Tube Forming: High Frequency Welded Tubes
Welded tubes are commonly produced of flat sheet material by continuous roll forming and a high frequency welding process. This type of tubes are widely used for automotive applications like seat structures, cross members, side impact beams, bumpers, engine subframes, trailing arms, twist beams, etc.
Tube manufacturing involves a chain of processing steps (roll forming, welding, calibration, etc), that influence the mechanical properties of the tube. Generally speaking during the tube manufacturing both the Yield Strength and the Ultimate Tensile Strength are increased while the Elongation is decreased.
Published data on technical characteristics of AHSS tubes (tubes made of AHSS) is limited. For example the ULSAB-AVC program deals only with those tubes and dimensions applied for the actual body structure. The ULSAC study resulted in design and manufacturing of Demonstration Hardware, which included AHSS tubes made of DP 500/800 material. The ULSAC Engineering Report provides the actual technical characteristics of those two tube dimensions used in the study, 55x30x1.5 mm and Ø 34×1.0 mm. (see http://www.worldautosteel.org/ulsac/ for more information).
The work hardening, which takes place during the tube manufacturing process, increases the Yield Strength and makes the welded AHSS tubes appropriate as a structural material. It clearly confirms that welded AHSS tubes provide excellent engineering properties. In comparison with HSLA steel tubes the AHSS tubes offer an improved combination of Strength and Formability. This combination together with good weldability makes AHSS tubes suitable for structures, where the competitive advantage is gained through high energy absorption, high strength, low weight and cost efficient manufacturing.
The most common tube forming operations include flaring, flattening, expansion, reduction, die forming, bending and hydroforming. It is essential to notice that the degree of work hardening and consequently the formability of tube depends both on the Steel Grade and the Tube Diameter Thickness Ratio D/T. Depending on the degree of work hardening the formability of tubular materials is reduced to some extent when comparing to the virgin sheet material.