developing time after verification, 8. number of wipes

Test Process :

Surface cleaning : Careful cleaning of the test surface and inside of the discontinuities is an extremely important step for success of the penetrant inspection because the penetrant method works by allowing a liquid to enter a

defect which is open to the surface. If there is dirt in the crack, there is no room for the penetrant to enter it, and the process will not work. Surfaces to be examined are cleaned so that they are free from dirt, scales, oil, grease, paint, rust and corrosion and discontinuities are free inside of water, oil or other contaminants. This is known as pre cleaning. Following cleaning, the parts shall be thoroughly dried.

Penetrant application : After drying, the entire surface to be tested must be wetted with a layer of the penetrant and kept wet during the entire dwell time.

This can be achieved by immersion, flow - on, spraying or brushing the liquid.

The temperature of the test surface should be between 10° C and 52° C.

Rubbing the surface with a penetrant soaked cloth is not permitted.

Penetrant Dwell : The penetrant is left on the test surface for sufficient time to allow penetration into the discontinuity openings. The time involved depends on the viscosity of the penetrant, temperature of the part surface and the tightness of the discontinuity opening. Penetration time used is generally between 10 and 30 minutes. At low temperatures, longer penetration time shall be used because the viscosity of penetrant increases. This time is known as dwell time.

Emulsification : For post emulsification type penetrant, the emulsifier is applied after the completion of the dwell time. Emulsification makes the penetrant water washable. Emulsification time varies between 1 to 3 minutes, depending on the type of emulsifier, the penetrant , the surface condition and actual time shall be determined by practical tests.

Removal of excess penetrant : The purpose of removing excess penetrant is to free the surface of penetrant so that the penetrant, which has entered a discontinuity, will be readily visible when it re-emerges onto the surface.

After the dwell time, excess surface penetrant is to be removed with a solvent wipe, water spray rinse, or the penetrant has to be emulsified so that it can be washed-off with water rinse. Excess penetrant removal depends on the removal characteristic of the penetrant being used. The removal of excess

penetrant is a delicate procedure because it is essential not to remove penetrant from the discontinuities. Penetrant is so easily removed from smooth polished surfaces that special procedures may be required to prevent over - removal. Rough surfaces reduce removability by retaining penetrant in the indentations or recesses by preventing the emulsifier from evenly combining with the surface penetrant. A completely cleaned surface may remove some or all of the penetrant trapped in discontinuities. To make sure that the surface has not been over - washed, the cleaning may allow a very low level penetrant background to remain. A slight shading of penetrant should be visible in the developer layer. During this cleaning operation, the surface has to be checked for residual penetrant using suitable illumination.

Developer application : To make the penetrant indication clearly detectable a developing medium is used. Developer may be applied by dusting or dipping for dry powder or spraying or immersion for water base developers. Non aqueous wet developers should be best applied by spraying only. The developer should then be allowed to dwell on the part surface for sufficient time [ usually 7 to 30 minutes ] to permit it to draw penetrant out of any surface flaws to form magnified visible indications of such flaws. This is known as developing time. Longer times may be necessary for tight cracks.

Inspection of indications : Inspection is then performed under ordinary light for color contrast penetrants or under ultraviolet illumination for fluorescent penetrants to detect any flaws which may be present.

Post Cleaning : After the completion of examination, the test surface is cleaned to remove developer and penetrant. In some industries, such as nuclear, the failure to post clean a part can have very detrimental results. For parts that will be in contact with liquid oxygen systems, the failure to post clean a part could result in a fire and possible serious injury.

If penetrants and developers that contain halogen and sulphur products are used, can have detrimental effects on some metals and must be cleaned according to approved technical data. Penetrant and developer residue tend to absorb moisture and cause corrosion. The removal of dry developer and non aqueous developer is easy. Wet aqueous developer are difficult to remove because they are baked onto the part during the drying process. Soluble developers can be removed by water rinse. The longer the developer remains on a part the harder it is to remove. The use of steam with detergents is probably the most effective of all methods. Some parts, such as those used

in liquid oxygen systems, require total penetrant removal after the inspection.

Entraped penetrant in discontinuities can be removed most effectively by hot tank solvent cleaning or ultrasonic cleaning.

Penetrant inspection normally leaves the part’s surface clean and exposed and thus making it susceptible to corrosion. All efforts should be made to protect the parts surfaces from corrosion effectively after the penetrant inspection.

Penetrant Testing using fluorescent and visible dyes

Glossary of Terms of Liquid Penetrant Inspection

Background: (fluorescent or colour contrast) residue of penetrant over general surface of part during inspection.

Background Colouration: colouration remaining after incomplete removal of dye penetrant from the test surface.

Background Fluorescence: fluorescence remaining after incomplete removal of dye

penetrant from the test surface.

Bath: refers to penetrant materials into which parts are dipped or totally immersed.

Black Light: light radiating with a wavelength ranging between 360 to 400 nm. i.e. just below visible light and used to excite fluorescence.

Black Light Filter: a filter passing black light but opaque to visible and far ultraviolet wavelengths.

Bleedout: the action of penetrant exuding from discontinuities on to the surface of a part.

Blotting: the soaking action developer has on penetrant as it bleeds from a discontinuity, increasing sensitivity and contrast.

Capillary Action: the action by which the level of a liquid in contact with a solid

surface is elevated or depressed to a level different from the liquid not in contact with the solid's surface.

Clean:free of solid or liquid contaminants.

Colour Contrast Dye: dye used in penetrant to give sufficient colour intensity to allow good contrast to the background being viewed under white light.

Colour Contrast Penetrant: a penetrant utilizing colour contrast dye.

Dark Adaptation: adjustment of the eyes when one goes from a bright area to a darkened area allowing better visibility in dim light.

Degreasing Fluid: agents used to clean oil and grease from a part prior to applying penetrant.

Developer: a white powder applied to a part to draw excess penetrant from

discontinuities after initial removal of penetrant from part. Also, provides a contrasting background to view against.

Developer, Dry: light, fluffy, dry absorbent powder.

Developer, Nonaqueous: absorbent powdered materials suspended in a nonaqueous liquid.

Developer, Wet: absorbent powdered materials suspended in water.

Development Time:

time a developer is allowed to remain on the surface of a part being inspected.

Dragout: The loss of fluid as a result of carry-over.

Drain Time: time allowed for excess penetrant to flow off part after it has been immersed in a bath.

Drying Time: time in which a washed or wet-developed part is in the hot air drying oven.

Dwell Time (Penetration time): time that a penetrant is in contact with a part's surface.

Dye Penetrant: a penetrating fluid containing a dye which is visible under normal (white) light.

Emulsifier: an agent that when added to an insoluble penetrant renders it soluble allowing the mixture to be washed off the part.

Emulsion: a stable mixture of water, oil and emulsifier.

Emulsification Time (soak time): period which emulsifier is allowed to mix with the liquid penetrant prior to water rinsing.

Evaluation: process of deciding on the severity of a discontinuity after it has been interpreted, and involves acceptance or rejection of the part.

False Indications: irrelevant indications. Family:

the complete series of materials of a specific manufacturer to perform a specific type or process of LPI.

Flash Point: lowest temperature at which a substance will decompose to form a flammable gaseous mixture.

Fluorescence: property of emitting light as a result of and only during absorption of radiation of a shorter wavelength.

Fluorescent Penetrants:

penetrants used to reveal surface discontinuities which become visible when irradiated with black light.

Hydrophilic: refers to type of emulsifier that is water based.

Indication:marks a discontinuity. In LPI it is the presence of bleed out.

Interpretation:same as evaluation.

Irrelevant Indication: indication resulting from poor technique and not associated with a material discontinuity.

Leak Testing: LPI technique where penetrant is applied to one side of a part and observations are made from the other side to locate any through-wall discontinuities.

Lipophilic: refers to type of emulsifier that is oil based.

LOX-Safe Penetrant: penetrant system designed to be compatible or nonreactive in the presence of liquid oxygen.

Monochromatic Light: light of one wavelength.

Nonrelevant Indications: LPI indications resulting from conditions not associated with a material discontinuity.

Penetrability: the property of a penetrant that allows it to enter very fine openings.

Penetrant: a liquid possessing properties enabling it to enter very fine openings such as cracks.

Polar Attraction: electrostatic attraction between positive and negatively charged ions.

Reproducibility: the reproducing of one or more LPI indications (primarily for statistical data).

Reference Pieces: test pieces containing controlled artificial discontinuities or natural discontinuities, used for checking the efficiency of the penetrant testing method and materials.

Seeability: the ability of an indication to be seen by an observer under adverse conditions e.g. poor background contrast or outside light (fluorescent method).

Soak Time: same as emulsification time.

Solvent Cleaning: process of removing excess penetrant from a surface by wiping or

washing with a solvent.

Solvent Removable Penetrant: a penetrant that must be removed by a suitable solvent.

Standard Cracked Test Panel: an intentionally cracked test panel having separate areas for the application of different penetrant materials so a direct comparison can be obtained.

Water Washable Penetrant: a penetrant with suitable emulsifying agents incorporated to render it directly water washable.

Surface examination (such as with liquid penetrant examination - PT) is not performed for damage mechanisms known to initiate from the inside surface as it would not be effective in these situations. A disadvantage of dye penetrant testing is that cracks not open to the surface can not be detected.

The ABC's of Nondestructive Weld Examination

Introduction

The philosophy that often guides the fabrication of welded assemblies and structures is "to assure weld quality." However, the term "weld quality" is relative. The

application determines what is good or bad. Generally, any weld is of good quality if it meets appearance requirements and will continue indefinitely to do the job for which it is intended. The first step in assuring weld quality is to determine the degree

required by the application. A standard should be established based on the service requirements.

"Whatever the standard of quality, all welds should be inspected."

Standards designed to impart weld quality may differ from job to job, but the use of appropriate examination techniques can provide assurance that the applicable standards are being met. Whatever the standard of quality, all welds should be

inspected, even if the inspection involves nothing more than the welder looking over his own work after each weld pass. A good-looking weld surface appearance is many times considered indicative of high weld quality. However, surface appearance alone does not assure good workmanship or internal quality.

Nondestructive examination (NDE) methods of inspection make it possible to verify compliance to the standards on an ongoing basis by examining the surface and subsurface of the weld and surrounding base material. Five basic methods are commonly used to examine finished welds: visual, liquid penetrant, magnetic

particle, ultrasonic and radiographic (X-ray). The growing use of computerization with some methods provides added image enhancement, and allows time or near real-time viewing, compar ative inspections and archival capabilities. A review of each method will help in deciding which process or combination of processes to use for a specific job and in performing the examination most effectively.

Visual Inspection (VT)

Visual inspection is often the most cost-effective method, but it must take place prior to, during and after welding. Many standards require its use before other methods, because there is no point in submitting an obviously bad weld to sophisticated inspection techniques. The ANSI/AWS D1.1, Structural Welding Code-Steel, states,

"Welds subject to nondestructive examination shall have been found acceptable by visual inspection." Visual inspection requires little equipment. Aside from good eyesight and sufficient light, all it takes is a pocket rule, a weld size gauge, a

magnifying glass, and possibly a straight edge and square for checking straightness, alignment and perpendicularity.

"Visual inspection is the best buy in NDE, but it must take place prior to, during and after welding."

Before the first welding arc is struck, materials should be examined to see if they meet specifications for quality, type, size, cleanliness and freedom from defects.

Grease, paint, oil, oxide film or heavy scale should be removed. The pieces to be joined should be checked for flatness, straightness and dimensional accuracy.

Likewise, alignment, fit-up and joint preparation should be examined. Finally, process and procedure variables should be verified, including electrode size and type,

equipment settings and provisions for preheat or postheat. All of these precautions apply regardless of the inspection method being used.

During fabrication, visual examination of a weld bead and the end crater may reveal problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld detects that can be recognized visually are cracking, surface slag in

inclusions, surface porosity and undercut.

On simple welds, inspecting at the beginning of each operation and periodically as work progresses may be adequate. Where more than one layer of filler metal is being deposited, however, it may be desirable to inspect each layer before depositing the next. The root pass of a multipass weld is the most critical to weld soundness. It is especially susceptible to cracking, and because it solidifies quickly, it may trap gas

and slag. On subsequent passes, conditions caused by the shape of the weld bead or changes in the joint configuration can cause further cracking, as well as undercut and slag trapping. Repair costs can be minimized if visual inspection detects these flaws before welding progresses.

Visual inspection at an early stage of production can also prevent underwelding and overwelding. Welds that are smaller than called for in the specifications cannot be tolerated. Beads that are too large increase costs unnecessarily and can cause distortion through added shrinkage stress.

After welding, visual inspection can detect a variety of surface flaws, including cracks, porosity and unfilled craters, regardless of subsequent inspection

procedures. Dimensional variances, warpage and appearance flaws, as well as weld size characteristics, can be evaluated.

Before checking for surface flaws, welds must be cleaned of slag. Shotblasting should not be done before examination, because the peening action may seal fine cracks and make them invisible. The AWS D1.1 Structural Welding Code, for example, does not allow peening "on the root or surface layer of the weld or the base metal at the edges of the weld."

Visual inspection can only locate defects in the weld surface. Specifications or

applicable codes may require that the internal portion of the weld and adjoining metal zones also be examined. Nondestructive examinations may be used to determine the presence of a flaw, but they cannot measure its influence on the serviceability of the product unless they are based on a correlation between the flaw and some

characteristic that affects service. Otherwise, destructive tests are the only sure way to determine weld serviceability.

Radiographic Inspection (RT)

Radiography (X-ray) is one of the most important, versatile and widely accepted of all the nondestructive examination methods - Fig. 1.

Fig. 1 - Radiography is one of the most Fig. 2 - Thicker areas of a specimen

important, versatile and widely accepted examination methods.

being x-rayed or higher density material absorbs more radiation and the corresponding areas on the radiograph will be lighter

X-ray is used to determine the internal soundness of welds. The term 'X-ray quality,"

widely used to indicate high quality in welds, arises from this inspection method.

Radiography is based on the ability of X-rays and gamma rays to pass through metal and other materials opaque to ordinary light, and produce photographic records of the transmitted radiant energy. All materials will absorb known amounts of this radiant energy and, therefore, X-rays and gamma rays can be used to show

discontinuities and inclusions within the opaque material. The permanent film record of the internal conditions will show the basic information by which weld soundness can be determined.

"Radiography is one of the most widely accepted NDE methods."

rays are produced by high-voltage generators. As the high voltage applied to an X-ray tube is increased, the wavelength of the emitted X-X-ray becomes shorter, providing more penetrating power. Gamma rays are produced by the atomic disintegration of radioisotopes. The radioactive isotopes most widely used in industrial radiography are Cobalt 60 and Iridium 192. Gamma rays emitted from these isotopes are similar to

X-rays, except their wavelengths are usually shorter. This allows them to penetrate to greater depths than X-rays of the same power, however, exposure times are

considerably longer due to the lower intensity.

When X-rays or gamma rays are directed at a section of weldment, not all of the radiation passes through the metal. Different materials, depending on their density, thickness and atomic number, will absorb different wavelengths of radiant energy.

The degree to which the different materials absorb these rays determines the

intensity of the rays penetrating through the material. When variations of these rays are recorded, a means of seeing inside the material is available. The image on a developed photo-sensitized film is known as a radiograph. The opaque material

absorbs a certain amount of radiation, but where there is a thin section or a void (slag inclusion or porosity), less absorption takes place. These areas will appear darker on the radiograph. Thicket areas of the specimen or higher density material (tungsten inclusion), will absorb more radiation and their corresponding areas on the

radiograph will be lighter - Fig. 2.

Whether in the shop or in the field, the reliability and interpretive value of

radiographic images are a function of their sharpness and contrast. The ability of an observer to detect a flaw depends on the sharpness of its image and its contrast with

radiographic images are a function of their sharpness and contrast. The ability of an observer to detect a flaw depends on the sharpness of its image and its contrast with

In document Penetrants TESTING (Page 60-75)