Radiographic testing of welds

Non destructive test ,radiography test,generation of x ray

1. Radiographic Testing of welds Mechanical Engineering Department Sant Longowal Institute of Engineering and Technology B y - P A W A N K U M A R P G W L F 2 1 5 5 6

2. Radiography Radiography is one of the most useful non-destructive tests which can be applied for assessing the quality of the welds. It can be employed from minute electronic welds to welds up to half meter thick in heavy fabrications. It can detect cracks, porosity, blow holes, slag, flux inclusions, lack of fusion and incomplete penetration.

3. Characteristics of X and Gamma Rays • They penetrate matter depending on the radiation energy, material density, and material thickness. • They travel in straight lines. • They travel at the velocity of light (186,262 miles per second, 344,472 km/sec). • They cause fluorescence in some materials. • They ionize matter and they expose film by ionization. • Their energy is inversely proportional to their wavelength. • They are invisible and undetectable by human senses. • They are not particulate. • They have no electrical charge. • They have no rest mass or weight. W a v e l i k e p r o p e r t i e s e x p e c t r e f r a c t i o n . A l s o n o t e f e c t e d b y e l e c t r i c o r m a g n e t i c f i e l d .

4. F i r s t t i m e t h e s e r a y s w h e r e p r o d u c e b y r o e n g t o n i n 1 9 8 5 a f t e r s i x y e a r a l s o a w a r d e d w i t h n o b e l p r i z e T h e u s e d a d i s c h a r g e t u b e i n h i s e x p e r i m ent i n w h i c h t h e u s e d h e a v y m e t a l a n o d e a n d o t h e r s i d e i s c a t h o d e . T h e b o m b a r d f a s t m o v i n g e l e c t r o n s o n a n o d e o f h e a v y m e t a l s t h e s e e l e c t r o n s a f t e r c o l l i d i n g w i t h a n o d e p r o d u c e s o m e u n k o w n r a y s c a l l e d x - r a y s G e n e r a l l y p o w e r require 5 t o 2 4 k v t o p e n e t r a t u u p t o 5 m m t h i c k s t e e l P R O D U C T I O N OF X - R A Y S

5. X-Rays Principle – Radiography by x-rays technique is based upon exposing the components to short wavelength X-rays (less than 0.001X 10 -8 to about 40X 10 -8 cm ) from a suitable source such as X- ray tube. These rays have high penetrating power, X-rays operating at 400,000 volts can inspect steel objects having thickness up to 62 mm.

6. X-Rays and Gamma Rays Radiations

7. X-Rays Generation X-rays are produced when electrons, traveling at high speed, collide with matter. In the tube of a conventional static x-ray machine, an incandescent filament supplies the electrons and forms the cathode (negative electrode). The tube target is made the anode (positive electrode). A high voltage potential is applied across the cathode and anode providing an accelerating force to the electrons produced by the filament. The sudden stopping of these fast moving electrons near the surface of the target anode results in the generation of x-rays. The higher the temperature of the filament, the greater is its emission of electrons and the larger the resulting tube current. Other conditions remaining the same, the x-ray output is proportional to the tube current. Most of the energy applied to the tube is transformed into heat at the focal spot on the target anode, with only a small portion being transformed into x-rays .

8. X-Ray Tube K  - cathode (electron source) A  - anode (target electrons) C  - cooling Water U H  - heating voltage U A  - accelerating voltage X  - X-radiation (X-rays )

9. A N I M A T I O N

10. Methodology The method is based on the same principle as medical radiography in a hospital. A piece of radiographic film is placed on the remote side of the material under inspection and radiation is then transmitted through from one side of the material to the remote side where the radiographic film is placed. The radiographic film detects the radiation and measures the various quantities of radiation received over the entire surface of the film. This film is then processed under dark room conditions and the various degrees of radiation received by the film are imaged by the display of different degrees of black and white, this is termed the film density and is viewed on a special light emitting device.

11. Methodology Discontinuities in the material affect the amount of radiation being received by the film. Qualified inspectors can interpret the resultant images and record the location and type of defect present in the material. Radiography can be used on most materials and product forms, e.g. welds, castings, composites etc. Radiographic testing provides a permanent record in the form of a radiograph and provides a highly sensitive image of the internal structure of the material.

12. Methodology The amount of energy absorbed by the object depends on its thickness and density. Energy not absorbed by the object causes exposure of the radiographic film. These areas will be dark when the film is developed. Areas of the film exposed to less energy remain lighter. Therefore, areas of the object where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film. Inclusions of low density, such as slag, will appear as dark areas on the film, while inclusions of high density, such as tungsten, will appear as light areas.

13. Methodology All discontinuities are detected by viewing the variations in the density of the processed film. This permanent film record of weld quality is relatively easy to interpret if personnel are properly trained. Only qualified personnel should conduct radiography and radiographic interpretation because false readings can be expensive for productivity, and also because invisible X-ray and gamma radiation can be hazardous.

14. Gamma Rays Gamma rays are emitted from the disintegrating nuclei of radioactive substances of which the quality and intensity of the radiation cannot be controlled by the user. Radium and its salts decomposes at a constant rate, giving out gamma rays which are of much shorter wavelength and have more penetrating power than ordinary x-rays. Cobalt 60 an isotope is more convenient and economical as compare to Radium is also used. Most cobalt 60 sources are cylindrical, with dimensions of 3X3 to 6 mm and sealed in an appropriate container or capsule.

15. Gamma Rays Gamma rays are emitted in all directions therefore; a number of separate welded objects with film fastened at the back of each object can be inspected simultaneously. Overnight exposures may be given without continuous supervision.

16. Advantages and Disadvantages Advantages of gamma rays compared with X-rays No electrical or water supplies needed Equipment smaller and lighter More portable Equipment simpler and more robust More easily accessed Less scatter Equipment initially less costly Greater penetrating power Disadvantages of gamma rays compared with X-rays Poorer quality radiographs Exposure times can be longer Sources need replacing Radiation cannot be switched off Poorer geometric unsharpness Remote handling necessary

17. T H A NKS