On the alloy steel tubes of heating systems and steam lines, from P22 to the more demanding P91, cold cutting is the correct choice so as not to compromise the area that will then be welded. GBC responds with two families of machines: pipe cutters, when it is necessary to cut and chamfer the pipe on site, and pipe bevelers, when it is only necessary to prepare the joint with the chamfer. The choice between the two depends on the processing required.
Why cold cutting is the correct choice on alloy steels
In steam circuits, boilers and high-temperature lines, chromium-molybdenum alloy steels work close to the limits of the material: the welded joint must maintain the same mechanical properties as the base tube, and any alteration in the cutting zone affects the quality of the subsequent weld. For this reason, on existing steam lines and new plant sections, boiler shops prefer to cut and bevel cold, with machines such as GBC pipe cutters and pipe bevelers, which work by mechanical removal instead of concentrated heat.
Pipe cutters such as MCA Cutter, FAST and Supercutter all work cold, as well as the entire pipe beveling machines range: no heat-affected areas to manage before welding. Flame cutting remains an option on non-critical carbon steels, but it is not optimal on the alloy material of a thermal system.
Pipe cutters or pipe bevelers: how to choose according to processing
The main factor to consider in the workshop concerns the machining required on the tube, even before the grade of the steel. If the intervention involves cutting on site, for example on an existing steam line or during the modification of a system, the ideal solution is a pipe cutting machine. Depending on the work and the specific needs of the customer, the pipe cutter can perform only the cut, only the bevel or perform combined cutting and beveling in a single phase, thanks to the use of dedicated tools. If, on the other hand, the pipe is already sectioned and only needs to prepare the edge for welding, the correct choice is a pipe beveling machine, which performs a chamfer of the desired geometry, in addition to the facing.
For on-site cutting on alloy steels, the pipe cutter range covers different diameters depending on the model: MCA Cutter is split frame, it is divided into two halves for mounting on pipes from 33.4 to 1095 mm; FAST is also split frame but with integrated self-centering, unique in the GBC range, for diameters from 153 to 1545 mm; Supercutter is clamshell, hinged but not separable, indicated where access to the pipe is more limited. For the large diameters of distribution lines, TAF comes into play, a chain system suitable for large wall thicknesses.
When the machining required is only chamfering, the choice falls on pipe beveling machines: the machines of the Boiler series cover medium-small internal diameters typical of boiler pipes and exchangers, while the Supermaxi series, with interchangeable tools for bevels of any shape and angle, extends to larger diameters. In both cases, the machine is offered in the appropriate range for the diameter and thickness of the tube to be processed.
Tools and power drive for the most demanding case: the P91
Among the alloy steels for thermal plants, P91 is the one that requires the most attention during processing: it is a ferritic-martensitic steel with 9% chromium, with molybdenum and microalloys such as vanadium and niobium, developed for power plants, refineries and chemical plants operating at high temperatures, where it replaces more traditional grades such as P22.
For P91, GBC recommends tools made of HSS 5% cobalt coated with TiCN, while carbide inserts are only rarely used on this type of material. On the power supply front, the recommendation is for pneumatic or hydraulic machines: if the customer still requires the electric version, the machine is supplied with thickness limitations depending on the diameter of the pipe. It is a distinction that is worth clarifying before choosing the model, because it affects both productivity and tool life over time.
The pipe thickness: a key factor when choosing the machine
On pipes with high thicknesses and tough materials, such as alloy steels typical of heating systems, thickness is a fundamental parameter to be evaluated together with the diameter and material of the tube. To identify the most suitable pipe cutting or pipe milling machine for a given job, it is necessary to analyze all this data. In the presence of particularly large thicknesses, the correct choice involves the use of special tools — defined in shape and profile to better manage even wear in operation, so as to configure the machine and tool on the real machining and not only on the dimensional range of the range.
This approach applies both to pipe cutters used on site cutting and to pipe bevelers dedicated to chamfering only: in both cases the thickness determines the most suitable tool configuration.
The bevel profile is dictated by the thickness
The geometry of the chamfer comes from the welding project: the profiles applied to these tubes are unified and comply with precise regulations that change according to the wall thickness. On alloy steels, the issue weighs more than on carbon, because steam lines work with thick walls: more thickness means more filling passes, and each pass is added heat on a material that is sensitive to heat. As long as the wall remains contained, a simple V-bevel is sufficient; when it grows, widening the V would increase the volume of filler metal, and we move on to composite geometries that contain that volume.
The threshold is not at the discretion of the workshop. ASME B31 standards apply to piping of heating systems and steam lines, and GBC’s ASME B31 in-depth study on joint preparation describes how ASME B16.25 indicates V-bevel up to about 22 mm wall thickness and compound geometry beyond that value. On high-temperature alloy steels, it is a boundary that is often crossed.
Choose the right machine for your alloy steel
On alloy steel tubes for heating systems, the useful decision in the workshop concerns the processing to be carried out: cutting on site or chamfering only, with tools and feeding chosen according to the material and wall thickness. For cutting, the GBC pipe cutter range covers different diameters and configurations, from the MCA Cutter split frame to the Supercutter clamshell; for chamfering only, the pipe bevelers range offers the same attention to thickness and tool wear on each joint profile. To find the right model and tool configuration for you, contact GBC.