Research on Bending Forming Limit of BIK700 High-Strength Steel Pipe

Literature Overview

The paper investigates the bending forming limit of BIK700 high-strength steel pipe, which is a hot-rolled high-strength low-alloy (HSLA) steel grade with a minimum yield strength of 700 MPa. Bending forming is a critical process for producing pipe elbows, tees, and other fittings from straight pipe sections. The research focuses on determining the critical bending parameters that govern the onset of failure, including minimum bending radius, wall thickness reduction, and the influence of material anisotropy on the forming capability.

Core Technical Analysis

The bending of a steel pipe involves complex stress states: the outer fibers experience tensile stresses while the inner fibers undergo compressive stresses, with the neutral axis shifted toward the inner surface due to the non-uniform stress distribution. For BIK700 steel, the high yield strength provides excellent resistance to deformation but simultaneously reduces the ductility margin available for forming. The research identifies the D/t ratio (bending diameter to wall thickness) as the primary parameter controlling the forming limit. Below a critical D/t ratio, the outer wall experiences excessive thinning, and the inner wall may buckle or fold.

Forming Parameter	Experimental Value	Failure Mode
Minimum D/t ratio	6.0 to 8.0	Outer wall thinning
Maximum wall thinning	15 to 20 percent	Tensile fracture
Inner wall buckling threshold	D/t < 5	Local buckling
Springback angle	2 to 5 degrees	Shape deviation
Forming temperature (cold)	20 to 25 degrees C	Room temperature forming

Process Parameter Optimization

The study employs both experimental and numerical methods to map the forming window for BIK700 pipe. Finite element simulations using the Johnson-Cook constitutive model capture the strain hardening behavior and the strain rate sensitivity of the material. The simulations reveal that the strain localization at the outer bend region follows a predictable pattern, with the maximum strain reaching approximately 0.15 to 0.20 at the onset of failure. The research also examines the effect of material texture on the forming limit: pipes with a pronounced rolling direction exhibit anisotropic forming behavior, with the transverse direction showing better ductility than the longitudinal direction.

A key finding is that the preheating treatment before bending significantly improves the forming limit. Preheating to 200 to 300 degrees Celsius reduces the yield strength by approximately 15 to 25 percent while maintaining the post-bend mechanical properties within acceptable limits. This approach effectively expands the forming window without requiring a larger bending radius, which is particularly valuable for producing tight-radius elbows in compact piping systems.

Defect Analysis and Countermeasures

The paper catalogs the primary defects observed in BIK700 pipe bending and proposes corresponding countermeasures. Outer wall cracking occurs when the tensile strain exceeds the material's elongation capacity, typically at D/t ratios below 6. The countermeasure involves increasing the bending radius, using a mandrel to support the inner wall, or applying preheating. Inner wall wrinkling results from compressive instability and is mitigated by using a plug or mandrel that fills the tube interior. Springback causes dimensional inaccuracy and is controlled by over-bending compensation in the tooling design.

Defect Type	Root Cause	Countermeasure
Outer wall cracking	Excessive tensile strain	Increase D/t, preheat, use mandrel
Inner wall wrinkling	Compressive instability	Insert plug or mandrel
Springback deviation	Elastic recovery	Over-bend compensation
Thickness variation	Non-uniform strain	Optimize tooling geometry
Surface scratches	Tooling contact damage	Lubrication, tool surface finish

Engineering Practice Implications

For engineers working with BIK700 grade pipes in high-pressure applications, the bending forming limit data presented in this literature is directly applicable to the design of on-site pipe bending operations. The critical D/t ratio of 6.0 to 8.0 must be respected in the selection of bending equipment and tooling. The material's high strength means that the bent sections retain good mechanical properties, but the reduced ductility requires careful process control. I recommend that quality inspection protocols include ultrasonic thickness measurement at the outer bend region and visual inspection for surface cracking, with acceptance criteria based on the minimum allowable wall thickness specified in the relevant standard.