Welding encyclopedia

1. occupational health and safety

a. Hazards.

As a general rule, welding is almost always associated with strong currents or explosive gases, toxic fumes, dangerous light and heat generation, and splashes of liquid metal. The hazards depend on which welding process is used. Often the welding fumes contain carcinogenic substances. This is always the case, especially when welding high-alloy materials. The use of welding consumables containing chromium and/or nickel in the form of chromates and/or nickel compounds also produces carcinogenic fumes. Acute poisoning by inhalation of dusts with a very high manganese content can lead to inflammatory reactions in the lungs. This toxicity manifests as bronchitis and may develop into fibrosing lung disease. If the extraction system is used properly, the limit value for manganese and its compounds is not exceeded. Nevertheless, a special health examination of the lungs is prescribed for welding personnel - according to (G39)- at regular intervals.

In Germany, the TRK limits for heavy metals must be observed. Many other components are also harmful and must be assessed accordingly (TRGS403, MAK values). TRGS 528, which has replaced BGR 220 (welding fumes), regulates, among other things, the requirements for the welding workplace.

b. Measures.

A risk assessment must be carried out for welding workplaces. All constituents of welding fumes must be taken into account, including titanium dioxide, fluorides, magnesium oxide, calcium oxide, iron oxides and their alloying components such as nickel, cobalt, chromium and manganese. In the case of high-alloy steels, electrode welding should be dispensed with if possible and gas-shielded welding or automated processes should be used instead, because the lack of a sheath around the electrode means that fewer chromates are released. Appropriate expert instruction is mandatory for all dependent employees according to the Occupational Health and Safety Act (ArbSchG); furthermore, proof of training (skilled worker's certificate or course examination of a chamber of crafts) is common. In many industrial sectors, in railway applications, a welding supervisor is required.

Protective glasses are required for oxyacetylene welding to prevent glowing parts or sparks from getting into the eyes. The glasses are coloured so that the welding environment can be observed without glare.

Arc welding produces ultraviolet radiation, which damages the skin, but especially the eyes.

Furthermore, infrared radiation (heat radiation) is produced, which not only causes burns on unprotected parts of the body, but can also damage the retina.

Therefore, protective glasses must be used that shield these two types of radiation. The protection classes for such glasses are defined in the European standard EN 169. For example, protection classes 2 to 8 are provided for oxyacetylene welding, whereas classes 9 to 16 are provided for open arc welding. The protective glasses bear a label characterising the properties of the glass. The information is as follows: Protection class, manufacturer's abbreviation, optical class 98, DIN standard. The modern substitute for protective glasses are automatic welding protection filters.

Since UV radiation also damages the skin, a screen is used that covers the entire face. In front of the actual almost black glass is usually a normal glass that keeps the sparks out and is cheaper to replace. In order to have both hands free, the umbrella can be hinged to a protective helmet or a device worn on the head. In addition, special flame-retardant welding clothing must be worn that safely covers all skin surfaces. Many welding processes are very noisy, so adequate hearing protection is necessary.

Welding also produces very fine dust particles that must be extracted to prevent them from entering the welder's lungs and diffusing from there into the bloodstream. For this purpose, mobile or stationary welding fume filters are used to extract and filter this fine dust. The state of the art today are so-called ePTFE filters (surface filtration). If no effective extraction of the welding fumes can be ensured, the welder must be protected by personal protective equipment in the form of a blower filter device(PAPR). These devices do not protect against oxygen deficiency or harmful gases in shafts and containers. If adequate ventilation is not possible, self-contained breathing apparatus must be worn. Particular care must be taken when flame straightening and preheating with gas burners, in inadequately ventilated confined spaces, as the flame consumes some of the oxygen in the breath

When welding, people in the vicinity must also be protected from the radiation and noise. Welding louvres, welding curtains and soundproof partition systems are available for this purpose. In manual arc welding, special attention must be paid to the electrical hazard to the welder. Although the arc voltage is below the - generally - hazardous range, a number of precautionary measures must be observed, especially when working under special electrical hazards, i.e. for example when working in confined electrically conductive spaces (boilers, pipes, etc.), which are suggested, among other things, in the BGI 553 bulletin of the German Metalworkers' Accident Insurance Association.

In laser welding, the laser beam itself is an additional source of danger. It is usually invisible. While radiation in the near infrared (solid-state lasers, fibre lasers, diode lasers) penetrates the skin and the eye and causes retinal damage even at low intensities (scattered radiation), the radiation of the CO2 laser (mid-infrared) is absorbed on the surface (skin and cornea of the eye) and causes superficial burns. Skin burns from near-infrared lasers are dangerous partly because the radiation is absorbed in deep areas under the skin where there are no temperature-sensitive nerves. Laser welding equipment is usually safely housed (locked safety doors, laser safety windows), it then falls under laser class I and can be safely operated without laser safety goggles.

2. electrode welding and arc welding

Manual arc welding (manual electric welding EN ISO 4063: process 111) is one of the oldest electric welding processes for metallic materials that is still used today. In 1891, Nikolai Gawrilowitsch Slawjanow replaced the carbon electrodes that had been used for arc welding until then with a metal rod that was both an arc carrier and a welding filler. Since the first rod electrodes were not coated, the welding point was not protected against oxidation. Therefore, these electrodes were difficult to weld.

An electric arc between an electrode melting as a filler metal and the workpiece is used as a heat source for welding. The high temperature of the arc melts the material at the welding point. Welding transformers (stray field transformers) with or without welding rectifiers, welding converters or welding inverters serve as welding power sources. Depending on the application and electrode type, welding can be done with direct current or alternating current.

Coated stick electrodes, for example for unalloyed steels according to ISO 2560-A, develop gases and welding slag during melting. The gases from the coating stabilise the arc and shield the weld pool from oxidation by atmospheric oxygen. The welding slag has a lower density than the molten metal, is washed onto the weld and provides additional protection of the weld against oxidation. Another desirable effect of the welding slag is the reduction of the welding shrinkage stresses due to the slower cooling, as the component has more time to redevelop the plastic deformation.

Due to the electron bombardment, the anode (positive pole) heats up more. In most welding processes, consumable electrodes are used as anodes, i.e. the workpiece is used as the cathode (negative pole). In the case of coated stick electrodes, the polarity depends on the electrode coating. If the coating consists of poorly ionisable components, as is the case with basic electrodes, the electrode is welded on the hotter positive pole, otherwise on the negative pole because of the lower current load.

The main field of application for manual arc welding is steel and pipeline construction. Electrode welding is preferred in the assembly area because of the significantly lower welding speeds, as the machine effort is relatively low compared to other processes. Electrode welding can also be carried out faultlessly under unfavourable weather conditions, such as wind and rain, which is particularly important for outdoor work. Another advantage is that - in contrast to other processes - the welding can often still be carried out without defects even if the weld joint is not completely metallically bright.

3. MIG - MAG welding (metal inert gas welding)

Partially mechanised gas metal arc welding (MSG), optionally referred to as MIG (metal arc welding with inert gases, EN ISO 4063: process 131) or MAG welding (metal arc welding with active, i.e. reactive gases, EN ISO 4063: process 135), is an arc welding process in which the melting welding wire is continuously fed by a motor at a variable speed. The common welding wire diameters are between 0.8 and 1.2 mm (rarely 1.6 mm). Simultaneously with the wire feed, the shielding or mixed gas is supplied to the welding point via a nozzle at a rate of approx. 10 l/min (rule of thumb: shielding gas volume flow 10 l/min per mm welding wire diameter). This gas protects the liquid metal under the arc from oxidation, which would weaken the weld. Metal active gas welding (MAG) uses either pure CO2 or a mixed gas of argon and small amounts of CO2 and O2 (e.g. "Corgon"). Depending on their composition, the welding process (penetration, droplet size, spatter losses) can be actively influenced; in metal inert gas welding (MIG), argon is used as the noble gas, and less frequently the expensive noble gas helium. The MAG process is primarily used for steels, the MIG process preferably for non-ferrous metals.

Optionally, flux cored wires, also called tubular wires, can be used for gas metal arc welding (with active gas welding EN ISO 4063: process 136, with inert gas EN ISO 4063: process 137). These can be provided with a slag former and possibly alloying additives on the inside. They serve the same purpose as the coatings of the stick electrode. On the one hand, the ingredients contribute to the welding volume, on the other hand, they form a slag on the weld bead and protect the seam from oxidation. The latter is especially important when welding stainless steels, as oxidation, the "tarnishing" of the seam must be prevented even after the torch has been moved on and thus the shielding gas bell has been moved on.

History of MIG-MAG processes
MIG-MAG welding was first used in the USA in 1948 in the inert gas or noble gas variant, at that time it was also called SIGMA welding (shielded inert gas metal arc).

In the Soviet Union, from 1953 onwards, an active gas was used for welding instead of the expensive noble gases such as argon or helium, namely carbon dioxide (CO2). This was only possible because wire electrodes had been developed in the meantime to compensate for the higher burn-off of alloying elements in active gas welding.

In Austria, by 2005, CMT (Cold Metal Transfer) welding had been developed for series production, in which the welding current is pulsed and filler wire is moved back and forth at high frequency to achieve targeted droplet detachment with low heat input.

4. plasma cutter

The plasma cutter consists of a power source, handpiece, ground cable, power supply line and compressed air supply line. A plasma is an electrically conductive gas with a temperature of about 30,000 °C. The arc is usually generated by a plasma arc. The arc is usually ignited with a high-frequency ignition and constricted at the outlet by an insulated, usually water-cooled, copper nozzle. Some systems also use lift-arc ignition, which is also used in TIG welders. With these units, the torch is placed on the workpiece at the interface and a small current flows that is not sufficient to damage the torch. The gas flow pushes the torch off the workpiece surface, the arc ignites and the electronics of the welding power source increase the current to the strength required for the cut. The high energy density of the arc melts the metal and it is blown away by a jet of gas, creating the kerf. Compressed air is often used as the gas for blowing out. For a better kerf, protective gas mixtures are also used, which prevent or weaken oxidation. A characteristic feature of plasma cutting joints is a rounding of the edge at the entry point.

The process has a number of advantages over other fusion welding processes. In combination with TIG pulse welding and TIG AC welding, any material suitable for fusion welding can be joined. TIG welding produces practically no welding spatter; the health risk from welding fumes is relatively low. A particular advantage of TIG welding is that it does not use a melting electrode. The addition of filler metal and the current intensity are therefore decoupled. The welder can optimally adjust his welding current to the welding task and only has to add as much filler metal as is required at the time. This makes the process particularly suitable for welding root passes and for welding in constrained positions. Due to the relatively low and small-scale heat input, the welding distortion of the workpieces is lower than with other processes. Due to the high weld seam quality, the TIG process is preferably used where welding speeds are less important than quality requirements. These are, for example, applications in pipeline and apparatus construction in power plant construction or the chemical industry.
The TIG welding system consists of a power source, which in most cases can be switched to DC or AC welding, and a welding torch, which is connected to the power source by a hose package. The hose assembly contains the welding power line, the shielding gas supply, the control line and, in the case of larger torches, the supply and return of the cooling water.

5. Plasma welding

In plasma welding (plasma metal inert gas welding, EN ISO 4063: process 151), a plasma jet serves as the heat source. Plasma is an electrically conductive gas that is highly heated by an arc. In the plasma torch, the plasma gas (argon) flowing through is ionised by high-frequency pulses and an auxiliary arc (pilot arc) is ignited. This burns between the negatively poled tungsten electrode and the anode formed as a nozzle and ionises the gas column between the nozzle and the plus poled workpiece. This makes it possible to ignite the arc without contact. Gas mixtures of argon and hydrogen or argon and helium are commonly used as plasma gas to protect the melt from oxidation and to stabilise the arc. The small addition of helium or hydrogen strengthens the penetration and thus increases the welding speed. The constriction of the plasma through the water-cooled copper nozzle to an almost cylindrical gas column results in a higher energy concentration than in TIG welding, which allows higher welding speeds. Distortion and stresses are therefore lower than with TIG welding. Due to the stable burning plasma arc even at the lowest currents (less than 1 A) and the insensitivity to changes in the distance between the nozzle and the workpiece, the process is also used in micro welding technology. With the micro plasma welding process (welding current range 0.5-15 A), sheets with 0.1 mm can still be welded. Plasma pinhole or keyhole welding is used from a sheet thickness of 3 mm and, depending on the material to be welded, can be used up to a thickness of 10 mm for single-layer welding without seam preparation. The main areas of application are tank and apparatus construction, pipeline construction and aerospace.

6. tungsten inert gas welding (TIG)

Tungsten inert gas welding (TIG welding, EN ISO 4063: Process 141) originated in the USA and became known there in 1936 under the name Argonarc welding. It was not until the early 1950s that it began to gain acceptance in Europe. In English-speaking countries, the process is called TIG or GTAW. TIG stands for Tungsten Inert Gas Welding and GTAW for Gas Tungsten Arc Welding. Both abbreviations contain the word "tungsten", which is the English term for tungsten.

There are two ways of igniting the arc, contact ignition and high-frequency ignition:
In historical contact ignition (strike or scribe ignition), similar to electrode welding, the tungsten electrode is briefly struck against the workpiece - like a match - and thus a short circuit is created. After the electrode is lifted from the workpiece, the arc between the tungsten electrode and the workpiece burns. A major disadvantage of this process is that each time the tungsten electrode is ignited, some material is left behind in the molten bath as a foreign body due to the higher melting temperatures of tungsten. Therefore, a separate copper plate, lying on the workpiece, was often used for ignition.
High-frequency ignition has practically completely replaced brush ignition. In high-frequency ignition, a high-voltage pulse generator that applies a high voltage to the tungsten electrode ionises the gas between the electrode and the workpiece, igniting the arc. The high-voltage pulse generator has a harmless current intensity.
A variant of contact ignition is lift-arc ignition. The electrode is placed directly on the workpiece at the welding point. A small current flows, which is not sufficient to damage the electrode. When the torch is lifted, the plasma arc ignites and the electronics of the welding machine increase the current to welding amperage. The advantage of this method is the avoidance of electromagnetic interference that can occur with high-frequency ignition.

Usually the noble gas argon is used for welding, more rarely helium or a mixture of both gases. The relatively expensive helium is used because of its better thermal conductivity in order to increase the heat input. In the case of austenitic stainless steels, small amounts of hydrogen in the shielding gas can reduce the viscosity of the melt and increase the welding speed (it is no longer an inert but a reducing gas, see planned amendment to EN ISO 4063.

The shielding gas is fed through the gas nozzle to the welding point. The rule of thumb is: gas nozzle inner diameter = 1.5 × weld pool width. The amount of shielding gas depends, among other things, on the seam shape, material, welding position, shielding gas and nozzle diameter; information on this can be found in the manufacturer's data sheets.

TIG welding can be done with or without filler metal. As with gas fusion welding, rod-shaped filler metals are usually used for manual welding. However, confusion with gas welding rods must be avoided at all costs, as the chemical compositions differ.

In TIG welding, a distinction is made between DC and AC welding. DC welding with a negatively poled electrode is used for welding all kinds of steels, non-ferrous metals and their alloys. In contrast, AC welding is mainly used for welding the light metals aluminium and magnesium. In special cases, light metals are also welded with direct current and with a positive electrode. Special welding torches with a very thick tungsten electrode and helium as shielding gas are used for this. The positive polarity of the tungsten electrode is necessary for light metals, as these usually form a hard oxide layer with a very high melting point (as with aluminium oxide, magnesium oxide) on their surface. This oxide layer is broken up when the workpiece has a negative polarity, as the workpiece now acts as an electron-emitting pole and negative oxygen ions are discharged.

BGI 746 (Handling of tungsten electrodes containing thorium oxide for tungsten inert gas welding (TIG)) contains information on the safe handling of tungsten electrodes containing thorium oxide for tungsten inert gas welding and describes the necessary protective measures that must be taken in order to exclude possible hazards from handling these electrodes or to minimise them to an acceptable level. This is necessary because of the low radioactivity of thorium and the harmful dusts of the heavy metal. Due to the availability of tungsten electrodes alloyed with lanthanum or rare earths, the use of thorium-alloyed tungsten electrodes can be dispensed with today.

TIG - impulse welding

A further development of TIG welding is welding with pulsating current. In TIG pulsed welding, the welding current pulses between a base and pulse current with variable frequencies, base and pulse current heights and widths. The pulse frequency, pulse width and pulse height can be adjusted separately. TIG pulsing with variable current can only be carried out with special welding equipment (welding inverters). The finely adjustable heat input in TIG pulse welding enables good gap bridging, good root welding and good welding in constrained positions. Welding defects at the beginning and end of the seam, as with tube welding, are avoided.

All descriptions refer to manual or partially mechanised TIG welding with filler metal mainly ø 1.6 mm. With impulse welding of light metals (namely: AA6061), melting on the surface can be achieved and thus through-melting can be avoided with thin sheets < 1.0 mm. Especially with fillet welds, the corner is captured sooner than with standard welding with constant current. Sheets with a thickness of 0.6 mm were also butt welded perfectly, as the stability of the arc as well as the concentrated heat input allow a small defined melt pool. Tacking is the main problem when there is a gap and thus oxygen has access on the root side. The influence of the tungsten electrode alloy and the composition of the shielding gas is important; these parameters influence the process significantly.

7. purpose of welding

In the definition, a distinction is made between joint welding and build-up welding according to the purpose of welding. Joint welding is the joining (DIN 8580) of workpieces, for example with a longitudinal pipe seam. Deposition welding is the coating (DIN 8580) of a workpiece by welding. If the base material and the coating material are different, a distinction is made between hardfacing, cladding and buffer layers.

Fusion welding is welding with localised melt flow, without the application of force with or without filler metal of the same type (ISO 857-1). In contrast to soldering, the liquidus temperature of the base materials is exceeded. In principle, all materials that can be transferred to the molten phase can be joined by welding. Welding is most frequently used for the cohesive joining of metals, thermoplastics or glass, both for consumer products and for the joining of glass fibres in communications technology. Depending on the welding process, the connection is made with a weld seam or a spot weld, and in the case of friction welding also over a wide area. The energy required for welding is supplied from the outside. The term path welding is used for automated welding when robots are used.

a. Influence of the welding on the base material.

The base material can have adverse properties due to the welding heat and the subsequent relatively rapid cooling. Depending on the material and the cooling processes, for example, hardening or embrittlement can be caused. In addition, high residual stresses can occur at the transition between the weld seam and the base material. This can be countered by a variety of countermeasures in production. These include technical welding measures, such as the selection of suitable welding processes, filler materials and post-weld treatment processes, preheating of the workpiece, as well as design and production measures, such as the correct welding and thus assembly sequence, selection of suitable seam shapes and, if available, the selection of the correct base material.

b. Service life extension through post-treatment methods.

The operational strength and service life of dynamically loaded, welded steel structures is in many cases determined by the weld seams, especially the weld seam transitions. By targeted post-treatment of the transitions by grinding, blasting, shot peening, high-frequency hammering, etc., the service life can be considerably increased by simple means in many constructions.

c. Weldability of the steel.

Steels with a carbon content of more than 0.22% are only considered weldable to a limited extent; additional measures such as preheating are required. However, the carbon content of the steel alone does not make any statement about the weldability, as this is also influenced by many other alloying elements. The carbon equivalent (CEV) is therefore taken into account for assessment. For many components, depending on the design and material, additional measures are required to prevent cracking and fractures (terrace fractures), preheating or slow cooling, stress relieving or buffer welding. In general, high-alloy or higher-alloy steels are more difficult to weld and require special knowledge and controls from the fabricator. For this reason, among others, a responsible welding supervisor is appointed in all companies in addition to the mandatory certified welders. Without an appointment, the company owner is automatically liable welding supervisor. From class B onwards, specially trained welding personnel, such as welding engineers/technicians, must be employed to ensure the necessary technical supervision of the welding work.