Patent GB-777881-A (GB)
Title: A furnace for heat treating metal parts
IPC: C21D1/76
Filed: 1955-03-30
Abstract: 777,881. Cementation. IPSEN, H. N. March 30, 1955 [Oct. 18, 1954], No. 9283/55. Class 72 [Also in Groups XI, XII and XXXV] In a furnace for heat treating metal parts in a non-oxidizing gaseous atmosphere, an apparatus is provided for measuring the dew point of the gaseous atmosphere and, through a control circuit, controls the supply of dew point control gas to the gaseous atmosphere of the furnace. In the embodiment schematically illustrated in Fig. 1, the dew point measuring apparatus is incorporated in a control system of a furnace used for the heat treatment of steel. A gas sample is withdrawn from the non-oxidizing gaseous atmosphere in the furnace, a dew point control gas is supplied to the atmosphere and the supply of this control gas through a valve is regulated by control means responsive to variations of the dew point of the sample, whereby the carbon potential of the gaseous atmosphere is maintained substantially constant. Steel work pieces 10 to be case-hardened are mounted in chamber 11 of a furnace heated by suitable gas or electric heaters 12 disposed adjacent a baffle structure 13 filled from a gas generator 14 with a non- oxidizing gas of a suitable composition throough pipe 14a for producing casehardening of the work piece; some of the gas being permitted to escape continuously from the chamber in which the gas is maintained in motion by a fan 15. The temperature of the furnace is regulated by suitable control means 21 which supplies energy to the heaters 12 in response to the closure or opening of switch 22. Changes of temperature are sensed by thermo-couple 20 and produce a voltage signal on the input of a normally balanced bridge 23 including a balancing potentiometer 24 actuated from a shaft 25a driven from a reversible electric motor 25 the selection of different temperatures to be maintained in the furnace being made by adjusting a control potentiometer 26. The dew point of the chamber atmosphere may be reduced by increasing the amount of hydrocarbon gas in the chamber supplied from natural gas source 31. through pipes 30, the flow being increased or decreased by the opening and closing of solenoid operated valve 32 in response to the closure of switch 34 which may be actuated by cam 35 on the output shaft of an electronic recording controller 36 which includes a reversible motor 37 energized from an amplifier 38 and drives the cam 35 as well as slider 39 of the balancing potentiometer 40. A voltage signal corresponding in magnitude to the prevailing dew point of the furnace atmosphere is derived from a mechanism 64 which continuously samples the atmosphere, alternately cools and heats the sample and measures the temperature at which condensation occurs. If, as by manual adjustment of the potentiometer 26, the furnace temperature is changed, the compensating potentiometer 84 will, as the result of the ensuing rebalancing of the network 23, correspondingly upset the balance of the network 36 which, as before, will be re-balanced at a different dew point temperature as required for the maintenance of the selected carbon potential at the new furnace temperature. Provision is also made for rendering the automatic control apparatus inoperative for a predetermined period when the furnace door is opened for recharging purposes, a timer 90 being emto operate switch 91 controlling: the supply of electrical energy to theapparatus.