1. Occupational safety.

a. Driven.

As a general rule, welding almost always involves strong currents or explosive gases, toxic exhaust fumes, dangerous light and heat generation, and splashes of liquid metal. The risks depend on which welding process is used. Welding fumes often contain carcinogenic substances. This is always the case, especially when welding high-alloy materials. The use of welding filler materials containing chromium and/or nickel in the form of chromates and/or nickel compounds also produces carcinogenic fumes. Acute poisoning caused 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 progress to fibrous lung disease. If the extraction system is used correctly, the limit value for manganese and its compounds will not be exceeded. Nevertheless, a special health examination of the lungs is required for welding personnel on a regular basis in accordance with (G39).

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

b. Measures.

A risk assessment must be prepared for welding workplaces. All components of the welding fume must be taken into account here, including: Titanium dioxide, fluorides, magnesium oxide, calcium oxide, iron oxides and their alloy components such as nickel, cobalt, chromium and manganese. With high-alloy steels, electrode welding should, if possible, be avoided and switch to inert gas welding or automated processes, as fewer chromates are released due to the lack of a coating on the electrode. According to the Occupational Safety and Health Act (ArbSchG), appropriate expert instruction is mandatory for all employees; Furthermore, proof of training (skilled worker certificate or course examination from a chamber of crafts) is common. In many industrial sectors, for railway applications, welding supervision is required.

When welding oxyfuel, you need protective glasses to ensure that no glowing parts or sparks get into your eyes. The lenses are colored so that you can observe the welding environment without glare.

Arc welding produces ultraviolet radiation that damages the skin, 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 both types of radiation. The protection classes for such glasses are specified in the European standard EN 169. Protection classes 2 to 8 are required for oxyfuel welding, while classes 9 to 16 are intended for open arc welding. The protective glasses have a label that characterizes the properties of the glass. The information is as follows: protection class, manufacturer's code, optical class 98, DIN standard. The modern replacement for protective glasses are automatic welding protection filters.

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

Welding also creates the finest dust particles that have to be sucked out so that they don't get into the welder's lungs and from there diffuse into the bloodstream. For this purpose, mobile or stationary welding smoke filters are used to extract and filter this fine dust. The state of the art today are so-called ePTFE filters (surface filtration). If effective extraction of welding fumes cannot be ensured, the welder must be protected by personal protective equipment in the form of a forced air filter device (PAPR). These devices do not protect against a lack of oxygen or harmful gases in shafts and containers. If adequate ventilation is not possible, self-contained breathing apparatus must be worn. Particular caution is required when setting the flame and preheating with gas burners in inadequately ventilated, confined spaces, as the flame consumes some of the oxygen you breathe

When welding, people in the surrounding area must also be protected from radiation and noise. There are also welding slats and welding curtains as well as soundproof partition systems. When carrying out manual arc welding, particular attention must be paid to the electrical hazard to the welder. Although the arc voltage is below the - generally - dangerous range, a number of precautionary measures must be observed, especially when working where there is a particular electrical hazard, for example when working in narrow electrically conductive spaces (boilers, pipes, etc.), which are suggested, among other things, in the information sheet BGI 553 of the metal workers' liability insurance association.

When laser welding, the laser beam itself is an additional source of danger. He is usually invisible. While near-infrared radiation (solid-state laser, fiber laser, diode laser) penetrates the skin and the eye and causes retinal damage even at low intensities (scattered radiation), the radiation from the CO2 laser (mid-infrared) is absorbed on the surface (skin and cornea of ​​the eye) and causes superficial burns. Skin burns caused by near-infrared lasers include: also dangerous because the radiation is absorbed in deep areas under the skin where there are no temperature-sensitive nerves. Laser welding machines are usually safely housed (locked protective doors, laser protective windows), they then fall under laser class I and can be operated safely without laser safety glasses.

2. Electrode welding and arc welding.

Manual arc welding (MAR welding EN ISO 4063: Process 111) is one of the oldest electrical welding processes for metallic materials that is still used today. In 1891, Nikolai Gavrilovich Slavyanov replaced the carbon electrodes that had previously been used for arc welding with a metal rod that was both an arc carrier and a welding filler. Since the first stick electrodes were not covered, the welding point was not protected from oxidation. Therefore, these electrodes were difficult to weld.

An electric arc between an electrode that melts as filler material 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 carried out with direct current or alternating current.

Covered stick electrodes, for example for unalloyed steels according to ISO 2560-A, develop gases and welding slags when melted. The gases from the coating stabilize the arc and shield the weld pool from oxidation by atmospheric oxygen. The welding slag has a lower density than the melt, is floated onto the weld seam and provides additional protection of the weld seam against oxidation. Another desired effect of the welding slag is the reduction in welding shrinkage stresses due to the slower cooling, as the component has more time to reverse the plastic deformation.

The electron bombardment causes the anode (positive pole) to heat up more. In most welding processes, consumable electrodes are operated as anodes, i.e. the workpiece is operated as a cathode (negative pole). For covered stick electrodes, the polarity depends on the electrode coating. If the coating consists of components that are difficult to ionize, as is the case with basic electrodes, the electrode is welded on the hotter positive pole; otherwise, because of the lower current load, on the negative pole.

The main area of ​​application for manual arc welding is steel and pipeline construction. Electrode welding is preferred in the assembly sector due to the significantly lower welding speeds, as the mechanical effort is relatively low compared to other processes. Electrode welding can still be carried out without errors even under unfavorable 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 welding joint is not completely bare metal.

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

Semi-mechanical gas metal arc welding (MSG), either as MIG (metal welding with inert gases, EN ISO 4063: Process 131) or MAG welding (metal 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 tracked by a motor with a variable speed. The common welding wire diameters are between 0.8 and 1.2 mm (less commonly 1.6 mm). Simultaneously with the wire feed, the protective or mixed gas is supplied to the welding point via a nozzle at approx. 10 l/min (rule of thumb: protective gas volume flow 10 l/min per mm of welding wire diameter). This gas protects the liquid metal under the arc from oxidation, which would weaken the weld. Metal active gas welding (MAG) works with 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), the noble gas used is argon, and more rarely the expensive noble gas helium. The MAG process is primarily used for steels, the MIG process is preferred for non-ferrous metals.

Alternatively, cored wires, also known as tube 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, if necessary, alloy additives on the inside. They serve the same purpose as the coverings of the stick electrode. On the one hand, the ingredients contribute to the weld volume, on the other hand, they form a slag on the weld bead and protect the seam from oxidation. The latter is particularly important when welding stainless steels, as oxidation, the “tarnishing” of the seam, must be prevented even after the torch has been moved on and the protective gas bell has been moved on.

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

In the Soviet Union, from 1953 onwards, an active gas, namely carbon dioxide (CO2), was used for welding instead of expensive noble gases such as argon or helium. This was only possible because wire electrodes have now been developed that compensate for the higher erosion of alloy elements during active gas welding.

In Austria, CMT (Cold Metal Transfer) welding was developed for series production by 2005, in which the welding current is pulsed and filler wire is moved back and forth at high frequency in order 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 around 30,000 °C. The arc is usually ignited with a high-frequency ignition and is constricted at the exit by an insulated, usually water-cooled, copper nozzle. Some systems also use lift-arc ignition, which is also used in TIG welders. With these devices, the torch is placed on the workpiece at the interface and a small current flows that is not enough to damage the torch. The gas stream pushes the torch away from the workpiece surface, the arc ignites, and the welding power source electronics increase the current to the level required for the cut. Due to the high energy density of the arc, the metal melts and is blown away by a gas jet, creating the kerf. Compressed air is often used as the gas for blowing out. To achieve a better kerf, protective gas mixtures are also used, which prevent or weaken oxidation. Characteristic 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 conjunction with TIG pulse welding and TIG AC welding, any material suitable for fusion welding can be joined. TIG welding produces virtually no weld spatter; The health impact of welding fumes is relatively low. A particular advantage of TIG welding is that it does not work with a consumable electrode. The addition of welding filler metal and the current strength are therefore decoupled. The welder can optimally adjust his welding current to the welding task and only has to add as much welding filler as is currently required. This makes the process particularly suitable for welding root passes and for welding in forced positions. Due to the relatively low and small-scale heat input, the welding distortion of the workpieces is less than with other processes. Because of the high quality of the weld seams, the TIG process is preferred where welding speeds are less than the 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 direct or alternating current welding, and a welding torch that is connected to the power source via a hose package. The hose package contains the welding power line, the protective gas supply, the control line and, for 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 electric arc. In the plasma torch, the plasma gas (argon) flowing through it is ionized by high-frequency pulses and an auxiliary arc (pilot arc) is ignited. This burns between the negatively polarized tungsten electrode and the anode designed as a nozzle and ionizes the gas column between the nozzle and the plus-polarized workpiece. This makes it possible to ignite the arc without contact. Gas mixtures of argon and hydrogen or argon and helium are used as plasma gas, which protect the melt from oxidation and stabilize the arc. The small additions of helium or hydrogen increase the penetration and thereby increase the welding speed. The constriction of the plasma by the water-cooled copper nozzle into an almost cylindrical gas column results in a higher energy concentration than with TIG welding, allowing higher welding speeds. The distortion and stresses are therefore lower than with TIG welding. Due to the plasma arc burning stably even at the lowest current intensities (less than 1 A) and the insensitivity to changes in the distance between the nozzle and the workpiece, the process is also used in microwelding technology. With the micro plasma welding process (welding current range 0.5-15 A), sheets with a thickness of 0.1 mm can still be welded. Plasma taphole or keyhole welding is used for sheet metal thicknesses of 3 mm and above and, depending on the material to be welded, can be used for single-layer welding without seam preparation up to a thickness of 10 mm. The main areas of application are container and apparatus construction, pipeline construction and space travel.

6. Tungsten - Inert Gas Welding (TIG).

Tungsten inert gas welding (TIG welding process, EN ISO 4063: Process 141) comes from the USA and became known there in 1936 under the name Argonarc welding. It was only at the beginning of the 1950s that it began to become established in Europe. In English-speaking countries the process is called TIG or GTAW. TIG stands for Tungsten Inert Gas Welding and GTAW stands for Gas Tungsten Arc Welding. The word “tungsten” can be found in both abbreviations, which is the English term for tungsten.

There are two ways to ignite the arc, contact and high frequency ignition:
In historical contact ignition (strike or scribe ignition), similar to electrode welding, the tungsten electrode is briefly struck on the workpiece - like a match - and thus a short circuit is created. After the electrode is lifted from the workpiece, the arc burns between the tungsten electrode and the workpiece. A major disadvantage of this process is that every time it is ignited, some material gets stuck on the tungsten electrode, which remains as a foreign body in the melt pool due to the higher melting temperatures of the tungsten. Therefore, a separate copper plate lying on the workpiece was often used for ignition.
High-frequency ignition has practically completely replaced string ignition. With high-frequency ignition, the gas between the electrode and the workpiece is ionized using a high-voltage pulse generator that applies a high voltage to the tungsten electrode, thereby igniting the arc. The high-voltage pulse generator has a safe current level.
A variant of contact ignition is the lift arc ignition. The electrode is placed directly on the workpiece at the welding point. A small current flows, which is not enough to damage the electrode. When the torch is lifted, the plasma arc ignites and the welding machine's electronics increase the current to the welding amperage. The advantage of this method is that it avoids electromagnetic interference that can occur with high-frequency ignition.

The noble gas argon, more rarely helium or a mixture of both gases is usually used for welding. The relatively expensive helium is used to increase the heat input due to its better thermal conductivity. For austenitic stainless steels, small amounts of hydrogen in the protective gas can reduce the viscosity of the melt and increase the welding speed (this is no longer an inert gas, but a reducing gas, see planned change to EN ISO 4063.

The protective gas is directed through the gas nozzle to the welding point. The rule of thumb is: gas nozzle inner diameter = 1.5 × melt pool width. The amount of protective gas depends, among other things, on the shape of the seam, material, welding position, protective gas and nozzle diameter; Information about this can be found in the manufacturer’s data sheets.

When TIG welding, you can work with or without filler metal. For manual welding, rod-shaped additives are usually used, as is the case with gas fusion welding. However, confusion with gas welding rods must be avoided at all costs, as their chemical compositions differ.

When it comes to TIG welding, a distinction is made between direct current and alternating current welding. Direct current welding with a negatively polarized electrode is used for welding all types of steel, non-ferrous metals and their alloys. In contrast, alternating current welding is primarily used to weld the light metals aluminum and magnesium. In special cases, light metals are also welded with direct current and a positive electrode. Special welding torches with a very thick tungsten electrode and helium as a protective gas are used. 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 aluminum oxide, magnesium oxide) on their surface. This oxide layer is broken when the workpiece is polarized to negative, as the workpiece now acts as an electron-emitting pole and negative oxygen ions are removed.

BGI 746 (Handling tungsten electrodes containing thorium oxide in 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 to exclude possible hazards from handling these electrodes or minimize them to an acceptable level. This is necessary because of the low radioactivity of thorium and the harmful dust 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 now be dispensed with.

TIG - impulse welding

A further development of TIG welding is welding with pulsating current. In TIG pulse welding, the welding current pulsates between a basic and pulse current with variable frequencies, basic and pulse current heights and widths. The pulse frequency, pulse width and pulse height can be adjusted separately. TIG pulsing with a variable current curve can only be carried out with a special welding system (welding inverter). The finely adjustable heat input in TIG pulse welding enables good gap bridging, good root welding and good welding in awkward positions. Weld seam defects at the start and end of the seam, as in pipe welding, are avoided

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

7. Purpose of welding.

When defining the term, a distinction is made between joint welding and build-up welding depending on 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 applied material are different, a distinction is made between deposition welding of armor, cladding and buffer layers.

Fusion welding is welding with a localized melt flow, without the application of force, with or without a similar welding consumable (ISO 857-1). In contrast to soldering, the liquidus temperature of the base materials is exceeded. In principle, all materials that can be converted into the molten phase can be connected by welding. Welding is most often used for the material connection of metals, thermoplastics or glass, both in consumer products and for connecting glass fibers in communications technology. Depending on the welding process, the connection is made with a weld seam or a welding point, or with friction welding also over a flat area. The energy required for welding is supplied from outside. The term web welding is used when using robots for automated welding.

a. Influence of the weld on the base material.

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

b. 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. Through targeted post-treatment of the transitions by grinding, blasting, shot peening, high-frequency hammering, etc., the service life of many constructions can be significantly increased using simple means.

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 provide any information about weldability, as this is also influenced by many other alloying elements. The carbon equivalent (CEV) is therefore taken into account for the assessment. For many components, depending on the design and material, additional measures such as preheating or slow cooling, stress-relieving annealing or buffer welding are required to avoid cracking and fractures (terrace fractures). In general, high or higher alloy steels are more difficult to weld and require special knowledge and controls from the fabricator. This is one of the reasons why, in addition to the mandatory certified welders, a responsible welding supervisor is appointed in all companies. Without an order, the company owner is automatically liable as the welding supervisor. From class B onwards, specially trained welding specialists, such as welding engineers/technicians, must be employed to ensure the necessary technical support for the welding work.