Welding – making welded assemblies
In our firm we have a separate department for welding assemblies up to a total weight of 50 tonnes. Welding can be carried our using different methods such as MIG, MAG, TIG and spot welding. Our employees have excellent training and regularly attend continued vocational development courses. Rime is certified to DIN 18800-7 and therefore fulfils all requirements for the welding and construction of structural members. We have a wide-ranging offer of metal sheet and plate machining and can produce in-house component parts required for the welding of assemblies. This saves unnecessary transport and as a result, you receive your order more quickly.
Rime is certified to DIN 18800-7, Class C!
Welding is the word to describe the creation of a joint between two individual metal parts. The action of extreme heat causes the material edges that are to be joined to liquefy and they are joined in the melted condition. When the metal cools the components bond together inseparably.
The early beginnings
In this epoch of human history, the development of the different cultures could not have been more different. In a time in which in Central Europe the first farmers were settling and were able to fire clay vessels, in Egypt and Mesopotamia the first advanced cultures were flourishing, and already had the knowledge necessary to make weapons and cult objects out of metal. Initially, only soft metals could be worked. Archaeological findings show that jewellery objects made of copper and gold were joined by spot heating. Soldering was the first method used to join together two metal parts.
The technique of getting metal parts made of iron to fuse permanently has a history lasting many millennia. The Egyptians under Pharaoh Thutmosis I already knew and used the technique of forge welding. Joining two metals with only a forge fire and simple tools required a great deal of experience. It took a lot of skill to take the metal out of the fire at the precise time at which it had reached the right temperature for welding. Another difficulty was the fact that the parts had to be protected from the effect of air. Metal heated until it glows oxidises very quickly in air. For many centuries, fine sand was used to prevent air contact.
Later the blacksmiths managed to achieve improved shielding against air by using the mineral borax, which contains sodium. The technique of forge welding spread across the whole world in time and survived for more than 3,500 years until the beginning of the 20th century. This process is still used today to produce Damascus steel.
At the end of the 19th century the first steel alloys were produced, which could no longer be worked with the conventional welding method. Even higher temperatures had to be generated to weld these steels. Industrialisation also demanded faster production methods, as it was no longer economical to heat complete metal parts in a forge fire in order to weld them. The growing size of the steel parts also required other welding procedures.
In 1782, the German naturalist Georg Christoph Lichtenberg discovered the electric arc, which he generated using the electrostatic generator that he had developed. But it took almost a further 85 years before this light phenomenon could be used specifically for welding.
In 1892, the ethine gas was discovered, which is better known as acetylene. Acetylene has a calorific value of over 57,000 kJ per cubic metre and generates a flame with almost 3,200 °C. Acetylene therefore achieves the highest flame efficiency of all available welding gases, which also include propane and methane.
Arc and acetylene welding were very effective welding procedures; however, due to the huge amount of heat produced, they caused severe oxidation of the materials. In 1940, a shielding gas was used for the first time, which prevents the molten metal from reacting with the surrounding air. The first shielding gas to be used was helium. This was the birth of TIG welding, still commonly used today.
What welding symbols are there?
Ideally, the best case is when a weld is not needed. However, in many constructions, joining material parts together by welding is unavoidable. Therefore, good and thorough Weld preparation is essential. The weld must be prepared so that the material edges are fused and the weld cross-section if completely filled.
Equally, structural calculations are necessary to determine the load bearing capacity. This is the only way to produce high-quality welded assemblies.
Symbols were developed, so that the welders in the production department receive all the necessary instructions. With this symbolic language, weld joints, the type of welds and the welding procedure can be marked. This means that all necessary information can be passed on perfectly to the production department.
The symbol for a weld is an arrow. A separate arrow must be drawn for each weld. The tip points directly to the joint. The horizontal part shows symbolically the workpiece from the front (1) and the rear (2).
By adding a symbol, it is possible to define which weld is to be made. In total there are almost 30 such symbols. The most important ones are summarised in the following diagram. With these symbols the welder knows which weld is to be made for the visible and concealed side of the workpiece.
The shape of the weld can also be represented symbolically. The shaping or form requirements differ from workpiece to workpiece. In our CAD department the optimum weld shape is determined and specified for the welders. The optical appearance is also important and must be noted.
The depth of the weld is also defined as part of the weld preparation. This dimension is abbreviated with the letter “s”. The weld depth can be given either as the height “a” or as the leg length “Z”.
What welding positions are there?
It is obvious that not every weld can be welded horizontally and in the flat position. Occasionally other welding positions can be necessary for the design or construction of assemblies and Adapters, as the workpiece cannot be placed in the required position due to its size or shape.
In order to inform the welders, in which position the workpiece has to be welded, abbreviations were introduced in the EN 6947 and EN 287 standards, from which the welding position can be clearly read.
This diagram clearly illustrates the welding positions:
What is a back support?
In bridge and shipbuilding, a welding procedure was developed, which enables particularly stable welds to be made. There are several advantages to jointing metal parts using a back support (also called backing).
Back supports are strips of steel, copper or ceramic, which are fixed under the weld gap, in order to support the liquefied metal. If such support strips are used, a far higher welding current can be used, which penetrates the material through to the root run. This means that more material is liquefied and the metal pars therefore bond completely across the whole workpiece thickness.
A back support can be used to level out air gaps up to 10 mm wide. In this way, differences in the alignment of the sheets or plates can be corrected in a time-saving way. The welds welded with a back support are of excellent quality. After removing the support the underside of the weld becomes visible; it must almost never be reworked and satisfies all requirements, both qualitatively and visually. In this way, valuable working time can be saved.
How does arc welding work?
Arc welding is the general term used to describe all welding procedures, in which the material is fused on by means of an arc. There are two prerequisites for arcs to occur. The voltage level and the distance between the electrodes play a decisive role. The greater the voltage, the greater the distances that can be covered by the arc.
In welding the arc is generated between an electrode and the workpiece. By moving the welding electrode towards the material, from a certain distance the gas is ionised. This makes the air conductive and the voltage can jump across the distance between the electrode and the workpiece. The ionisation produces a plasma, which is hot enough to melt steel and therefore to weld. Initially the electrode used for this welding procedure was made of simple steel, but this melted off due to the strong heat during the welding process.
The direct contact with air was a disadvantage, because string reactions occurred with the oxygen in breathable air (atmospheric air??). The oxygen led to slag formation and burn-off and also reduced the quality of the weld, because during welding oxygen was introduced into the weld pool. This oxygen then reacted with the material over time and formed rust, which could cause welds to break.
As this procedure was developed, additives were used in the weld electrodes, which evaporate as an effect of the plasma and formed a shielding gas around the weld pool. Due to the melting the protective substances were also introduced into the liquid metal, which increased the quality of the welds significantly.
There are advantages in welding with electrodes. For example, this procedure can be used in virtually any weather conditions and can even be performed under water.
Welding with electrodes is slower than other welding procedures, but this also means that the weld pool can cool more slowly. This results in less strain and increases the subsequent dimensional accuracy of the component.
What is a Demmeler welding table and what are its advantages?
We have used the patented Demmeler company clamping system for several years. These “Demmeler tables” are modular systems, with which we can clamp workpieces extremely quickly and efficiently.
Advantages of the table
The advantages are manifold and diverse. The tables have a regular pattern of holes in which matching clamping elements can be fixed. This is an advantage especially in series production. The table only has to be adjusted to the workpiece once. After that, each further part can be fixed within a few seconds.
Even complicated shaped parts can be fixed on the table quickly. Among other things, this system includes suitable stops/gauges, setting angles, vices and assembly lifting forks, which make welding significantly easier.
With the latest Demmeler tables the tabletop can even tilt three dimensionally up to 180° and also freely rotate. The advantage of this is that the welder can always weld in the most favourable flat position (PA).
By using the Demmeler clamping technology we have been able to reduce our welding processing times to a minimum and are now far faster and more precise that other firms when it comes to the construction of welded assemblies.