Orbital Welding – The Optimum Solution for Joining Tubes and Pipes.
Our third welding processes and applications tutorial brings us onto a more specialist area of high-quality welding – orbital welding. Tom our welding instructor gives an introduction and advises Paul on some of the key elements of the orbital welding process.
This guide will cover:
What is orbital welding?
Orbital welding has become the accepted method of producing very high-quality joints in pipes.
What is the orbital welding process used for?
To produce a high-quality weld in overhead and down hand positions, frequently in situations of restricted access.
What are the benefits of orbital welding?
Orbital welding was developed to specifically overcome the problems of achieving high weld quality on a repetitive basis on small diameter pipes and tubes, particularly in situations of difficult access and for applications where very high quality of welding is guaranteed.
How does an orbital welder work?
Orbital welding utilises the Gas-Tungsten- Arc-Welding (GTAW) or TIG welding process. The TIG welding power source provides and controls the pre-set welding parameters according to a specific welding programme created and/or stored for each specific job. In this way, a fully repeatable weld is produced every time. In addition to controlling the welding parameters, the power source provides the power to drive the welding head and switches the shielding gas or gases on or off as required
Orbital Welding problems.
Providing that the trials have been done to establish and prove the welding details, then, basically the only problems that should arise are those associated with maintenance of the equipment.
Tom: Today we are going to look at a more specialised process but one which is widely used to produce very high-quality welds on a repeatable basis. What do you think we mean when we talk about Orbital Welding?
- Paul: My feeling is that in this case we are talking about some sort of process whereby welding is carried out on tubes and pipes, which must be more difficult than the normal fabrication that we have discussed so far.
Tom: You are right. Orbital Welding was developed some 50 years ago during attempts to solve problems of poor quality, leaking welds in the fabrication of the X-15 rocket research plane. Over the intervening period, it has become the accepted method of producing very high-quality joints in pipes as newer welding equipment was developed that offered portable combination power supply and computer control system that operated from 110V AC and hence, which could be used on site as opposed to only in a major workshop.
- Paul: Why do you need to go down this route?
Tom: The problem arises, particularly when dealing with small pipes, to produce a high-quality weld in overhead and down hand positions, frequently in situations of restricted access. The answer to routinely producing high quality welds on tubes and pipes is to maintain a good balance between gravitational force and the surface tension of the molten weld metal. Using orbital welding, these problems are overcome with totally enclosed welding heads on a 100% repeatable basis.
- Paul: Is the equipment a lot more complicated and, hence more expensive?
Tom: Orbital welding utilises the Gas-Tungsten- Arc-Welding (GTAW) or TIG welding process, which we talked about when we discussed welding of stainless steel. The main components of an orbital welding system are:
A power supply with integrated computer control.
A welding head – generally this is a closed head but may be an open head on larger (over 200mm diameter pipe).
The TIG welding power source provides and controls the pre-set welding parameters according to a specific welding programme created and/or stored for each specific job. In this way, a fully repeatable weld is produced every time. In addition to controlling the welding parameters, the power source provides the power to drive the welding head and switches the shielding gas or gases on or off as required. Using a closed welding head ensures that an inert gas shield is maintained at all times during welding. In general, standard welding heads are available in a closed format for tubes from 1.6mm to 152mm and with wall thicknesses up to 3.9mm. If it is required to weld large diameters, then this is generally done by using an open head design.
- Paul: If the head is closed, how do you feed the filler wire in?
Tom: In most cases, particularly in the smaller tube sizes, the TIG process is an autogenous one, i.e., no filler is added, the joint being made by using the arc from the non-consumable tungsten electrode to melt both edges of the joint under a gas shield, this is known as a fusion weld.
- Paul: Obviously, pipes and tubes come in a variety of metals. What shielding gas is used?
Tom: Pure argon with a purity level of 99.995% is used (known as 4.5, i.e., “four nines five”) unless you need to weld the more “delicate” metals, such as titanium, tantalum, zirconium, and their alloys which require a purity of 4.8, i.e., 99.998%. If you need to increase the weld energy, 2-5% of hydrogen can be added to the argon. As well as increasing the heat input by 10-20%, this gas mixture offers better penetration and faster welding speeds. These mixtures can also help to protect the molten weld metal from any oxygen. It is not just that simple, however, for when welding mild and carbon steel tubes, these metals absorb hydrogen leading to possible porosity and cold cracking. For this reason, hydrogen mixtures are not recommended for these metals while, for the welding of aluminium and titanium, the admixture of hydrogen is strictly forbidden.
- Paul: So, what would you use in these cases?
Tom. If you need to increase the weld energy then instead of hydrogen, you would add helium to the argon – 20%, 50%, 70% or even pure helium. Helium has no adverse effects on pure titanium or titanium alloys. There is one other problem area and that is Duplex or Super Duplex steels. In these cases, mixtures of argon, helium and nitrogen are used.
- Paul: How does this work?
Tom: Unlike argon, helium is a good heat conductor. The arc voltage under helium is much higher than under argon, so the energy content of the arc is greatly increased. The arc column is wider and allows deeper penetration. Helium is applied for the welding of metals with high heat conductivity like copper, aluminium and light metal alloys. As helium is a lightweight gas, compared to argon its flow rate for identical gas coverage must be increased two to three times.
- Paul: Is the shielding gas from the torch sufficient to protect the weld?
Tom: That’s a good question and I am glad you asked it. While the orbital welding process can thus be relied upon to produce high quality joints, the need to provide protection of the weld underbead by using inert gas purging techniques is often overlooked. The presence of oxygen, nitrogen, and other contaminants can not only affect appearance, but also corrosion resistance and mechanical properties. Properly developed welding procedures thus address the issues by specifying weld purging and, in most applications, even defining the purging equipment.
- Paul: How do you do this weld purging?
Tom: At the very basic level, welders will often make plugs out of carboard discs but, as you can imagine, this is neither a realistic nor totally practical solution – after all, how do you remove the cardboard after welding? No, the correct way to do this is to use professional plugs that can be expanded to fit tightly into the tube. However, the same problem can arise with removing the plug on the far side of the weld without damaging the plug. Another answer is the use of rubber or silicone discs connected by a flexible tube, developed specifically for the application as they are simple to remove after welding. On very small diameter pipes, continuous inert gas flow through the pipe is often used to prevent problems.
- Paul: So, we have chosen to use orbital welding, we know what metals we are welding, and we have the equipment, what next?
Tom: If we are looking at welding small diameter pipes and tubes, the welding process is, as I mentioned earlier, an autogenous process. That means that, in contrast to the situations we have looked at previously, with orbital welding we do not add filler metal. It is therefore essential that you prepare the tube ends to provide a perfect fit and that the pipes/tubes are correctly located and securely fixed. The welding head is then fitted around the joint and clamped securely. If weld purging is being used, then this must be fitted before welding begins. Assuming that this is a job that has been done before, the operator will select the pre-set welding conditions and welding will commence. If it is a new job, then trials will have been made to develop the necessary conditions for that job.
- Paul: How would you summarise the advantages of orbital welding?
Tom: Orbital welding was developed to specifically overcome the problems of achieving high weld quality on a repetitive basis on small diameter pipes and tubes, particularly in situations of difficult access and for applications where very high quality of welding is guaranteed. After all, the majority of applications for this process are in safety critical industries – pharmaceutical, food, beverage, nuclear and semiconductor sectors are particularly demanding of weld quality – where weld failure could be disastrous.
- Paul: We have talked specifically about welding of small diameter tubes and pipes – is this the only area of application?
Tom: For many years, the process was developed for and applied to small diameter using closed heads but gradually over the last 40 years or so, the potential for producing high quality welds on a repetitive basis in larger diameters has been taken up and the process has been widely used in applications such s offshore rigs and pipelines. Here much more complex welding heads are required which constantly rotate around the pipe, adding filler material as they go, but that is for another time.
- Paul: Are there many problems when carrying out orbital welding?
Tom: Providing that the trials have been done to establish and prove the welding details, then, basically the only problems that should arise are those associated with maintenance of the equipment. The tungsten electrode – which is the key to the operation, should be checked and changed regularly to ensure that the welding tip is in good shape. All welding cables and the cable to the remote controls likewise should be regularly checked, so to avoid any accidents and to ensure proper performance. The welding head should be cleaned out and checked and, finally, simple as it sounds, if welding is being supplied with gas from a cylinder, ensure that there is enough gas to complete the job.
- Tom: There we go, a good introduction to a vitally important welding process. I hope you found it interesting and I look forward to your next session.
If you want to find out more about the orbital welding process, the Tulsa Welding School in the USA has a great article, click here to take a look.
Now you know all about the process, if this is something you feel will benefit your business then you will need some equipment! We can offer a range of equipment for sale or hire, give us a call or click here to view our current stock.