Why do cracks appear when a pipe bending machine bends a pipe?

Cracks are a common and troublesome defect in pipe bending. Once they appear, they can affect the appearance or even lead to structural failure. The causes of cracks in bent pipes differ from those in sheet metal bending, involving the unique mechanical state of the pipe and process details.

Insufficient material plasticity or excessively thin wall thickness

The elongation, yield strength ratio, and wall thickness of the pipe directly determine its bending limit. When the material itself is too hard (such as high-strength steel or some stainless steel) or the wall thickness is too thin relative to the outer diameter, the tensile deformation of the outer fibers exceeds the limit, causing cracks to appear along the outer side of the bend.

Insufficient Bending Radius

The ratio of the bending center radius (R) to the outer diameter of the pipe (D), R/D, is a key indicator. If R/D is too small (generally recommended to be R/D ≥ 3, depending on the material and process), bending deformation will be highly concentrated, the outer wall thickness reduction rate will exceed the standard, and cracks will be easily triggered.

Lack of mandrel or improper die matching

When bending pipes with a pipe bending machine, if a mandrel is not used, the mandrel is positioned too far back, or the dies (such as bending dies, clamping dies, and anti-wrinkle dies) are improperly matched, the inner side of the pipe will lack effective support, which will lead to cross-sectional distortion (flattening, wrinkling) and further aggravate the concentration of tensile stress on the outer side, inducing cracks.

Improper weld placement

For welded pipes, if the weld is located in the tension zone on the outer side of a bend, it is highly susceptible to becoming a crack initiation point due to the difference in plasticity between the weld area and the base material, and the potential presence of incomplete penetration or residual stress. The weld should generally be deflected to the vicinity of the neutral layer.

Insufficient lubrication and surface damage

Scratches or indentations on the pipe surface, or excessive friction between the mold and the pipe due to poor lubrication during bending, can all create localized stress concentrations, from which cracks can initiate and propagate.

Prevention Recommendations

Select pipes with excellent plasticity; for hard materials, consider bending after annealing.

Ensure the bending radius R/D is not less than the minimum allowable value for the material.

Select a suitable mandrel (e.g., a ball-head mandrel) based on the pipe diameter, wall thickness, and bending radius, and adjust it to the correct position.

Rotate the weld seam of the welded pipe to the inside of the bend or the neutral layer area.

Check the wear condition of the die and use a special pipe bending oil to ensure adequate lubrication.

Inspect the surface of the pipe blank before bending, removing burrs, scratches, and other defects.

Pipe bending is a process that is highly coupled with stress and mold conditions. Only by precisely controlling every step can we obtain smooth, full, and crack-free bent pipe quality.