Desulfurization of welded metals
Welding is a metalworking process that uses fusion to join two or more metal components together. It is one of the most common ways to fabricate structures and metal products for various applications. In the process of welding, a clean and pure metal surface is required. However, immersion of metals in a welding bath or heat welding can cause the metal to pick up elemental sulfur (S). The presence of sulfur in metallic welds can have detrimental effects on the mechanical and structural properties of the metal. This calls for the desulfurization of welded metals.
The use of desulfurization processes on welded metals aims to eliminate elemental sulfur from the metal surface by converting it into other compounds. The most common methods used for this are vacuum degassing/vacuum arc remelting (VAR), plasma arc remelting (PAR) and chemical de-sulfurization processes. Each of these methods has distinct advantages and disadvantages; some of these are discussed in the following sections.
Vacuum degassing is done in a vacuum chamber with a pressure of 10–3 torr. This process works by removing residual solids and gases from the weld by evaporation and condensation. As the pressure is reduced, atoms occupying the high-pressure spaces within the structure of the weld can break away and escape, taking with them gases dissolved in the molten weld. Vacuum degassing is a relatively cheap and straightforward method and can be used as a prelude to the VAR process.
Vacuum arc remelting (VAR) is a specialized technique used to remelt materials in a vacuum. It works by using an electrical arc between two electrodes, with one electrode carrying a sliver of the material. This arc melts the sliver and its surface can then be cleansed of sulfur. VAR is more effective than vacuum degassing; however, it is also more expensive and time-consuming to set up and operate.
Plasma arc remelting (PAR) works similarly to VAR, but with a more powerful electric arc generated by a plasma. It has the same advantages and disadvantages as VAR, but is even more expensive and time-consuming to set up and operate.
Finally, chemical desulfurization processes involve the use of chemical reagents to separate sulfur from the welds. This is usually done with two steps: first, the sulfur is converted into a chemical compound (the reagent) and second, it is then separated from the weld. However, this method can also be corrosive to the weld and it is generally more expensive and time consuming than either of the other methods.
In conclusion, desulfurization of welded metals is essential to ensure the structural integrity and performance of metallic components. Vacuum degassing, VAR, PAR and chemical desulfurization are all viable methods for this, but each has its own benefits, drawbacks, and costs associated with it. It is important to consider the needs and dynamics of a metallurgic project when selecting the most appropriate desulfurization process.
