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Aviation History
1919
1919 - 0221.PDF
FEBRUARY 13, 1919 #_I?_J^^5^ THE HEAT TREATMENT OF AERO ENGINE PARTS IN no engineering sphere has the heat treatment of metals been brought to a finer or more scientific degree than in the aeronautical industry. The enormous strain placed upon modern aircraft renders it imperative that every metal component should be of the highest possible tensile strength compatible with lightness. The times when the hardening or annealing of steel could be left to the experienced eye of an operator are past, and much as we admire the wonderful skill of those men who can judge temperature by the " dull red " or other shades the metal assumes during the process of heating-up, we are bound to confess that it is entirely unscientific and inadequate to meet the modern demand for either output or efficiency, in which the limit of error in heating-up should be not greater than 50 C. ^ Fig. 1.—Cross section of a small " Richmond ' * L.P.G.A. furnace The remarkable achievements of the modern gas-fired furnace will come as a revelation to those unfamiliar with the reliability of gaseous fuel for the heat treatment of steel and other metals used in aeroplane construction. When the demand for increased production became insistent, the numerous advantages of the gas furnace at once appealed. Speedily installed and connected up, a constant supply of fuel at a standard calorific value was always available. Compact, self-contained and mobile, the choice of position was practically unlimited allowing the furnace to be brought into close proximity to the machine worker. Accurate temperatures speedily attained, and readily maintained for any length of time, created the necessary conditions with which to successfully face the ever-increasing and exacting demand brought about by the War. Among the many furnaces that have acquired distinction during the past four years, the " L.P.G.A." (low pressure gas and air) furnace, manufactured by the Richm nd Gas Stove and Meter Co., Ltd., of Warrington, deserves special mention. This type of furnace is " over-fired," i.e., the gas supply at normal town's pressure is led through ports at one or both sides, where it comes into contact with the air supplied (at approximately 2-in. water gauge) obtained from a small fan, which air has been previously well pre-heated by being taken through the opposite side of the furnace in fire-clay tubes, and then passed along the bottom of the furnace in close proximity to the hot waste products. Combustion takes place inside the working chami er round the furnace walls, the usual separate combustion chamber being absent. The names produced keep well up and sweep round the arch. The products of combustion then pass to the opposite side 'f the furnace, are carried under the floor and up the other s,tic, away to the flue. AH the waste heat possible is uti.ised in pre-heating the air. The expense of a positive pressure blower is eliminated, a fan only being required to induce the necessary draught. Fig. 1 is a sectional view of this furnace, and it will be noticed that the " over-fired " principle of construction allows of a considerably thicker and stronger floor than in the case of an " under-fired " furnace, in which the heat has necessarily to pass through a comparatively thin floor tile. The thick floor permits of a very heavy charge being treated at one time, and Fig. 2 shows a " Richmond " L.P.G.A. furnace for heating 6 to 7 ton loads of bar steel. This par ticular furnace has during the past two years annealed J,200 tons of steel (valued at ^1,000,000) at a temperature of i,ooo° C. for the aeronautical and allied industries. It is claimed for this type of furnace that its special con struction effects an economy in gas consumption of 25 per cent. Fig. 3 shows twj of these furnaces, both of which are employed exclusively on aeroplane work. The processes employed in the heat treatment of aero engine parts, etc., naturally vary according to the b~and of steel provided. To procure the required strength and qualities, an exact heating formula must be either supplied by the steel makers or arrived at by a qualified metallurgist employed on the premises. In cases where large supplies of steel of numerous brands are constantly being used, the latter course is adopted by the large manufacturers of to day. To give some idea of the work " Richmond " furnaces are performing, and the importance with which heat treatment is now regarded, it will be of interset to examine the opera- Fig. 2.—Large " Richmond " low-pressure gas and air furnace for annealing 6-7 ton charges of high-speed steel bars tions involved in the preparation of a propeller cam-shaft, prior to being passed into the machine shop for finishing. The stamping, which is of special steel, is first rough- machined, and a hole is bored through the shaft to ensure the oil reaching all parts when quenching takes place. Normalising is the first process; the shaft is heated to 7800 C. and quenched in oil, again heated to 8300 C, and quenched in oil. The final heating is at 560° C. and the shaft is then quenched in water. The stamping is made of sufficient length to allow of test pieces being cut from either end, one of which is cut in half, one piece being examined for tensile strength, and the other subjected to an impact test; the remain ing piece is tested for fracture. AH requirements having been 221
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