スチーフェルマンネスマン式穿孔機によるパイプの外面疵發生と管材表面疵との關係について

抄録

In the manufacture of seamless steel tubes by the use of the Stiefel-Mannesmann piercing mill, we are often annoyed by cracks on the outer surfaces of tube blanks. These cracks are mainly due to seams on the surface of round billets, but the standard of judging their harmful nature has not been yet made clear. The authors collected many examples of them, and studied the microstructures and depth variations along their axis after classifying them into 6 groups, and then heated these specimens in the heating-furnace, checking the changes of their crosssectional form and depth. On the other hand, the authors pierced about 70 examples of round billets and compared the cracks of tube blanks with the duepth of seams. Making use of these results obtained, the authors concluded as follows:
(1) Cracks appeared on the outer surface of hollowed pieces are due to seams on the surface of round billets. If the seams are removed before piercing, no crack appears. The kinds and appearance of seams have no bearing upon the cracks of hollowed pieces and the dominant factor is the depth of the seams.
(2) The boundary depth on which the harmfulness of the seams of billets may be decided as follows:
a. 0.7mm under is harmless.
b. From 0.7mm to 1.4mm is harmful or harmless according to circumstances, inasmuch as the scale-off amount of billets varies from place to place between these amounts.
c. 0.7mm up is always harmful.
The above conclusions were confirmed by actual experiments.