Piercing is employed to produce seamless tubing which is the natural form from which is made any thin-walled round objects. Seamless tubing is a popular and economical raw stock for machining because it saves drilling and boring of parts. The process of making hot-pierced tubing consists of passing a hot-rolled billet between two conical-shaped rolls and over a mandrel which assists in piercing and controls the size of the hole as the billet is forced over it.
The solid billet is uniformly heated to about 1100*C, and the piercing action is actually started previous to placing it between the rolls by drilling, punching, or piercing with a oxygen a hole to about 25mm deep. It it is then pushed into the two piercing rolls which impart axial as well as rolling movement to the billet and force it over the mandrel. The alternate squeezing and bulging of the billet open up a seam in its center. The first pass makes a rather thick-walled tube, which is again passed over a tapered plug and through grooved rolls in a two - high mill where the thickness is decreased and length is increased. While it is still up to a working temperature, it is passed onto a reeling machine which has two rolls similar to the piercing rolls, but with flat surfaces. The reeler further straightens and sizes the tubes. If more accuracy and better finish are desired, the tube may run through sizing dies or rolls. After cooling, the tubes are usually placed in a pickling bath of dilute sulphuric acid to remove the scale and oxide.
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