Patent GB-619039-A (GB)
Title: Improvements in or relating to furnaces and the production of controlled atmospherestherein
IPC: C21D1/76
Filed: 1944-03-21
Abstract: 619,039. Metal-heating furnaces. STEVENS, A. H. (Metallurgical Processes Co.). March 21, 1944, No. 5262. [Class 51 (ii)] [Also in Group II] In furnace plant for the heat-treatment of metals in a lithiated atmosphere, the latter is produced externally of the furnace chamber by passing a gaseous medium, e.g. a mixure of propane and air which has been treated in a cracking chamber, after the removal of a substantial part of the contained moisture, into contact with a lithium compound at a high temperature. The lithium compound preferably consists of a mixture of 60 per cent lithium carbonate and 40 per cent lithium chloride which have been fused together at 1800‹ F; during the passage of the cracked gaseous mixture the fused mass is maintained at a temperature of about 1400‹ F. As shown in Fig. 1, the furnace consists of a muffle 10 extending through a heating chamber 12 fired by burners 28, the front end of the muffle being fitted with a door 16 operated by a pedal 17 and the rear end communicating with a conduit 106 through which the lithiated atmosphere is supplied. A mixture of propane and air, in proportions depending upon the type of heat-treatment to be carried out, is passed through a pipe 43 to a cracking apparatus 42 arranged in a chamber 39 heated by burners 40 and, after cracking, is led to a water-jacketted condenser 87 which communicates by a pipe 91 with the lithiating device 38. The latter is heated by burners 28<1> and includes a lithium compound containing pot 108 above which is arranged a vertical baffle 104 having an aperture 105 of such size that a sufficient quantity of the gaseous stream is deflected downwardly into the pot to entrain the required amount of lithium vapour. Means are provided to supply to the cracking apparatus 42 mixtures of propane and air in varying proportions, the apparatus shown being designed to give the following alternatives:- (1) 18 parts of air to 1 of propane for a neutral atmosphere, (2) 6.5 parts of air to 1 of propane for carburizing to produce a case of from 0.9 to 1 per cent of carbon, and (3) 8 parts of air to 1 of propane to give a case of from 0.7 to 0.8 per cent of carbon. Mixtures in these proportions are ensured by the provision of electrically controlled valves 74a, 74b, and 74c in branches 71, 72, 73 from the propane supply line 69 and similar air admission valves 76a, 76b, 76c, the opening of the appropriate pair of valves being effected by the setting of a manually or automatically operated switch 81. The articles to be heated are carried on plates 109 resting on the floor of the muffle 10, the innermost plate 110 being attached to a rod 111 extending through the door 16 and serving to draw the train of plates outwardly towards the shoot 25 which delivers into an oil quenching-bath 26. The upper end of the shoot 25 is fitted with a pivoted plate 115 which is moved to open and close the shoot by a pneumatic system having electrically-controlled valves 123, 124 operated by mercury switches 125, 126 on the pedal 17, the arrangement being such that when the door 16 is open the shoot 25 is closed and vice versa. In a modified consruction, particularly suitable for a long muffle, the latter is supported on rollers to permit expansion and the atmosphere supply conduit 106 communicates with a perforated tube housed in a recess in the roof of the muffle and extending the entire length thereof. As shown in Fig. 11 the quenching-bath 26 is fitted with oil-circulating propellers 151, 152 rotated at different speeds from a common motor 153 while, to prevent stagnation of the oil, deflecting plates 159, 162 are provided on the lower end of the shoot 25 and the wall of the charge-receiving basket 161 respectively. An automatic timing mechanism is described by which control of the furnace atmosphere is effected to give alternate neutral and carburizing periods, these periods being, if necessary, repeated a number of times to give the desired depth of case.