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MARINE BOILERS, Construction Under Proposed Rules FTER welding, completed A drums are prepared for X- ray examination, which is the most important nondestructive test and was adopted because it provides a ready means of locating defects. Such examination gives both trained and untrained observers an easily interpreted and permanent record of the condition of the weld. In the preparation of drums for X-ray ex- amination the first step is to se- cure lead markers to the drums alongside each seam. The exact lo- cation of each section explored by the X-ray is thus identified by the numbers that are printed on the film. The inside of the drum is also marked off in suitable lengths to facilitate the placing of each film in its proper location. Modern X-Ray Apparatus The X-ray apparatus consists of 300,000-volt equipment, the most powerful used at present for com- mercial purposes, and is capable of exploring seams in plate 4% inches thick on a production basis. The time of exposure depends, of course, upon the thickness of the plate or the weld explored, but some idea of the rate at which exposures may be made can be had from the fact that under usual conditions of focal dis- tance, etc., an X-ray photograph made through plate 2% inches thick, requires about 20 seconds. The in- spection of a drum with 4%-inch wall thickness, for a mercury boiler, made recently with this equipment, consumed about an hour for a length of two feet of weld, because of the exceptionally thick section of metal. This time included that required to change the films. The films used are generally 10 inches long, so that, in exploring a longitudinal seam of a drum 20 feet long, twenty-four films are used. In order to check the recording on the film of small defects, ‘‘thickness gages,’ consisting of thin steel strips, each with a %-inch hole in the cen- ter, are placed on the side of the weld next to the X-ray tube. These strips are of various gages and one *This article was prepared, at the re- quest of the editor, by the staff of The Babcock & Wilcox Co., and is published in two parts. The first part appeared in the. June issue. 10 PART II* Radiograph of sound weld in 1-inch plate. LMacess weld metal not removed is selected that has a thickness of 2 per cent of the total thickness of the plate to be explored. Thus, when the X-ray film is developed, the hole in the thickness gage is recorded as a defect, equal in depth to 2 per cent of the plate thickness. The clarity with which this artificial or simulated defect appears shows how clearly real defects of the same mags nitude will appear on the film. With proper technique in the ex- amination of plates less than 1 inch in thickness, defects as small in depth as 1 per cent of the thickness of the plate can be readily detected. Annapolis test boiler. Sectional express type marine watertube boiler MARINE REVIEW—July, 1934 For routine examination, however, a sensitivity of 2 per cent is gen- erally considered satisfactory and is the degree specified under the pro- posed rules for the construction of marine boilers. Defects of certain sizes have been established as being safe, first by recording such defect on an X-ray film and, later, by testing the welded joint containing the defect for ten- sile strength and other physical char- acteristics, The presence of such defects is easily and positively identified with the thickness gage just described. The location of the defect with re- spect to the outer and inner sur- faces of the seam, however, is not shown. To find the depth of such a defect, so as to facilitate its re- moval, a triangulation method is used in which the defective section is ex- posed to the X-ray from two differ- ent angles, using the same film. By noting the distance between the two images or two shadows of the defect on the film and comparing this dis- tance with that between the two shadows of a lead marker placed on the outside of the plate, the depth of the defect is easily calculated. Stress-Relieving Furnace When the welding has passed the X-ray examination, the drum is placed in a stress-relieving furnace and brought to a temperature suit- able for neutralizing any _ stresses that may have been set up, by the welding process, in the metal in and adjacent to the seams, Small drums may be thus treated in segmental furnaces, with the required number of furnace sections joined together to form a length of furnace suffi- cient to accommodate the drums to be heated, Large drums, or. several small drums, are heated in a large stress- relieving furnace erected in the shop yard. This furnace is of the car- bottom type, is 60 feet long by 14 feet 6 inches wide, and is 14 feet 6 inches to the spring of the arch. It is fired by 48 natural-gas burn- ers, and is equipped with automatic temperature-control equipment. The multiplicity and arrangement of the burners, together with the furnace lining of light-weight Babcock &