GB1011972A - Improvements in or relating to the heat treatment of elongate metal material - Google Patents

Abstract

<PICT:1011972/C6-C7/1> In a process and apparatus for patenting steel rod or wire, the latter is austenitized and is then quenched in a molten metal bath to a temperature between the lower critical temperature and the martensite start temperature the coolant being circulated through the bath in a direction opposite to that of the passage of the metal. Heating to austenitizing temperature is preferably by a fluidized bed furnace, the fluidizing bed, which may be inert, being recirculated. Prior to austenitizing the material may be preheated by passage through a liquid metal bath which is recirculated in a closed path including the quench bath, the liquid flowing through the preheat bath in a direction opposite to that of the material treated. After the first quenching the material may pass through a second quench bath of molten metal, which may be connected in closed circuit with a further preheating bath in advance of that mentioned above, the molten metal flowing in a direction opposite to that of the material treated in both the quenching and preheating baths. The molten metal in each circuit may be lead-bismuth alloy, possibly the eutectic, and there may be a temperature holding zone between the two quench baths whereby a fine pearlitic structure is developed in the steel. As shown in Fig. 1, material passes through first and second quench baths 11 and 12 and austenitizing furnace 13 before being quenched in baths 14 and 15. Molten metal is circulated in closed circuit in the arrowed direction between 14 and 12 and between 15 and 11 respectively by pumps 17 and 19. The apparatus may be enclosed by hood 20 for inert gas, the treated material entering and leaving by molten metal seals 21. In the embodiment of Fig. 4, material leaving furnace 31 passes through quench bath 33 within which the coolant is circulated in counterflow to the material by pump 35, pipe 36 and nozzles 37. A tube 39 carrying cooling liquid may pass through the entry end of the bath, while the latter may be provided with baffles 38 when arranged for multi-strand operation.