Small diameter steel pipe is mainly used in oil, natural gas pipeline. The small diameter steel pipe has one side welding and welding by both sides, welded pipe shall ensure that the water pressure test, the tensile strength and cold bending properties of the weld in accordance with regulations. Small diameter steel pipe is made by high frequency welding machine to certain specifications of strip steel coil into circular tube and straight seam welded steel pipe. The shape of the small diameter steel pipe can be round, also can be square or aliens, straight seam pipe depends on the sizing rolling after welding.

High frequency welding of small diameter steel pipe is using skin effect and proximity effect of alternating current (ac), steel (strip) after rolling forming, form a cross section of broken circular tube billet, straight seam tube billet in near the center of induction coil rotating one or a set of impedance (magnet) and impedance and pipe opening form a electromagnetic induction loop, under the action of skin effect and proximity effect, the pipe opening edges produce powerful and concentrated heat effect, make the weld edge rapidly heated to required after roller extrusion temperature, welding metal molten state to realize the joint between the crystal, straight seam formed by cooling a solid tube butt weld.

The small diameter steel pipe welding seam shrinkage induced local strain often reach the yield point several times, which is much bigger than the load caused by strain; The residual stress caused by uneven cooling. Residual stress is without external force within the phase equilibrium under the action of stress and all sorts of cross section of hot rolled steel has this kind of residual stress, larger cross-sectional dimension generally steel, the residual stress is greater. Although the residual stress is the phase equilibrium, but the performance of the steel members under the action of external force still has certain influence. Such as the deformation, stability, fatigue resistance, etc, are likely to produce adverse effect.