Fabrication and Characterisation of Thin Walled Seamless Tubes

摘要

The classification of tubes, into thick and thin walled, is done based on the wall thickness of the tubes in relation to the diameter. This is typically described by the diameter (D) to wall thickness (t) ratio V. While the thick walled tubes are characterized by r values up to 10, thin walled tubes are characterized by r values of 30 to 60. The tubes with r values greater than 60 can be categorized as 'extremely thin walled tubes'. The last group of extremely thin walled tubes is conventionally produced by the 'welded route' to meet the dimensional tolerances on wall thickness. The welded route of fabrication involves extrusion of ingots into slabs followed by a series of hot and cold rolling process steps, thus forming the slabs into sheets and finally these sheets are pressed into O-shape & welded at the seam to close the gap.rnAn alternative and superior method of fabricating these extremely thin walled tubes is through the seamless route of cold pilgering after hot extrusion/rolling of blanks. It is rather difficult to meet the tolerances on wall thickness in this route. However, this process has several advantages of substantially improved material recovery, lower cycle time and superior mechanical & metallurgical properties compared to the welded route. These advantages are listed below.rnIt is relatively easier to adopt pilgering process for the fabrication of tubes with D/t ratios in the range of 10 to 60. However, pilgering of thick walled tubes with D/t ratio up to 10 and extremely thin walled tubes with D/t ratio over 60 can be very tricky. During pilgering of thick walled tubes, the mandrels being slender due to the small bore size of the hollows are likely to break frequently and sometimes cause damage to the rolling dies. In addition, control of the inside diameter is very difficult and large variation on the inside diameter is generally associated with the production of thick walled tubes. These details are summarized in Fig.1.