ORBITAL WELDING OF STAINLESS STEEL USING THE TIG PROCEDURE1>

Orbital welding is a process in which the electric arc travels in a circle around the workpiece describing a circle, that is, where the electric arc travels for an angle of at least 360 degrees around the workpiece without interruption. Orbital welding was first used in the 60s of the last century when a group of engineers working for a large airline company developed a system that enabled automatic rotation of the TIG torch around the pipe, which was a significant improvement over manual TIG welding at the time. Orbital welding is primarily used when the demands on the quality of the welded joint are high in the mechanical, metallurgical and geometric sense. These requirements are not only focused on mechanical properties and quality control requirements, such as ventilation, but also on corrosion properties and the aesthetic appearance of the welded joint. The main reasons for using orbital welding are for most users: flat, uniform and homogeneous root weld.

Welding procedures used today in orbital weldingeither as mechanized or semi-automated processes are:/p>

1. TIG welding process with or without additional material (cold and hot wire)
2. TIG procedure in a narrow groove (hot wire)i>
3. MIG/MAG welding process
4. Plasma welding procedure (only in special cases in HV position - wall)

Due to the need for good controle weld melts, the orbital welding process is used mostly in combination with TIG welding process/strong>with cold wire or without additional material while respecting all the rules of the TIG welding procedure by selecting the welding gas, cleanliness and preparation of the joint. The weldability of the material and the structural requirements of the weld metal, which are important for the mechanical and corrosion properties of the welded joint, must also be taken into account. The rotation around the workpiece is performed by a part of the equipment for orbital welding, which we call orbital welding heads. The orbital welding head consists of a fixed part that serves to fix the head on the workpiece and a rotating part that carries the welding torch. In this way, the welding torch can describe a circle around the workpiece with its movement. In practice, we can encounter a method of welding that could be called orbital welding, where the workpiece rotates while the torch is stationary, which contradicts the definition of orbital welding.

Howeverim, in this case it is still a welded joint that forms a circle and is created by a minimum rotation of 360O, but now of the workpiece, not of the torch or the electric arc. In this case, the welding torch can be placed in the most favorable position regarding the influence of the force of gravity on the weld melt and the configuration of the workpiece. The advantage of this system is that the results can be more constant and it is possible to achieve higher welding speeds, but it is largelack of flexibility of application.
 

APPLICATION, POSSIBILITIES AND LIMITATIONS

The areas of industry where orbital welding is primarily used are:/p>

• Chemical and oil industry
• Pharmaceutical industry
• Food industry
• Aviation and space industry
• Nuclear industry
• Semiconductor industry/li>

In addition to the abovestrong>orbital welding is often used/strong>and in the following industrial branches and production:

• Pneumatic parts
• Auto industry
• Construction of main pipelines
• Shipbuilding, etc.

The application of orbital welding, as can be seen from the above, is wide. The logical conclusion is that the possibilities of use are enormous, but they are limited both in the financial sense due to the high price of the equipment, as well as in the thickness and type of material, accessibility to the welding joint, preparation possibilities, etc. Due to the specificity of orbital welding, these are circular elements that are join by welding (pipes, elbows, elbows, T-pieces, flanges, various pipe connectors and similar connections).

OPERAMA FOR ORBITAL WELDING

Orbital welding equipment consists primarily of welding power sources and orbital welding heads. One of the tasks of the welding current source is to control and manage the movements of the heads for orbital weldingdigging. Heads can be of different designs and construction solutions. Various additional devices can certainly be part of the equipment for orbital welding.

Power source for TIG welding
Power sources for welding can be direct current (DC) sources intended primarily for welding structural steel, stainless steel and titanium, but they can also be alternating current (AC) sources that can then be used for welding aluminum and aluminum alloys.

Modern power sources for orbital welding must possess the ability to manage and control the welding currents, the rotation speeds of the welding heads, the control of the addition of additional material, the control of the gas flow for welding and root protection, the control of the coolant cooling of the welding heads, etc. Today, the power sources for welding must have the ability to monitor important welding parameters that affect the end result of the quality of the welded joint, print them via an internal or external printer, transfer data to a PC for processing, program memorization, autoprogramming, warning messages, etc.

ORBITAL WELDING HEADS

There are three common types of orbital welding heads that cover almost the entire field of application and needs of orbital welding. These are fully closed welding heads, open welding heads and tube-to-wall welding heads.

Closed welding heads
Closed heads for orbital welding are intended for performing the welding process in a completely closed chamber system. The chamber (closed) part of the head is filled with shielding welding gas before welding so that the entire welded joint is completely protected from atmospheric influences during the welding process. These heads are primarily used for welding stainless steel and titanium, precisely for the reason of protecting the surface (face and root) of the welded joint from oxidation.

ONve orbital welding heads are used exclusively for welding without additional material, and are therefore used for material thicknesses up to 3 mm, (max. up to 4 mm), and for external pipe diameters from (1.6) 2 to approx. 170 mm depending on the manufacturer.

Open heads for orbital welding
Open heads for orbital welding are usually used for welding materials that need to be welded with the use of additional material for technological reasons, for wall thicknesses above 3 mm and for multi-layer welding. Depending on the manufacturer, the welding torch can be rotated between 35O and 45O, which gives the possibility of welding corner welds. Maintaining the height of the distance of the non-fusible tungsten electrode from the work piece is maintained mechanically.

Amongtherefore, in multi-layer welding, it is necessary to achieve oscillation and automatic control of the net distancefree tungsten electrodes from the workpiece. In this case, the welding torch must be mounted on a motorized cross support. The motors must be controllable by the welding power source. When creating a welding program, in addition to the standard parameters of swing width, swing frequency, holding in certain positions, the height, i.e. the distance of the non-fusible tungsten electrode from the work piece, must be maintained electronically, i.e. there must be automatic welding voltage control (AVC).

Orbital heads for welding pipes into pipe walls
These heads are used for the manufacture and repair of heat exchangers of various designs and purposes. It is common for pipe rocks to have hundreds to tens of thousands of pipes that must be welded (pipe bundle). For welders, it is an extraordinary effort if they have to weld manually, but the size of the pipes, the small distance between them, etc. greatly complicate the work of the welder, who, even with the best skill and will, cannot provide uniform quality and high productivity in this case. As all welded joints are equal, the operator only has to follow the work rules that will give constant quality. Welding heads can be of different designs and for special purposes, such as welding pipes inside the pipe wall, with or without additional material.

GASES FOR ORBITAL WELDING

Argon, Helium, Hydrogen and Nitrogen and theirsmixtures are gases that are often used during the welding process as shielding gases for welding and shielding gases to protect the root of the weld joint. However, in Europe, argon gas and mixtures of argon and hydrogen, and argon and nitrogen are more often used. The reason for this is the high price of helium.

Waspsm that the function of the shielding gas is to protect the weld melt from the influence of the environment (oxidation), by using the mentioned gases and their mixtures, we can additionally influence the geometry of the weld, penetration and heat input into the welded joint, but we can also influence the chemical composition and structure of the welded joint, which must not be neglected when choosing a gas for a specific material to be welded.

Up the mentioned gases and their mixtures are used for the TIG and plasma welding process, while for the orbital MAG process a mixture of argon and carbon dioxide is used.>

It can be said that the gases used for orbital welding do not differ in composition from the usual gases for manual welding for a certain process and for a certain material, but they are still used with a proven chemical composition and a high degree of purity.

Gases used for the purpose of orbital welding:

NETAL PREPARATIONMERCURY TUNGSTEN ELECTRODES

The non-fusible tungsten electrode is the source of the electric arc and is one of the most important factors in orbital TIG welding that is often overlooked by the user. The non-fusible tungsten electrode must be ground by machine. A controlled and reliable start depends on the quality of the grinding and the composition of the infusible tungsten

SELECTION OF WELDING PARAMETERS

During the pipe welding process, one extremely important factor that we must keep in mind is the progressive heating of the pipe in the area of the welded joint zone. This area heats up progressively and accumulates energy from the electric arc during the rotation of the electric arc around the tube.

If the energy input is not reduced during the welding process and the formation of the welded joint, the volume of the weld metal melt will continuously increase, and metallurgical changes will be inevitable. In all positions (9, 12, 3 and 6 o'clock and in between) the balance of the weld metal melt is important, i.e. it is important to maintain a controlled balance between the force of gravity and the forces of surface tension of the weld metal melt with the aim of achieving a uniform height of the root of the welded joint and the geometry of the face of the welded joint connection.

This the balance and control of the size of the weld metal melt can be successfully solved through the appropriate welding parameters, namely: pulse current values, pulse frequency, welding speed in such a way that a separate subprogram is created for each critical area, i.e. parameters for the so-called sectors, for example, from 9 to12 o'clock, from 12 to 3 o'clock, from 3 to 6 o'clock and finally from 6 to 12 o'clock. Reducing the energy intake is usually achieved through the value of the impulse and basic current, but also through the welding speed for certain sectors.>

Common welding parameters that are programmed are: gas before and after, ignition current, heating current, impulse and basic current, welding speed, speed and method of adding additional material, AVC control value, oscillation parameters, and all associated times.

The important fact is that through the use of an orbital welding system, we can easily achieve the ability to produce high-quality welded joints with flawless reproducibility of that quality. It can only be achieved if we follow all the rules of STAINLESS STEEL WELDING and keep all input parameters constant. The rules of use of orbital welding equipment and the way of work must never be violated, changed or shaped at will during operation. Such a way of working will not achieve the desired results, and the equipment for orbital welding will be completely useless.

UkrainianAtko, there are several reasons for using orbital welding equipment:/p>

1. Productivity

2. Quality and repeatability

3. Investing in the level of welding knowledge and skills

4. Flexibility