by Bill Atkinson
Over the years, the equipment used for welding, testing, and certifying welds has become more complex. This is evident in the use of computers by welders for the study, design, and control of welding processes. Technology has provided us with an intimate understanding of the metallurgical processes of metal joining. In many industries, one way to help obtain repeatable and reliable high-integrity welds is to use machinery specifically designed to generate accurately machined tube and pipe ends.
Ever since the advent of early welding equipment, welders have relied primarily on personal skill to cut and grind suitable ends on pipes or fittings. Their ability, combined with hard-earned welding experience, allowed them to recognize incongruities and deviations in the weld and compensate for them accordingly as the weld was performed. Each weld was unique. Each joint took its toll on time expended.
As time moved on however, the specifications and demands placed on welds became more exacting. The needed welding skills were elevated, requiring complex or critical welds to be performed by journeyman welders with expert knowledge of their trade.
Today, automatic welding equipment has changed the rules and ushered in a new way of looking at tube and pipe welding. It was probably inevitable that since the computer has found a home in so many occupations, it is now making a significant impact on advanced welding processes.
Automation in Tube Welding
Automation has greatly affected the science of high-purity welding. It has brought about pipe and tubing systems which have carried the liquids and solutions that have allowed numerous medical, electronic, energy, aerospace, chemical, pharmaceutical, environmental, and scientific breakthroughs.
In the semiconductor field alone, the introduction of precision tube squaring equipment has simplified the procedure of autogenous welding of microfittings and tubing, contributing to the achievement of the high level of purity required for the production of silicon microchips.
Tube squaring models have been designed to be matched to the size range and capabilities of automatic welding machinery. While able to generate near-perfect metal joining, automatic welders lack the logic and intuition to see the welding take place. The machine merely steps through a list of instructions and has no conscious awareness of the weld being performed.
It could be said that the automatic welder is a case of reversal, you take the pipe to the weld, instead of the traditional technique of taking the weld to the pipe.
Whether the welder is clamped to the pipe, or the pipe is clamped into the welder, all aspects of the weld (upslope, pulse rate, power level, rotation speed, oscillation, and downslope) are performed by the automatic welder in response to a detailed set of programmable instructions supplied to the welding head and power supply by a microprocessors or computer. The programming can only direct the welding head to operate orbitally in 3-D space around a theoretically existing diameter and axis of rotation.
At this point, the weld is defined and exists solely as power and positioning coordinates in virtual space, those coordinates being determined by the intended pipe material and dimensions, and by the weld diagram. In other words, the programmable welder will perform its steps including rotation, power control stages, upslope, oscillations and downslope, whether or not a tube is located in the machine.
Actual welding takes place, when the tube or pipe sections are introduced into the weld plane in exact relation to this spatial model of the weld and when the pipe’s presence is in relation to the welding tip for the current to complete its circuit. The more perfectly the physical pipe can conform to the theoretical pipe used as a basis for programming the weld, the more reliable and predictable the results. Remember that the theoretical pipe sections for which the welder is programmed are perfectly flat, perfectly square, perfectly beveled, and in perfect relation to themselves, and the position of the weld plane. Precision pipe machining equipment works to help ensure that real tube sections accurately match, the theoretical tubing dimensions of the weld model.
Some of the primary end prep factors which are addressed to place the tube sections into proper relation to the weld are:
- end squareness;
- end flatness;
- mismatches in tube/pipe dimensions; and
- precise bevel angles.
The angular deviation or squareness of the tube end to its centerline produces a directional change in the centerline. This angular deviation, however slight, can compound in an arbitrary manner to produce layout and welding fit-up problems for the tubing system. Unintentional angular change can induce unforeseen stresses on tubing, fittings, flanges or elbows, or may affect the flow of materials in critical systems.
Tube squaring machinery helps ensure squareness in the process by means of precision collets or saddles which both grip and align the tube for cutting.
The flatness of the tube ends can be mismatched. When attempting to weld without proper end preparation, these irregularities can produce minute gaps between the two tube sections. These gaps would not be perceived or compensated for by an automatic welder and could cause blow-through of the weld or inconsistent flow results throughout the weld pass.
Tube squaring machinery helps ensure that the ends butt up perfectly flat and flush to each other which is critical to autogenous pipe welding processes. In this way, the amount of weld material is consistently present and matches the programming expectations for the individual weld.
Bevels can also be used intentionally to produce desirable weld prep profiles for different welding situations, which will be discussed later.
Tubing that has not been end prepped by a squaring machine may introduce unintentional bevel angles on the tube ends that differ from the amount of base metal being calculated into the weld power requirements. Squaring machinery preps the pipe with an end perpendicular to the centerline, helping to ensure that flush fits have the expected amount of metal for each weld.
Mismatches in Tube/Pipe Dimensions
Mismatches in dimensions (inside diameter [ID] or wall thickness) can produce stress collection or contamination points in high purity systems and turbulence or flow restrictions in other piping systems. In addition, nondestructive testing (NDT) and certification procedures used on critical tube welds, such as X-ray and ultrasonic testing, require that the joint have a continuous, smooth baseline through the region of the weld for measurement.
X-ray testing procedures, which are performed as a series of scans from directly above and rotated angularly to both sides of vertical in relation to the weld plane, expect to have a similar amount of pipe material on either side of the weld.
A counterbore cutting module fitted to a pipe cutting machine can provide this constant reference ID by using a special counterbore bit which reaches into and cuts the ID of the tube instead of the face, as is the case in squaring. In this way, the IDs of the pipe in the area of the weld can be blended into each other for testing or to remove a restriction or buildup point which may become a problem.
By being able to control the ID in the area of the weld, the welder can control land thickness, which affects how heat is applied to the weld. While heat produced by the welding process is required to melt the metals together, the heat must be managed to control the heat input. The areas of the tube or pipe that have had heat buildup sufficient to fuse nearby metal, may also have altered temper or other physical properties. Heat affected zones (HAZs), which can be critical in certain situations, can be reduced or are more controllable when the land thickness is controlled and calculated into the weld programming. Land thickness, while important to heat dissipation, is also important in creating a good root pass, which is the foundation of an excellent weld. When land thickness is inconsistent, weld material is more likely to intrude into the inside bore of the pipe or porosity in the root pass. Using a machine to control land thickness can help ensure uniform heat distribution and consumption.
Precise Bevel Angles
In the case of larger tube and pipe, precise bevel angles are used intentionally to open up the weld to allow the introduction of the welding tip below the outside diameter (OD) for proper root pass penetration and for oscillation movement as filler metal is built up into the weld profile.
In these instances, bevels can be machined with form tooling that has a variety of specialized bits with predetermined weld prep angles designed into their cutting faces. Unlike the squaring bits which cut perpendicular to the centerline, these bits provide end preps with the exact bevel angles desired.
Bevel angles can be cut at the usual 37.5 degrees, at a standard compound angle such as a 37.5-10 degrees, or at any other specific angle to fit any specification or requirement.
Another procedure used for the end preparation of pipes (especially very heavy-wall pipes) is the use of a single-point cutting accessory in conjunction with the standard end prep machinery. A single-point device allows for the machining of precision bevels, compound bevels, counterbores and unique prep configurations.
Because of the latitude of cutting the single-point cutter provides, precision prep configurations can be cut on pipes of almost any size, including extra heavy-wall pipe for use in industrial high-pressure piping systems for powerplants, oil refineries, offshore oil platforms, petrochemical plants, etc.
Single-point machinery can produce narrow gap weld preps and J-preps which are used to reduce the amount of filler metal used on large, heavy-wall pipes (thus reducing HAZs) and to achieve special details such as seal grooves, steps, etc.
End Preparation Machine Options
End preparation equipment can be powered by electric, pneumatic, or hydraulic power supplies depending on the environment or available power where they will be used.
While some end preparation equipment is bench-mounted, a variety of portable equipment is available to take precision machining to the pipe regardless of its location. This is helpful in power plants, ships, factories, and other sites where existing piping systems must be welded or where the construction of new piping systems would benefit from on-location pipe machining.
Machinery is available for the cutoff, squaring, and finishing of large-diameter thin-wall stainless steel tubing, which is helpful for the aerospace industry. Full support collets provide positive grip for machining without deforming the pipe.
Other machinery is available for locating end prep equipment into large elbows and fittings by using a miter mandrel that does not require a deep insertion for mounting. An end prep machine can also perform extra duty as a flange facer for valves and flanges.
Tri Tool Inc. maintains a staff of qualified and experienced on-site machining technicians who can bring the latest equipment to your job site and perform any required end preparation machining operations for you. As an alternative to regularly scheduled maintenance outages, emergency shutdowns, or special operations. We also provide leasing and rental of end prep equipment. In addition, special engineering and custom application services are available to design or modify equipment for particular purposes beyond the standard range of the machinery. Bits are available in thousands of configurations or can be produced for any special application.