What Is Hot Expanded Steel Pipe?
Hot expanded steel pipe is produced by heating a mother pipe into its hot-working range and then pushing it over a conical mandrel so that the diameter increases to the finished size. The process is used to obtain large-diameter pipe from a smaller, more readily available mother pipe, and it is also a way of improving the mechanical properties of the pipe. Thermal expansion is not simply a sizing operation: the combination of controlled temperature and controlled deformation refines the grain structure, closes certain internal defects and redistributes residual stress, which is why the process is often specified for line pipe and pressure pipe in demanding service.
Residual Stress and Why It Matters
Residual stress, also called internal stress, is the stress that remains locked inside metal after the external forces have been removed. It is caused by uneven deformation and uneven volume change during casting, forming, welding or cooling. Its effect on a finished pipe is significant: when the local stress exceeds the yield strength of the material the pipe distorts, and when it exceeds the tensile strength the pipe can crack, changing its dimensions and sharply reducing its service life. Residual stresses also add to the stresses imposed in service, and they are a common contributor to premature failure at welded joints and cold-formed areas. Reducing them is therefore a key objective of the expansion process.
The Hot Expansion Process
In a typical line the mother pipe is heated uniformly in a controlled furnace into the hot-working range for the grade, then advanced over a conical inner mandrel by a pushing or drawing force while the outside diameter is supported and gauged. Heating is applied only to the section being deformed, which keeps energy consumption low and the temperature uniform and controllable. The conical mandrel can be changed quickly, so the same line can produce a range of finished diameters and wall thicknesses. After expansion the pipe is cooled under controlled conditions to avoid a hard or cracked microstructure. Two process variables decide the final quality: the temperature at which the deformation is carried out, and the expansion ratio, which is limited so that wall thinning does not take the pipe below the minimum wall required by the product specification.
Effect on Microstructure and Properties
Hot expansion breaks down the as-cast or as-rolled structure of the mother pipe and promotes deformation at a controlled rate and temperature. The grains recrystallise and refine, so the finished pipe has a denser structure and better mechanical properties; porosity, cracks and shrinkage looseness left from casting can be closed by the combination of high temperature and compressive stress. Strength and ductility improve along the direction of the expansion, which means the pipe is no longer fully isotropic, and this directional effect must be taken into account when the pipe is used in a pressure or bending application. The thermal cycle also reduces hardness and increases plasticity, which makes subsequent cutting, bevelling and cold forming easier, and it substantially relieves the internal stress that would otherwise encourage distortion or cracking during fabrication.
Specifications, Applications and Inspection
Hot expanded pipe is normally supplied to the same product specifications as the mother pipe, including ASTM A106, ASTM A53 and ASTM A333 for carbon and low-temperature service, ASTM A335 for chromium-molybdenum service, and API 5L PSL1 or PSL2 for line pipe, with EN 10216 and EN 10217 and GB/T 9711 used for European and Chinese projects. Typical applications are long-distance oil and gas transmission lines, water transmission mains, piling and structural tube, and steam or process piping where large diameters are required. Because the process changes diameter and wall thickness permanently, the purchaser should specify the finished dimensions and the minimum wall, not the mother pipe size. Inspection of hot expanded pipe normally includes dimensional and ovality checks, ultrasonic wall inspection to ASTM E213, a hydrostatic test to the product specification, and tensile and flattening or bend tests on the finished pipe. Where the pipe is to be welded in the field close attention is paid to the dimensional tolerance of the ends, because alignment and fit-up of a large expanded pipe govern the quality of the girth weld.
FAQ
Q: What is the difference between hot expanded pipe and seamless pipe?
Hot expanded pipe is made from a smaller seamless mother pipe whose diameter is increased by hot deformation. The finished product still has no longitudinal weld, but its dimensions and properties are achieved through the expansion step.
Q: Does hot expansion reduce the wall thickness?
Yes. Increasing the diameter with a fixed amount of metal thins the wall, so the expansion ratio is controlled to keep the finished wall above the minimum required by the specification.
Q: Why is residual stress a problem in pipe?
Residual stress adds to the stresses applied in service and can cause distortion or cracking when it exceeds the yield or tensile strength of the material. Relief of that stress is one of the main benefits claimed for the thermal expansion process.
Q: Can the properties become directional after expansion?
Yes. Because the deformation runs mainly along the axis of the pipe, the improvement in mechanical properties is greatest in the rolling direction and the material becomes somewhat anisotropic. Design and testing should take this into account.
Q: How is hot expanded pipe inspected?
Dimensional and ovality checks, ultrasonic wall testing to ASTM E213, a hydrostatic test to the product specification, and mechanical tests on the finished pipe. Order the pipe by finished size and minimum wall thickness.
