Through-Thickness Deformation Gradient in a Part-Pilgered Zirconium Tube: Experimental Measurements and Numerical Valida
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COLD pilgering is an intricate tube making process, which reduces the diameter and the wall thickness of the tube simultaneously.[1–4] The Mannesmann brothers came up with an innovative process of pierce rolling in 1885,[2,4,5] which can be used to make a mother hollow tube from a cast ingot. Later in 1890, they invented the pilger-rolling (also known as pilgering) process[5] to manufacture seamless steel tubes. In this process, the mother hollow is passed between a pair of grooved rolls with repeated rolling over a tapered mandrel.[2–4,6] The grooved roll rotates along its axis and also moves forward and backward on the outer surface of the tube. After each stroke (i.e., forward plus backward movement), the part-pilgered tube is advanced by a predetermined amount (feed rate) and is turned by a particular angle. This pattern of forward and backward movement continues until the required tube dimensions are achieved. The synchronization between the die rotation and reciprocating movement is maintained by a rack-and-pinion arrangement.[7] The outer diameter of the tube is formed according to the caliber or roll gap, which decreases from the inlet to outlet of the outer surface. The tapered mandrel, on the other hand, decides the inner shape of the tube. In other words, the respective reductions in diameter and wall thickness GULSHAN KUMAR, JAIVEER SINGH, and I. SAMAJDAR are with the Department of Metallurgical Engineering and Materials Science, IIT Bombay, Powai, Mumbai 400076, India. Contact e-mail: [email protected] SANDIP BALO, ASHISH DHOBLE, and RAMESH SINGH are with the Department of Mechanical Engineering, IIT Bombay, Powai, Mumbai 400076, India. D. SRIVASTAVA and G. K. DEY are with the Materials Science Division, Bhabha Atomic Research Center, Trombay, Mumbai 400085, India. Manuscript submitted September 14, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
are obtained by the tapered mandrel and the grooved rolls. The Indian Nuclear Industry uses seamless zirconium tubes manufactured through the pilgering process. For example, pilgered Zircaloy-4 fuel clads are used to encase uranium dioxide (UO2) fuel pellets.[8,9] The cladding needs to be manufactured with a close dimensional tolerance and an excellent surface finish, optimized microstructure/crystallographic texture, and favorable state of residual stresses.[4,10] The fabrication method is comprised of steps of cold pilgering in conjunction with intermediate annealing.[4,6,11] Though the fuel clads are typically thin walled (0.4-mm wall thickness), there are other zirconium pilgered seamless tubes (pressure tube, for example) with higher wall thickness. The pilgering was shown to introduce significant through-thickness gradients of residual stresses even in the thin walled fuel clads.[12] Such gradients, in turn, were shown to influence the burst ductility, which is an important safety parameter. Consequently, the ability to comprehend the deformation and residual stress gradients (both experimentally and numerically) during pilgering is vital to the N
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