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Induction bending for pipe is widely used.

Capabilities and Advantages

Large induction bending machines can bend pipe from 3 to 66 inches in diameter with wall thicknesses up to 4 inches and bend angles up to 180 degrees (90 degrees

Bend radius and bend length are measured in pipe diameters Bend angle is in degrees

D = nominal pipe diameter, ft

Bend radius 57.3°×bend length bend angle

---=

Cold forming strain D×100%

2×bend radius

---=

for pipe diameters greater than 34 inches). Small bending machines (e.g., Cojaflex PB Special) can bend 2- to 12-inch diameter pipe. The bend radius can be as low as 1.5 times the pipe diameter (3D bends are routine) for small diameter pipe.

Figure 300-8 gives some basic induction bending terminology.

Induction bending overcomes most of the deficiencies of furnace hot bending and has several additional advantages:

• High dimensional accuracy.

• Various bend angles and multiple plane bends.

• Off-the-shelf seamless and ERW pipe may be induction bent to avoid small quantities of special order pipe. The weld metal in SAW pipe can be a problem.

Description of the Induction Bending Process

The induction bending process uses a medium frequency induction coil to heat the pipe (to 1500-2000°F for carbon and low alloy steels, and 1900-2100°F for stain-less steel), while a hydraulic ram pushes the pipe around a radius (see

Figure 300-9). A water or air quench ring is placed closely behind the induction coil, so that the width of the heated zone is typically only twice the pipe wall thick-ness. Restricting the heating and bending to this small zone helps maintain dimen-sions and avoid wrinkling. As a result, most of the residual stresses are

compressive. A water quench provides the best dimensional properties and is preferred, except for ASME P4 and P5 materials. ASME P4 and P5 materials are easily hardened, and they can crack as a result of water quenching.

Fig. 300-8 Basic Induction Bending Terminology

Note The induction bending temperature cycle can significantly affect the mechan-ical properties (strength and/or impact toughness) of the pipe as detailed below.

Metallurgical Effects of Induction Bending

The thermal cycle associated with induction bending can significantly affect the yield and tensile strength (of all steels), impact properties (of carbon and low alloy steels), and corrosion resistance of austenitic stainless steels. The effects of induc-tion bending vary with the chemical composiinduc-tion and the prior heat treatment of the pipe to be bent.

• Carbon steel pipe (e.g., ASTM A106 & A53, API 5L Gr B, X-42, etc.) is strengthened primarily by carbon and manganese. It will often strengthen significantly during the induction bending thermal cycle. For example, API 5L Gr B pipe has met X-70 strength requirements following induction bending. To reduce the excessive strength and hardness in the as-bent condition, these grades require tempering following induction bending.

• High strength steel pipe (e.g., X-56 and above) is strengthened by chemistry (carbon and manganese), thermal mechanical working, and microalloying.

Grades of pipe that gain a significant amount of their strength by microalloying and thermal mechanical working often do not retain their original strength after induction bending. Hence, these steels may have problems meeting specified minimum strength requirements following induction bending. Bends should also be tempered following induction bending, which may further reduce their strength. They must be tempered for sour service.

Fig. 300-9 Induction Bending Machine

• Low alloy steel pipe (e.g., ASME P4 & P5) will be strengthened significantly by induction bending and is normally cooled by air quenching rather than water quenching during induction bending. The air quench will minimize the hardness and the risk of cracking. These grades require tempering following induction bending.

• Austenitic stainless steel pipe (e.g., Type 316), which receives some of its strength from cold work, will have its strength reduced by the induction bending thermal cycle. In addition, the beginning and end of the bend will have reduced corrosion resistance similar to that experienced in weld heat affected zones. If corrosion resistance or resistance to stress corrosion cracking is required, only low carbon or stabilized grades (e.g., 304L, 316L, 317L, 321, or 347) of stainless steel should be induction bent.

In summary, all induction bent pipe (except austenitic stainless steels—Type 3XX) should be tempered following bending to reduce the strength and hardness and to improve the impact toughness of the pipe. Tempering should be waived only on certain nonsour-service, high-strength grades when tempering would be detrimental to the final strength and/or impact toughness—and then only after prototype bends are made and shown to meet the service requirements.

Selection of Materials for Induction Bending

Successful induction bends have been made in Carbon, Low Alloy, And Line Pipe Steels with carbon equivalents (CE) from the high 0.20%’s through the 0.50%’s. However, carbon equivalents in the mid- to high-0.30%’s represent the optimum chemistry for both bendablity and weldability. Carbon equivalents are defined by the equation:

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15

(Eq. 300-1) A potential weld toughness problem exists when SAW pipe, particularly pipe for low temperature service, is induction bent. SAW wire and flux combinations devel-oped to give good impact toughness in the as-welded condition may undergo a dramatic decrease in toughness after being exposed to a stress-relieving (tempering) or quenching and tempering cycle similar to that encountered during induction bending. Welding consumables are available which will respond more favorably to heat treatment, but they will typically not be the pipe mill’s standard consumable.

Model Specification PPL-MS-4737 requires testing of the weld for SAW pipe and weld impact testing when the original pipe is impact tested.

If corrosion resistance or resistance to stress corrosion cracking is required, only low carbon or stabilized grades (e.g., 304L, 316L, 317L, 321, or 347) of Austenitic stainless steel should be used as bent. The corrosion resistance would not normally be a factor for service temperatures above 850°F or below 32°F.

Model Specification PPL-MS-4737

Model Specification PPL-MS-4737, Induction Bending, presents requirements for induction bending carbon and low allow steel pipe (ASME P1, and P3 through P5

pipe, CAN3-Z245.1 line pipe, and API SPEC 5L line pipe). Submerged arc welded, seamless, or electric resistance welded (ERW) pipe may all be bent according to this specification.

Austenitic stainless steels (e.g., Type 3XX) may also be bent by induction bending.

They have been excluded from the model specification for simplicity. A hard copy and PC disk copy of PPL-MS-4737 are contained in this volume.