Surface Inspection Methods

In this article, we will touch on the three surface inspection methods commonly available for examining welds, visual inspection, magnetic particle and liquid penetrant inspection.

Visual Welding Inspection 

Visual welding inspection is of great importance in the fabrication industry because it constitutes the principal basis of acceptance for many types of weldments. 

For an example of this, we can refer to clauses 11.5.4.1 and 12.5.4.1 of CSA W59, the Canadian Standard for Welded Steel Construction, which states that all welds shall be visually inspected

It has become one of the most extensively used inspection methods because it is relatively easy to apply and is fast and inexpensive compared to other Non-Destructive Evaluation (NDE) methods. In Canada the requirements for a Visual Welding Inspector are laid out in CSA W 178.2 “Certification of Welding Inspectors”. This standard gives the requirements for three Levels of Certification.  Generally, the standard defines the minimum needs for vision and the maintenance of certification via additional vision tests as time progresses and further lays out other general requirements viz:

Welding inspectors at any level shall be capable of

• reviewing welding inspection procedures in conjunction with the related standards, codes, and drawings to a degree consistent with the level of certification;

• monitoring and examining the work performed by tackers, welding operators, or welders;

• recording inspection results in such a way that the work personally performed may be identified; and

• preparing clear and adequate reports.

The standard then defines additional requirements for Level 1, 2 and 3 inspectors which includes much additional knowledge relative to materials, welding procedures, welding symbols, general knowledge of Standards, steel and welding metallurgy, welding consumables and other NDE methods etc.

The certification agency for visual inspectors, acting as a third party,  is the Canadian Welding Bureau (CWB)

Magnetic Particle Inspection. (MPI)

Magnetic particle inspection is a step up from visual inspection. The method utilizes a magnetic field, with north and south poles and that are induced in steel or other magnetic materials.

If a magnetic field is interrupted by a discontinuity in the materials surface, the field becomes distorted. This is termed a flux leakage field and new north/south pole will be formed at each point of material separation as shown in Figure 1 below.

If fine magnetic particles are applied onto the test object, the flux leakage field formed at the surface of the weld attracts and hold the particles, which will form a visible bridge across the gap and showing this indication.

Figure 1 Illustrating Flux Leakage at a Surface Breaking Discontinuity

The most common way to induce the field in the testing of welds is by using the AC/DC magnetized yoke method using alternating current or direct current. Alternating current (AC) is the most common method used for surface examination of welds. To perform a proper examination, at least two different orientations of the magnetic field are required because discontinuities show up the best when oriented 90° to the field as shown in Figure 2

Figure 2. Longitudinal Discontinuities are found with the Yoke placed as per the Figure. To find Transverse Discontinuities the Yoke must be reapplied at 90 degrees to that in the Figure

MPI is an excellent tool to disclose weld defects and it can be used in all positions including overhead. In most cases, inspection standards call for certified MPI technicians. In Canada NDE technicians are Certified by a third party, the CGSB, the Canadian General Standards Board, in three levels, the requirements of which are laid out by the governing Standard

Figure 3 below shows MPI indications close to the toe areas of a repair weld

Figure 3. MPI Indications at the toe of a Repair Weld

3.0 Liquid Penetrant Inspection (LPI)

The Liquid Penetrant Inspection method is based, on a very simple process. Initially, the surface of the parts to be inspected should be cleaned prior to application of the penetrant materials to ensure removal of all dirt, grease, oil and other surface contaminants

A coloured penetrant is then applied to the surface of an object, and is permitted to “dwell” (remain on the surface) for a specific period of time during which it us drawn into any discontinuities open to the surface by capillary action. This capillary action allows the process to be used in all positions. 

When the dwell time is completed, the excess penetrant remaining on the surface is carefully removed from the surface without removing it from any potential discontinuities it has been drawn into. 

An absorbent, light-colored powder (developer) is then applied to the surface. This developer draws out the penetrant that has seeped into cavities. As the penetrant is drawn out, it spreads into the thin layer of developer, forming visible indications.

Figure 4. Steps in the LPI Process

The six basic steps involved in performing LPI are illustrated in Figure 4 above and a bleed out from a weld toe defect is shown in Figure 5.

Figure 5  A bleed out illustrating a linear indication at the weld toe. Red bleed out against the white contrasting developer

The two basic types of penetrants available are: 

• Visible dye: the penetrant is coloured to provide a contrast and viewing in this method is done under white light as per Figure 5.

• Fluorescent dye: viewing in this method is done under ultra-violet or “black light” (near ultra- violet light). 

The penetrant testing methods are reasonably easy to apply and obtain information. The results, however, are not available as quickly as with magnetic particle testing. Although penetrant testing is performed on both austenitic (non-magnetic) and carbon steels, it is more efficient to use MPI on the carbon steels. 

When choosing the type of penetrant materials with non-magnetic austenitic stainless steels, some specifications limit the halogens (chlorides etc.) that can be present in the penetrant materials. Halogens can cause stress corrosion cracking under certain conditions. Therefore, in the inspection of certain components, (e.g., for aerospace and nuclear), the materials used in penetrant testing must be certified as to halogen content. 

As with MPI, most fabrication standards call for certified LPI technicians. In Canada this is again through the CGSB and requirements are laid out in the Standard CAN/CGSB-48.9712/ISO 9712.

The text here has covered the surface inspection methods and techniques, future articles will cover the radiography and ultrasonic methods. 

In the meantime, please check out the attached short (very) YouTube videos describing both MPI and LPI.

https://www.youtube.com/watch?v=N1emHJD0-1E&t=28s&ab_channel=CWBGroup

https://www.youtube.com/watch?v=QpU5JyhNVgQ&t=5s&ab_channel=CWBGroup