limits specified in the acceptance criteria over the whole of the joint.
Any undercut is within the limits indicated in the acceptance criteria.
Any imperfections such as cracks or porosity, detected using optical aids when necessary, in the weld surface or heat-affected zones comply with the appropriate acceptance criteria.
Any attachments temporarily welded to the object to facilitate production or assembly and which are prejudicial to the function of the object or the ability to examine it are removed so that the object is not damaged; the area where the attachment was fixed shall be checked to ensure freedom of cracks.
Any arc strikes are within the limits of the acceptance criteria.

The role and duties of a Weld Inspector^[3]
Quality control/inspection may be carried out at any time throughout the welding process at the inspector’s discretion. The Welding Inspector should always ensure that all welding and associated actions are completed in accordance with the WPS, which is relevant to the contract.

Ideally, Welding Inspectors should work to specific quality plans when available. This is a detailed production and inspection route submitted by the manufacturer and approved by the customer.

Duties prior to welding
Before assembly, the Welding Inspector should check the following against the contract requirements:

Specification or standard;
Weld procedure has been approved;
Welder qualification;
Drawings;
Material composition/certification/cast numbers;
Condition of the material;
Preparations required;
Consumables;
Welding process.

After assembly, but still prior to welding:

Observe tolerances, clearance of plates and general assembly.
Check for alignment, frequency of any tack welds and any bridging pieces or stiffeners used.
Check joint cleanliness.
Check the pre-heat temperature (if any required).

Duties during welding

Check all welding parameters, such as polarity, voltage and amperage.
Check the welding technique and run sequence.
Ensure inter-run cleaning is carried out.
Check maximum/minimum inter-pass temperatures.
Check for compliance to the WPS.
Check back-grinding/gouging for shape and cleanliness.

Duties after welding

Check for dimensional accuracy.
Carry out visual inspection of all welds, paying particular attention for weld defects such as:

– undercut;
– overlap;
– porosity (surface);
– arc strikes.

The Weld Inspector should check all contours of welding and width (acceptable regarding appearance):

Observe any post-weld heat treatment.
Report on welds and confirm repairs if required.
Confirm any NDT requirements.

Repairs
The Welding Inspector should identify and positively mark the weld area or part of the final assembly to be repaired. The Weld Inspector should check that a method is used and understood, that it is acceptable and should relate to a repair weld procedure:

Check when weld partially removed (visual and NDT).
Check when weld fully removed (visual and NDT).
Monitor during weld repair and re-inspect when complete.
Arrange for final inspection.

References

The American Society for Nondestructive Testing (ASNT)Nondestructive Testing Handbook: Visual Testing, 3rd edition, Volume 9, 2010.
J Blitz, W G King and D G Rodgers, Electrical, Magnetic and Visual Methods of Testing Materials, Butterworth, 1969.
Lavender International, Training Notes: Weld Inspection.
PD CEN ISO/TS 25107, ‘Non-destructive testing – NDT training syllabuses’, 2019.
BS EN ISO 17637, ‘Non-destructive testing of welds – Visual testing of fusion-welded joints’, 2016.
BS EN 13018, ‘Non-destructive testing – Visual testing – General principles’, 2016.
BS EN ISO 9712, ‘Non-destructive testing – Qualification and Certification’, 2012.
ASNT, SNT-TC-1A, ‘Personnel qualification and certification in non-destructive testing’, 2020.

Visual Inspection, Part 2 will appear in the September 2021 issue and will describe the certification requirements and give details of the visual inspection syllabus.

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