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Heat Treatment of welded joints Part 4 - Precipitation or age hardening

There are several methods that may be used to increase the strength of a metal; alloying, quenching of steel, work hardening, and one very specific form of heat treatment, that of precipitation or age hardening (the two terms are synonymous). Many ferrous and non-ferrous alloys are capable of being age hardened and, as the name suggests, this method of increasing the strength relies upon the formation of precipitates. To achieve the optimum combination of mechanical properties the heat treatment cycles must be very closely controlled.
Unfortunately, to understand how the precipitates affect the mechanical properties it is necessary to introduce some fundamental metallurgy.
The precipitation hardening mechanism requires the solubility of the alloying element, the solute, in the metal, the solvent, to increase as the temperature increases as shown in the phase diagram in Figure 1 where the solvus line shows decreasing solubility of alloying element B in the solvent A as the temperature falls.
An analogy is that of salt in water; as the temperature increases more salt can be dissolved but the converse happens as the solution is allowed to cool when salt crystals begin to form or precipitate.
The same process occurs in suitably alloyed metals except that the processes of dissolving and precipitating take place in the solid and are hence much slower as atoms find it more difficult to move in a solid than a liquid solution.
A consequence of this is that once the precipitates have been dissolved by taking the metal alloy to a sufficiently high temperature, ie above the solvus line, they can be prevented from re-forming by rapid cooling or quenching.
This heat treatment is known as solution heat treatment and is carried out to form an unstable super-cooled solid solution which, if reheated to a lower ageing or precipitation hardening temperature, will begin to re-form the precipitates, these growing in size as the heat treatment proceeds. A schematic of such a heat treatment cycle is also given in Figure 1 for alloy N.

Figure 1: Phase diagram showing decreasing solubility of B in A and heat treatment cycle

In the solution heat treated metal the atoms of the alloying element, the solute, are randomly distributed throughout the matrix but once the temperature is raised the precipitates begin to form by a nucleation and growth process. At relatively low temperatures and in a short timescale the solute atoms begin to cluster together to form extremely small and very finely dispersed precipitates known as Guinier-Preston (GP) zones, named after the two metallurgists who first identified them. The GP zones are so small that they are not visible using normal optical microscopes but can be seen using electron microscopy at magnifications of around x100,000.
The GP zones are described as coherent, in other words they have the same crystal structure as the solvent metal. However, they distort the crystal lattice, the framework on which the atoms are positioned. This makes it more difficult for dislocations to move through the lattice and it is dislocation movement that enables metal to deform; tensile strength and hardness therefore increase but ductility and toughness decrease. As the ageing treatment continues or the temperature is raised the tensile strength also continues to increase as the precipitates grow and coarsen whilst still remaining coherent. At some point, however, the precipitates begin to lose their coherency; they become incoherent, forming separate particles within the metal with a different crystal structure from the solvent and at this stage they become visible using an optical microscope.
Just before this point is reached is when the alloy has the very highest tensile strength. As these incoherent particles form and grow in size the tensile strength progressively decreases. The alloy then is said to be overaged although the precipitates still contribute towards the tensile strength of the alloy. The high strength low alloy (HSLA) steels are a good example of this where incoherent, overaged precipitates are used to give a substantial increase in the tensile strength.
In order to achieve the best combination of properties the precipitates need to be evenly distributed throughout the grains of the alloy and of an optimum size. The ageing temperature and/or time can obviously be changed to tailor the distribution and size of the precipitates; longer times and/or higher temperatures generally result in a reduction in strength but an increase in ductility, an overaged structure giving the lowest tensile strength but the highest ductility.
Typical heat treatment times and temperatures of a range of different alloys are given in Table 1. The ferritic and nickel based alloys are generally used in the overaged condition in order to ensure a reasonable degree of ductility. It can be seen that with some alloys, eg 17/4PH stainless steel, the precipitation mechanism is sufficiently sluggish that the component can be cooled in still air or, as with the A286 stainless steel, long ageing times are required.
On the other hand the aluminium-copper alloy 2219 is capable of ageing at room temperature if left for a couple of days. Some of the 6000(Al-Si-Mg) and series 7000 (Al-Zn-Mg) alloys will similarly age at ambient temperature. This is known as natural ageing; – aging at an elevated temperature is known as artificial ageing.

Table 1: Typical ageing heat treatments and properties of a range of age hardening alloys

The close control of heat treatment times, temperatures and cooling rates is therefore essential if the required properties are to be obtained. For the solution treatment of aluminium alloys a salt bath is frequently used, artificial ageing taking place in a forced air circulation furnace. Illustrated in Figure 2 is the effect of varying the time and temperature on the ultimate tensile strength of an Al-4%Cu alloy such as alloy 2025 where it can be seen that a difference as small as 40OC in the ageing temperature can have a major effect on the strength. The higher temperature needed by the nickel and ferrous alloys generally requires the use of gas fired or electrical furnaces with sufficient thermocouples to ensure the correct temperatures are consistently achieved throughout the component.

Figure 2: Effect of varying ageing times and temperatures on tensile strength

This article was written by Gene Mathers.

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Radiography Part 3

The previous two articles dealt with what may be termed conventional radiographic techniques employing either gamma- or X-ray sources and photographic film. The development of electronics, in particular the increase in computing power over the last 20 or 30 years, has enabled what were laboratory based radiographic methods - real time radiography and computerised tomography (the “CT” scan that is perhaps more familiar in a medical context) -to be implemented both on the shop floor and on site.

The fundamental difference between film radiography and real time radiography is the way in which the radiographic image is handled - the image in real time radiography being produced electronically rather than on film. This means that the image can be viewed, as the name suggests, in real time – an instant result rather than the significant time delay that occurs when radiographic film is carried to a darkroom, developed and dried before viewing can take place. The real time image is viewed on a monitor screen and the results can be stored and transmitted electronically. An additional difference from film radiography is that the image is a positive rather than the negative image of film radiography, denser materials transmitting less radiation and therefore appearing darker and hence voids, slag entrapment, porosity etc show as light areas in a welded joint.

The early real time radiographic equipments used a fluorescent screen which interacted with the x-ray radiation to produce an image that was then passed through an image intensifier. This enabled a video film to be produced and the image to be displayed on a monitor screen in real time. Developments in computing power now enable the image to be digitised and enhanced and then analysed. Comparison with a set of pre-programmed parameters e.g. an acceptance standard, enable the process of inspection and acceptance/rejection to be automated.

Fluorescent screens to a great extent have been replaced by flat or curved screen photodiode arrays based on Si sensors with other alloys e.g. PbI and HgI, being researched in order to improve further the sensitivity of the examination method. Photo-conductors such as selenium or cadmium telluride are also used in what is termed “direct conversion” to give a sharper image than that from the photo-diode panels.

The X-ray source has been progressively reduced in size such that the focal spot is now as small as 2 or 3 microns and with micro-focus units to as small as 0.1mm, again resulting in improved image quality. These features also enable the image to be enlarged with magnifications of over 1000 times being available with little or no loss in image quality.

Real time radiography is employed in applications where a rapid in-production inspection is required. It has found extensive use in the electronics industry for the in-line examination of circuit boards, in the aerospace and automotive industries for the examination of castings and, often using gamma ray sources, in the process industry for the detection of corrosion. By moving the component between the X-ray source and the detector screen seam welded tube and tube butt welds can be inspected.

Robust, portable manipulating equipment is commercially available for the on-site examination of process pipework and cross country pipelines. Orbiting heads carrying both an X-ray source and, diametrically opposed, the detection screen, are capable of radiographing pipe butt welds with a substantial reduction in time compared with a conventional film radiograph, particularly where multiple exposures are required to give full coverage of the weld. The head rotates around the pipe to give a DWSI (double wall single image) with very high resolution and good contrast. Internal crawlers equipped with a rotating X-ray head and an external detector can be used to provide a SWSI (single wall single image) radiograph. The examination results are instantly available for display on a laptop screen, the images being stored on disc or memory stick. No film processing or darkroom are required resulting in a further reduction in cost. A typical commercially available unit is illustrated in Fig 1.

Fig 1. Portable Real Time Radiographic unit for pipeline inspection. Courtesy Shaw Pipeline Services.

Computed tomography (CT) is a process that uses the techniques of real time radiography to produce three dimensional, rather than two dimensional, images of components. Both the external surfaces and the internal structures of an item can be imaged. All materials – metals, plastics, composites, rocks, fossils, the human body – can be scanned, making CT scanning a very powerful investigatory and diagnostic tool. Industrial CT scanning is used in many applications for the internal inspection of components, metrology, and for reverse engineering.

The earliest CT units utilised an X-ray beam collimated to create a fan shaped beam of radiation which is scanned across the item, the beam being collected at the detector screen. The signal is then manipulated by a powerful software programme, enabling a 3D image to be created. The technique was further improved when the fan shaped beam was replaced by a cone shaped beam. Rotating the part beneath the beam enables a sequence of 2D images to be obtained, generally between 360 (one image per degree) to 3,600 images, depending on the desired resolution, with scanning of the object being completed in as little as a few seconds. The images are subsequently combined using complex software to provide a 3D image of the item, the image containing all of the information relating to both the external and internal surfaces of the component. A typical commercially available 225Kv microfocus unit is shown in Fig. 2.

Fig. 2 CT scanning equipment showing control console, viewing screen and radiation proofed cabinet containing the X-ray source, turntable and detector screen. Courtesy Nikon Metrology UK Ltd.

The 3D image can be manipulated to provide slices through the object and these slices can be rotated so that the precise size and position of internal features can be accurately determined. Assembly or machining errors can be identified. Similarly, flaws within a welded joint or casting can be categorised and the size and position of inclusions, lack of fusion, cracks etc accurately measured, thus allowing an accurate Engineering Critical Assessment (ECA) to be carried out. The method can also be used to examine non-metallics – a couple of examples being the inspection of composite wind turbine blades for delaminations and the positioning of the cores in lost wax casting. The technique can also be used for accurate measurement of the internal and external surfaces of a component, the image being capable of being overlaid on a CAD drawing of the object. This is particularly useful where parts are provided by a number of suppliers to a manufacturer for subsequent assembly. The items can be checked against a CAD drawing before despatch and, if required, the manufacturer instantly provided with the results, thus reducing the risk of being supplied with non-fitting parts. 

Unfortunately, unlike real time radiography that has moved from being a non-portable method to construction site use, CT scanning requires the use of a protective enclosure containing the X-ray source, the detector screen and a precision turntable. The method is currently therefore confined to within a factory or laboratory environment. 

The final radiographic method is neutron radiography. Neutrons, like X- and gamma-radiation, will pass through solid materials and can be used to produce an image on photographic film or to react with a detector screen. Neutrons, however, unlike X- and gamma radiation, are strongly affected and absorbed by certain elements such as hydrogen, carbon and boron but to a far lesser extent by iron, nickel and aluminium. Neutron radiography is therefore useful in detecting the presence of material containing hydrogen or hydrocarbons – this includes corrosion products, water, explosives, oil and plastics. Composite items e.g. a combination of metal and plastic, can be non-destructively examined for assembly faults or manufacturing defects; explosive ordnance can be examined for complete filling and fluid flow analysis can be carried out in real time – oil flow in vehicle engines, fluid flow in piping systems.

Neutron radiography can be used with conventional film with real time techniques and for CT scanning. The neutrons are produced by particle accelerators or, with poorer image definition, from certain isotopes, primarily californium²⁵². The method is currently not portable but development work is in progress to address this limitation.

This article was written by Gene Mathers.

RADIOGRAPHY TESTING PART 2

The previous article dealt mostly with the basic principles of radiography – this part will cover the methods of ensuring that a radiograph is of an acceptable quality and capable of showing relevant imperfections. As mentioned in the previous article the quality of a radiograph is assessed using three factors: density, contrast and definition or sharpness of the image. Density and contrast have already been covered but there also has to be some method by which the sensitivity (the ability to reveal imperfections) can be measured. To do this devices known as image quality indicators (IQI’s), formerly called, are used. These can be of several forms as illustrated in Fig.1

Fig. 1. Image Quality Indicators

The wire type is the most frequently used IQI in radiography-by-film. The design of the IQI is given in EN ISO 19232 Part 1 or ASTM E747. Both specifications list a series of IQIs containing six or seven wires of increasing diameter, from 1-8mm, from 10-50mm in length and in a range of metals – iron, nickel, aluminium, magnesium, copper and titanium. The wires are mounted side by side in a flexible plastic sheath which also carries appropriate identification, generally lead letters that will be clearly seen on the radiograph. The IQI is selected with respect to the metal type and the component thickness; the thicker the component the thicker the IQI’s wires. The IQI carries an identification and serial number so that it can be confirmed at a later stage that the correct IQI has been used.

Ideally the IQI is placed on the source side of the component and, in the case of a weld, transversely across the joint although this is not always possible when radiographing pipe and tube butt welds. The sensitivity is taken as the smallest diameter wire that can be seen divided by the component thickness, expressed as a percentage. Most application codes specify a sensitivity of between 2-4% ; this is a maximum, the smaller the figure the greater the sensitivity of the radiograph. Alternatively an actual wire diameter that must be visible is specified.

The step hole IQI is used less frequently. It is a stepped wedge with a hole drilled in each step, the hole diameter matching the thickness of the step. As with the wire IQI, the material and dimensions of the step wedge are selected to match the application. The diameter of the smallest hole visible on the radiograph determines the sensitivity, this being calculated as hole diameter divided by component thickness expressed as a percentage. The sensitivity measured by the use of a wire IQI is not the same as the sensitivity using a step wedge IQI.

As with any film, the method of processing will affect the quality of the image. Care must be taken to ensure that there is no light contamination, the processing chemicals are at the correct concentrations and temperatures and that drying the film does not leave marks and stains that would leave spurious indications and would make accurate interpretation difficult.

Interpretation of the radiographs must be undertaken by trained and experienced radiographers. In addition to being fully conversant with radiographic techniques such individuals should also have a comprehensive knowledge of welding processes, joint design and the various imperfections that may occur.  Many application specifications require such individuals to be independently certified to a suitable certification scheme such as PCN, administered by the British Institute for NDT or CSWIP, administered by TWI Certification Ltd. Viewing should be carried out in a darkened room, allowing a period of time for the viewer’s eyes to be accustomed to the conditions. The luminance of the viewing screen will need to vary with the density of the radiograph – there is generally a rheostat control (a dimmer switch) on the viewer to enable the luminance to be varied. The light itself should be white and diffuse and there should be as little light as possible leaking around the edges of the radiograph. 

Radiography of flat plates and cylinders large enough to permit entry for placement of the film is a relatively simple operation, as shown in Fig. 1 of the previous article. Lead numbers are placed at fixed intervals along the plate adjacent to the weld or around the circumference of a pipe to enable the position of any imperfections to be accurately located. The weld also carries a unique identification number reproduced on the radiograph by the use of lead figures placed adjacent to the weld.

Radiography of pipes, however, where access to the bore to place the film is not possible presents some problems and terms such as SWSI and DWSI are used as shorthand to identify the various techniques that may be used.

Single wall, single image (SWSI) is a technique whereby the radiographic source is placed inside the pipe by some suitable method, the film wrapped around the outside of the pipe and the exposure made as shown in Fig. 2. This may also be known as a panoramic exposure. The IQI is placed on the outside of the pipe immediately beneath the film. Both X- and gamma-radiography can be used, the source being placed in position by the use of a pre-placed spider or by means of a crawler unit. This method is most commonly used for the inspection of pipelines where the weld can be radiographed in one exposure, making the technique rapid and cost effective.

Fig. 2 Single wall, single image (SWSI) or panoramic radiographic technique

Where access to the bore is not possible or the pipe diameter is too small to permit the use of an internal source then the double wall, single image (DWSI) technique is used. Here the film is placed on the outside of the pipe on the farthest side from the radiographic source, as shown in Fig. 3. The source may be offset slightly to avoid an image of the upper part of the weld to be projected onto the film or directly in line. The source may be close to or a substantial distance from the pipe, the location being a compromise between a less sharp image but short exposure time for a small stand-off and sharper image but longer exposure time for a large stand-off. The need to penetrate two wall thicknesses means that the sensitivity will be poorer than with the single wall single image technique. The technique also requires multiple exposures to enable the complete circumference of the pipe to be examined – specification or contract requirements frequently specify the minimum numbers of exposures to ensure complete coverage and images of an acceptable quality. The technique is generally used on pipes over 80mm in diameter.

Fig. 3 Double wall, single image

The last technique is double wall, double image (DWDI), generally used only on pipes less than 75-80mm in diameter. By offsetting the source from the weld centre line and using a long source to film distance it is possible to project an image onto the film of both the upper and the lower parts of the weld as shown in Fig. 4. As with the DWSI technique multiple exposures are required to achieve complete coverage.

Fig. 4. Double wall double image radiograph of a pipe butt weld. Note the IQI, identification numbers and position markers.

Part 3 will look at some of the more sophisticated radiographic techniques and the advantages and disadvantages of radiography.

This article was written by Gene Mathers.

Radiography Testing PART 1

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Radiography

Fig. 1. Principles of radiography

The previous two Connect articles dealt with the defect detection techniques of MPI, LPI and eddy current testing. These methods are capable of detecting surface or very near surface imperfections and some process is therefore required to enable buried imperfections to be reliably detected – a so-called volumetric detection method. The first such method to be used in manufacturing to determine the quality of fabricated components is radiography. X-rays generated from a cathode ray tube were discovered in the late 1890’s, soon followed by the discovery of gamma radiation from radioactive isotopes.

The ability of this radiation to penetrate both living and inert objects with the amount of transmitted radiation depending on the density, thickness, and atomic number of the object was eventually understood.  The transmitted radiation was found to produce images on photographic film and these findings resulted in the development of industrial radiography as illustrated in Fig. 1.

Fig. 2 Schematic of an X-ray tube

X-radiation is produced from a vacuum tube - this contains a cathode which produces a beam of electrons when an electric current is passed, the higher the voltage the more intense is the stream of electrons and the deeper the penetration – see Table 1. Industrial X-ray tubes use voltages between 20kV and some 450kV but up to 30MV in linear accelerator and betatron equipment. The stream of electrons impacts an anode, some 1% of the energy being emitted as a beam of X-rays, the remainder being released as heat, requiring the anode to be cooled, often with water or oil. A schematic of an X-ray tube is shown as Fig. 2. The anode can be designed to provide a beam as illustrated in Fig. 2 or a 360O panoramic beam, typically used for pipe butt weld inspection with the X-ray source inside the pipe.

kV	Maximum Thickness (mm)
	Steel	Aluminium
50	1	12
100	20	60
150	30	80
200	50	100
300	80	150
400	100	200
2000(2MV)	200	500
25000(25MV)	500	1400

Table 1 Applied kV and maximum thickness

Gamma radiation is naturally occurring and is produced by the decay of a radioactive isotope which, when it decays, emits three types of radiation, alpha, beta and gamma. Alpha and beta radiation are short range and very easily absorbed – gamma radiation, however, is very energetic, typically over 100keV, and can easily pass through a metal, the thickness that can be penetrated depending on the type and size of isotope. The isotopes are stored in shielded containers from which they can be wound to expose the isotope, as shown in Fig 3.

Fig 3. Typical arrangement of gamma ray NDE equipment

Each isotope has a “half life”- a length of time by which half (50%) of the radioactive isotope has decayed into a stable element – two half lives means the source has only 25% of its original strength, three half lives 12.5%. As the source decays and becomes less energetic the length of the exposure time must be increased to achieve the same density of image on the radiographic film. There is thus a point at which use of the gamma ray source is discontinued and replaced with a fresh isotope. In addition, because the gamma radiation emitted by an isotope cannot be varied in quality there is a range of material thickness for which each source will give acceptable results. The commonest isotope in regular use is iridium192 with cobalt60  being used for very thick components.

Isotope	Half Life	Typical Thickness Range – steel  (mm)
Iridium 192	74 days	10 - 50
Cobalt 60	5.26 years	25 - 200
Ytterbium 169	32 days	< 10
Thulium 170	128 days	< 10
Selenium 75	119 days	5 - 20
Caesium 137	30 years	20 - 80

Table 2 Half life and typical thickness range of industrial isotopes.

The film is a fine grained photographic film with a coating of a light sensitive emulsion on both sides which is loaded in a darkroom into a flexible cassette. The cassette contains an intensifying screen, a card mounted thin lead foil, 0.05 mm to 0.5mm thick, held in close contact with the film. This screen absorbs stray radiation and emits electrons that enable the exposure time to be substantially reduced without affecting the radiographic quality.

The quality of a radiograph is assessed using three factors – density, contrast and definition or sharpness of the image. Most specifications require film densities in the range 1.8 to 2.5  (film density = 1, 1/10th of light is transmitted: film density = 2, 1/100th of light is transmitted). The density of the film can be measured using a densitometer; films outside of the specified range of density would be rejected. The density is affected by the exposure time and metal thickness – this can make the radiography of components with dissimilar thicknesses problematic – thin sections being over-exposed, thick sections under-exposed.

Fig 4. Formation of a penumbra

The contrast is determined by the differences in absorption between the metal and the defect and by the type of film used. The speed of the film in particular affects the  contrast of the image. Most  weld defects are less absorbing than the surrounding metal – slag, porosity, lack of fusion or penetration etc therefore appear dark against a lighter background. The only weld defects that are more absorbingr than the parent metal are tungsten inclusions that appear as bright white specks on the film.

Sharpness of the image is a function of a number of factors, a radiograph with poor sharpness being somewhat similar to an out-of-focus photograph with fine details being blurred or not visible. Film with a very fine grain size is preferred for high quality radiography, being capable of resolving fine details. There is a geometric unsharpness caused by the size of the radiation source, known as the “focal spot”. A large diameter source will cause a penumbra which means that the edges of an image become blurred as shown in Fig. 4. The further away from the item being radiographed than the less obvious is this effect – unfortunately the further away the source is from the object then the longer is the exposure time – twice the distance quadruples the time. To minimise the penumbra and increase sharpness the source should be the smallest diameter possible; the film should be as close to the back of the object as possible; the source should be as far from the object as possible, bearing in mind the lengthening of exposure time; fine grained film should be used.

Additional unsharpness is caused by the release of electrons within the film emulsion that darken the adjacent area.

To ensure that radiographs are of an acceptable quality with respect to image sharpness, contrast etc it is a requirement that an image quality indicator (IQI) is used as can be seen in Fig. 1. This topic will be covered in the next article, as will the techniques for radiography of a variety of joint types and component configurations.

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Non-destructive Examination (NDE) Part 1 Liquid Penetrant and Magnetic Particle Inspection

Fig. 1 Principles of LPI

In achieving high quality defect free welds there is no substitute for experienced and qualified welders and competent supervision. However, no matter how skilled the welder, during the process of depositing weld metal, imperfections of various types may be formed. It is therefore necessary to have methods of ensuring that the weld is of an acceptable quality, hence the development of a range of non-destructive inspection techniques capable of both detecting and sizing buried and surface breaking imperfections, enabling a decision to be made regarding acceptance or otherwise.

Note that both the ISO specifications and the ASME codes differentiate between an ‘imperfection’ (ISO) or a ‘discontinuity’ (ASME) and a ‘defect’. It is accepted that all welds contain features or imperfections but it is only when an imperfection exceeds the relevant acceptance standard does an imperfection become a defect – as ASME states “..... this term designates rejectability...”   

Liquid penetrant (LPI) and magnetic particle (MPI) inspection techniques are methods that supplement visual inspection, revealing defects such as fine cracks or micro-porosity that would be invisible or difficult to detect by the naked eye.

LPI is a simple, cheap and easily portable inspection method that requires no equipment apart from spray cans. It can detect surface breaking imperfections only and relies on a coloured or fluorescent dye, sprayed on the surface and penetrating the imperfection. About15 minutes is generally specified to enable the dye to penetrate any very fine imperfections. After cleaning the excess the dye is drawn to the surface by spraying on a developer in the case of the colour contrast dye or exposing the surface to ultra-violet light in the case of a fluorescent dye, the imperfection being revealed by the dye staining the developer or fluorescing, as shown schematically in Fig. 1 and in Fig. 2, a liquation crack in an aluminium alloy.

The fluorescent dye gives greater sensitivity than the colour contrast dye and does not require the use of a developer. It does however require the use of an ultra-violet light source and preferably a darkened room which makes it a less portable inspection method than the contrast dye technique.

The dye used as a penetrant must be able to penetrate tight cracks but must not be capable of being removed from more open imperfections during the cleaning operation carried out prior to applying the developer.

Fig. 2 LPI test result – cracking in the HAZ of an aluminium alloy weld

Careful surface preparation and thorough cleaning of the item before applying the penetrant is important. Swabbing with or immersion of the item in a proprietary degreasant is generally sufficient – cleaning in an ultrasonic bath is the best method but can be used only for small portable components. Grinding or wire brushing, particularly of materials such as copper and aluminium alloys, should be avoided if possible as such cleaning methods can smear over imperfections, making them undetectable. If such is the case an acid etch may be required to remove the smeared metal and enable the dye to penetrate the imperfection
Inspection in positions other than flat can be a problem but penetrants are available with a jelly like consistency that can be used in the vertical and overhead positions. It is possible to automate the process with small components loaded into baskets and processed on a conveyor line. Fluorescent dyes are better than contrast dyes in this application due to their greater sensitivity.

Although a simple inspection process to use, interpretation can be a problem if the surface is naturally rough – coarsely ground or rough machined for example - or contains acceptable geometric features that trap the dye. Training of operatives to recognise genuine and spurious indications is therefore essential. Other limitations are that it can be used at room temperature only and it is not possible to indefinitely retest components as the imperfection becomes filled with dry dye. Health and safety may also be an issue with irritation of unprotected skin and fumes from some of the cleaning and solvent chemicals, particularly when the process is used in confined spaces.

The process can however be used to inspect both ferrous and non-ferrous metals, large areas can be examined very quickly and it can be used on components with complex geometry.

Magnetic particle inspection (MPI) is also a simple-to-use inspection method but, as the name suggests, is limited in use to magnetic materials- in other words ferritic (NOT austenitic) steels. The basic principle is that the component is magnetised, producing a flux within the metal as shown in Fig 3 An imperfection is non-magnetic and therefore cuts the lines of flux producing a leakage field around the imperfection –a localised “magnet”. A magnetic powder sprayed or dusted onto the surface will be attracted to this “magnet” forming a line of powder. The strength of the magnetising current should be specified in a written examination procedure and the adequacy of the magnetic field verified by the equipment being capable of lifting a specified weight. 

[ Zoom ]

Fig. 3 Principles of MPI

Conventionally the item to be inspected is spray painted with a thin coat of rapid drying white paint, the magnetic field is applied and a black magnetic ink is sprayed onto the surface, forming a black indication against the white background. The maximum imperfection sensitivity is when the imperfection cuts the magnetic flux at 90O. In order that both longitudinal and transverse oriented imperfections are detected, examination of a weld must therefore be carried out with the magnet applied twice at 90O along and across the weld . 

Magnetisation may be by prods, electromagnetic yokes as shown in Fig 4, or permanent magnets. Inspection by the use of prods supplied with high amperage low voltage alternating or direct current is often used in the workshop, the local magnetisation being achieved by two prods connected to a transformer or transformer/rectifier. The prods are pressed onto the metal surface, a trigger pulled to initiate a current in the component and the magnetic ink applied. This is generally a two man operation. Care must be taken not to initiate the current before the prods are in firm contact with the surface as arcing between the prod tips and the component can occur, resulting in a feature similar to a welding arc strike. The use of rectified half wave single phase direct current has an advantage over alternating current and the yoke in that the process is capable of detecting imperfections up to perhaps 1mm below the surface, depending on their size and orientation.

[ Zoom ]

Fig. 4 MPI using an AC yoke

The yoke method, illustrated in Fig 4, has several advantages over the prod method. The equipment is relatively small and lightweight; can be battery powered and is readily portable, making it ideal for site inspection. The yoke can be operated in one hand and the magnetic ink sprayed on from the other making this a one man operation. In addition no electrical current is transferred into the component.

Spurious indications can be produced where there is a difference in magnetic properties within the joint, perhaps in the HAZ where it may be mistaken for unacceptable undercut. Two metals of different magnetic characteristics when joined together can give a well defined indication suggesting the presence of a crack – a careful dressing of the surface followed by LPI is helpful in deciding whether or not the indication is genuine. Ferritic/austenitic dissimilar metal joints cannot be MPI’d. Residual magnetism can also cause problems in interpretation. As with LPI rough uneven surfaces may also give rise to spurious indications.

Neither technique gives a permanent record of the inspection but where this is necessary photographs of the affected area are very useful. Conveniently positioned reference markers and a scale are helpful for the accurate recording of the size and position of indications, particularly if repairs are required. It is also possible to transfer the indication onto transparent sticky tape by carefully pressing the tape onto the surface and then applying this to a sheet of white paper.

This article was written by Gene Mathers.

JOB VACANCIES UT SUPERVISOR

There are vacancies for Advance UT Supervisor in a Pressure Vessels Manufacturing company in Saudi Arabia. At least five years of experience in (TOFD/PAUT) on pressure vessels is mandatory. Interested candidate may apply on shaikhimranamo@gmail.com

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Eddy Current Testing

Job Knowledge 123

Eddy current testing is an inspection method that can be used for a variety of purposes including the detection of cracks and corrosion, material and coating thickness measurement, material identification and, in certain materials, heat treatment condition. The process relies upon a material characteristic known as electro-magnetic induction. When an alternating current is passed through a conductor – a copper coil for example – an alternating magnetic field is developed around the coil, the field expanding and contracting as the alternating current rises and falls. If the coil is then brought close to another electrical conductor the fluctuating magnetic field surrounding the coil permeates the material and induces a circulating or eddy current to flow in the conductor. This eddy current, in its turn, develops its own magnetic field. This ‘secondary’ magnetic field opposes the ‘primary’ magnetic field and thus affects the current and voltage flowing in the coil. Any changes in the conductivity of the material being examined such as near surface defects or differences in thickness will affect the magnitude of the eddy current and this change can be detected using either the primary coil or a second detector coil.  This forms the basis of the eddy current inspection technique.

As with any inspection method there are both advantages and disadvantages to eddy current testing. The method can be used only on conductive materials and, although all metals can be inspected, the depth of penetration of the eddy currents varies. Eddy current density is higher and defect sensitivity greatest at the surface and decreases with depth, the rate of the decrease depending on the “conductivity” and “permeability” of the metal. The conductivity of the material affects the depth of penetration with a greater flow of eddy current at the surface in high conductivity metals and a subsequent decrease in penetration in metals such as copper and aluminium.

Permeability is the ease with which a material can be magnetised. The greater the permeability the smaller will be the depth of penetration. ’Non-magnetic’ metals such as austenitic stainless steels, aluminium and copper have very low permeability whereas the ferritic steels have a magnetic permeability several hundred times greater.

The depth of penetration may be varied by changing the frequency of the alternation current – the lower the frequency the greater is the depth of penetration. Unfortunately, as the frequency is decreased to give this greater penetration the defect detection sensitivity is also reduced. There is therefore, for each test, an optimum frequency to give the required depth of penetration and sensitivity.

A parameter known as the “standard depth of penetration”, taken as the depth at which the eddy current value has reduced to 37% of that at the surface, can be calculated from the magnetic permeability, the metal’s conductivity and the frequency of the alternating current in the probe. The standard depth of penetration is generally regarded as the criterion by which the efficiency of detection can be judged, although changes in the eddy current can be detected at depths of up to three times this figure. A simple calculation may be used to select the optimum probe frequency.  
 

Fig 1 Pen type probe being used to examine bolt holes for cracks.

For any particular inspection the accuracy of the measurement of defect size, material thickness, heat treatment condition etc. is largely determined by the design of the coil (or coils) used in the examination whilst detection capability is also determined by material properties and the equipment characteristics. The selection of the probe is therefore critical for accurate results.

Some inspections involve sweeping through multiple frequencies to optimize results, or inspection with multiple coils to obtain the best resolution and penetration required to detect all possible flaws. It is always important to select the right probe for each application in order to optimize test performance.

 The eddy current operator is therefore faced with a material whose conductivity and permeability are physical properties and outside of the operator’s control. The parameters that can be selected are probe size, probe type and frequency of the alternating current, the selection depending upon the test requirements i.e crack detection, corrosion depth, coating thickness, heat treatment condition etc. Some equipment is designed to operate using multiple frequencies or with multiple probes in order to optimize the test performance and achieve the best detection performance and depth of penetration. The results are displayed either as a digital read-out for the more simple examinations such as thickness measurements or displayed on an oscilloscope screen as an X-Y display of resistance versus the inductive reactance. This gives a characteristic curve, the shape and size of which can be used to detect and size a defect as illustrated in Fig.2 to determine heat treatment condition or, as a quick sorting test, to establish the type of alloy.

Fig. 2 Schematic of Screen Display

In addition to selecting the optimum frequency the size of the probe can be varied – a large diameter coil will inspect a larger volume of metal and therefore reduce the inspection time – a small diameter probe, however, is more sensitive and better suited to detecting small flaws. The large diameter probes are often used for the detection of large sub-surface flaws in castings and forgings and for the detection of corrosion; the small diameter pencil type probes for detecting cracks. Weld examination requires special probes to reduce noise from the permeability change across a weld.

As mentioned earlier eddy current testing can be used for a variety of inspection tasks. Chief amongst these is the inspection of welded joints using pencil probes as a replacement for the more conventional magnetic particle or liquid penetrant inspection techniques. A major advantage is that the process may be used underwater and can be used to scan welds through paint and other coatings. With respect to detection of linear defects such as cracks and lacks of fusion the defect should break the lines of the eddy currents ideally at right angles – as with magnetic particle inspection defects parallel to the eddy currents are likely to remain undetected. It is important therefore that the weld is scanned in the correct direction. Cracks as small as 0.5mm deep and 5mm in length are capable of being detected.

Encircling coils are used for automated in-line tube inspection with welded tube lines using localised probes for weld examination.

By measuring the conductivity of a metal it is possible to identify and sort both ferrous and non-ferrous metal and with certain alloys - in particular the aluminium alloys - it is also possible to establish the heat treatment condition. Low frequency probes are used to detect generalised corrosion, particularly in the aerospace industry for the examination of aircraft skins. Specially designed “bobbin” probes can be used to inspect the bore of tubes in service for signs of pitting or corrosion and there are also probes specially designed to examine the bores of bolt holes for cracks.

Measuring the proximity of a component to the probe can also be used to determine coating thickness provided the coating is non-conductive. The “lift-off”, the distance of the probe tip from the conductive surface, causes a change in eddy current flow which is measurable.

All of the systems must be calibrated using appropriate reference standards – as for any NDT method, this is an essential part of any eddy current examination procedure. The calibration blocks must be of the same material, heat treatment condition, shape and size of the item to be tested. For defect detection the calibration block contains artificial defects simulating defects; for corrosion detection a calibration block of different thicknesses is used.

The eddy current method requires more skill on the part of the operator than, say, MPI and penetrant inspection – it goes without saying that operator training is essential.