TECHNICAL ARTICLE:

Introduction

When selecting precision welding equipment there are two types of system that may be taken into consideration. To start with we will define these two types of system.

1) Rotary Welding Equipment – These systems rotate the component while the welding torch or electrode remains stationary. The advantage of these systems is that you can achieve more consistent results and higher speed. This is because the effects of gravity on the weld pool do not need to be considered.

2) Orbital Welding Equipment – These systems clamp the tube or component being welded and the welding torch or electrode rotate around the component. It is usual to have to alter the welding parameters to take into account the effects of gavity on the weld pool.

In this article we will concerntrate on the orbital wleing process. Orbital welding was first seen during the 1960's when a group of engineers working for a large aircraft company developed a system which automatically rotated a tungsten electrode around a tube. This gave far improved results over manual welding methods. In the 1980’s these fundimental ideas were taken further and designed in to dedicated welding systems. The early systems had analogue controls and programs had to be recorded and an entire program re-entered each time a specific task was performed. This equipment is used to produce excellent repeatability in production and high specification applications.

Typical OD Weld Profile

The TIG Welding Process

Orbital welding makes use of the TIG (Tungsten Inert Gas) Welding Process with DCEN (Direct Current, Electrode Negative) or AC (Alternating Current). DCEN has a constant heat distribution with two thirds of the head being in the work piece and the other third in the electrode. With AC the heat is balanced with polarity switching at a pre-set rate of between 50-200 Hz. The AC balance can then be adjusted in order to control the arc contour and in turn the weld profile.

The process creates an arc in an inert atmosphere between a non-consumable tungsten electrode and the work piece. This arc creates enough heat to melt the work piece and create a weld pool.

The whole process has to be surrounded by and Inert Gas, the most commonly used gas for this Argon. Some gas mixtures which will allow faster welding speeds, increased penetration and give a cleaner weld bead. Most common gas mixtures are Argon/Hydrogen and Argon/Helium in varying ratios. The gas type selected will depend on the material, application and economical factors.

Still the most commonly used Tungsten Electrodes for DCEN welding are Thoriated, despite the health issues with these electrodes. Other types of electrodes can be used, however it is generally considered that Thoriated Tungsten Electrodes perform better in a wide range of applications.

For AC applications the use of Zirconiated tungsten electrodes is most common. It is important to select and electrode type which will provide a stable balled end when welding. Because of the characteristics of AC welding the electrode will not sustain a pointed profile.
Orbital Welding Equipment

To create an Orbital Welding System there are two main components. An Orbital Welding Power Supply to provide the power and control of the system and an Orbital Welding Head which will rotate the torch or electrode and perform the weld cycle.

Power Supply

Modern day Orbital Welding Power Supplies have the capability of being used for all applications that may be required of them. The most up to date equipment, such as the Orbimat 160C will control Welding Current, Rotation Control, Wire Feed and Gas Flow with feed backs from the weld head to maintain a consistent weld profile. These power supplies also have a built in Cooling Unit which provides a liquid coolant to the sensitive parts of the weld head to eliminate heat build up. Additionally the units can store hundreds of weld procedures which include details of the Tungsten Electrode, Gas Type & Flow, Joint Preparation, Parent Material and Filler Materials used. They have extensive integral data acquisition capabilities, this allows weld data logs to be load to a PC Card and easily transferred directly to a PC for interrogation or storage.

Modern power supplies also include an Auto-Program facility which drastically reduces the amount of time needed to develop a suitable weld procedure for application. These systems will take tube size, material type and gas used to achieve near perfect results in seconds. Large well laid out display panels make the equipment welder friendly and easy to use. Other features such as integral printers, fault sensors, key switch and weld head recognition are also standard features of this type of equipment.

Orbimat 160C DC Power Source

For AC applications a AC/DC power supply is used which offers all the standard functions of a DC machine but contains a AC generation module and the operator is able to control AC frequency and AC balance in addition to the the standard controls.

Weld Head

There are three common types of weld head, these combined cover most applications you may encounter. The three types of weld head are Fully Enclosed Fusion Weld Head, Open Arc Wire Feed Weld Head and Tube to Tube Sheet Weld Heads.

Enclosed Weld Head – Fully Enclosed Weld Heads perform the weld in a localised welding chamber formed by the clamping system around the tube being welded. This chamber is filled with inert gas prior to welding which fully protects the outer surface of the tube and the tungsten electrode. These heads also have the additional advantage that when an internal gas purge is being used and gaps in the weld joint are encountered the internal quality of the weld is not degraded as no oxygen is allowed to enter the tube. These style of weld heads are available for tube outside diameters from 2mm to 170mm and it is possible to produce acceptable welds on materials of up to 4mm wall thickness.

Fully Enclosed Weld Head

Open Arc Weld Heads – Open arc weld heads are used for thick wall applications, generally materials with a wall thickness of more than 3.5mm and for materials which require the addition of a filler material. These heads allow the use of a additional filler material and multiple pass weld sequences. The weld torch block can be tilted to 45 degrees to allow fillet welding and the welding of short leg fittings. The tungsten gap is controlled by an arc length controller which ensures a consistent profile, even on oval pipes.

Open Arc Weld Head

Tube to Tube Sheet Weld Head – Tube to Tube Sheet weld heads are used in the manufacture and repair of heat exchangers. Usually a heat exchanger will have a tube sheet with several hundred tubes to be welded to them. The nature of this work is very mundane and repetitive and operator fatigue is a key factor. With the use of a tube to tube sheet weld head this operator fatigue can be eliminated with the welder only having to ensure the tube are fitted correctly and operating the machine. The operator then simply supervises the machine while in operation and can be preparing the next joint while the head is welding another.

Why Use 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:

1. Productivity. An orbital welding system will drastically outperform manual welders, many times paying for the cost of the orbital equipment in a single job.
2. Quality. The quality of a weld created by an orbital welding system with the correct weld program will be superior to that of manual welding. In applications such as semiconductor or pharmaceutical tube welding, orbital welding is the only means to reach the weld quality requirements.
3. Consistency. Once a weld program has been established an orbital welding system can repeatedly perform the same weld hundreds of times, eliminating the normal variability, inconsistencies, errors and defects of manual welding.
4. Skill level. Certified welders are increasingly hard to find. With orbital welding equipment you don't need a certified welder to operator the equipment. All it takes is a skilled engineer with some welding training or a semi-skilled welder.
5. Tracability. Equipment is now available to give complete traceability of all welding carried out. The power supplies used are capable of recording real time data log file which record deviations from the set parameters. These data log files can be either printed with the machines internal printer of recorded on standard PC memory cards and transferred directly onto a PC via a PCMCIA slot of Floppy drive.
   

Quality Assurance For the purpose of quality assurance the above mentioned features are available as standard features of the most up to date equipment and no complicated and expensive interface equipment is required to carry out these operations. Processing plants are often asking contractors for weld mapping on pipe work being installed and this is easier to control with this modern equipment. Each weld performed can be given a unique weld number. Alternatively accumulative data logging may be used. Each log file will detail the date, time, weld number, comment and real time parameter details for easy referencing on completion of a contract. The log files can be printed weld by weld or stored on a PC card and transferred to a PC for interrogation and filing. It is now becoming common for contractors to store all procedures for a whole contract and then copy the log files to a CD to present to their client on completion. The machines contain not only weld schedules but full weld procedures which detail not only welding parameters but also details of the application specification. Details of Gas Type, Gas Flow Rate, Tungsten Electrode Type, Tungsten Electrode Grind Angle and Filler Materials used can be be stored with each welding procedure.

Sample Procedure Print Out

Industries and Applications for Orbital Welding

Orbital welding equipment is suitable for use in any application where a round component is to be welded. However in certain types of application this type of equipment comes into its own. The two main applications where orbital welding offers significant advantages are where large and bulky items need tubes and fittings welded to them and also in the installation of service pipe work.

Industries which use orbital welding equipment include aerospace, pharmaceutical, semi-conductor, vacuum, food & beverage, offshore and automotive.

Internal Gas Purge

Most applications which call for orbital welding equipment also require the introduction of an inert gas to the back of the weld joint. This gas purge will eliminate the degradation of the internal surface of the material. It is also important to control the internal gas purge tightly as this also can effect the quality of the weld produced. There are a range of products available to control the purity of gas used and to minimise the amount of gas used.

For Ultra High Purity (UHP) applications very effective gas filters can be fitted to gas lines to provide a clean and pure gas to the weld area. To minimise gas usage a range of products are available to localise the area of gas shielding around the weld joint. These products often pay for themselves very quickly. Also to allow the operator to ensure the internal gas level is low enough a range of gas analysers are available, these instruments offer two advantages. They let the operator know exactly when the weld cycle can commence and also they give a guarantee that gas levels are within specification.

Analysers are available which simply plug into the power supply and then take full control of the welding cycle. The analyser can be set to send signal to the power supply to start the welding cycle when the oxygen level is a suitably low level. These analysers will also monitor the gas during welding and will either sound an alarm or stop the welding cycle should the oxygen level rise to high during the welding cycle.

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. 400 series stainless steels are often weldable but may require post weld heat treatment.
Accommodation must be made for the potential differences of different material batches. The chemical composition of each 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 sulphur can vary the fluid flow in the weld pool thus completely changing the weld profile and also causing arc wander. Minor changes in sulfur content can change weld pool flow characteristics with a dramatic effect on penetration (the Maragoni effect)

The Effect of Sulphur

Low Sulphur Content (Ranges from 0.001 to 0.008% Sulphur ) Has a negative surface tension temperature coefficient resulting in wide shallow weld profile.
Normal Sulphur Content (Ranges from 0.009 to 0.030% Sulpur ) Has a positive surface tension temperature coefficient resulting in narrow deep weld profile.

Weld Joint Fit-Up

Weld joint fit-up is dependent on the weld specification requirements on tube straightness, weld concavity, reinforcement and drop through. If no specification exists the laws of physics will require that the molten material flow and compensate for tube mismatch and any gap in the weld joint.

Tubing is produced according to tolerances that are rigid or loose according to the application for which the tube was purchased. It is important that the wall thickness is repeatable at the weld joint from part to part. Differences in tube diameter or out-of-roundness will cause weld joint mismatch and arc gap variations from one welding set up to another.

Tube preparation equipment is available which will give a perfectly square end with a burr free inside and outside surface ready for orbital welding. This equipment is essential in applications which require guaranteed repeatability of the weld joint.

When two tubes are butted together for welding, two of the main considerations are mismatch and gaps. In general, the following rules apply:

• Any gap should be less than 5% of the wall thickness. It is possible to weld with gaps of up to 10% (or greater) of wall thickness, but the resultant quality of weld will suffer greatly and repeatability will also become a significant challenge.
• Wall thickness variations at the weld zone should be +/- 5% of nominal wall thickness. Again, the laws of physics will allow welding with mismatch of up to 25% of wall thickness if this is the only challenge but again, the resultant quality of weld will suffer greatly and repeatability will also become a significant issue.
• Alignment mismatch (high-low) should be avoided by using engineering stands and clamps to align the two tubes to be welded. This system also removes the mechanical requirement of aligning the tubes from the orbital weld head.