Apparatus for the zone-annealing of a workpiece consisting of a high-temperature material
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method and apparatus for zone-annealing of the generic type. 
     2. Description of the Prior Art 
     It is known that workpieces consisting of high-temperature materials, especially of dispersion-hardened high-temperature alloys, such as nickel-based superalloys, have to undergo coarse-grain annealing at the end of their production process. This heat treatment is necessary in order, in particular, to achieve good mechanical properties (high creep resistance) at high operating temperatures and to obtain grains which are elongated in the direction of tensile stress and which have increased strength and ductility. This can be accomplished by means of so-called zone-annealing if a temperature gradient in the appropriate direction is employed (see U.S. Pat. No. 221,979 of July 31, 1972, now abandoned, and No. DE-B-2,303,802). 
     The problem arising in all existing zone-annealing plants is that, particularly when the geometry of the workpiece is complicated, it is only rarely possible to force the temperature gradient to follow the desired longitudinal direction in an ideal manner. There is therefore the need to seek new methods and processes associated with them. 
     SUMMARY OF THE INVENTION 
     The object on which the invention is based is to provide an apparatus for the zone-annealing of workpieces consisting of heat-resistant materials, especially superalloys, and a specific process which can be carried out with the apparatus, which allow measures for increasing the longitudinal temperature gradient and lowering the lateral temperature gradient and which make it possible to vary the temperature gradient and the feed speed. It will also be possible to zone-anneal workpieces of complicated geometry in a simple way. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described with reference to the following exemplary embodiment explained in more detail by means of Figures. 
     FIG. 1 shows diagramatically a zone-annealing apparatus in elevation and in a longitudinal section, 
     FIG. 2 shows a further design of an apparatus with an especially designed cooling system, 
     FIG. 3a shows a covering consisting of ceramic material, 
     FIG. 3b shows an envelope consisting of heat-insulating material, and 
     FIG. 3c shows thermal insulating material. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates diagramatically an apparatus for the zone-annealing of workpieces consisting of heat-resistant materials partially in elevation and partially in longitudinal section. 1 denotes the workpiece which in the present case is stepped and has a shank part and a root part for example, a turbine blade consisting of a nickel superalloy with a thickened root and a slender shank. The workpiece 1 hangs via a hook on a suspension device 2 such as a chain or wire rope which can be actuated via a roller 3 driven by a drive motor 4 for the purpose of raising and lowering the workpiece 1. 5 denotes a salt bath which is maintained at the zone-annealing temperature and which is arranged in an insulating vessel 6 and is completely enclosed by the latter with the exception of the bath surface. 7 designates the heating system of the salt bath 5, which heating system is surrounded concentrically by a centrally symmetrical guide plate 8 and which serves for guiding a favorable flow of the fluid of the salt bath 5. This flow is indicated by arrows. Located on the surface of the salt bath is a floating insulating cover 9 which protects at least the round parts of the equipment from excessive thermal radiation and harmful effects. For the same purpose, there rests on the thicker root part of the workpiece 1 a plate-shaped covering screen 10 which is provided with an orifice and which moves along part of the vertical movement path of the workpiece 1. Both the workpiece 1 and the covering screen 10 are indicated in the Figure in the raised position (unbroken lines) and in the lowered position (broken lines). In the upper central part of the apparatus, there is a cooling body 11 through which the cooling air 14 flows axially. It possesses a passage 12 somewhat larger than the periphery (or diameter) of the covering screen 10, so that the latter can slide vertically in the cooling body 11 without friction. The cooling body 11 is provided on its lower end face with an inwardly projecting edge 13 which serves as a stop for the covering screen 10 during the downward movement of the workpiece 1. The orifice (not shown) in the covering screen 10 has such dimensions that the latter possesses sufficient play relative to the profile of the shank part of the workpiece 1, so that when the latter dips into the salt bath 5 it can slide through the orifice unimpeded. 
     FIG. 2 shows in elevation and in section a further embodiment of the apparatus with a specially designed cooling system. Reference numerals 1, 2, 3, 4, 10 and 11 refer to elements correspond exactly to those of FIG. 1. The cooling body 11 is provided at its bottom, and on its inner periphery, with a lateral orifice 15 which is intended for the outflow of the cooling air 14 and which makes it possible for the latter to flow radially against the workpiece 1. The amount of cooling air 14 is adjusted to satisfy the particular operating conditions required, as a function of the movement of elements 1, 2, 3 and 4 (defining the position and dipping rate of the workpiece 1), by a control unit 18 via a servo-motor 17 and a control valve 16. This interdependence is indicated by a dot-and-dash line in the Figure. 
     FIGS. 3a through 3c illustrates various means of reducing the heat flow in the transverse direction of the workpiece 1. 
     According to FIG. 3a, the workpiece 1 is provided on its shell surfaces, but not on its end faces, with a covering 19 consisting of ceramic heat-insulating material. The heat flow entering the lower end face of the workpiece 1 dipped into the salt bath can pass unimpeded through the workpiece 1 in a vertical direction (the arrows pointing directly upwards). In contrast to this, the heat flow directed transversely is throttled sharply (the horizontal broken arrows). 
     FIG. 3b shows a similar cladding in the form of an envelope 20 which consists of heat-insulating material and which has previously been slipped over the workpiece 1 to be treated. 
     According to FIG. 3c, the lateral thermal insulation of the workpiece 1 is effected by means of a more or less loose thermal insulating material held together and closed off by means of a tube 21. 
     PRACTICAL EXAMPLE 
     A gas-turbine blade consisting of a high-temperature material was subjected to a zone-annealing process by means of the apparatus according to FIG. 1. The workpiece 1 consisted of an oxide-dispersion-hardened nickel-based superalloy of the following composition: 
     Cr=15% by weight 
     Mo=2% by weight 
     W=4% by weight 
     Al=4.5% by weight 
     Ti=2.5% by weight 
     Ta=2% by weight 
     Y 2  O 3  =1.1% by weight 
     Ni=the remainder 
     The dimensions of the turbine blade were: 
     Shank 
     length=120 mm 
     width=100 mm 
     thickness=30 mm 
     Root 
     length=80 mm 
     width=120 mm 
     thickness=40 mm 
     The mean grain size of the untreated material before annealing was approximately 0.2 microns, equiaxial. 
     The salt bath 5 of the apparatus was heated to a temperature of 1260° C. by means of the heating system 7 and was maintained at this temperature as constantly as possible during the course of the process. An envelope 20 consisting of heat-insulating material (see FIG. 3b) and a covering screen 10 cut to a suitable size were slipped onto the workpiece 1. The workpiece was then successively lowered vertically into the hot salt bath 5 at a speed of 2 mm/min until it was completely immersed. After the workpiece 1 had been taken out and cooled, the grain size was determined. It was possible to detect longitudinally directed fringe crystals averaging a length of 40 mm, a width of 5 mm and a thickness of 2 mm. 
     In a further test, both the envelope 20 and the covering screen 10 were omitted. In this case, it was possible to detect a fine-grain edge zone 0.2 mm thick on the workpiece 1 after zone-annealing. Moreover, there was a considerable grain growth in the root part of the workpiece 1, and the fringe crystals were arranged obliquely relative to the longitudinal axis of the latter. 
     The apparatus and the process which can be carried out with it are not restricted to the exemplary embodiment. By means of the apparatus according to FIGS. 1 to 3c, other high-temperature materials which are superalloys of the class given in the example can also be zone-annealed successfully. Because of the use of appropriate heat-insulating claddings similar to those of FIGS. 3a through 3c, even complicated workpieces can be treated, and the various operating parameters can be coordinated with one another as efficiently as possible. 
     It should be mentioned, in particular, that the apparatus makes it possible to adjust the zone-annealing conditions according to the material and the workpiece in steps. The temperature gradient and feed speed of the workpiece can be varied in a suitable way during the annealing process. This is especially useful when the texture and structure are to vary over the length of the workpiece, for example where the root part has a different grain size and grain form from the shank part. This can be carried out in a simple way by regulating the drive motor 4 (stepping motor) and varying the amount of cooling air 14 (compressed air). 
     Obviously, numerous modification and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.