Patent Number: 051867648
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to FIGS. 1 to 3 these show a furnace suitable for nitriding the surfaces of thin apertured steel plates. The furnace comprises a central chamber 20 mounted within a furnace structure indicated generally at 21. The furnace has an outer housing 22 of refractory and heat-insulating material and a lining 23. Between the housing 21 and the lining 23 are heating elements, two of which are shown at 24. A pipe 25 is connected to the lining 23 and passes through the housing 22 and allows the introduction of gaseous treating medium into the interior of the lining. There is a space 26 between the lining 23 and the exterior of the chamber 20. The furnace structure carries a door 27 which fits into the left-hand end of the lining 23 to close the furnace. The door may be opened by moving it to the left in FIG. 1 and then lifting it by means not shown. A lower wall 28 of the furnace structure is pivoted at 29 so that it may move to the dotted line position shown at 28a which then places a duct 30 in communication with the interior of the lining 23. The duct 30 is connected to an evacuation fan 31 which enables gas to be drawn out of the furnace. The chamber 20 is defined by bottom and top walls 32 and 33, side walls 34 and end walls 35 and 36. The end wall 35 has a central aperture 37 in which is mounted a circulating fan 38 which is arranged to draw gas from the chamber 20 and discharge it into the space 26 between the chamber 20 and the lining 23. The fan is driven by an electric motor 39. The end wall 36 of the chamber has an aperture 40 which is partially closed by the furnace door 27 which thus forms part of the left-hand end wall of the chamber. The clearance 41 between the aperture and the door 27 communicates with the space 26 and the interior of the chamber. Each of the walls 32 to 34 is provided with three baffles. Thus the bottom wall 32 is provided with baffles 42, 43 and 44. The baffles are inclined to the right in the drawings towards the wall 35 and it will be seen that the baffles extend further from the wall 32 the nearer they are to the wall 35. Thus the baffle 44 extends further from the wall 32 than does the baffle 43 and the latter extends further from the wall 32 than does the baffle 42. The top wall 33 has three baffles 45, 46 and 47 and each of the side walls 34 has three baffles 48, 49 and 50. As will be seen from the drawings all the baffles are inclined to the right towards the wall 35 and they are all arranged as described with reference to the baffles 42 to 44 i.e. the baffles nearer the wall 35 extend further from the wall to which they are attached than do the baffles further from the wall 35. The bottom wall 32 of the chamber supports a roller conveyor 51 on which is received a fixture 52, described below, on which plates are mounted for treatment in the chamber. The fixture can be moved into and out of the chamber 20 when the door 27 is open. Referring now to FIGS. 4 to 11 the fixture 52 comprises a base 53, FIGS. 5 and 6, comprising opposed sides 54 in the form of square tubes which are held in spaced apart relation by six cross-pieces 55. Each side 54 of the base supports ten vertical columns 56, each column on one side of the base being aligned with a column on the other side of the base. Referring to FIG. 7 the base of each column 56 comprises a cylindrical tube 57 welded in aligned apertures in the upper and lower walls 58 and 59 of a side 54 to project upwardly from the side. A cylindrical re-enforcing bar 60 is received in the lower part of the tube while the upper part thereof forms a socket 61. Each column 56 is built up on a tube 57 with a number of components such as 62 and spacers shown in FIGS. 8 and 9 respectively. Each component 62 comprises a cylindrical tube 63 of the same diameter as the tube 57 and a cylindrical spigot 64 dimensioned to fit into the socket 61 in the tube 57 and a similar socket 65 in the tube 63 of another component 62. The tube 63 and spigot 64 are welded together at 66. Thus a column 56 is built on a tube 57 by inserting the spigot 64 of a component 62 into the socket 61 and then inserting the spigot 64 of another component 62 into the socket 65 of the component already in place on the column and so on. FIG. 10 shows a support bar 67 which fits between two aligned columns of the fixture, one on each side 54 of the base 53. The support bar has a central section 68 of Vee-section with opposed sides 69. Formed in each of the opposed sides 69 is a series of Vee-shaped notches 70, the notches in one side 69 being aligned with the notches in the other side 69. At each end the support bar has an apertured end fitting 71 of angle section with its flanges vertical and horizontal. The vertical flanges 72 are welded to the ends of the central section 68 and the horizontal flanges 73 are provided with apertures 74. The apertures 74 are of such size as to fit over the tubes 57 and 63 with a small clearance. As will be described below, the apertures 74 of the support bars are threaded over the tubes 57 and 63 and are held in vertical spaced relation by tubular spacers 75 shown in FIG. 9 which slide over the tubes 57 and 63. FIG. 11 shows a tie bar 76 for the fixture. The tie bar is of angle section having horizontal and vertical flanges 77 and 78. The flange 77 is apertured at 79 to receive the tubes 63 of the components 62 and so that it may be supported by spacers 75. The apertures 79 are spaced to receive the upper ends of the columns 56. The vertical flange is cut away at 80 to give clearance to the vertical flanges 72 of the support bars 67 as shown in FIG. 6. FIG. 4 is a section through the whole fixture assembled and carrying a multiplicity of apertured plates 81, the edges of the apertures being received in the notches 70 of the support bars 67. Each plate is received in one aligned pair of notches in a support bar and this holds the plates in fixed positions in spaced-apart relation. The fixture 52 is built up and filled with plates as follows. Starting with the base 53 with the attached tubes 57, one aligned pair of columns is built up at a time. Thus the spigots 64 of components 62 are inserted in the sockets 61 of the tubes 57 and short spacers 82, similar to the spacers 75 but shorter, are put on the tubes 57. A support bar 67 is then filled with plates 81 and the apertures 74 on the ends thereof threaded over the tubes 63 to rest on the tops of the spacers 82. Spacers 75 are then threaded over the tubes 63 of the lowermost components 62. Then another pair of components 62 has its spigots inserted in the sockets 65 of the lowermost pair of components 62. Then another support bar with its plates 81 is threaded over the tubes 63 of the uppermost components 62 and the sequence is continued until there are five support bars on each column as shown in FIGS. 4 and 6. The remaining columns on the base are built up in the same way. When all the columns have been built up a tie bar 76 is slipped over the upper ends of the columns on each side of the fixture to give a rigid assembly. The fixture 52 with its plates is then placed in the chamber 20 so that the support bars 67 extend parallel to the end walls 35 and 36 of the chamber as shown diagramatically in FIG. 2 so that the faces of the plates are parallel to the side walls 34 of the chamber and, as will be described, parallel to the general direction of gas flow through the chamber. Preferably the plates 81 are made of non-alloyed steel or fine grained structural steel containing niobium and vanadium or titanium and range from 0.4 to about 5 mm in thickness. When the fixture 52 with its plates has been inserted in the chamber 20, the door 27 is shut. The lower wall 28 of the furnace is pivoted down and air is evacuated from the interior of the furnace by the pump 31. An inert atmosphere e.g. nitrogen is introduced into the furnace through the pipe 25 and the lower wall 28 closed. The furnace is then heated by the heaters 24 to a temperature between 600 and 700 degrees C. The inert gas is then evacuated by the pump 30 and a gaseous medium capable of nitriding the surfaces of the plates is introduced as described in said above mentioned U.S. Pat. No. 4,793,871. During the heating of the furnace the inert atmosphere is circulated by the fan 38. Thus the inert gas is drawn by the fan 38 from the interior of the chamber and discharged to the space 26 from which it again enters the left-hand end of the chamber. The baffles 42 to 50 respectively direct the flowing gas inwardly towards the centre of the chamber and thus into the spaces between the rows of plates 81 on the support bars 67. This gives a substantially uniform flow of gas over all the plates and thus even heating of the plates. The gas flow is shown by the arrows in FIGS. 2 and 3. When the nitrogen-containing gas is introduced into the chamber to treat the plates, this also is circulated by the fan 38 and deflected by the baffles to give a substantially uniform flow of reactive gas over the surfaces of the plates and thus ensure a uniform coating of nitride on the plates. The provision of the baffles in the furnace ensures an even flow of gas over the surfaces of the plates both to heat and to treat them and the support of the plates in the fixture as described enables a large number of plates to be treated at one time and exposed to a uniform gas flow due to the baffles.