BENDING METHODS

Now Piping & Fabrication entering chapter of Bending Methods, and in this chapter we will discuss all about bending methods.

Bending Methods. Pipe is bent by a variety of methods, using bending tables or bending machines, with and without the application of heat. The selection of one method over another is a function of economics, materials properties, pipe size, bending radius, and equipment availability. The arc length of the bend may be heated in order to reduce the yield strength of the material. Higher bending temperatures result in lowering the yield strength and reduction of the bending energy required.

Cold bending normally infers bending at ambient temperature, while hot bending infers the application of heat. However, definitions given in B31.1 and ASME Section III create an exception to this for ferritic materials. These codes define cold bending of ferritic steels as any operation where the bending is performed at a temperature 100_F (55_C) below the lower critical or lower. Ferritic materials undergo a phase change on heating and cooling. On heating, this change starts at a temperature called the lower critical. (See Heat Treatment—Ferritic Steels).

Ferrous Pipe and Tubes
1. Cold bending: Where sufficient quantities of repetitive bends are required, ferrous pipes and tubes up to NPS 10 or 12 (DN 250 or 300) with wall thickness of 1⁄₂ in (12.7 mm) or less are most often bent at ambient temperature using some type of bending machine.

There are a great variety of cold bending machines available, with degrees of sophistication varying from simple manually operated single-plane bending devices to numerically controlled hydraulically operated machines capable of multiplane bends.
In ram-type bending, two pressure dies which are free to rotate are mounted in a fixed position on the machine frame. The pipe to be bent is positioned against these dies. A ram then presses a forming die against the pipe and the pressure dies wipe the pipe around the forming die. See Fig. A6.7. Ram bending is usually applied to heavier wall thicknesses.

In compression bending the pipe is clamped to a stationary bending die and wiped around it by a follower. As in all bends, the extrados thins and the intrados thickens or compresses. The degree of compression is greater than the thinning in this method. Compression bending is usually limited to heavier walls and larger bending radii. See Fig. A6.8 to compare compression and draw bending.

In rotary draw bending the pipe is clamped to a rotating bending form and drawn past a pressure die which is usually fixed. See Fig. A6.9. The degree of thinning of the extrados is greater than the compression of the intrados. This method permits bending of thinner wall pipe and tubes at smaller bending radii. To accommodate lighter walls and tighter radii it is often advisable to provide internal support to minimize flattening or buckling. Usually this takes the form of an internal mandrel. As the diameter-to-thickness ratio increases and the bending radius decreases, mandrels using follower balls are employed.

Roll bending is often used for coiling. One of its great advantages is that the bending radius is not dependent on a fixed radius die, and consequently there is great flexibility in choosing a bending radius. In roll bending three power-driven rolls, usually in pyramid form, are used. The pipe to be bent is placed between the two lower rolls and the upper roll. Bending is accomplished by adjusting the rolls relative to each other as necessary to attain the required diameter.

Pipe can also be cold bent on a bending table in the manner described for hot bending below, except that for ferritic materials the bending temperature is kept at least 100°F (56°C) below the lower critical. A postbending heat treatment for cold bends may be advisable for some alloys, degree of deformation, certain service conditions, or when mandated by code.