From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article on hard-facing welding techniques and the use of body jointing compounds.
| Identifier | ExFiles\Box 132\5\ scan0080 | |
| Date | 1st March 1939 | |
| MARCH, 1939 THE AUTOMOBILE ENGINEER 81 Hard-facing—contd. flame is directed at an angle of from 30 to 60° to the steel surface, the tip of the inner cone should be about ⅛in. from the steel. This position must be maintained until the steel under the flame suddenly becomes glazed, indicating that an extremely thin surface layer has melted. The extent of the “sweating” area will vary according to the size of the welding tip, but for a medium size tip, the steel should “sweat” for a distance extending about ¼in. around the excess acetylene feather. The part is then ready for the application of the hard-facing alloy. Manipulating the blowpipe Next the flame is withdrawn just sufficiently to enable the end of the hardfacing alloy welding rod to be brought between the inner cone of the flame and the hot steel, with the tip of the inner cone just touching the rod and the rod lightly touching the “sweating” area. The end of the rod will then melt and form a puddle on the “sweating” steel surface. If the first few drops form bubble, or do not spread uniformly, the steel is too cold, and should again be brought to the full “sweating” temperature. By removing the rod from the flame and directing the flame into the puddle, the molten alloy is spread over the “sweating” area. The rod is then returned to the flame and more alloy melted off into the puddle, as required. Following this the flame is manipulated so that part of it plays on the edge of the puddle to keep it molten and part plays on the steel surface adjoining the puddle. When the steel reaches the “sweating” heat, the puddle of hard-facing alloy will spread over this area. As it spreads, the rod is brought quickly into the flame again, and with the end of the rod touching the puddle, more alloy is melted into the puddle. These steps are continued steadily until the desired area is coated. With a little practice, the correct amount of hard-facing alloy can be added to make the deposit of the required thickness, and it is better to do this in one operation than to go back over the whole job to add another layer of alloy. During the operation, the flame can be repeatedly moved back to melt a thin surface layer of the deposited alloy in order to smooth out high spots just behind the molten puddle. This should be done quickly without allowing the front edge of the puddle to solidify and without interrupting the steady forward progress of the work. If desired a second pass can be made with the flame to smooth off the surface and to minimise grinding. The puddle should be flowed in the desired direction by means of the pressure of the flame, not by stirring with the rod. Usually the work progresses towards the hand holding the rod, but on steel which scales badly, or on very thin section the rightward method is sometimes used. In this case, the work progresses towards the hand holding the blowpipe. With practice, the welding operator can determine the better method for each job. If any dirt or scale appears on the steel or in the puddle it is floated to the surface with the flame. If it will not float, it is dislodged with the rod. In extreme cases it may be necessary to use a good cast-iron welding flux. Avoiding cracks and blowholes When the deposit has been built up to the desired size and thickness, the flame is removed slowly from the puddle to prevent the formation of shrinkage cracks or blowholes. If these are present the deposit is re-melted in their immediate vicinity, any particles of scale or oxide from the pool being flicked away with the rod and a little more hard-facing alloy is added from the welding rod. If the holes still appear, it may be necessary to grind off the alloy deposit down to the steel at the defective point, the area being warmed gradually with the flame for depositing additional hard-facing alloy. As soon as the deposit has cooled below a red heat, it is often advisable to place the entire part in a box of powdered mica, lime, ashes or other similar heat-insulating material so that it will cool slowly. In many cases, slow cooling is absolutely essential to produce a deposit free from cracks and internal stresses. Parts which show a strong tendency to crack, such as large gate valve wedges and seat rings, pump shaft sleeves or parts on which the deposit is circular or large in area, should preferably be placed in a heattreating furnace while still hot from welding, brought slowly to a low red heat (about 620 deg. C.) and then allowed to cool over-night in the furnace with the door closed and the heat turned off. No part should be cooled by dipping it in water. After the hard-faced part has cooled, it may be necessary to remove the high spots or grind the hard-faced surface to an exact size. This may be done by hand, or on a grinding machine. Any soft grade vitrified wheel, not coarser than 46 or finer than 60 in Grade I or J, is suitable for grinding Stellite. The wheel speed should not be less than 3,000 nor more than 4,500 surface ft. per min. Higher speeds are apt to check the work. The best blowpipe tip size to use for average work is one having a consumption of approximately 20 cu. ft. of oxygen per hour and 23 cu. ft. of acetylene per hour. For small parts, a tip one size smaller should be used, and for large, heavy parts, a size larger. The welding operator will be able to judge the proper tip size for each job very easily after he has hardfaced a few parts. It is better, however, to use a tip which allows slow, careful deposition without overheating the steel than to use a large tip in an attempt to speed up the work. Welding rods In some cases, especially when applying the cobalt-chromium-tungsten hard-facing alloys to small areas or parts, it is best to use a welding rod of ⅛in. or ³⁄₁₆in. diameter. If the smaller diameter rods are not available, ⁵⁄₃₂ or ³⁄₁₆in. rod can be drawn out to the smaller size. To do this, one end of the rod is melted with the usual excess acetylene flame, and the first drop laid on a cold plate of carbon, graphite, iron or steel. With the flame playing on the end of the rod and melting it, and the end of the rod touching the plate, the rod is drawn along the plate at a steady rate, leaving a newly formed rod of smaller diameter ready for use. Hard-facing materials may be applied by either the oxy-acetylene or the electric arc method. For the non-ferrous alloys, and many of the ferrous alloys, however, the oxy-acetylene method is usually to allow better control over the operation, and produce a smoother deposit, while edges and corners can be formed more accurately. Inter-alloying of the hardfacing materials, which are invariably closely controlled as to composition, with the base metal, is negligible. Practically all types of metals can be hard-faced with non-ferrous welding rod, provided these three important points are borne in mind: (1) the surface to be hardfaced should be thoroughly cleaned; (2) an excess acetylene flame should be used, and (3) the surface should be brought only to a sweating heat and not melted. BODY JOINTING COMPOUND TO the builders of composite bodies the possibility of squeaks developing in the timber framework is a constant source of worry. To-day the degree of silence in the chassis emphasises the slightest creak which may arise from any one of the large number of timber joints. Various compounds have been developed for the treatment of timber framing joints, and one which several automobile manufacturers have found to be satisfactory is Synthaprufe, which is marketed by Stephenson Clarke and Associated Cos., Ltd., of Cardiff. In addition to its use in connection with framing joints, Synthaprufe is also extensively employed at points where metal is in close contact with timber, and it is customary to treat the edges of the door pillars before panelling, as well as the surfaces upon which body plates rest. The material creates a strong joint which is also waterproof and it is, to some extent, elastic, permitting relative movement between adjacent members. Synthaprufe is unaffected by high temperature, and will, therefore, remain effective after the body shell has passed through the various painting operations. After application the compound sets into a rubberlike film, possessing sufficient elasticity to allow for normal expansion without cracking. To ensure that Synthaprufe smears should be entirely removed from exterior surfaces of the panels, it is customary to wash down the body before painting with a cleansing solution which incorporates Xylene or Xylol, either in effect neutralising the material. The compound is non-inflammable, both in its liquid and dry state, and at normal temperatures in the region of 65 deg. F.{Mr Friese} is resistant to all common dilute inorganic acids, whilst it is also resistant to weak solutions of alkalis. For its application, it is recommended that two soft bristle brushes be employed and used alternately. When it is found necessary to suspend the application of Synthaprufe for any length of time, care should be taken to see that the brushes are immersed in cold water. Creosote oil may be used as a solvent, and should be used for cleaning brushes. (493) | ||