Engineering X-Ray Inspection

for this is the radiographic room of an industrial x-ray inspection Laboratory this cast steel gear blank has just been x-rayed the X-ray film is in this holder when the film has been developed in the dark room it will reveal the internal characteristics of the metal this is the radiograph the shadow picture the x-rays prove Pro that the steel casting is free from internal defects now it may be machined to finish Dimensions with full assurance that it is internally sound but this radiograph of the drive shaft flange tells a different story the dark areas indicate gas pockets and extensive shrinkage thus x-ray inspection provides a reliable production control by providing a check at the early stages of production it avoids a waste of Machining prevents serious failures in service for efficient operation of X-ray equipment the nature and control of x-rays must be understood within this housing the generation of x-rays takes place when electrons traveling at a high speed strike a solid body some of their energy is converted into x-rays the source of the electrons is a wire filament when it is electrically heated negatively charged electrons are released now if we connect this filament as a cathode or negative pole and introduce a positive pole an anode the negatively charged electrons will speed toward the anode a focusing cup around the filament concentrates the flow of electrons on a small area of the anode this is called the focal spot the smaller the focal spot the sharper the X-ray image the impact of the electrons on the anode generates Heat this is carried off by means of coolants and an anode of heat conducting copper the tungsten disc fused into the end of the anode is called the target tungsten is used because of its hardness and extremely high melting temperature the filament cathode and the anode are sealed in a highly evacuated glass envelope this becomes the X-ray tube the heating current that releases electrons from the filament is supplied by a Step Down Transformer by regulating the filament current the quantity of released electrons can be controlled the higher the current the greater the number of electrons A Step up Transformer supplies the high voltage required direct current is attained through a rect ifier or through a self-rectifying x-ray tube in order to generate x-rays the electrons must strike the target at a very high speed by increasing the voltage across the tube that is by increasing the difference in potential between the filament and anode the electrons will travel faster toward the target generating X-rays of Greater intensity this voltage may run into hundreds of thousands of volts and is therefore measured in kilov volts the tube current is very small however and is measured in milliamp when the energy imparted to the electrons is converted into x-rays it travels in waves that are similar to ordinary light waves but of much shorter wavelengths this wave motion may be suggested in this manner although in this symbolic repres presentation only one segment of the total radiation is shown the higher the voltage used the shorter the wavelength of the x-rays the shorter the wavelength the greater their penetrative power but actually in any x-ray beam there may be rays of various wavelengths the beam of course is invisible to the eye the X-ray tube and trans Transformers are usually enclosed in a grounded shockproof housing if we place an object on photographic film and turn on the X-ray beam a shadow picture of the object will be formed on the film if the object is of uniform thickness the photographic density on the film will also be uniform any defect in an object usually has a lower density than the surrounding material more rays will therefore pass through the defects than through the adjacent area the film then gets more exposure under the defects and dark spots appear on the film if the same exposure is given to a piece of two different thicknesses fewer x-rays will penetrate the thicker section a defect in the thin part will be revealed by the film but the defect in the thick part may not if a longer exposure is made the defect in the thick section will eventually be revealed but the film under the thin part May then be Overexposed greater penetration of the thick side of the object is made possible by increasing the kilov voltage this shortens the wavelengths permitting more Rays to penetrate the object object this results in a faster exposure but the film May again be Overexposed on the thin side therefore Whenever there is a marked difference in thickness two exposures should be made one for each thickness unless special techniques are employed when the difference in thickness is only slight however one radiograph will accurately reveal all flaws in making a setup for radiography the tube should be placed as far as practical from the object in film to make the defect appear as near to actual size as possible if the source of the x-rays is too close to the object the shadow picture of the defect will appear larger than it really is because of the spread of the beam when the tube is further away from the object the defect will appear more nearly actual size some of the xrays which pass through the object are scattered and tend to overexpose the film to get a sharp picture these random Rays must be controlled as much as possible placing the film on a lead covered table helps to minimize this back scatter x-rays will not penetrate a thick sheet of lead to confine the x-rays which scatter and reflect in the room the walls of the radiographic room are also lined with lead since repeated exposure to X radiation is injurious to living tissue the operator always remains out of the room while the picture is being made remember that there are four major factors which must be considered in making a radio graph one the tube current two the voltage across the tube this controls the speed of the electrons and hence the wavelength of the x-rays the higher the voltage the shorter the wavelength the shorter the wavelength the more intensity and the greater the penetration three the Focus film distance this is adjusted to obtain the most accurate image of the defects in the peps four the exposure time this is arrived at in terms of the density and thickness of the piece when a radiograph is made the operator's procedure is at all times Guided by an understanding of the nature of X-rays and their control it is important to keep an accurate record of each radioraps not only the density of the material but its thickness must also be considered the operator is helped in his work by available information on the application of x-rays to various Metals this is a typical technique chart it shows for example that a piece of Steel 1 in thick requires a kilov voltage of 110 an exposure time of 1,800 milliamp seconds and that the distance from the focal spot to the film should be 36 in the kilov voltage the exposure time and focus film distance required for this particular piece are determined from the chart and entered on the worksheet in the dark room lighted only by a safety light the operator now loads a film holder he selects a film size large enough to cover the area to be radiographed he is careful not to buckle a large x-ray film the first step in making the setup is a check of the focus film distance there is a mark on the tube house which locates the focal spot on the tube in this case a slight adjustment is needed to bring the focus film distance to the required 36 in if the focus film distance is correct a better image will result to make C certain that the X-ray beam is directed to the center of the proper area a telescopic pointer is often used each radiograph is identified by means of lead symbols these are placed on the film holder and the object the preparations for radiography of this gear blank are now completed as a safety Factor this x-ray unit is wired so that it cannot be operated while the door is open and the X-ray control unit is always separated from the tube head by a lead lined wall this is the switch to turn on the main circuit the next step is to set the required voltage in this case 110 Kilts the major selector will adjust the kilov voltage to the approximate value the minor selector is for more precise adjustments the next step is to set the proper exposure time as given by the worksheet here the operator arrives at the exposure time by dividing 1,800 milliamp seconds by the tube current 15 milliamp the result 120 seconds 2 minutes he presets the length of exposure by a timer it shuts off the current automatically the X-ray is now on a rat is used to apply the power gradually these few seconds prevent a sudden high voltage from damaging the tube most x-ray units are operated at a given milliamperage but adjustments are possible as required by means of a filament control when the exposure is completed the X-ray shuts off automatically but the main line should also be shut off after each exposure the X-ray film is now developed the technique is similar to ordinary dark room procedure the operator's hands the room and the equipment should be clean it is important to use fresh Solutions and accurate temperature control for uniform development the film should be agitated in the bath it is then rinsed fixed washed and dried now the radiograph reveals the hidden faults these dark areas show gas inclusions this G blank is unsuitable for Milling other radiographs obtained through the same process reveal other internal characteristics note the feathery appearance of the defects in this specimen they indicate shrinkage of the material at this point there is also a large gas paracity this is an aluminum casting showing extens ensive gas paracity shrinkage and misrun these welded sections show one a good weld two paracity three a crack in the Weld and four lack of fusion of the plate edges this aluminum blower wheel shows extens ensive very fine paracity throughout the entire Rim also some shrinkage skill in the interpretation of radiographs and in the use of X-ray equipment can only be acquired by study and practice but the technici who has developed this skill can obtain information important to production control he can also prevent needless waste of valuable materials and Manpower he can prevent serious failure that may cost lives for