RESULTS There is a special program for relevant research. This program involved: – drawing of thin-walled tubes, both uncoated and coated, through dies of various diameters without in-process and finish anneals; and– mechanical tests of the tubes. The drawing conditions were standard, with spindle oil used as lubricant. All specimens of a nickel– chromium alloy were cut from one batch of as-received tubes 7.0 x 0.3 mm in diameter. The following sets of specimens were tested. Set A. As-received (initial) tubes. The test procedure was the following: As-received tubes were drawn sequentially through dies of different diameters. After each drawing, three specimens were chosen from the set. The cross-sectional strain (the ratio of the reduction in the initial tube diameter to the inner diameter of the die) was controlled. These specimens were subjected to tensile tests at 573 K. Set B. As-received tubes were coated by amorphous zirconia to a coating thickness of 1.5 µm. The drawing procedure, specimen selection, and mechanical tests were identical to those applied to Sets A. Set C. Set B tubes, each time after being drawn but before being tested, were additionally coated by amorphous zirconia to form a 1.5-µm-thick film. The subsequent testing procedure was as in other sets. We were also interested in achieving the highest possible reduction of the tube area. The mechanical test data were processed in terms of ultimate tensile strength, offset yield strength, and total strain at fracture versus prior drawing strain (reduction of area). Figures 1-3 present the results from mechanical tests at 573 K of tubes drawn to various extents. Tested were pieces of Set A (uncoated) tubes, Set B (zirconia-coated) tubes, and Set C tubes (Set B tubes with a zirconia coating overlaid after each drawing). Fig.1: The plot of the 573 K ultimate tensile strength vs. reduction of area after drawing for nickel– chromium alloy tubular specimens of Sets A, B, and C. The film is zirconia 1.5 µm thick
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