CLOSED SHAPE FORMING METHOD Russian patent published in 2007 - IPC B21C37/15 

FIELD: plastic working of metals, possibly manufacture of shaped tubes.

SUBSTANCE: method comprises steps of forming arc-shaped members of round tube blank by upsetting to flat faces by means of rolls forming grooved passes for receiving flat faces joined by rounding portions of predetermined radius. Upsetting is realized by value ▵h_Σ. Steel blank having limit strength σ_B equal to or less than 450 MPa is shaped during three passes and steel blank having σ_B more than 450 MPa is shaped during four passes. At such procedures equal angles are formed between adjacent flat faces of shape. At shaping blank during four passes upsetting values are set for different passes equal to ▵h₁ =(0.03 -0.04)D, ▵h₂ = ▵h₃ = (0.08 - 0.07)D and ▵h₄ = (0.02 - 0.03)D where D - outer diameter of blank. At shaping blank during three passes ▵h₁ = (0.06 -0.08)D, ▵h₂ = (0.12 - 0.10)D and ▵h₃ = (0.03 -0.04)D.

EFFECT: improved quality of triangular shaped tubes due to their enhanced geometry and elimination of cracking at shaping blanks.

3 cl, 1 ex