WHAT IS ORBITAL WELDING
The term Orbital-Welding is based on the Latin word ORBIS = circle. This has been adopted primarily by aerospace and used in terms of Orbit (n.) or Orbital (adj.) for the trajectory of a man-made or natural satellite or around a celestial body.
The combination Orbital and Welding specifies a process by which an arc travels circumferentially around a work piece (usually a tube or pipe).
The concept Orbital Welding is basically a loosely defined term that is usually used for process only, where the arc travels at least 360 degrees around the work piece without interruption.
Consequently, processes, which interrupt the full 360-weld sequence such as for better puddle control (often used for MIG/MAG welding, using the down-hand welding sequence in 2 half-circles), can not truly be called orbital welding.
Orbital Tube Welding
Understanding the basic principles behind orbital tube welding may help you arrive more rapidly at the optimum weld procedure for your specific application.
by Bernard Mannion and Jack Heinzmann III
Orbital welding was first used in the 1960s, when the aerospace industry recognized the need for a superior joining technique for aerospace hydraulic lines. A mechanism was developed in which the arc from a Tungsten electrode was rotated around the tubing weld joint. The arc welding current was regulated with a control system thus automating the entire process. The result was a more precision and reliable method than the manual welding method it replaced.
In the early 1980s, Orbital welding became practical for many industries when combination power supply/control systems were developed that operated from 110 VAC. These systems were physically small enough to be carried from place-to-place on a construction site for multiple in-place welds.
Modern day orbital welding systems offer computer control, where welding parameters for a variety of applications can be stored in memory and later called up for a specific application. Hence, the skills of a certified welder are thus built into the welding system, producing enormous numbers of identical welds and leaving significantly less room for error or defects.
Orbital Welding Equipment
In the orbital welding process, tubes/pipes are clamped in place, and an orbital weldhead rotates an electrode and electric arc around the weld joint to make the required weld. An orbital welding system consists of a power supply and an orbital weldhead.
The power supply/control system supplies and controls the welding parameters according to the specific weld program created or recalled from memory. This supply provides the control parameters, the arc welding current, the power to drive the motor in the weldhead, and switches the shield gas(es) on/off as necessary.
Orbital weld heads are normally of the enclosed type, and provide an inert atmosphere chamber that surrounds the weld joint. Standard enclosed orbital weld heads are practical in welding tube sizes from 1/16 inch (1.6 mm) to 6 inches (152 mm) with wall thicknesses of up to .154 inches (3.9 mm). Larger diameters and wall thicknesses can be accommodated with open style weld heads.
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Reasons for Using Orbital Welding Equipment
There are many reasons for using orbital welding equipment. The ability to make high quality, consistent welds repeatedly, at a speed close to the maximum weld speed, offer many benefits to the user:
Many other reasons exist for the use of orbital equipment over manual welding. For example, applications where inspection of the internal weld is not practical for each weld created. By making a sample weld coupon that passes certification, the logic holds that if the sample weld is acceptable, that successive welds created by an automatic machine with the same input parameters should also be sound.
General Guidelines for Orbital Tube Welding
For orbital welding in many precision or high purity applications, the base material to be welded; the tube diameter(s); weld joint and part fit-up requirements; shield gas type and purity; arc length; and Tungsten electrode material, tip geometry, and surface condition may already be written into a specification covering the application.
Each orbital welding equipment supplier differs slightly in recommended welding practices and procedures. Where possible, follow the recommendations of your orbital equipment supplier for equipment set-up and use, especially in areas that pertain to warranty issues.
Note that, this section is only intended as a guideline for those applications where no specification exists. The engineer responsible for the welding must create the welding set-up, and derive the welding parameters, in order to arrive at the optimum welding solution.
WELDING BASICS AND SET-UP
The Physics of the GTAW Process
The orbital welding process uses the Gas Tungsten Arc Welding process (GTAW), as the source of the electric arc that melts the base material and forms the weld. In the GTAW process (also referred to as the Tungsten Inert Gas process - TIG) an electric arc is established between a Tungsten electrode and the part to be welded. To start the arc, an RF or high voltage signal (usually 3.5 to 7 KV) is used to break down (ionize) the insulating properties of the shield gas and make it electrically conductive in order to pass through a tiny amount of current. A capacitor dumps current into this electrical path, which reduces the arc voltage to a level where the power supply can then supply current for the arc. The power supply responds to the demand and provides weld current to keep the arc established. The metal to be welded is melted by the intense heat of the arc and fuses together.
Material Weldability
The material selected varies according to the application and environment the tubing must survive. The mechanical, thermal, stability, and corrosion resistance requirements of the application will dictate the material chosen. For complex applications, a significant amount of testing will be necessary to ensure the long-term suitability of the chosen material from a functionality and cost viewpoint.
In general, the most commonly used 300 series stainless steels have a high degree of weldability with the exception of 303/303SE, which contain additives for ease of machining. Four hundred series stainless steels are often weldable, but may require post weld heat treatment.
Accommodation must be made for the potential differences of different material heats. The chemical composition of each heat batch number will have minor differences in the concentration of alloying and trace elements. These trace elements can vary the conductivity and melting characteristics slightly for each heat. When a change in heat number is made, a test coupon should be made for the new heat. Minor changes in amperage may be required to return the weld to its original profile.
It is important that certain elements of the material be held to close tolerances. Minor deviations in elements, such as sulfur, can vary the fluid flow in the weld pool, completely changing the weld profile and causing arc wander.
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