WELDING TECHNOLOGY

Introduction - Welding is a process for joining different materials. The large bulk of materials that are welded are metals and their alloys, although the term welding is also applied to the joining of other materials such as thermoplastics. Welding joins different metals/alloys with the help of a number of processes in which heat is supplied either electrically or by means of a gas torch. In order to join two or more pieces of metal together by one of the welding processes, the most essential requirement is Heat. Pressure may also be employed, but this is not, in many processes essential.The use of welding in today's technology is extensive. It had a phenomenal rise since about 1930; this growth has been faster than the general industrial growth. Many common everyday use items, e.g., automobile cars, aircrafts, ships, electronic equipment, machinery, household appliances, etc., depend upon welding for their economical construction.

History of Gas Welding - The oxyacetylene process was an outgrowth of to discovery of Henry Louis Le Chatelier, a French chemist, who in 1895 showed that the combustion of acetylene with oxygen produced a flame having a temperature far higher than that of any gas flame previously known.

The commercial success of the oxyacetylene process, however, depended upon the availability of oxygen and acetylene in sufficient quantities to make the process practical. In the early 1900's torches of a practical type were introduced and by 1903 the oxyacetylene process began to be used industrially.

History of Arc Welding - Electric arc was first described by Davy in England in the year 1809, but the beginning of arc welding could become possible only with the improvements in electric dynamos or generators between 1877 and 1880. Auguste de Meritens established arc welding process in 1881 which was applied to join certain components of electrical storage batteries.

The joining of metals, using a carbon arc was suggested by Moissan (a Frenchman) in 1881, but it was only between 1885 and 1887 when, in Russia, Bernardos and Olszewski got patented and used single carbon arc welding for joining metals. In 1889 Zerener processed an idea, which aided later on to develop Twin Carbon arc welding.

In 1892, in Germany, N.G. Slavianoff proposed the use of bare wire metallic electrodes for joining metals. The arc being unshielded in this case, satisfactory welds could not be produced. In 1907, in Sweden, Oscar Kjellberg got a patent for covered electrodes. The coating which he employed contained only arc stabilizing materials, and thus a good welded joint was not obtained.

In 1912, in USA Strohmenger obtained another patent on covered electrodes and the first good welded joint was produced. Strohmenger used a coating of blue asbestos with sodium silicate as a binder. Since then a lot of changes and developments have occurred as regards the constituents of flux coating and core wire compositions. Covered electrodes were commercialized in the year 1929.

History of Resistance Welding -Elihu Thomson, a Professor at Franklin Institute in Philadelphia, was the first to demonstrate the possibility of joining metals by resistance welding. His experiments which were completed in 1886, led to the very extensive, complex methods which are useful in the assembly of many of our modern structures.

Today multiple electrode machines are employed in high production units for completing hundreds of welds in fast moving production lines.At the present time, welding practice is divided into about 70% Arc welding with the

balance divided between Resistance welding and Oxyacetylene welding.

Classification of Welding Processes - There are about 35 different welding and brazing processes and several soldering methods in use by industry today.There are various ways of classifying the welding and allied processes. For example, they may be classified on the basis of : (i) Source of heat, i.e., flame, arc, etc. (ii) Type of interaction i.e. liquid/liquid (fusion welding) or solid/solid (solid state welding). In general, various welding and allied processes are classified as follows: 1. Gas Welding Airacetylene Welding Oxyacetylene Welding Oxyhydrogen Welding  Pressure gas Welding

2. Arc WeldingCarbon Arc Welding Flux Cored Arc WeldingTIG (or GTAW) WeldingPlasma Arc WeldingElectroslag Welding and Electro gas WeldingStud Arc Welding.Shielded Metal Arc WeldingSubmerged Arc Welding MIG (or GMAW) Welding

3. Resistance WeldingSpot Welding Seam WeldingProjection WeldingResistance Butt WeldingFlash Butt WeldingPercussion WeldingHigh Frequency Resistance Welding.

4. Solid State WeldingCold WeldingDiffusion WeldingExplosive Welding Forge WeldingFriction WeldingHot Pressure WeldingRoll Welding Ultrasonic Welding.5. Thermo Chemical Welding ProcessesThermit Welding Atomic Hydrogen Welding.

6. Radiant Energy Welding ProcessesElectron Beam Welding

Laser Beam Welding.

Commonly Welded Base Metals -  Metals can be classified as 1. Ferrous 2. Nonferrous. Ferrous materials contain iron and the one element people use more than all others is Iron. Ferrous materials are the most important metals/alloys in the metal lurgical and mechanical industries because of their very extensive use. Ferrous materials finding day to day welding applications are: 1. Wrought Iron.2. Cast Iron.

3. Carbon Steel (Low. Medium and High Carbon Steels).4. Cast Steels. 5. Alloy Steels. 6. Stainless Steels, etc. Nonferrous materials are those that are not iron based. Like ferrous materials, nonferrous materials also find extensive industrial applications. Nonferrous materials finding day to day welding applications are : 7. Aluminium and its alloys. 8. Copper and its alloys.9. Magnesium and its alloys. 10. Nickel and its alloys. 11. Zinc and its alloys, etc.

Advantages of Welding - (i) A good weld is as strong as the base metal. (ii) General welding equipment is not very costly. (iii) Portable welding equipments are available. (iv) Welding permits considerable freedom in design. (v) A large number of metals/alloys both similar and dissimilar can be joined by welding. (vi) Welding can join workpieces through spots, as continuous pressure tight seams, end to end and in a number of other configurations. (viii) Welding can be mechanized.

Disadvantages of Welding - (i) Welding gives out harmful radiations (light) fumes arid spatter. (ii) Welding results in residual stresses and distortion of the work pieces. (iii) Jigs and fixtures are generally required to hold and position the parts to be welded. (iv) Edge preparation of the workpieces is generally required before welding them. (v) A skilled welder is a must to produce a good welding job.(vi) Welding heat produces metallurgical changes. The structure of the welded joint is not same as that of the parent metal. (vii) A welded joint for many reasons, needs stress relief heat treatment.

Welding Compared to Rivetting and Casting - Bridges, ships and boilers which were previously riveted are now welded. Machine tool beds which were earlier cast are now fabricated using welding. In many fields welding has replaced riveting and casting processes. Some of the reasons, for the same, are as follows: 1. Welding is more economical and is a much faster process as compared to both casting and riveting.2. Fabricated mild steel structures are lighter as compared to (cast) cast iron ones.

3. Fabricated mild steel structures have more tensile strength and rigidity as compared to (cast) cast iron ones.

4. Welding can join dissimilar metals and thus in a complicated structure (depending upon strength or other criteria) different parts of the structure can be fabricated with different materials.5. For the same complexity of a component the design of a welded structure is simpler as compared to that of a cast part. Standard rolled sections help considerably in fabricating different structures by welding.6. Being noiseless as compared to riveting, welding finds extensive use, when making modifications, additions or extensions in hospital buildings.7. Cost of pattern making and storing is eliminated.8. As compared to casting and riveting fewer persons are involved in a welding fabrications.9. Welding fabrication involves inventory, much less as compared to casting and since no patterns are involved, the chances of obsolescence are negligible.10. Against riveted construction welding fabrication involves less cost of handling.

11. Structural shapes not easily obtainable with riveting or casting can be produced by welding without much difficulty.12. Welding design involves lower costs and it is very flexible also.13. Fabrication by welding saves machining costs involved in cast parts. 14. Welded pressure vessels are more (fluid and) pressure tight as compared to riveted ones. Moreover, for pressure tightness, the rivets must be calked.15. Ratio between weight of weld metal and the entire weight of structure is much lesser than the ratio between the weight of rivets and the entire weight of the structure. Welded structures are comparatively lighter than corresponding riveted ones.16. Cover plates, connecting angles, gusset plates, etc., needed in riveted construction are not required when welding the structures.17. Members of such shapes that present difficulty for riveting can be easily welded.18. Welding can be carried out at any point on a structure, but, riveting always requires enough clearance to be done.

19. A welded structure possesses a better finish and appearance than the corresponding riveted structure.20. Layout for punching or drilling of holes is not required in welding.21. Drilling holes in the plates in order to accommodate rivets, breaks material continuity and weakens a riveted structure.22. Cost of standard rolled sections is much less as compared to that of a casting with the result that welded structures involve less material costs.23. Making changes in an already cast or riveted structure is extremely difficult, if not impossible. On the other hand a welded structure can be modified or repaired without much difficulty.24. Welding can produce a 100% efficient joint which is difficult to make by riveting.

25. Riveting high strength steels presents the problem of acquiring high strength steel rivets.26. Old structures can easily be reinforced by welding.Whereas welding claims its supremacy, casting, however has got its own good points. For example,(i) a product is obtained as one piece, ,(ii) thermal effects as in welding are not there,(iii) very heavy and bulky parts like those of power plants and mill housings which are otherwise difficult to fabricate can be cast.

 Practical Applications of Welding - Welding has been employed in Industry as a tool for: (a) Regular fabrication of automobile cars, aircrafts, refrigerators, etc. (b) Repair and maintenance work, e.g., joining broken parts, rebuilding Worn out components, etc. A few important applications of welding are listed below:

1. Aircraft Construction: (a) Welded engine mounts.(b) Turbine frame for jet engine.(c) Rocket motor fuel and oxidizer thanks.(d) Ducts, fittings, cowling components, etc.

2. Automobile Construction: (a) Arc welded car wheels. (b) Steel rear axle housing.(c) Frame side rails.(d) Automobile frame, brackets, etc. 3. Bridges: (a) Pier construction.(b) Section lengths.(c) Shop and field assembly of lengths, etc. 4. Buildings: (a) Column base plates.(b) Trusses.(c) Erection of structure, etc.

5. Pressure Vessels and Tanks: (a) Clad and lined steel plates.(b) Shell construction.(c) Joining of nozzle to the shell, etc. 6. Storage Tanks: (a) Oil, gas and water storage tanks.7. Rail Road Equipment: Locomotive(a) Under frame.(b) Air receiver.(c) Engine.(d) Front and rear hoods, etc. 8. Pipings and Pipelines: (a) Rolled plate piping.(b) Open pipe joints.(c) Oil, gas and gasoline pipe lines, etc.

9. Ships: (a) Shell frames.(b) Deck beams and bulkhead stiffeners. (c) Girders to shells.(d) Bulkhead webs to plating, etc. 10. Trucks and trailers. 11. Machine tool frames, cutting tools and dies.12. Household and office furniture. 13. Earth moving machinery and cranes. In addition, arc welding finds following applications in repair and maintenance work: 14. Repair of broken and damaged components and machinery such as tools, punches, dies, gears, shears, press and machine tools frames. 15. Hard facing and rebuilding of worn out or undersized (costly) parts rejected during inspection.16. Fabrication of jigs, fixtures, clamps and other work holding devices.

Welding Quality - Quality is the degree of conformity to a certain predetermined standard. Standards result ultimately from the establishment of objectives. If the company has an objective of gaining a reputation for manufacturing a high quality product, standards will have to be high. In order to meet these high standards, there would have to be a very rigid quality control program. Increased quality generally results in higher prices. Quality is a relative term and is generally explained with reference to the end use of the product.

For example, a gear used in a sugarcane juice extracting machine though not of the same material and without possessing good finish, tolerance and accuracy as that of a gear used in the headstock of a sophisticated lathe may be considered of good quality if it works satisfactorily in the juice extracting machine. Thus a component is said to be of good quality if it works well in the equipment for which it is meant.

Compared to other common fabrication techniques, welding is a relatively new technology. Ever since its initial introduction there has been a keen interest in the resulting quality of welds produced. For this reason much of our present inspection technology exists because of the need to provide assurance that welds are suitable for their intended service.

Quality Assurance V/S Quality Control - In welding fabrication work, quality assurance is concerned with defining and planning all the operations necessary to manufacture a product which will perform satisfactorily in service but which has been completed in the most economical manner. It covers design, evaluation of the manufacturing procedures and definition of inspection criteria for each stage, including testing the final product. One can differentiate between quality assurance and quality control because the latter relates to the system established to monitor each stage of manufacture with a view to achieving the requirement specified by quality assurance and be of proper size. They may be painted/coated to prevent corrosion etc.

Factors to be Considered for Weld Quality - Weld quality relates directly to the integrity of a weld. It underlies all of the design fabrication and inspection steps necessary to ensure that a welded product will be capable of serving the intended function for the desired life.Factors to be considered for weld quality are related to :(i) Design,(ii) Fabrication,(iii) Inspection,(iv) Operation and maintenance,(v) Economy.

(i) DesignWeld quality includes weld design considerations, which means that each weldment should be :(a) adequately designed to meet the intended service for the required life,(b) fabricated with specified materials and in accordance with the design concepts,(c) handled and maintained properly. The design of a weldment should be consistent with sound engineering practices.Components of adequate size should be specified to ensure that stresses from anticipated service loads are not excessive. The intended service should be carefully analyzed to determine whether cyclic loading

might result in fatigue failure in highly stressed members. Environmental conditions leading to brittle fracture, creep, and corrosion of welds should be considered in the design. Brittle fracture is a possibility in low temperature service and creep is a consideration in high temperature service.

Corrosion and wear can reduce the section size and increase service stresses, as well as create sites for fatigue cracks to initiate and further reduce quality of the weldment.Materials selected for fabricating the weldment should have proper hardness, chemical composition and mechanical properties

Fabrication The fabrication procedures and practices selected should be such that they ensure that the weldment meets the design specifications.; The procedures should be rigorously followed and the weldment should be properly inspected to verify that the base metal and the weld joint are free from unacceptable defects.Majority of welded fabrication standards define quality requirements to insure reasonably safe operation in service.

Inspection Weld quality is verified by non-destructive examination (NDT). The acceptance standards for the welds are generally related to the method of non-destructive examination. All deviations are evaluated and the acceptance or rejection of a weld is usually based on well defined conditions. Repair of unacceptable or defective conditions is normally permitted so that the quality of the weld may be brought up to acceptance standards.However, they will not tell us if the weld is good enough for our purposes; equally important, they will not guarantee that welds will be made which are free of defects. In other words, we cannot control quality simply by specifying that the welds will be subjected to NDT on completion. Admittedly the knowledge that the welds will be tested may act as an incentive to do better work, but it is far more relevant to establish a system which encourages the production of defect free welds at all times.

Operation and maintenance Operating a welded product safely, requires periodic inspection. If failure at a facility would result in a public hazard or the destruction of property, then inspections should be more frequent and more rigorous. Power plants, chemical plants and refineries, dams, and bridges are examples of fixed facilities that may endanger facility employees, the public, and the surrounding neighborhoods. Automobiles, trucks, trains, and ships are mobile facilities that may also damage individuals and properties. Maintenance of these facilities is a requirement for continued safe operation. Some facilities can partially continue operations during inspection and maintenance, and others may require a complete shutdown. Often the loss of production is far more expensive than the direct cost of the repair. In such cases, premium materials and. costly fabrication practices can easily be justified, provided these precautions permit extended operating periods between inspections.

 Discontinuities in Welds, their Causes and Remedies - Discontinuities in welds break the continuity of the weld metal and with the parent metal. Discontinuities are classified as follows:(a) Related to welding process or procedure:(i) GeometricMisalignment ConcavityOverlapUndercutConvexity Burn throughIncomplete penetration Lack of fusion Surface irregularityImproper ReinforcementShrinkage

(ii) OthersSlag inclusionSpatterOxide FilmsArc Craters

(b) Metallurgical:(i) Cracks or fissuresHotStrainageColdLamellar Tearing

(iii) PorositySphericalWorm holeElongated

Heat affected zone (HAZ), microstructure alterationWeld metal and HAZ segregationBase plate laminations

(c) Design related(i) Changes in section and other stress concentrations(ii) Weld joint type

Characteristics of discontinuities to be considered:

(i) Their size,(ii) Acuity or sharpness.(iii) Orientation with respect to the principal working stress and residual stress.(iv) Location with respect to the weld, the exterior surfaces of the joint, and the critical sections of the structure.

Quality Conflicts - Ever since the introduction of welding as a fabrication technique, there has been a keen interest in the resulting quality of welds produced. For that reason, much of our present inspection technology exists because of the need to provide assurance that welds are suitable for their intended service and do not contain major discontinuities. However, as long as welds have been evaluated, there have been conflicts among those responsible for the design, production, and examination of those welds. Unfortunately, many of those differences still exist today.

The main reason for these conflicts has to do with the human element. As such, it is unlikely that they will ever be completely eliminated. While human factors represent the greatest contribution, there are also other factors that must be explored. The first step toward elimination of these differences is to identify their causes. This discussion will attempt to offer suggestions for reducing the occurrence of conflicts by providing an understanding of the reasons for quality disputes.

Reason for Quality Conflicts - The reasons for quality conflicts can be separated into four different categories:

1. Human factors.2. Poor understanding of quality requirements.3. Inexperience of inspection personnel.4. Lack of knowledge of inspection methods.

The discussion that follows describes how these various factors affect welded fabrication by contributing to the cause of many conflicts that occur in the quality arena.

Human factors(a) An initial reason why the human element affects weld quality assessment is that welding is not an exact fabrication process. This is not to say that welding cannot be used to produce accurate parts, because mechanized applications can provide a high degree of accuracy. However, compared to machining processes, welding is usually not considered to be as precise. While technological advances in welding result in more and more mechanized and robotic welding being done, most of the welding done today is still applied manually or semi automatically. Consequently, the quality of the resulting weld is heavily dependent upon the skill of the individual welder.(b) Another human factor contribution relates to the inspection process itself, because that operation involves judgment by the individual performing the examination. Most codes that provide acceptance criteria for visual weld examination present that information in text form.Consequently, requirements that may appear perfectly clear on paper may become quite vague when an individual attempts to apply those criteria to a physical part. There are two possible solutions to this dilemma. First, whenever possible, the standards describing visual weld quality requirements should be expressed graphically instead of just trying to specify the necessary details through the use of text. The most direct way to describe a condition that is to be evaluated visually is to show a picture or graphic illustration of that condition.When a text description is provided, the individual first has to mentally envision the condition and then compare that mental picture with the physical appearance of the weld. Interpretation errors could result if the wording is confusing. However, the use of pictures or illustrations would minimize interpretation errors.

Another possible remedy to this predicament is to utilize three dimensional "workmanship samples." These are actual welded samples, or plastic replicas of welded samples that depict actual weld conditions. There are currently several military welding standards that dictate the use of workmanship samples. A company is required, by these standards, to produce actual workmanship samples that illustrate minimum acceptable weld quality requirements. Then, if there is a question as to the acceptability of some production welds, the interested parties can compare the production welds with the workmanship sample. It then becomes easier to determine whether the production weld truly meets the applicable quality requirement.

Poor understanding of quality requirementsThe second influencing factor to be discussed here is described as poor understanding of quality requirements. While our discussion is aimed primarily at the individual performing the inspection operation, the concepts presented here could also apply to others who are involved as well. That is to say, everyone: the welder, designer, foreman, inspector, etc., must be aware of the quality requirements that exist. Not only should these individuals be aware of the requirements; they also must understand the requirements. Inexperience of inspection personnelThe next factor to be discussed relates to the experience and qualifications of weld inspection personnel. Welding inspection is not a simple task. Anyone who thinks that it is obviously has never tried to do it or has done it improperly.

 

Consequently, we desperately need individuals who are properly trained and experienced so that the best possible inspection can be performed. To present the effects of this aspect, the following should be considered (a) Improvement in the qualifications and competency of today's inspection personnel;(b) The need for more technically competent personnel to perform more sophisticated inspection methods; and(c) The cost associated with using inexperienced inspectors. An inexperienced inspector may reject acceptable welds or approve the unacceptable ones. In either case, the possible impact on the cost can be significant. An unacceptable weld if approved may fail in service and result in loss of human life. On the other hand, if acceptable weld is rejected, there will be an increased cost associated with the performance of an unnecessary repair.

Lack of knowledge of inspection methodsWhile it is important to use only those individuals who are properly qualified to perform inspection, it is also critical for other involved individuals to be aware of the advantages and limitations of the various inspection techniques. The real solution to the elimination of quality conflicts boils down to the use of common sense and the application of technical understanding. This burden must be borne by all involved parties, not just the quality control personnel. The statement, "quality cannot be inspected into the part," is extremely appropriate. Everyone is responsible for being 'aware of the quality requirements that exist, having an understanding of how those requirements are to be applied, and communicating with one another to accomplish the desired goal an acceptable product.

Stages of Weld Inspection and Testing - For fabricating a welded structure, inspection may be carried out in three stages: 1. Before starting welding.2. during fabrication by welding.3. After the welding is over.

1 Inspection before Welding (a) Check the drawings in respect of weld details, dimensional tolerances, process specifications, etc.(b) Examine the specifications given by the customer applicable to the class of work, quality required, end use of the product, etc.(c) Select a suitable welding process to obtain welds of the desired quality.(d) Only the tested and defect free materials and as per the approved specifications or drawing should be used for the structure to be welded.

(e) Consumables such as welding electrodes, flux, gases, etc., to be used shall be as per standards. Deteriorated or spoilt consumables should not be used.(f) Welding Procedure should be laid down. Preferably, it should conform to some standard, (e.g. ISI). The procedure should give preparations and tolerances to be achieved during production work.The procedure should ensure required quality of welds.(g) Welding equipment should be in satisfactory working condition and be able to produce right quality welds.(h) Welders and operators employed for the work shall be trained, tested and certified to the appropriate standards.(i) Weld Testing Equipment should be in good operating condition and be handled by the well trained staff.(j) Equipment for cutting, straightening, heat treatment, material handling, etc., should be in good working condition. Jigs, fixtures, manipulators etc., should be available depending upon the type of work. Adequate storage facilities should exist for storage of consumable and materials.

2. Inspection during Welding (a) The weld groove should be free from dirt, rust, oil, slag or any other foreign matter which may affect the quality of the weld.(b) Job edge preparation should be as per approved welding procedure.(c) The fitup, gap, orientation, welding position, method and sequence of assembly all should be as per approved welding procedure.(d) Tack welds should be of adequate size, length and pitch.(e) Fittings, clamps, fixtures, etc., should not interfere with welding.(f) Methods should be adopted to minimize distortion.(g) If during welding certain, consumables are found defective, they should be replaced by another brand of identical type.(h) Welding procedure may be modified if it is found during welding that welds of acceptable quality are not being produced.

(i) Slag should be thoroughly removed from each pass in multipass arc welds and spot welding electrodes should be dressed periodically.(j) Welds which would become inaccessible or more difficult to inspect at a later stage shall be inspected at this stage.(k) Inspection if carried out during welding will help identify potential sources of defects and thus eliminate them at the early stage.(l) Inspection during welding will help finding any deviation from approved procedures, consumables, etc. which if goes unnoticed may deteriorate weld quality.(m) Visual inspection during welding will reduce the chances of rejecting the weldment at the final stage.

Inspection after Welding

Testing and Inspection is carried out after the jobs have been welded, with a view to: 1. Assess the properties and quality of the welded joints.2. Asses the suitability of the weldment* for the intended purpose. All forms of testing and inspection of welds after fabrication can be grouped into two basic categories, namely:1. Destructive Testing.2. Non destructive Testing.

 Destructive Testing of Welds - Destructive tests are applied to samples representative of the welded joint under review, often made especially for test purposes. In a destructive test, the test piece or specimen is destroyed, in most cases by fracturing. After destructive testing the specimen remains no longer useful for further use.

Destructive tests as applied to welds are: (a), the Tensile Test,(b) The Bend Test,(c) The Impact Test,(d) The Nick-break Test, (e) The Hardness Test, and (f) The Etch Test.

Non destructive tests are applied to welded components to determine their suitability for the service conditions to which they will be subjected. These tests neither break nor alter the structure or appearance of the welded component. Non destructive tests have the ability to detect invisible sub surface defects. Non-destructive tests make components more reliable and safe. Although non-destructive tests do not

provide direct measurement of mechanical properties, yet they are extremely useful in revealing defects in components that could impair their performance when put in service.

Welded components if found satisfactory after being tested so, can be used for the purpose for which they were made. A welded component found defective after non-destructive testing can be rectified and subsequently used. Non-destructive tests as applied to welds are: Visual Inspection Stethoscopic TestX-Ray and y-Ray RadiographyMagnetic Particle Inspection

Tensile Test - It is one of the most widely used mechanical tests. A tensile test helps determining(a) Tensile properties such as tensile strength, yield point or yield strength and modulus of elasticity.(b) Ductility of a weld. Two standard measurements of ductility are the percent elongation and the percent reduction of area.

Preparation of Test Specimen 1. Transverse Tensile Test Using Radius Reduced SpecimenUsing Reduced Specimen

2. All weld Metal Tensile Test A transverse Tensile Test specimen is cut from a welded butt joint (at right angle to the weld direction and is used to determine its transverse tensile strength. Reduced transverse test specimens are not intended to give the tensile strength of the weld metal, but Radius reduced specimens do occasionally.

WidthGauge Length*

Minimum Parallel Length

Minimum Radius at Shoulder

Total Length

b Lo Lp r L

Plate thickness but not less than 25 mm

50 60 25 200100 110 25 300200 225 25 375

In an all weld metal tensile test, the specimen is prepared from all weld metal. This type of specimen is prepared by machining a groove in a plate of steel and then completely filling the groove with deposited weld metal. The surrounding steel is then machined away leaving a specimen of weld metal. The purpose of such a test is to test (i) Electrodes for their suitability for the job concerned,(ii) The quality of deposited metal in welded joint.

Cross-sectional Area,Ao,mm2

Diameter do

Gauge Length, Lo

Minimum Parallel Length, Lc

Minimum Radius, R

400 22.56 113 124 23.5200 15.96 80 88 15.0100 11.28 56 62 10.025 5.64 28 31 5.0

Test Procedure Tensile test is carried out by gripping the end E, E of the specimen in a tensile testing machine and applying and increasing pull on to the specimen till it fractures.

During, the test, the tensile load as well as the elongation of a previously marked gauge length in the specimen is measured with the help of load dial of the machine and extensometer respectively. These readings help plotting stress strain curve.

After fracture, the two pieces of the broken specimen are placed as if fixed together and the distance Lf between two gauge marks and the area Af at the place of fracture are noted.

 Bend Test - A Bend Test may be carried out on a tensile testing machine with the help of certain attachments as described later in this section.A bend test is an easy and inexpensive test to apply. The method is fast and shows most weld faults quite accurately. Bend tests may be used to find a number of weld properties such as (i) Ductility of the welded zone(ii) Weld penetration(iii) Fusion(iv) Crystalline structure (of the fractured surface)(v) Strength.

The bend test assists in determining the soundness of the weld metal, the weld junction and the heat affected zone. The test shows the quality of the welded joint. Any cracking of the metal will indicate false fusion or defective penetration. The stretching of the metal determines to some extent its ductility. Fractured surface shows the crystalline structure. Large crystals usually indicate wrong welding procedure or poor heat treatment after welding. A good weld has small crystals.To conclude, the bend test is an easy and useful method of comparing one welded joint with another of the same type and of revealing abnormalities and defects at or near the surface in tension.

Types of Bend Tests

Bend tests may be categorized as (a) Free Bend Test(b) Guided Bend Test

Bend tests may be further classified as(i) Transverse bend testRoot bend testFace bend test(ii) Longitudinal bend test(iii) Side bend test.

Free Bend Test - Free bend test determines the ductility of weld metal. Free bend test may be conducted on a tensile testing machine or a vise capable of exerting a sufficiently large compressive force. For bend test, the test pieces are cut from the plate so as to include the weld.

Dimensions of test Specimen (mm)

T 10 20 25 40 50W 15 30 38 60 75L,Min 200 275 300 375 450

B,Min 30 50 50 50 50

The top of the weld is ground or machined so that it becomes flush with the base metal surface. The specimen (after the gauge length has been scribed on the face of the weld) may be bent initially by using a device.After giving an initial bend on to the specimen, it is placed in another fixture. The bending is continued until a crack or open defect exceeding 1.5 mm in any direction appears on the convex surface of the specimen. If no crack appears, the specimen shall be bent double.

Guided Bend Test - A guided bend test shows surface imperfections near and in the weld bead. A guided bend test is performed on the specially designed jig. The specimen to be tested is first ground smooth so that all weld reinforcement is removed. The specimen is then placed across the die supports and bent by depressing the plunger until it forms the shape of a U.

The specimen is bent by the movement of a plunger or former. Suitable gauge marks if scribed at the outside surface of the specimen help estimating % elongation. In a Face bend test the specimen is placed with its face down . The former is depressed until the piece becomes D shaped in the die (a guided bend test). If upon examination, cracks greater than 3 mm appear in any direction, the weld is considered to have failed. Face bend tests are used to inspect the degree of fusion, the absence or presence of inclusions and the weld porosity, if any.In a Root bend test the specimen is placed in the jig with the root down or in just the reverse position of the face bend test. The results must show no cracks to be acceptable. Root bend tests are used primarily to determine the degree of weld penetration.

Transverse Bend Test - A transverse bend test is useful in qualifying welders because it quite often reveals the presence of defects that are not detected in tension test. However, in this test, non-uniform properties along the length of the specimen can cause non-uniform bending. An over matching weld metal strength may prevent the weld zone from conforming exactly to the bend die radius and may force the deformation out into the base metal, causing less than the desired elongation of the weld. With under matching weld strength, the specimen may tend to kink in the weld and there occurs more severe elongation in the weld. The specimen for transverse bend test is of the full thickness of the material at the welded joint and the upper and lower surfaces of the weld are dressed flush with the base metal surface.

Longitudinal Bend Test - The problems of weld mismatch (as described in transverse bend test) can be avoided by using longitudinal bend specimens in which the weld runs the full length of the bend specimen; the bend axis being perpendicular to the weld axis.In longitudinal bend test, all zones of the welded joint (i.e., weld, heataffected zone and the base metal) are strained equally and simultaneously. This test is generally used for evaluations of joints in dissimilar metals. Specimens for longitudinal bend test are prepared in the same manner as for transverse bend tests.

Side Bend Test - Side bend specimens strain the entire weld cross section and are thus especially useful for exposing defects near mid thickness that might not contribute to failure in face or root bend tests. A side bend test determines the soundness of the welded joint in crosssection. This test is used for relatively thick sections (over 19 mm), since in this test the entire weld thickness may be included in the test.The width of the test specimen shall be the full thickness of the material at the welded joint and the upper and lower

surfaces of the weld shall normally be dressed flush with the surface of the base metal. The specimen is placed over the supports and it is bent by the downward movement of the former. The specimen is examined after the test and type and location of flaws, present, if any, are noted.

Impact Test - Impact testing becomes essential in order to study the behavior of welded objects under dynamic loading. An impact test determines the behavior of welds when subjected to high rates of loading, usually in bending. An impact test gives an indication of the relative toughness of the material. Toughness is defined as the resistance of a metal to fracture after plastic deformation has begun. The purpose of impact testing is to determine the amount of impact a specimen will absorb before fracturing. In an impact test, a specimen machined or surface ground and notched is struck and broken by a single blow in a specially designed testing machine. The quantity measured is the energy absorbed in breaking the specimen by a single blow. The ideal impact test would be one in which all the energy of a blow is transmitted to the test specimen.

Types of Impact Test - The two basic types of Impact Tests are

(i) The Charpy (Beam) Test. (ii) The Izod (Cantilever) Test. The Charpy specimen is placed in the vise so that it is just a simple beam supported at the ends whereas Izod specimen is placed in the vise such that it is in the form of a cantilever. The dimensions of Charpy test specimen.

Test Procedure 1. The swinging pendulum weight is raised to standard height depending upon the type of specimen to be tested.2. With reference to the vise holding the specimen, the higher the pendulum, the more potential energy it has got. 3. As the pendulum is released, its potential energy is converted into kinetic energy until it strikes the specimen.4. The Charpy specimen is hit behind the V notch while the Izod specimen, placed with the V notch facing the pendulum, will be hit above the V notch.5. A portion of the energy possessed by the pendulum is used to rupture the specimen and the pendulum rises on the other side of the machine to a height lower than its initial height on the opposite side of the impact testing machine.6. The energy consumed in breaking the specimen is the weight of the pendulum times the difference in two heights of pendulum on either side of the machine.7. This energy in foot pounds or metre kg is the notched impact strength and can be read from the dial of the impact testing machine.

Reporting of Results: The following results shall be reported after the test:

1. Nature of specimen, i.e. Charpy or Izod.2. Testing Temperature.3. The energy absorbed.4. Appearance of fractured surface and defects, if any, present hereover.

Nick Break Test - A nick break test involves breaking the weld joint to examine the fractured surfaces for internal defects such as: (i) Gas pockets (ii) Slag inclusions(iii) Porosity.

The test also determines weld ductility and the degree of fusion.

Procedure The test specimen shall be cut transversely to the welded joint and shall have the full thickness of the plate at the joint. The excess weld metal and penetration bead shall be left intact. Slots are sawed at each end of the specimen to be tested.

The specimen is then placed upright on two supports and the force on the weld is applied either by a press or by the sharp blows of a hammer until a fracture occurs between the two slots. A visual inspection of the fractured surfaces is carried out in order to find defects (as mentioned earlier), if any. If any defect exceeds 1.5 mm in size or the number of gas pockets exceeds one per square cm, the piece has failed the test.

Hardness Test - The hardness test gives an idea of the resistance to wear of the weld metal. This is important with respect to the components which have been built up and have to withstand abrasive wear. Hardness values can give in formation about the metallurgical changes caused by welding.

In the case of premium and high carbon steels and cast iron, the heat affected zone or weld junction may become hard and brittle because of the formation of marten site. Hardness values in a welded joint are usually sensitive to such conditions of welding, as(i) The process used.(ii) Heat input.(iii) Preheat or interpass temperature.(iv) Electrode composition(v) Plate thickness.

Hardness values indicate whether the correct welding technique and pre and post heat treatments have been carried out. The hardness of welds is particularly important if the welds must be machined.

Methods of

Various commonly used test methods for finding hardness are: (a) The Brinell Test(b) The Rockwell Test.Vickers pyramid and Shore Scleroscope* methods may also be used to find hardness. The welded specimen in whose case hardness is to be tested is ground, polished or polished and etched to show clearly the weld metal area. Hardness is determined on specific areas of interest, including the weld center line, face or root regions of the weld deposit, the heat affected zone and the base metal.

Brinell Hardness Test - It consists of pressing a hardened steel ball into a test specimen. According to ASTM specifications, a 10 mm diameter ball is used for the purpose. Lower loads are applied for measuring hardness of soft materials and vice versa.

Procedure of Hardness Testing Specimen is placed on the anvil; the hand wheel is rotated so that the specimen along with the anvil moves up and contacts with the ball. The desired load is applied mechanically (by a gear driven screw) or hydraulically (by oil pressure) and the ball presses into the specimen.

The diameter of the indentation made in the specimen by the pressed ball is measured by the use of a micrometer microscope, having a transparent engraved scale in the field of view. The indentation diameter is measured at two places at right angles to each other, and the average of the two readings is taken. The Brinell hardness number (BHN) which is the pressure per unit surface area of

the indentation in kg per square metre, is calculated as follows:BHN = W / (π D / 2) (D – root of D 2 – d 2) W is load on indenter, kgD is diameter of steel ball, mmD is average measured diameter of indentation, mm

Rockwell Hardness Testing - Rockwell hardness testing differs from Brinell testing in that the indenters and the loads are smaller and therefore the resulting indentation on the specimen is smaller and shallower. Rockwell testing is suitable for materials having hardness beyond the scope of Brinell testing. Rockwell testing is faster as compared to Brinell testing because diameter of indentation need not be measured; the rockwell machine gives arbitrary direct reading. Unlike Brinell testing, rockwell testing needs no surface preparation (polishing, etc.) of the specimen whose hardness is to be measured.

Vickers Hardness Test In Vickers hardness test, a known load (P) (from 1 to 120 kg) is applied for a specified time to the surface of the material through a square base pyramid diamond having 136° between opposite faces. The two diagonals of the resulting square indentation on the test piece are measured with a micrometer microscope and averaged, (D, mm). The Vickers hardness number is calculated as follows VHN = 1.854 P / D2 Before conducting Vickers hardness test, the surface of the specimen should be flat and of sufficient polish so that any remaining scratches do not cause difficulty in locating the corners of the indentation when diagonals are measured. The impression of Vickers indenter on the specimen being very small, peak (and not average) values of hardness can be determined on the weld from root to face. In the same length of the specimen, more hardness readings can be taken with Vickers hardness test than with Brinell or Rockwell hardness tests.

Etch Test - An etch test involves inspecting the welded test specimen after polishing and etching the same with a chemical reagent e.g., a dilute acid.

Types of There are two types of etch tests, namely(i) Macro etch examination,(ii) Micro etch examination.

Concept and Purpose (i) Macro etch examination: After preparing the specimen by polishing and etching, it is examined either by the naked eye or by low power magnification up to X15. Macro examination gives a broad picture of the specimen by studying relatively large sectioned areas.Macro examination reveals in welded specimen (i) Cracks,(ii) Slag inclusion,(iii) Blowholes,(iv) Shrinkage porosity,(v) Penetration of the weld,(vi)The boundary between the weld metal and the base metal, etc

(ii) Micro etch examination: After preparing the specimen by polishing and etching, it is examined under a microscope at magnifications from X20 to X2000.

Micro etch examination involves areas much smaller than those considered in macro etch examination and brings out information that can never be revealed by macro examination.

Micro examination determines in a welded specimen (i) Cracks and inclusions of microscopic size.(ii) Grain boundaries and solidification structures of weld metal, heat affected zone and the base metal.(iii) Distribution of micro constituents in the weld metal.(iv) The quality of heat treatment, etc.

Preparation of Test Specimen (i) The specimen shall be the full thickness of the material at the welded joint and the weld reinforcement and penetration bead shall be left intact. The specimen shall contain a length of the joint of at least 10 mm and shall extend on each side of the weld for a distance that includes the heat affected zone and some base metal portion.Given below are a few etching reagents: 1. Hydrochloric Acid. The reagent contains equal parts by volume of concentrated HCI and water. Specimen is immersed in this reagent at or near the boiling point. This will usually enlarge gas pockets and dissolve slag inclusions, enlarging the resulting cavities.2. Nitric Acid. One part of concentrated nitric acid is added to three parts of water by volume. The reagent may be applied to the surface of the weld either with a glass stirring rod at room temperature or the weld be immersed in boiling reagent provided the room is well ventilated. This reagent is used on polished surfaces only to show the weld metal zone as well as the refined zone. Nital contains 2CC HNO3 Conc. + 98CC absolute methyl alcohol.

 

3. Ammonium persulphate. Mix one part of ammonium persulphate (solid) to nine parts of water by weight. The reagent thus prepared is rubbed vigorously on the surface of the weld with cotton saturated with this reagent.4. Iodine and potassium iodide. One part of powdered iodine (solid) is mixed with twelve parts of a solution of potassium iodide by weight. The latter solution should consist of one part of potassium iodide to five parts of water by weight. The reagent is brushed at room temperature on the surface of the weld.

Non Destructive Visual Inspection Testing of Welds -Visual inspection is the simplest, fastest, economical and most commonly used test for detecting defects on the surfaces of the welded objects. The weld surface and joint is examined visually, preferably with the help of a magnifying lens.

Careful examination of what can be seen on the surface of a welded joint can assist in determining the ultimate acceptability of the weldments. Visual examination can help detecting the following flaws on the surface of the welded structure:

(i) Porosity, blowholes, pipes, exposed inclusions, unfused welds, unfilled craters etc.(ii) Surface cracks in the weld metal, heat affected zone or the parent metal.(iii) Undercutting, burning or overheating of the base metal adjoining weld metal.(iv) Improper profile and dimensional inaccuracy of welds. There may exist excessive convexity or concavity, overlap, excessive reinforcement, unequal leg length, excessive penetration bead, shrinkage grooves, incompletely filled grooves etc.Door weld appearance, i.e., irregular ripple marks, weaving faults, chipping and peening marks, spatter, surface roughness etc.

Leak OR Tightness Test on Welds - Leak refers to an actual discontinuity or passage through which a fluid flows or permeates. Leak testing is the determination of the rate at which a liquid or gas will penetrate from inside a tight component or assembly to the outside as a result of pressure differential

between the two regions.

Purpose To test welded pressure vessels, tanks and pipelines to determine if leaks are present. Absolute tightness of all the welded joints can be tested this way.

Procedure The welded vessel, after closing all its outlets, is subjected to internal pressure using water, oil, air or gas (e.g. CO2). Hydraulic pressure, using water as the fluid, is the usual medium employed in this test. Oil if it is thin/hot will penetrate leaks that do not show up with water under an equal pressure. Air will leak out more readily than water and gas (e.g. Hydrogen) will escape where air will not. Where feasible, it is better to use water or oil because there will be very less tendency for the parts to be violently thrown out in case of a sudden release of pressure. When using air/gas, failure of vessel can cause injuries to persons around.

The internal pressure may be raised to two times the working pressure.When under pressure, the weld may be tested as follows for detecting the leak:

(i) Pressure on the gauge may be noted immediately after applying the internal pressure and after, say, 12 to 24 hours. Any drop in pressure reading indicates a leak.(ii) After generating air pressure in the vessel, soap solution may be painted on the weld seam and carefully inspected for bubbles which would indicate leak.(iii) The welded surface is coated with a lime solution. After the lime has dried, pressure is built up in the vessel. Where the lime flakes from the metal, a flaw is indicated as being present.(iv)In another method an aluminium foil is laid over a wider strip of water soluble paper and both are stuck with a tape over the welded seam of a water filled pressure vessel. If a leak exists, the water soluble strip will dissolve, indicating the leak location and the aluminium foil strip will be in electrical contact with the vessel. A corresponding change in resistance indicates the pressure of leak.

Stethoscope Sound Test - The principle of this test is that, defect free weld metal gives a good ringing note when struck with a hammer whereas weld metal containing defects (such a cracks, lack of fusion, slag inclusion, etc.) gives a flat note. An ordinary physician's stethoscope may be used to magnify and identify the sound. Structural welds and welds in pressure vessels have been successfully tested using this method.

Radiography Using X-Ray and Gamma γ Ray on Welds - Radiography is one of the most useful of the non-destructive tests which can be applied for assessing the quality of the welded joints. Radiograph has been used for the inspection of welds of all types and thicknesses ranging from minute welds in electronic components to welds upto half metre thick employed in heavy fabrications.

Radiography can detect flaws or discontinuities in welds such as:(i) Cracks.(ii) Porosity and blow holes.(iii) Slag, flux or oxide inclusions.(iv)Lack of fusion between the weld metal and the parent metal(v) Incomplete penetration.Principle Radiography technique is based upon exposing the components to short wavelength radiations in the form of X-rays (wavelength less than 0.001 x 10-8 cm to about 40 x 10-8 cm) or gamma (y) rays (wave length about 0.005 X 10-8 to 3 X 10-8 cm) from a suitable source such as an X-ray tube or cobalt-60.

The characteristic feature of X-ray and y-ray which makes them to work is their power to penetrate matters opaque to light. X-rays operating at 400,000 volts can inspect steel objects having thickness up to 62 mm.γ -rays given off by radium and radioactive isotopes such as cobalt-60, lridium-192 and caesium-167 can penetrate and thus inspect joints of bigger thickness than examined by X-rays.

X-Ray Radiography Procedure - X-rays are produced in an X-ray tube where a (cathode) filament provides electrons which proceed towards the target (anode); strike and are suddenly stopped; a part of their kinetic energy is converted to energy of radiation or X-rays. The portion of the weldment where defects are suspected is exposed to X-rays emitted from the X-ray tube. A cassette containing X-ray film is placed behind and in contact with the weldment, perpendicular to the rays. During exposure, X-rays penetrate the welded object and thus affect the X-ray mm.

Since most defects (such as blow holes, porosity, cracks, etc.) possess lesser density than the sound parent metal, they transmit X-rays better than the sound metal does; therefore the film appears to be more dark where defects are in line of the X-ray beam. The exposed and developed X-ray film showing light and dark areas is termed as RADIOGRAPH (or precisely known as an EXOGRAPH). The radiographs of sound metal and metals containing blow holes and porosity respectively.

Gamma γ-Ray Radiography Procedure - (a) Gamma-ray radiography differs from X-ray radiography in following respects: 1. Gamma-rays are used for detecting defects in welded plates thicker than those inspected by X-rays. 2. Owing to less scatter, gamma rays are more satisfactory than X-rays for examining objects of varying thickness, where as X-rays provide better results for welded plates of uniform thickness. 3. X-rays are better than gamma-rays for detecting small defects in weldment sections less than about 50 mm. 4. X-ray method is much more rapid than gamma-ray method, because unlike gamma-ray method, it requires seconds or minutes instead of hours.

5. Unlike X-ray method, gamma-ray technique can inspect a number of welded objects at one time. 6. Gamma-ray equipment being small possesses better portability and convenience of use for certain field inspections. (b) Gamma radiations, a product of radioactive decay, are extensively used in the testing of welded objects. Radium and its salts decompose at a constant rate, giving out gamma rays which are of much shorter wavelength and more penetrating (than ordinary X-rays). Radium and Radon were originally employed as gamma-ray sources but more convenient sources are available at present in the form of isotopes, for example cobalt 60. It is an isotope produced by neutron irradiation and can be used in place of radium; it is much cheaper as well.

The apparatus necessary for gamma-ray radiography is very simple. Most cobalt-60 sources are cylindrical, with dimensions of 3 by 3 to 6 mm and sealed in an appropriate container or capsule.Unlike X-rays, gamma-rays from its source are emitted in all directions; therefore a number of separate welded objects having cassette containing film, fastened to the back of each object, are disposed in a circle around the source placed in a central position. This way many welded objects can be radiographed simultaneously and overnight exposures may be taken without continuous supervision.

Advantages, Disadvantages and Applications of X-Ray Radiography - Advantages 1. A permanent record of defects in a welded object is obtained. 2. Reference standards for defects are available.

Disadvantages 1. The equipment is costly.2. Trained operator is required. 3. The method involves radiation hazards.

Applications 1. Pressure vessels and boilers. 2. Penstocks. 3. Aircraft and ship structures.

Advantages, Disadvantages and Applications of Gamma γ-Ray Radiography - Advantages 1. A permanent record of defects, in a welded object is obtained. 2. Reference standards for defects are available. 3. Low initial cost. 4. This is a very good method for testing at the site.

Disadvantages1. Trained operator is required. 2. The method involves radiation hazards. 3. γ-ray source loses strength continuously. 4. γ-ray radiography possesses lower sensitivity and definition than X-ray radiography.

Applications 1. Pressure vessels and boilers. 2. Penstocks and pipe-work. 3. Ship building. 4. Structural steel work.

Magnetic Particle Inspection - Introduction For example, radiography ordinarily cannot detect small cracks, especially when they are too small to be seen with human eye.

This method of inspection is used on magnetic ferrous weldments for detecting invisible surface or slightly subsurface defects*. Deeper subsurface defects are not satisfactorily detected because the influence of the distorted lines of magnetic flux (owing to a discontinuity) on the magnetic particles spread over the job surface becomes weaker with the distance, so that sensitivity falls away rapidly with the depth. The defects commonly revealed by magnetic particle inspection are quenching cracks, thermal cracks, seams, laps, grinding cracks, overlaps, non-metallic inclusions,

fatigue cracks, hot tears, etc. Magnetic particle inspection is a relatively simple and easy technique. It is almost free from any restriction as to size, shape, composition and heat-treatment of a ferromagnetic specimen. This method of non-destructive testing tends to supplement rather than displace radiography.

Principle of the methodWhen a piece of metal is Placed in magnetic field and the lines of magnetic flux get intersected by a discontinuity such as a crack or slag inclusions in a job, magnetic poles are induced on either side of the discontinuity. The discontinuity causes an abrupt change in the path of magnetic flux flowing through the job normal to the discontinuity, resulting a local flux leakage field and interference with the magnetic lines of force. This local flux disturbance can be detected by its effect upon magnetic particles which are attracted to the region of discontinuity and pile up and bridge over the discontinuity.

A surface crack is indicated (under favourable conditions) by a line of fine particles following the crack outline and a subsurface defect by a fuzzy collection of the magnetic particles on the surface near the discontinuity. Maximum sensitivity of indication is obtained when the discontinuity lies in a direction normal to the applied magnetic field and when the strength of magnetic field is just enough to saturate the section being inspected.

Technique or Procedure: Procedural steps involved are, (a) Magnetising the component part,*(e.g., a welded plate).(b) Applying magnetic particles on the Component part. (c) Locating the defects.

(a) Magnetising the Welded Plate Different methods employed for magnetisation may be classed as follows: 1. Continuous method. 2. Residual method. 1. Circular magnetisation. 2. Longitudinal magnetisation. 1. A.C. magnetisation. 2. D.C. magnetisation. In continuous method, the current inducing the magnetic flux in the workpiece to be inspected is allowed to flow while the powder is applied. The job is placed between two contacts (in the form) of solid copper clamps. The induced magnetic field runs in the transverse direction; producing conditions favourable to the detection of longitudinally disposed cracks.

Residual method relies upon the residual magnetism in the welded job. The job may be magnetised by any method but the magnetising source is removed first and then the magnetic particles are applied over the job. Continuous method is much mare sensitive than the residual method and especially far steels having law magnetic retentivity, only continuous method is used. Circular magnetisation is produced by circular fields. A can duct or carrying an electric current is surrounded by a magnetic field which farms closed circles in plane at right angles to the direction .of current flaw Circular magnetisation may be produced: (i) By passing current through the part itself.(ii) By passing current through a conductor placed axially inside the hollow abject.(iii) with the help of prods or cant acts.

 

Prods are used to inspect small areas .of large weldments. By changing the position of prods systematically, the entire surface .of the large weldment can be surveyed. Longitudinal magnetisation is produced by passing current through a solenoid coil .or several turns of conductor surrounding the jab, the jab serving as the core of the solenoid. Cable wrappings are commonly used an large objects such as tanks, bailers, large crankshafts, etc. The cracks running in the transverse direction are best revealed in longitudinal fields. A.C. magnetization is preferred far maximum surface sensitivity and offers special operating advantages, including straight forward demagnetisation after testing, whereas D.C. appears to permit the detection of defects lying mare deeply in the objects.

(b) Applying Magnetic (particles or) Powder Magnetic particles may be applied during the passage of magnetising current or following the same when the residual magnetism is made use of in detecting cracks in welded objects; this method avoids the danger of oversaturatian. The magnetic powder of iron or black magnetic iron oxide base and having elongated individual particles is used far the purpose Metallic iron particles are coated to prevent oxidation and sticking. The powder is available in two colours red and black; a colour which shows up to best advantage an the part to be inspected is selected. Magnetic particles prepared with a fluorescent coating and inspected under ultraviolet light are also used. In this case the cracks are marked by glowing indication. Ordinary magnetic powder is applied the job in Dry or Wet farm. Dry powder is applied in form of a cloud or spray. The dry powder is better far locating near surface defects; moreover all the powder can be recovered after the test. Dry powder is not so messy to work with as is the powder wet in oil. In wet method, the powder is suspended in law viscosity non-corrosive fluid such as kerosene and is sprayed aver the job; alternatively jab may be immersed in the liquid far the purpose. Wet method is better far locating minute surface defects; moreover wetted powder can be applied bath an vertical and underside of horizontal surfaces far detecting cracks.

(c) Locating the Defects In the magnetized bar if a crack or avoid interrupts a magnetic field, the magnetic field gets distorted. The magnetic permeability of air being too law in comparison with iron, the magnetic flux spreads out to get around the avid. Same of the magnetic flux lines extend outside of the metal in the air over the discontinuity and the discontinuity is noticed or located distinctly because the magnetic particles collect and pile at any discontinuity or crack.

Liquid Dye Penetrant Test - A liquid penetrant test is non-destructive type. It detects flaws that are open to the surface e.g., cracks, seams, laps, lack of bond, porosity, cold shuts, etc. It can be effectively used not only in the inspection of ferrous metals but is especially useful for non-ferrous metal products and on non-porous, non-metallic materials such as ceramics, plastics and glass. The principle of liquid penetrant test is that the liquids used enter small openings such as cracks or porosities by capillary action. The rate and extent of this action are dependent upon such properties as surface tension, cohesion, adhesion and viscosity.

They are also influenced by factors such as the condition of the surface of material and the interior of the discontinuity. For the liquid to penetrate effectively, the surface of the material must be thoroughly cleaned of all foreign matter that would obstruct the entrance of the liquid into the defect. After cleaning, the liquid penetrant is applied evenly over the surface and allowed to remain long enough to permit penetration into possible discontinuities. The liquid is then completely removed from the surface of the component and either a wet or a dry developer is applied. The liquid that has penetrated the defects will then bleed out onto the surface, and the developer will help delineate them. This will show the location and general nature and magnitude of any defect present. To hasten this action,

the part may be struck sharply to produce vibrations to force the liquid out of the defect. The oil-whiting test is one of the older and cruder penetrant tests used for the detection of cracks too small to be noticed in a visual inspection. In this method, the piece is covered with penetrating oil, such as kerosene, then rubbed dry and coated with dry whiting.

In a short time the oil that has seeped into any cracks will be partially absorbed by the whiting, producing plainly visible discolored streaks delineating the cracks. The Dye penetrant test (DPT) based on liquid penetrant is a sensitive extremely versalite and a very reliable method of test. It is quite inexpensive, does not require any special apparatus and is quite simple in application. Only a moderate skill is required. In this test, the strongly coloured red penetrant fluid (or dye) has a property of seeping into surface flaws when applied on an impervious surface. The steps involved in dye penetrant test are (1) Clean the surface of the component free of dust and dirt with a piece of cloth.(2) Brush the surface of the component to remove scale, rust, paint etc., by a soft wire brush.

(3) Spray the cleaner to remove oil, grease, etc. (4) Apply the dye penetrant (by spraying) adequately to cover the area to be tested. Allow 3 to 5 minutes or more for dye to penetrate into the cracks. (5) Wipe off the excess penetrant on the surface with a rag. (6) Again spray the surface with the cleaner to remove the remnants of the red dye. (7) Spray the developer evenly on the surface to give a thin even layer. This layer absorbs the penetrant from the cracks and red spots or lines appear on t e stir ace to give a visible indication of the flaws. (8) The crack if any will be indicated with the red dye absorbed by the white absorbent.

Fluorescent Penetrant Inspection -(Zyglo Process)

Like magnetic particle inspection, fluorescent penetrant inspection is also carried out to detect small surface cracks, * but it has the advantage that it (i.e., penetrant inspection technique) can be used for testing both ferrous and nonferrous welded jobs. Zyglo is the registered trade mark of the Magnaflux Corporation applied to its equipment and material for fluorescent penetrant inspection.This method is sensitive to small surface discontinuities such as cracks, shrinkage and porosity open to the surface which tend to retain penetrant in spite of the rinse. Smooth or machined job surfaces provide more satisfactory conditions for the test.

Operational Steps (i) Clean the surfaces of the object to be inspected for cracks etc. (ii) Apply the fluorescent penetrant on the surface by either dipping, spraying or brushing. Allow a penetration time up to one hour. The fluorescent penetrant is drawn into crack by capillary action. (iii) Wash (the surface) with water spray to remove penetrant from surface but not from crack. (iv)Apply the developer. The developer acts like a blotter to draw penetrant out of crack and enlarges the size of the area of penetrant indication. (v) The surface is viewed under black light [having a wavelength of 3650 Angstrom (A) units, which is between the visible and ultraviolet in the spectrum. Black light causes penetrant to glow in dark.

Applications(i) Besides locating cracks and shrinkage in ferrous and especially non-ferrous castings, fluorescent penetrant inspection is used to determine cracks in the fabrication and regrinding of carbide tools, cracks and pits in welded structures, cracks in steam and gas turbine blading and cracks in ceramic insulators for spark plugs and electronic applications. (ii) Besides metals, penetrant inspection can also be carried out on parts made up of other materials such as plastics, ceramics, glass, etc.

Ultrasonic Inspection -Introduction (i) Ultrasonic inspection is employed to detect and locate internal defects such as cracks, porosity, inclusions, lack of fusion and incomplete penetration. Wall thickness can be measured in close vessels or in cases where such measurement cannot otherwise be made. (ii) Ultrasonic vibrations can be used to locate defects in ferrous and non-ferrous metallic objects as well as in plastics and ceramics. (iii) Ultrasonic inspection for flaw detection makes use of acoustic waves with frequencies in the range between 20 kHz and 20 MHz, which can be transmitted through solids (even liquid and air as well) and get reflected by the subsurface defects. Ultrasonic waves form a basis for detection, location and size estimation of defects.

Principle of Operation Ultrasonic waves are usually generated by the Piezoelectric effect which converts electrical energy to mechanical energy. A quartz crystal is used for the purpose. When a high frequency alternating electric current (of about 1 million cycles per second) is impressed across the faces of the quartz crystal, the crystal will expand during the first half of the cycle and contract when the electric field is reversed. In this manner the mechanical vibrations (sound waves) arc produced in the crystal. The surface of job to be inspected by ultrasonic is made fairly smooth either by machining or otherwise so that ultrasonic waves can be efficiently transmitted from the probe into the job and even small defects can be detected properly. Ultrasonic inspection employs separate probes (or search units), one for transmitting the waves and other to receive them after passage through the welded jobs.

Alternatively, since the ultrasonic waves are transmitted as a series of intermittent pulses, the same crystals may be employed both as the transmitter and receiver. Before transmitting ultrasonic waves, an oil film is provided between the probe and the job surface; this ensures proper contact between them and better transmission of waves from the probe into the surface of the object to be tested. For operation, ultrasonic wave is introduced into the metal and the time interval between transmission of the outgoing and reception of the incoming signals is measured with a cathode ray oscilloscope (CRO). The time base of CRO is so adjusted that the full width of the trace represents the section being examined. To start with, as the wave is sent from the transmitter probe, it strikes the upper surface of the job and makes a sharp (peak) or pips (echo) at the left hand side of the CRO screen.

If the job is sound, this wave will strike the bottom surface of the same, get reflected and indicated by a pip towards the right-hand end of CRO screen. In case a defect exists in between the top and bottom surfaces, most of the beam striking this defect will get reflected from the defect, reach the receiver probe and indicate a pip (echo) on the CRO screen before the pip given by the waves striking the far end of the job and returning. The distance of the defect from the surface where transmitter probe is applied, can be determined with the help of a time distance scale in the form of a square wave constantly shown on the oscilloscope. The distance scale may be changed as per convenience and one cycle of square wave may indicate 1 cm or 25 cms, etc. Quantitative assessment of defects can be made on the basis of the use of comparison standards containing real or artificial defects such as drilled holes in metal blocks. The interpretation of defect (echo) signal may be assisted by post-mortem sectioning of the job containing serious flaws; correlation with radiography is also useful.

Advantages, Disadvantages and Applications of Ultrasonic Inspection - Advantages 1. It is a fast and reliable method of non-destructive inspection. 2. This method of locating flaws with metal objects is more sensitive than radiography. 3. The minimum flaw size which can be detected is equal to about 0.1 % of the distance from the probe to

the defect. 4. Big weldments can be systematically scanned for initial detection of major defects. 5. Ultrasonic inspection involves low cost and high speed of operation. 6. The sensitivity of ultrasonic flaw detection is extremely high, being at a maximum when using waves of highest frequency.

Limitations 1. Surface to be tested must be ground smooth and clean.2. Skilled and trained operator is required. 3. It is not suited to the examination of weldments of complex shape or configurations.

Applications 1. Inspection of large weldments, castings and forging, for internal soundness, before carrying out expensive machining operations. 2. Inspection of moving strip or plate (for laminations) as regards its thickness. 3. Routine inspection of locomotive axles and wheel pins for fatigue cracks. 4. Inspection of rails for bolt-hole breaks without dismantling rail-end assemblies.

Eddy Current Testing - Principle of Operation

An AC. coil is brought up close to the weldment to be tested. The AC. coil induces eddy currents in the welded object. These eddy currents produce their own magnetic field which opposes the field of the AC. coil. The result is an increase in the impedance (resistance) of the AC. coil. Coil impedance can be measured. If there is a flaw in the weldment, as soon as the coil passes over the flow, there is a change in the coil impedance which can be wired to give a warning light or sound and thus the flaw and its location can be determined.

Flaws Indicated

Flaws at or close to the surface such as cracks, weld porosity, poor fusion or any linear discontinuity can be detected.

Procedure For generating eddy currents, the test piece is brought into the field of a coil carrying alternating current. The coil may encircle the part, may be in the form on probe, or in the case of tubular shapes, may be wound to fit inside a tube or pipe. The eddy current in the metal test piece also sets up a magnetic field, which opposes the original magnetic field. The impedance of the exciting coil or of a second coil coupled to the first and in close proximity to the test piece is affected by the presence of the induced eddy currents.

A second coil is often used is a convenience, and is called a sensing or pickup coil. In the case of a crack or an unwelded seam, the discontinuity must be oriented nearly normal to the eddy current flow to disturb it. The change in coil impedance caused by the presence of a discontinuity can be measured, and is used to give an indication of the extent of defects. Subsurface discontinuities may also be detected, but eddy currents decrease with depth.

Advantages, Disadvantages and Applications of Eddy Current Testing -

Advantages 1. The coil or probe does not require contact with the surface to be tested. 2. The method can be used to test both ferrous and non-ferrous weldments.

Disadvantages 1. The method is limited to materials with good electrical conductivity.2. The method is difficult to set and interpret.

Applications

EDDY CURRENT TESTING is primarily used for continuous inspection of seamless and welded piping and tubing during production. Testing of ferromagnetic steel, austenitic stainless steel, copper alloy, and nickel alloy tubular products are covered, by ASTM specifications.

Method Defects detected Advantages LimitationsVisual Inaccuracies in size and

shape. Surface cracks and porosity, under-cut, overlap, crater faults.

Easy to apply at any stage of fabrication and welding. Low cost both in capital and labour.

Does not provide a permanent record. Provides positive information only for surface defects.

Dye- penetrate

Surface cracks which may be missed by naked eye.

Easy to use. No equipment required. Low cost both in materials and labour.

Only surface cracks detected with certainty. No permanent record.

Method Defects detected Advantages LimitationsMagnetic-particle

Surface cracks Which may be missed by naked eye. May give indication of sub-surface flaws.

Relatively low cost. Portable. Gives clear indication.

Only surface cracks detected with certainty. Can be used only on ferromagnetic metals. Can give spurious indications. No permanent record.

Radiography Porosity, slag inclusions, cavities, and lack of penetration. Cracks and lack of fusion if correctly orientated with respect to beam.

Can be controlled to give reproducible results. Gives permanent record.

Expensive equipment. Strict safety precautions required. Better suited to butt joints -not very satisfactory with fillet-welded joints. Requires high level of skill in choosing conditions and interpreting results.

Ultrasonic All sub-surface defects. Laminations.

Very sensitive can detect defects too small to be

Permanent record is difficult to obtain. Requires high level of skill

discovered by other methods. Equipment is portable. Access required to only one side.

in interpreting cathode-ray-tube indications.