Method of making pipes by powder metallurgy and pipe blanks (semi-finished products) made in accordance therewith
The invention is directed to a method of making pipes by powder metallurgy, in which powder of metal and/or metal alloys is filled into a thin-walled casing consisting of an inner casing wall, an outer casing wall and a bottom, whereupon the casing is closed by an annular cover on the side remote from the bottom and is subjected to a cold-isostatic pressure, thereby compacting the power within the casing so as to produce a solid or dimensionally stable pipe blank which is then hot-worked, viz., extruded while being heated. 
     Such a method has been known generally. However, it has been found that cracks due to thermal stresses frequently occur chiefly on the inner pipe or inner pipe wall, respectively, particularly on the two end regions, i.e., near the casing bottom and/or the casing cover. The cracks may extend either across the entire circumference of the inner pipe or inner pipe wall, respectively, or only across part thereof, depending on the magnitude of the thermal stresses. 
     It has been found that the cause of such crack formation is non-uniform heating of the pipe casing to the hot-working temperature. Such non-uniform heating occurs especially when large furnaces and relatively large pipe blanks or compacts are employed. For instance, it has been found that the thermal stresses may result in elongations especially of the inner pipe or inner pipe wall, respectively, amounting to 0.5 to 1% of the length of the pipe blank or compact. 
     The present invention is based on the object of preventing such crack formation during heating of the pipe blank to the hot-working temperature. 
     In accordance with the invention this object is solved in a surprisingly simple way by the characterizing measures of patent claim 1, wherein the features described in the subclaims, which relate to engineering and shaping measures, have been found to be especially advantageous. 
     Due to the bulge according to the invention it is possible to compensate or neutralize thermal stresses, wherein the bulge or bulges are provided at locations of maximum thermal stress, i.e., preferably in the end regions of the inner pipe or inner pipe wall, respectively. 
     The size of the bulge(s) according to the invention depends on the dimensions of the pipe blank and also, of course, on the hot-working temperature. The bulge(s) should be dimensioned such that the inner pipe or inner pipe wall, respectively, is practically completely smoothed out upon heating of the pipe blank or compact to the hot-working temperature. The expansion or elongation of the inner pipe caused by the differences in temperature may be calculated from the start on the basis of the following equation: ##EQU1## where ΔT: temperature difference expressed in °C., 
     L: length of the pipe blank or compact expressed in (mm), and 
     α: coefficient of thermal expansion. 
     Preferably, the bulges extend radially outwardly and are formed by means of rollers or rolls or the like, wherein both male and female moulds may be employed. 
     It is highly significant that the bulges should have a &#34;gentle or shallow curved shape&#34;, especially in the transitional regions between bulge and inner pipe or inner pipe wall, respectively. Thus, the bulge cross-section is somewhat trough-shaped, the radii of transition being approximately in the range of the radius of the inner pipe wall. The formation of folds should be prevented at all costs. 
     As explained above, the bulges according to the invention permit stretching or elongation of the inner pipe or inner pipe wall upon heating of the pipe blank or compact to the hot-working temperature without any problems, so that the inner pipe wall will be smoothed out. 
     With especially large pipe blanks it will be appropriate prior to the formation of the bulges to anneal the corresponding locations, e.g. the end regions of the inner pipe, and to remove the oxide layer formed thereby. 
    
    
     Below, a preferred embodiment of a pipe blank or compact configured and employed according to the invention will be described with reference to the accompanying drawing, which is a cross-section of a pipe blank or compact prior to hot-working. In the drawing 
     10 indicates the pipe blank or compact, 
     11 indicates the casing outer wall, 
     12 indicates the casing inner wall which is concentric with the casing outer wall, 
     13 indicates the annular casing bottom, and 
     14 indicates the annular casing cover. 
     Each end of the inner pipe or inner pipe wall 12 is formed with a radially outwardly extending bulge 15, 16 which permits thermal expansion of the inner pipe upon heating of the tubular casing 10 to the hot-working temperature such that the inner pipe wall 12 will be smoothed out. With longer and larger-sized tubular casings 10 it is possible to provide at each end two or more bulges corresponding to the bulges 15, 16 and extending across the circumference of the inner pipe or inner pipe wall. As will also be apparent from the drawing, the bulges 15, 16 have relatively shallow cross-sections, i.e., the radii of transition are relatively large and are within the range of the radius of the inner pipe wall 12. 
     The pipe blank or compact 10, which is heated to the hot-working temperature and has its inner pipe wall 12 smoothed out, may then be extruded in a conventional manner to form the final piping. 
     The depth and the radii of transition r 1 , r 2  and r 3  depend on the dimensions of the pipe blank or compact 10, on the powder material used, on the material of inner pipe wall 12 and outer pipe wall 11, and on the hot-working temperature. Preferably, the bottom radius r 2  is twice as large as the radii of transition r 1  and r 3  in the transitional region between bulge and inner pipe. 
     All of the features disclosed in the application papers are claimed as being essential to the invention insofar as they are novel over the prior art either singly or in combination.