What Is Hot Induction Bending?

Quick Answer

Hot induction bending is a pipe forming process that uses a localized induction heating coil to heat a narrow band of pipe to 850-1100°C while a mechanical arm pushes the pipe through the coil at a controlled speed. The heated zone becomes plastic, bends to the desired radius, and is immediately quenched with water spray. The result is a smooth, full-body bend with no weld seam at the bend point.

The process produces bends from 3D to 40D radius (where D = nominal pipe diameter) in sizes from NPS 3 to NPS 64. It is governed by standards including ASME B16.49, CSA Z245.11, EN 15872 (ISO 15590-1), and API 5L Appendix D for line pipe bends.

Process Parameters

Parameter	Typical Range
Heating temperature	850-1100°C (grade-dependent)
Heated band width	25-50 mm
Bending speed	10-40 mm/min
Quench medium	Water spray (immediate)
Bend radius	3D to 40D (5D most common)
Bend angle	1° to 180°
Pipe OD range	NPS 3 (88.9 mm) to NPS 64 (1626 mm)
Wall thickness	4.8 mm to 100+ mm
Ovality limit	5% max (3% for subsea)
Wall thinning	Typically 10-15% at extrados

Bend Radius Designations

The bend radius is expressed as a multiple of the pipe’s nominal diameter (D).

Designation	Bend Radius	Application
3D	3 x OD	Tight bends, process piping
5D	5 x OD	Standard pipeline bends
7D	7 x OD	Piggable pipelines
10D	10 x OD	Large-radius directional changes

A 5D bend on NPS 24 (OD 610 mm) pipe has a centerline radius of 3050 mm. The 5D radius is the most common for cross-country pipelines because it allows pipeline pigs to pass through without restriction.

Wall Thinning and Thickening

During bending, the extrados (outside of the bend) stretches and thins, while the intrados (inside) compresses and thickens. ASME B16.49 limits wall thinning to ensure the minimum wall thickness after bending meets design requirements.

Typical practice: start with pipe one schedule heavier than required, or specify a minimum post-bend wall thickness on the purchase order.

Heat Treatment After Bending

Material	Post-Bend Treatment
API 5L Gr. B, X42-X52	Typically none (quenched from bending temp)
API 5L X60-X70	Quench + temper, or normalize
ASTM A106 Gr. B	Normalize or stress relieve
Cr-Mo alloys (P11, P22)	Normalize + temper, or PWHT
Stainless (TP304, TP316)	Solution anneal at 1040°C + water quench

Pro Tip

For sour service bends per NACE MR0175, the post-bend hardness must not exceed 248 HV10 (22 HRC). Quench and temper heat treatment is mandatory. Specify hardness testing at the extrados, intrados, and neutral axis of each bend.

Applications

• Cross-country oil and gas pipeline route changes
• Subsea pipeline field joints and risers
• Process piping directional changes (as an alternative to elbows)
• Penstock and water conveyance systems
• Power plant steam piping (Cr-Mo bends)

Read the full guide to pipe types