TPT September 2009

Wall thinning during tube bending By B Lynn Ferguson, PhD Metallurgical; Zhichao (Charlie) Li PhD Mechanical of Deformation Control Technology (DCT); Tim Kreja, manager of new product development, Pines Technology; and Dan Auger, director of engineering, Pines Technology

Introduction Applying lateral pressure and axial push to the outside of a pipe during bending can minimise distortion of the cross section and reduce the amount of wall thinning along the outer radius of the bend. Many pipe bending machines are equipped with Pressure Die Assist (PDA) that apply the lateral pressure and axial push to meet this requirement; an example is shown in Photo 1. The axial push or boost relies on friction between the pressure die and pipe to work effectively. However, in heavy wall tube or pipe, a greater axial pushing force is necessary.

a No. 4 Bender equipped with a 12-ton booster, Dial-a-bend SE machine control and a Pines CNC 150 HD CNC using the TS 2000 machine control and a 30-ton PDA booster. Tooling was designed and made by H&H Tooling (a division of Pines Technology). To make an effective simulation model: • Pines bent several pipes (2.5 schedule 40 pipe made from SA213T22 material) through 180° without boosting. The dimensions of the pipe were measured before and after bending at several locations and recorded. • Tube material was sent to a commercial testing lab to establish the mechanical properties of the specific material (yield strength, tensile strength etc). • A computer model was developed to simulate the conditions found in the practical tests. • A series of simulations were run to determine the effect of various boost schedules (pressure vs bend angle) on the bend geometry and wall thickness. • Using the range of boosting pressures examined in the models, bending tests were run using a Pines No. 4 Bender fitted with a 12-ton booster, Dial-a-bend SE machine control. • Boundary conditions in the bending models, such as friction between the tooling and tube, were adjusted so that simulation results accurately predicted bend geometry and wall thickness as measured in the bending. • The computer model was validated in terms of accurate material property data and accurate process boundary conditions, and it can be used to simulate bending of other dimensions and geometries, and to determine booster bend schedules. The finite element analysis (FEA) used to simulate tube booster bending is described in the following paragraphs. The simulation results include prediction of the wall thinning of the bend outer diameter (OD), the tube wall thickening of the inner diameter (ID) of the bend, and the tube ovality around the bend diameter. The bending conditions such as clamping pressure, the use of a boost load, the geometries of the clamp die and the bend die, and friction can all be accommodated in the model. An example application of FEA is described, which involved the use of a boost load to bend a steel tube of 2.5 inch diameter and a wall thickness of 0.23 inches. The FEA model setup is shown in Figure 1. The pressure die, the clamp die, and bend die were assumed to be rigid, and they were modelled using rigid surfaces. A friction coefficient of 0.15 was applied to all the interfaces between tooling components and the tube being bent. The ‘D’ of bend was 1.5 for this example. D is the ratio of the diameter of the tube bend axis (3 inches in this case) and the bend die radius (2 inches). The angle of the bend was 180 degrees. The tube was modelled as elasto-plastic material Finite element simulation of the tube booster bending process

 Photo 1 :

Pines No. 4 equipped with a 25,000lb open top booster

Pines first introduced booster bending in the 1960s when the Navy ship yards started to use steam pipes with minimum wall thickness to save weight, bend tighter radius to save space and reduce the overall cost. More recently, booster bending has been introduced to CNC bending machines using the carriage to push. Carriage booster bending provides a positive method of pushing on the end of the pipe without necessarily relying on friction. The ability to control the axial load or push for this type of boosting is critical for CNC operations to provide predictability and repeatability. However, carriage boosting generally cannot provide sufficient boosting forces to achieve the less than 10% wall thinning and the corresponding ovality requirements demanded in less than 1D bends in schedule 40 and 80 pipe and tube required by high pressure pipe applications. Pines, in cooperation with Deformation Control Technology (DCT) has developed a computer simulation of the bending process that can accurately determine the conditions necessary to effectively booster bend heavy wall pipe or tube while minimising wall thinning. Objectives of the research • To accurately predict the wall thinning of heavy wall pipe or tubing during bending • To accurately predict the percentage ovality relevant to the amount of wall thinning • To determine how much ‘boost pressure’ is necessary to minimise wall thinning Methodology DCT used ABAQUS/STANDARD finite element software to model the tube bending process. The models were run on a desktop computer, with each simulation taking around two hours. Pines used

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S eptember 2009

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