Furnace For Heat Treating Of Metallic Workpieces

Abstract

Wall means defines a chamber having an inlet and an outlet and including a first chamber section and a second chamber section which are respectively nearer to the inlet and to the outlet. Elongated tubular guide members extend through the chamber from the inlet towards the outlet for supporting metallic workpieces which are advanced from the former to the latter, each of these guide members having a longitudinally extending upwardly directed wall portion. Cooling means supplies internal cooling for the tubular guide members. Rows of wear-resistant first workpiece engaging elements are arranged in the first chamber section supported on and projecting upwardly from the respective wall portions by a predetermined height. Rows of second workpiece engaging elements are provided in the second member section supported on the respective wall portion and each comprises a holding portion straddling the respective wall portion, a thermally insulating portion supported on the holding portion and a heat-resistant portion supported on the associated thermally insulating portion and having a workpiece-contacting upper surface.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to furnaces, and more particularly to furnaces for the heat treatment of metallic workpieces. Still more specifically the present invention is concerned with so-called pusher furnaces wherein workpieces rest on supports which extend through the furnace chamber and are advanced on these supports through the chamber.

It is known that in pusher furnaces contact between the metallic workpieces to be treated and the supports on which they rest and along which they are pushed from the inlet to the outlet of the furnace, results in the formation of so-called dark strips in the workpieces, that is striplike undercooled zones in the region of the workpiece surface which contacts the support. It must be kept in mind in this connection that the supports are cooled, usually by circulating cooling fluid through the tubularly configurated supports. The existence of the inadequately heated zones or dark strips in the workpieces is highly undesirable because it adversely influences the material characteristics, particularly the behavior of the material of the workpiece during subsequent rolling. Therefore, it is necessary to attempt to eliminate these undercooled zones and for this purpose it is conventional to provide an equalizing zone or equalizing chamber in which heat is directed against the side of the workpiece where the undercooled zones are present. However, this is not entirely satisfactory for two reasons, namely because on the one hand it is not possible to completely eliminate the undercooled zones in this manner and on the other hand because the necessary heating of only one side results in significant temperature gradients within the workpiece between opposite sides, that is between the side having the undercooled zones and the opposite side. This latter fact, also, is disadvantageous with reference to the subsequent behaviour of the workpiece during rolling, especially if the workpiece is relatively thick, i.e., if it has a thickness of 200- 300 mm. or more.

Attempts have of course already been made to overcome this problem. Thus, German Auslegeschrift 1,094,778 teaches a support comprising a tubular member through which cooling fluid may be circulated, and on which so-called "riders" or workpiece-engaging elements are mounted which extend over the entire length of the tubular member and on which the workpiece is supported for the purpose of preventing undercooling of portions of the workpiece by direct contact with the tubular member.

A further approach is taught in German Auslegeschrift 1, 193,528 . Here, an internally cooled tubular support is provided which carries over its entire length highly thermally insulating elements, with the latter in turn carrying the actual heat-resistant workpiece-engaging elements. This construction has been found to be highly advantageous in that it eliminates almost completely the formation of undercooled zones in the workpieces and makes it possible to omit the heretofore conventional equalizing zone or chamber.

However, this construction cannot be used under all circumstances. One detrimental consideration is concerned with costs. The various different types of guide members, as the supports for the workpieces will hereafter be designated in toto, are of differing complexity and accordingly the cost of their construction varies widely. The more effective the particular guide member is in preventing the development of undercooled zones in the workpiece, the more complex must be its construction and the more it is necessary to use expensive highly heat-resistant material; from this it follows that such constructions are significantly more expensive than others.

It is therefore the current industry practice to equip pusher furnaces with differently constructed guide members, depending upon the requirements made of the evenness to which the material of the workpieces must be heated.

A further consideration with respect to these known constructions is the fact that the workpieces must be heated to temperatures of substantially 1,250.degree. C. This temperature also represents the approximate limit to which the currently available heat-resistant metal alloys have a sufficient resistance to heat so as to be usable as workpiece engaging elements. Now it is well known that in the downstream region of the furnace, that is in the region through which the workpieces pass last before they leave the outlet, it is desired to obtain as even as possible a temperature throughout the respective workpiece. In fact, the upper and under sides of the workpiece should be hotter than the center of the workpiece by only approximately 10.degree. - 20.degree. C. when the workpiece leaves the outlet of the furnace. In order to obtain such equal temperatures also at the area of contact of the workpiece with the guide members, the workpiece engaging elements must have the workpiece temperature, i.e., approximately 1,250.degree. C. at their interface with the respective workpieces in the region just upstream of the furnace outlet, that is the region where the temperature equalization is to be achieved. Because the furnace temperatures may be only slightly higher than the workpiece temperature which is demanded, in order to obtain in the equalization region a good temperature equalization, the requirement for identity or substantial identity of temperature between the workpiece and the contacting surface of the workpiece engaging elements can be fulfilled only by a thermally well-insulated workpiece engaging element.

However, in the actual heating zone, that is in the heating zone where the workpieces undergo their actual heat treatment, conditions are different from those in the equalization zone. In order to obtain high-operational capacities of the furnace, the latter may be so constructed that in the main heating zone of the furnace chamber the temperature can be raised to and substantially above 1,300.degree. C. Depending upon the temperature to which the chamber is heated in the main heating zone, the position of the workpieces in the main heating zone and the quantity of workpieces which are in the main heating zone at any given time, it is possible for the surfaces of the workpieces to be heated to temperatures in excess of 1,250.degree. C. This is not necessarily impermissible, but if continued movement of the workpieces through the furnace is interrupted for any reason whatsoever, for instance as a result of a breakdown or other difficulties in a rolling mill which receives workpieces from the outlet of the furnace, the workpieces are in danger of being heated in the main heating zone to such an extent-- even if the burners are throttled, because of the heat retention and radiation of the furnace walls-- of being heated at their surfaces to such a temperature that they are close to the softening and melting point of steel, that is the material of which the workpieces would normally consist.

It will be appreciated that in the main heating zone it is neither possible to achieve a significant temperature equalization in the workpieces, nor that this is particularly important or desirable because the following equalization zone provides for this. Also, at furnace temperatures in excess of 1,300.degree. C. the heat resistance of a highly thermally insulating workpiece engaging element is overtaxed.

SUMMARY OF THE INVENTION

It is, accordingly, an object of the present invention to overcome the outlined disadvantages and to provide an improved construction.

A more particular object of the invention is to provide such an improved construction which is relatively simple and less expensive than what is known from the prior art.

In pursuance of the above objects, and others which will become apparent hereafter, one feature of the invention resides, in a furnace for heat treating of metallic workpieces, in a combination which briefly stated comprises wall means defining a chamber having an inlet and an outlet and including a first chamber section extending from the inlet towards the outlet and a second chamber section extending from the first chamber section to the outlet. Elongated tubular guide members extend through the chamber from the inlet towards the outlet for supporting metallic workpieces which are advanced through the chamber from the inlet through the outlet, and each of these guide members has a longitudinally extending upwardly directed wall portion. Cooling means is provided for internally cooling the tubular guide members.

Rows of wear-resistant first workpiece engaging elements are arranged in the first chamber section supported on and projecting upwardly from the respective wall portions by a predetermined height. Rows of second workpiece engaging elements are arranged in the second chamber section supported on the respective wall portions, and each of these second elements comprises a holding portion straddling the respective wall portion, a thermally insulating portion supported on the holding portion, and a heat-resistant portion supported on the associated thermally insulating portion and having a workpiece-contacting upper surface.

Thus, the rows of wear-resistant first workpiece engaging elements are of less complicated and less expensive construction than the second elements, and the predetermined height by which they project upwardly from the respective wall portion is determined in dependence upon the temperatures in the first chamber section. The height is so selected that the cooling influence of the internally cooled tubular guide members upon the first workpiece engaging elements is just large enough--even at the highest temperatures and the most disadvantageous operating conditions envisioned for the furnace-- that the hottest point of the respective first element can reach but not exceed a temperature at which the strength and resistance of the material from which the respective first element is made is still sufficient for the operational requirements.

This being the case the invention further suggests that the first chamber section--that is the main heating zone--be a high-temperature zone in which temperatures on the order of 1,350.degree. C. can be routinely achieved.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a somewhat diagrammatic vertical longitudinal section through a pusher furnace incorporating the present invention;

FIG. 2 is a section taken on the line II--II of FIG. 1; and

FIG. 3 is a section taken on the line III--III of FIG. 1; and

FIG. 4 is a section taken on the line IV--IV of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference numeral 1 identifies in FIG. 1 the pusher furnace in toto. Its walls surround and define an internal chamber extending from the inlet 1a to the outlet 1b of the furnace 1. The first chamber section extends from the inlet 1a towards but short of the outlet 1b, and the second chamber section extends from the first chamber section to the outlet 1b. The approximate line of division--which is evidently not sharply defined--is indicated at A.

Extending through this chamber from the inlet 1a to the outlet 1b in the illustrated embodiment are the tubular guide members 5. Only one of these is visible, but there will of course be at least two or more of them which are transversely spaced from one another, that is spaced in direction normal to the plane of the drawing (in FIG. 1). The workpieces to be treated, which are not illustrated because their showing is not thought necessary for an understanding of the information, enter through the inlet 1a and are supported on the tubular guide members 5, being pushed along the tubular guide members in the direction of the arrow A' until they reach and pass through the outlet 1b. The tubular guide members 5 are hollow and are connected in known manner with a source of coolant (compare FIG. 1) which internally cools them. They are supported by the upright supports 7 and the transverse supports 9, which all may be tubular and which all may themselves be internally cooled.

Reference numeral 3 diagrammatically identifies heating means-- of any type conventional in such furnaces-- which are arranged above and below the guide members 5 so as to subject workpieces on the same to heating from above and below. The fact that double the number of heating means 3 is shown in the first chamber section than in the second chamber section indicates that in accordance with the present invention the first chamber section is intended to be a high-temperature zone. Of course, this is illustrated in this manner only for explanatory purposes, because evidently different relationships with respect to the number of heating means 3 could obtain, depending upon the particular type of heating means employed, and other factors.

In the region just downstream of the inlet 1a the upwardly directed wall portions of the tubular guide members 5 carry wear-resistant workpiece-contacting guide rails 23 which rest directly on the tubular guide members 5 and are suitably secured thereto, for instance by welding as shown in FIG. 4.

Downstream of this region, that is in the actual high-temperature zone of the first chamber section, the upwardly directed wall portions of the respective tubular guide members 5 each carry a row of wear-resistant first workpiece-engaging elements 19 which directly rest on the respective tubular guide member 5 (compare FIG. 3) and are prevented from lateral displacement by the presence of retaining ribs 21 located at opposite lateral sides and secured as by welding to the tubular guide members 5. A further interlock against lateral displacement may be provided, as shown in FIG. 3, by keying the elements 19 to the respective tubular guide member 5. The height of the elements 19, that is the distance between the upper surface of the respective tubular guide member 5 and the upper free surface of the respective element 19 on which the workpieces to be treated will slide, is determined in accordance with the earlier discussed considerations, namely such that the cooling influence of the tubular guide member 5 upon the respective element 19 even under the highest temperatures and most disadvantageous operating conditions is just so large that the hottest point of the element 19 can assume only a temperature which the material of the element 19 is still of sufficient strength and resistance to fulfill its intended function. Because the other contact surfaces of the elements 19 and 23 should be located in a common plane, and because the height of the elements 23 is obviously considerably less than that of the elements 19--both to conserve material and because the considerations with respect to the elements 19 do not obtain with respect to the elements 23 which are not subjected to such high temperatures-- the tubular guide members 5 may be stepped where the transition between the elements 19 and 23 takes place, that is where they carry the elements 23 the tubular guide members 5 may have a larger cross-sectional dimension in vertical direction.

In the second chamber section, that is downstream of the line A, the tubular guide members 5 are provided with rows of second workpiece-engaging elements, as shown in FIG. 2. These second elements each comprise a holding portion 15 straddling the respective tubular guide member 5 and being secured thereto, for instance with the bolts 17 illustrated in FIG. 2. The holding portions 15 in turn each support a thermally insulating portion 13, for instance of ceramic material, and this in turn supports a heat-resistant portion 11 on whose upper surface the workpieces slide. The portions 13 are pressure resistant, and because of the construction of the second element only forces acting normal to the elongation of the tubular guide members 5 in vertical direction, that is the weight of the workpieces, will be transmitted to the portions 15 and the tubular guide members 5, whereas other forces resulting from the pushing of the workpieces in the direction of the arrow A' , will not be transmitted.

The construction of the workpiece supports, that is the tubular guide members 5 and their associated workpiece-engaging elements, in accordance with the present invention is in contrast to the practice which heretofore has prevailed. Heretofore the assumption has been that the workpieces continue to become warmer during their movement from the inlet to the outlet of the furnace chamber, and that therefore the thermal resistance of the material of the workpiece support becomes more critical in direction from the inlet to the outlet. However, as has been shown above the contrary is the case and the use of thermally insulating workpiece-engaging elements in the equalization zone or second chamber section is desirable and also possible. By contrast, the thermally insulating effectiveness of the workpiece-engaging elements in the main heating zone can be varied as needed by accommodating the height of the elements to the prevailing temperatures.

By utilizing the present construction, that is by utilizing highly thermally insulating workpiece-engaging elements in the second chamber section or equalization zone, it becomes possible contrary to previous practice to continue heating of the workpieces from both above and below in this equalization zone, also. This eliminates the heretofore common stationary hearth which was used in the equalization zone, and consequently avoids the high expenses for maintaining and the difficulties in operating such a hearth.

It is emphasized that in contrast to what has been shown in the drawing it is possible to have the elements 19 extend all the way to the inlet 1a , that is to eliminate the elements 23. However, the construction illustrated by way of example, and utilizing the entirely conventional elements 23, still further reduces the expense involved in construction of the workpiece supports.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in a furnace for heat treating of metallic workpieces, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

Claims

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

1. In a furnace for heat treating of metallic workpieces, in combination, wall means defining a chamber having an inlet and an outlet and including a first chamber section extending from said inlet towards said outlet and having an upstream and a downstream zone, and a second chamber section extending from said first chamber section to said outlet; elongated tubular guide members extending through said chamber from said inlet towards said outlet for supporting metallic workpieces which are advanced through said chamber from said inlet through said outlet, each of said guide members having a longitudinally extending upwardly directed wall portion; cooling means for internally cooling said tubular guide members; wear-resistant workpiece-contacting guide rails supported on said upwardly directed wall portions in said upstream zone; rows of wear-resistant first workpiece-engaging elements in said downstream zone supported in heat-exchanging contact on and projecting upwardly from the respective wall portions by a predetermined height so selected that the cooling influence of said cooling means on said first workpiece-engaging elements under all operating circumstances is just sufficient to prevent heating of said first elements beyond the upper limit of their ability to support said workpieces; and rows of second workpiece-engaging elements in said second chamber section supported on the respective wall portions, each of said second elements comprising a holding portion straddling the respective wall portion, a thermally insulating portion supported on said holding portion, and a heat-resistant portion supported on the associated thermally insulating portion and having a workpiece-contacting upper surface.

2. In a furnace as defined in claim 1, wherein said insulating portion is supported on said holding portion in such a manner that the former transmits to the latter only stresses acting normal to the elongation of said guide members.

3. In a furnace as defined in claim 2, said holding portion comprising two downwardly extending arms straddling the respective wall portion at opposite lateral sides of the respective guide member, and two upwardly extending arms extending laterally of said thermally insulating portion and said heat-resistant portion.

4. In a furnace as defined in claim 1; further comprising heating means for heating said first chamber section to temperatures in excess of 1,350.degree. C.

5. In a furnace as defined in claim 1, wherein said guide members extend from said inlet to said outlet.

6. In a furnace as defined in claim 5; and further comprising heating means above and below said guide members for heating workpieces advancing thereon from above and from below from said inlet to said outlet.

7. In a furnace as defined in claim 1; further said guide rails being welded to the respective wall portions in said first chamber section intermediate said first elements and said inlet.