VISUAL TESTING
IV. APPLICATIONS AND TECHNIQUES
General Applications
Visual examinations and other nondestructive test methods cover the spectrum of exam- ining materials from raw product form to the end of their useful lives. Initially, when raw material is produced, a visual examination is conducted to locate inherent discontinuities.
As the material is further transformed through the manufacturing process, a product re- sults. At this stage, the visual examination method is used to find discontinuities that are produced during the primary processing steps. When the product is further developed into its final shape and appearance, the secondary processes that give the product its final form can also introduce new discontinuities. Finally, the product is placed into service and is subject to stresses, corrosion, and erosion while performing its intended function. The process concludes when the material has reached the end of its useful life and is removed from the source. At every stage, the visual examination method is applied using various techniques to ascertain the physical condition of the material that became the component, system, or structure serving the needs for which it was intended.
After material is produced, visual examination is used to assure that a product will meet the specification requirements prior to processing into a product form for use in its intended service.
The technology associated with visual testing (VT) and remote visual testing (RVT) includes a spectrum of applications, including various products and industries such as:
앫 Tanks and vessels 앫 Buildings
앫 Fossil-fuel power plants 앫 Nuclear power plants 앫 Turbines and generators 앫 Refinery plants
앫 Aerospace
Tanks and vessels usually contain fluids, gases, or steam. Fluids may be as corrosive as acid or as passive as water, either of which can cause corrosion. Tank contents are not al- ways stored at high pressure. Conversely, vessels usually contain substances under sub- stantial pressure. This pressure, coupled with the corrosive effects of fluids and thermal or mechanical stresses, may result in cracking, distortion, or stress corrosion of the vessel material.
Buildings also serve as a source for a myriad of RVT applications. These applica- tions include location of clogged piping; examination of heating and cooling (HVAC) heat exchangers; and looking for cracking, pitting, blockages, and mechanical damage to the components. Structural damage that may be present in the support systems, beams, flooring, or shells, such as cracking, corrosion, erosion, or warpage can also be detected.
Fossil-fuel power plants have piping, tubing, tanks, vessels, and structures that are ex- posed to corrosive and erosive environments as well as to other stresses. These compo- nents may require RVT.
Turbines and generators, existing at both fossil-fuel and nuclear power plants, are vul- nerable to damage due to high temperatures, pressures, wear, vibration, and impingement of steam, water, or particles. Accessing the small openings and crevices to reach damaged turbine blades becomes a very tedious job and a serious challenge, but the effort of per-
forming remote inspections through limited access ports reduces the need and cost of downtime and disassembly of major components.
VT and RVT technologies and techniques are used in nuclear power plants as well.
Water used for shielding and cooling is exposed to both ionizing radiation and radioactive surface contamination. The use of water as a coolant and radiation shield in a nuclear en- vironment places additional requirements on RVT evaluation. The equipment must not only be waterproof, but also tolerant of radioactive environments.
Due to process requirements in refineries, the containment of pressure and tempera- ture is a necessity of paramount importance, as is the containment of hazardous materials.
These same materials can be a source of corrosion to piping, tanks, vessels, and struc- tures, all of which are in constant need of monitoring.
Standards, Codes, and Specifications
The above applications all require VT and RVT to detect surface anomalies. A common source for material specifications is the American Society for Testing and Material (ASTM) standards. ASTM was founded in 1898 and is a scientific and technical organi- zation formed for “the development of standards on characteristics and performance of materials, products, systems and services; and the promotion of related knowledge.” At this time, the annual ASTM Standards fill 75 volumes and are divided into 16 sections.
An ASTM standard represents a common viewpoint of producers, users, consumers, and general interest groups intended to aid industry, government agencies, and the general public. Two metal material sections are Section 1—Iron and Steel Products and Section 2—Nonferrous Metal Products. These standards provide guidance on the material condi- tions that must exist in order to be considered satisfactory for use.
Additionally, when material is fabricated and formed into a product, other standards, specifications, and codes delineate visual testing requirements. The American Society of Mechanical Engineers (ASME) publishes the ASME Boiler and Pressure Vessel Code, Sections I through XI. The Material section is Section II. The Design sections are I, III, IV, VIII, and X. Section V contains the methodology for nondestructive examination, and Section IX addresses welding and brazing. Section VI deals with heating boilers, Section VIII, pressure vessels, and Section XI, in-service inspection of nuclear power plant com- ponents. Scattered throughout these sections are visual examination requirements.
The American National Standards Institute (ANSI) is a consensus-approval-based or- ganization. ANSI’s B31.1—Power Piping and B31.7—Nuclear Power Piping also pro- vide visual examination requirements for materials, fabrication, and erection.
The American Petroleum Institute (API) has developed approximately 500 equipment and operating standards relating to the petroleum industry that are used worldwide. An example is the API standard for Welding of Pipelines and Related Facilities (API 1104).
The aerospace and military standards are being replaced with the more commonly ac- cepted industry codes and standards.
Visual Detection of Discontinuities
Chapter 2 addresses and illustrates many of the following discontinuities in greater detail.
Inherent Discontinuities
The VT technique most often used to detect inherent discontinuities is direct visual. The human eye assisted by measuring devices, auxiliary light sources, visual aids (e.g., mag-
nifiers and mirrors), and the recording media of photographs and sketches are most com- monly used for this application.
Applications. Raw material may be checked for inherent discontinuities and com- pared with the requirements of material specifications for acceptance. Frequently, the first examinations performed on new materials and products are dimensional checks. Attribut- es such as thickness, diameter, out of round, roughness or smoothness, etc., are examples of the first round of visual checks made on new materials. Checking for discontinuities that interrupt the normal physical structure of the part is also conducted at the material fabrication stage. The inspector might look for blemishes or imperfections that are gener- ally superficial and could be located by observing a stain, discoloration, or other varia- tions on the surface. Discontinuities on the surface could be indicators of other anomalies in the material. Discontinuities or lack of some element that is essential to the usefulness of the part should be of importance to the inspector. Any of these attributes would be evaluated against the specified code, acceptance criteria, standard, or customer require- ments. The following is a typical sequence of observation steps an inspector or examiner might follow for a general condition visual examination.
1. The observation of an anomaly—something different, not normal, or irregular.
2. The visual indication is evaluated and determined to be relevant or not.
3. The relevant indication is compared to the appropriate standard, code, specification, or customer requirements.
4. The relevant indication is judged acceptable, not recordable, recordable, or unaccept- able and rejected.
Discontinuities may start out as something small. They may result from corrosion, a slight scratch, or other discontinuities inherent in the material such as porosity, cracks, or inclu- sions. These seemingly harmless anomalies may develop into cracks due to stress concen- tration under varying loads and propagate with time. Eventually, there may no longer be sufficient solid material left to carry the load.
A defect is a discontinuity of such size, shape, type, number, or location that results in a condition that will cause the material to fail in service or not be used for its intended purpose.
Ingots. Common discontinuities detected visually in the ingot are cracks, scabs, pipe, and voids.
Cracks may occur longitudinally or in the transverse plane in the ingot. Although cracks may be observable in the ingot stage, it is more likely that they will appear in sub- sequent processing operations.
Scabs are surface conditions on ingots caused by the hot molten metal splashing on the mold surface, solidifying and oxidizing such that it will not re-fuse to the ingot. Again this condition may be more detrimental to the product at later stages of processing.
Pipe is a cavity formed by shrinkage during the last stage of solidification of the molten metal. It is an elongated void with a “pipe like” shape and occurs in the center of the ingot. This discontinuity can result in a lamination after the ingot is rolled into plate or sheet. If ingot is processed into bars, blooms, or billets, the pipe will become elongated as a result of the processing and generally remain in the center.
Voidsare produced during the solidification of the ingot, caused by insufficient de- gassing. The gas, which becomes entrapped, often forms spherical cavities.
Nonmetallic inclusionsin ingots are chiefly due to deoxidizing materials added to the molten steel in the furnace, ladle, or ingot mold. Oxides and sulfides constitute the bulk of nonmetallic inclusions.
All of these dicontinuities can occur during the pouring of molten metal into the ingot
mold. The resulting discontinuities are the same as those found in the processing stage when pouring metal into a mold and creating a casting.
Techniques. The types of anomalies found in ingots are typically gross in nature.
These can generally be found with direct visual examination techniques. A magnifying lens, auxiliary light, and possibly a mirror should suffice for this application.
Primary Processing Discontinuities
The visual technique most often used to detect primary processing discontinuities is the direct visual technique. Measuring devices, auxiliary light sources, visual aids (e.g., mag- nifiers and mirrors), along with the recording media of photographs and sketches, are the most common techniques used for this application.
Applications
Materials are formed into a final or near-final shape as a result of the various primary processes. The forming processes of wrought products include forging, rolling, drawing, extruding, and piercing. Discontinuities caused by these methods of forming the product shape are known as primary processing discontinuities. When steel ingots are worked down into usable sizes and shapes such as billets and forging blanks, some of the inherent discontinuities described earlier may appear. These primary-processing discontinuities may ultimately become defects. Working the metal down into useful forms such as bars, rods, wire, and forged and cast shapes will either change the shape of existing discontinu- ities inherent to the product or possibly introduce new ones.
Forging. Typical forging discontinuities include forging bursts, laps, and cracks.
Bursts are internal or external forging discontinuities. Forging bursts appear as scaly, ragged cavities inside the forging. (Note: Internal bursts are more likely to be detected with the use of volumetric inspection methods such as radiography or ultrasonics. For ad- ditional information refer to Chapter 2.)
Forging laps are folded flaps of metal that have been forced into the surface of the part during the forging process. The lap can vary from shallow to very deep on the forg- ing surface. A lap can appear to be tight and irregular, straight, crack-like and linear, or even wide and U-shaped. Forging laps are difficult to detect without magnification.
Cracksin a forging are usually referred to as “forging bursts.” Many forging bursts open to the surface are detectable visually; however, the smaller ones may require other methods such as PT or MT.
Technique. The direct visual technique is best suited for this application. This can be best accomplished by examination using a 5× to 10× magnifier with an auxiliary light source held at an angle so as to cast a shadow on any irregularities.
Rolling. Rolling of product forms can create a variety of discontinuities. The more common ones are seams, stringers, cracks, and laminations. These can appear in product forms such as bars, rods, channels, beams, angle iron, and strip metal.
Seams and stringers generally appear on the outside surface of the product parallel to the direction of rolling and are very difficult to detect visually. Seams contain nothing other than entrapped oxides. A seam is usually produced when two unbonded surfaces are pressed together during the rolling process. Stringers are inclusions that get “strung out”
during rolling and may be observable if exposed to the surface. Complex structures may be formed from plate or rolled shapes, making some surface indications difficult to detect.
Additionally, internal discontinuities such as laminations are best detected using UT.
Techniques. The direct visual technique is best suited to detect these conditions in this application. This can be best accomplished by examination using a 5× to 10× magnfi-
er with an auxiliary light source held at an angle so as to cast a shadow on any irregulari- ties. Indirect visual examination with a borescope or fiberscope may be appropriate for viewing internal areas of complex product forms. The limiting factors will be access and distance.
Drawing, Extruding, and Piercing. Drawn, extruded, or pierced products may exhibit similar types of surface discontinuities. Any irregularities on the surface that are gross in nature could result in problems during the forming process. Common observable anomalies may be scabs, slugs, or scoring.
Scabs are loosely adhering material that can be removed from the final product. These can result from dirty dies that press in the loose foreign material during the drawing process or from oxide layers that are not bonded to the material.
Slugsare usually larger pieces of material that are firmly impressed into the product form and may not come off easily.
Scoringappears as deep scratching of the surface of the product by foreign material that may be adhering to the inside of the die or piercing mandrel during processing.
Technique. The direct visual technique is best suited to detect these conditions. This can be best accomplished by examination using a 5× to 10× hand-held magnifying lens with an auxiliary light source held at an angle so as to cast a shadow on any irregularities.
Castings. Casting is a primary process. Castings may exhibit various types of discon- tinuities that are inherent in the metal forming process, including nonmetallic inclusions (slag), hot tears, gas, unfused chills and chaplets, cold shuts, and misrun. Some of these discontinuities may also occur internally. The casting process can produce some very complex shapes. Access for visual examination can be a real challenge.
Inclusionsexposed to the surface may be akin to the “tip of the iceberg” in that the visible area may be merely a small percentage of the inclusion contained within the cast- ing. The visual examiner can only detect and measure the exposed area of the discontinu- ity.
Hot tearsare more likely to be visible on the surface, since they occur at locations where changes in thickness occur. Hot tears are the result of different coefficients of ex- pansion and contraction at a change of thickness. The abrupt change from thick to thin material sections provides a stress riser. The different rates of cooling (contraction) could cause a rupture of the metal under stress at these junctions.
Gasis a spherical void condition. It may occur both internally and at the surface of the casting. Surface gas typically has a hemispherical shape, since if exposed to the surface an incomplete sphere is trapped in the casting. There may be one or many of these anom- alies distributed randomly or in clusters at any location.
Unfused chaplets and chillswill be visible only if they protrude through the surface.
Remember that the chaplet is usually metal with a low melting point that is used to sup- port the internal mold cores until the molten metal enters the mould and supposedly melts the chaplet. The examiner may note the “footprint” of the metal form that is exposed to the surface. The chill can be likened to a nail with a head. The “head” may be all that the examiner observes, with the remainder of the “nail” encased in the casting.
Cold shutscould result from balls of molten splattered metal that adhere to the mold wall prior to the remainder of the molten metal rising over it and entrapping it on the in- side of the mold and thus the outside of the casting. It can appear as a circle around a ball of trapped metal. Cold shuts also result when two regions of the casting do not metallurgi- cally fuse; one surface begins to solidify before the remainder of the molten metal forms around or over the already solidified metal. This condition is generally not detectable by standard VT techniques.
Misrunis a surface condition in which the casting mold does not completely fill with
molten metal, thereby leaving a noticeable depression, which is readily detectable with VT.
Techniques. Either the direct visual or remote visual techniques are necessary for examining castings for exposed inclusions—hot tears, surface voids, unfused chaplets or chills, cold shuts, and misrun. Accessible surfaces can best be examined by using a 5× to 10× hand-held magnifying lens with an auxiliary light source held at an angle so as to cast a shadow on any irregularities. The surfaces that are internal to the casting may require examination using a rigid borescope or flexible articulating end fiberoptic borescope, the latter being more versatile. Either one would bring the remote surface under close exami- nation and provide light to the examination site. Extra care is required to assure full cov- erage and proper orientation when performing internal exams. Knowing where the dis- continuity is located and accurately measuring its dimensions are requirements for proper evaluation of the indication. If a visual record is desired, a fiberoptic video camera may be appropriate. The distance to the area of interest as well as the size of the access port are important factors to be considered when choosing the proper instrument. Additionally, it is difficult to obtain accurate measurements at the remote site. Some instruments have been outfitted with detachable scales that are visible in the field of view. Other instru- ments have remote measuring devices that rely on shadow casting and geometric calcula- tion techniques. These devices require calibration on surfaces similar to the orientation plane of the examination surface in order to be accurate.
Metal Joining Processes
Metal joining processes include a number of welding and allied processes. Each major process must be considered for unique as well as common discontinuities. Metalworking industries generally use soldering, brazing, and a broad variety of welding processes. A generic definition of the basic welding process is “a materials joining process that pro- duces coalescence of materials by heating them to suitable temperatures, with or without the use of filler metal.”
Solderingutilizes filler material that becomes liquid at temperatures usually not ex- ceeding 450° C (±840° F). The solder is heated and distributed between two surfaces of a joint by capillary action.
Brazingproduces coalescence of materials by heating them to a suitable temperature and using a filler metal that becomes liquid above 450° C (±840° F). The filler metal is distributed between the closely matching surfaces of the joint by capillary action. In either case, when completed, the only visible signs of completed wetting, and hence bonding, is the micro fillet at the intersection of the two materials. The normal configuration for join- ing is a lap joint of two materials overlapping each other. When heated and wetted with filler material, the resultant fillet should be uniform the entire length of the joint.
Arc welding produces coalescence of metals by heating them with an arc, with or without the application of pressure, and with or without the use of filler metal. Shielded metal arc welding (SMAW; Figure 3-15), submerged arc welding (SAW; Figure 3-16), gas tungsten arc welding (GTAW; Figure 3-17), and gas metal arc welding (GMAW; Fig- ure 3-18) all have two things in common. Each utilizes an electric arc as the heat source for melting the base and filler metal, and each provides a means of shielding the arc to block out harmful elements found in the air (oxygen).
SMAW and SAW both use the combustion of flux as the means of consuming oxygen at the weld site, thus preventing the oxidation (burning) of the metals being joined. The common by-product of burning flux is slag.
GTAW and GMAW both utilize inert gas cupped around the weld site to eliminate the oxygen portion of the fire triangle (oxygen, heat, and fuel) and thus prevent the metals from burning at high temperatures. The electrode bearing the heat for GTAW is the tung-