The submerged arc welding process is a high productive and highly flexible process for welding steel plate. Discuss the validity of the above statement.
Submerged arc welding (“SAW”) is a fusion welding process (Jeffus, 2004 p.736). The main feature of this welding process is the production of high heat from an arc between the work “and a continuously fed filler metal electrode” (Jeffus, 2004, p.736). A more technical definition is given in the following statement, “An arc welding process which produces coalescence of metals by heating them with an arc or arcs between a bare metal electrode or electrodes and the work” (Miller Welds, 1982, p.1)
The following reasons help explain why SAW enables the worker to have more flexibility and therefore achieve a higher productivity rate. The reasons are listed as follows: a) higher deposition enhances welding speed or production; b) deep penetration in some cases may eliminate join preparation; c) excellent mechanical properties for high quality code and X ray requirements; and d) improves welding operator comfort and appeal (Miller Welds, 1982, p.1).
High productivity is not only limited in the speed of completion but also in the quality of the work that is made possible by this technology. Consider the following benefits. SAW enables the welder to fabricate large, thick sections, from 10 mm to more than 508 mm thick (Jeffus, 2004, p.735) SAW also enables the welder to “cover large areas on tanks and pressure vessels with welded covering of special alloys” (Jeffus, 2004, p.735). As a result it is possible to lower the total cost of fabrication because the main components are made of less expensive metal but the surface of the finished work is made of expensive alloy. It produces high quality work because it enables the welding of two different types of metal.
Another major advantage of SAW is that it requires minimal operator protection. This is possible because of the heavy blanket of granular flux which is a major component of the SAW process. The heavy granular flux covers all the arc light and the only evidence that someone is welding two pieces of metal together is the occasional flash that can be seen from time to time. As a result, many welders can work close to each other without having to worry about the effect of arc flash. Furthermore, the welder need not wear a helmet. Thus, workers can see better and this improves the safety of the work environment. The need to wear a helmet can easily affect vision and movement. The heavy protection required in ordinary welding processes can restrict movement. The cumbersome clothing can significantly reduce the performance of the welder as it can contribute to fatigue.
In conventional welding methods there is a need for “forced ventilation” in order to protect the workers. But with SAW this precaution is virtually eliminated (Jeffus, 2004, p.738). As a result high productivity is achieved because it is possible for a greater number of welders to work together in a confined space. The technology also enables businessmen to reduce their expenses when it comes to heating and cooling costs because they can design their building without the need for extensive ventilation. Ventilation must be top priority even with SAW. In a workplace that uses equipment and other synthetic materials it is always prudent to have adequate ventilation. But in the case of welding, the by-products of the welding process can be hazardous to the health of the workers. But with this technology the fusion of the metal can be achieved without the need to release unwanted gas and materials to the immediate environment.
The high deposition rate refers to the amount of welding material that can be deposited. This is made possible by the use of large diameter wires. The standard system can generate more than 40 lb/hr. It is considered as one of the highest deposition rates in this type of technology. It is important to have a high deposition rate because it greatly affects the work area that is covered. In large structures the welding of the joints requires the continuous deposition of weld material. A small deposition rate can only cover a small part of the requirement. But with a high deposition rate a substantial amount of weld material is delivered into a particular area and therefore the work can be completed at a much faster rate.
High productivity can also be measured on the basis of material use. The SAW is a more efficient use of material because there is no spatter. In addition, there is no reason to cleanup the mess afterwards due to the fact that majority of the electrode has been transferred and became part of the weld deposit. Furthermore, the unused granular flux can be retrieved and reused. One can just imagine how much time can be saved if there is no need to clean up the work area. Thus, workers can continue work and focus on one project after the other without the need to worry about the accumulation of unwanted materials in the work area.
The SAW technology can produce higher quality welds. In fact, many industries use it on “structural iron, pressure vessels, cryogenic cylinders and in many critical applications” (Jeffus, 2004, p.741). The importance of the SAW can be seen in its ability to satisfy the requirements of the most demanding client. Consider the safety requirements of pressure vessels and cryogenic cylinders and one can appreciate the technology behind SAW.
Briefly describe the problem related to the submerged arc welding of stainless steel plate to carbon steel.
There are various problems encountered with welding stainless steel and carbon. The first issue is the low electrical conductivity of stainless steel. When compared to carbon steel, the reverse is true. Due to the low electrical conductivity of stainless steel there is faster generation of heat when using the same current. Thus, in welding stainless steel there is a need to utilize a lower welding current. Another issue is thermal conductivity. Welders are able to conclude that “there is lower thermal conductivity for stainless steel when compared to carbon steel” (American Society for Metals, 1976, p. 273). As a consequence, heat is conducted away from the weld zone in slower manner as compared to carbon steel.
Another issue is the melting temperature of stainless steel. It is important to consider the melting temperature of the metal because it is necessary to produce fusion for welding. In comparison to carbon steel, it requires a lower temperature range to melt stainless steel. Furthermore, it requires greater electrode force to bring the “work-metal surfaces together in the required intimate contact at points of welding than is required for carbon steel” (American Society for Metals, 1976, p. 273).
Another issue relates to thermal expansion. Stainless expand and contract as a result of changing temperature more readily than carbon steel. The expansion and contraction and the slower heat diffusion because of thermal conductivity can lead to greater thermal stress and warping. As a result care is needed to adjust the techniques used for the stainless steel and the carbon steel. Adjustments must be made based on the different reactions of the different types of steel. Failure to do so will result in unwanted structure variations.
The flux cored welding and manual metal arc welding of steel share the same technology. Discuss this statement.
The flux cored arc welding (“FCAW”) is a type of fusion welding. The FCAW is a process in which “weld heating is produced from an arc between the work and continuously fed filler metal electrode” (Jeffus & Bower, 2010, p.114). Manual arc welding or shielded metal-arc welding (“SMAW”) is one of the most commonly used welding process in fabrication (Kaushish, 2008, p. 301). In this process the arc is developed “between a consumable coated metal electrode and work piece” (Kaushish, 2008, p.301). Using high temperatures the small part of the base metal of the work piece melts. At the same time, the end-part of the electrode melts. The result is tiny globules or drops of molten metal that passes through the arc and reaches the joint wherein coalescence will occur. The FCAW and the SMAW share the same technology and this can be seen in the function of the flux. Welders are able to conclude that “the function of the flux in FCAW is similar to the electrode covering SMAW” (Kou, 2003, p.22). In the case of SMAW the electrode covering protects the molten metal from air (Kou, 2003, p.22). The same thing occurs with the flux core of FCAW (Kou, 2003, p.22).
FCAW is deemed to be more productive than SMAW (Singh, 2012, p.159). However, equipment cost for FCAW is higher. The setup and the operation are more complicated with the FCAW as compared to the SMAW. At the same time the FCAW is limited when it comes to the operating distance from the electrode wire feeder. In addition, FCAW may be limited by the availability of suitable filler wire and flux combinations for various metal alloys (Sing, 2012, p.22).
Briefly discuss the structure variations in the heat affected zone when two pieces of steel are butt welded.
In welding it is important to develop technology that does not only fuse two metal pieces together but also to provide a by-product that can handle the stress of the work load applied to it. It is therefore important to know the strength of the butt weld. The welder must determine if it is defined by the weld metal, the heat affected zone or the parent metal. In most cases, through the use of common structural steels, the weld metal is usually stronger (Hicks, 1999, p.80). However, its width is usually small. The insignificant size of the area that it covers is the reason why its presence does not influence the strength of the whole joint. Nevertheless, in the case of higher strength steels there is a possibility that the welder can obtain weld metals of lower strength when compared to the parent metal. In the same manner, the heat affected zone may be weaker than the parent metal.
The heat affected zone (“HAZ”) is developed adjacent to the fusion zone. In the HAZ the parent metal has not melted yet. However, it has been subjected to elevated temperatures. The exposure to high temperatures has occurred in a brief period of time. Due to the variations in the temperature as well as the duration of exposure, the welder must expect problems. It is like having a metal casting in a metal mould with an abnormal and widely varying heat treatment (DeGarmo, Black & Kohser, 2012, p.852).
The metal adjacent to the HAZ may experience sufficient heat. As a result the welder can observe structure and property changes. An example of a structure and property changes are 1) phase transformation; 2) re-crystallization; 3) grain growth, precipitation or precipitate coarsening; 4) brittle parts; and 5) cracking. It is easy to understand why these structural variations occur. It is due to the fact that the parent metal was exposed to various temperatures in different durations. It is therefore important to point out that due to the structural variations mentioned earlier there is a possibility that the HAZ no longer posses the desirable qualities of the parent material (DeGarmo, Black and Kohser, 2012, p.852). This is important because the strength of the metal and the integrity of the design are important factors when considering the safety of those who use that particular equipment or finished product.
In the case of the non-melted metal the welder must realize that it does not assume the properties of the parent material. As a consequence the HAZ is often considered as the weakest area in the weld joint. In butt welds this is the area that one can find the weakest link in the whole structure. Unless there were obvious defects in the weld deposit, brought about by the use of defective materials, the common source of problems can be traced to the HAZ. In most cases, the failure originates from the HAZ.
References
American Society of Metals 1976, Source book on stainless steel, ASM Press, New York.
De Garmo, P, Black, J & Kohser R 2012, Degarmo’s Materials and processes manufacturing, John Wiley & Sons, New Jersey.
Jeffus, L, 2004, Welding: principles and applications, Delmar Learning, New York.
Jeffus, L, & Bower L 2010 Welding skills, processes and practices, Cengage Learning, Ohio.
Hicks, J 1999, Welded joint design, Abingdon Publishing, Cambridge.
Kaushish, J 2008, Manufacturing processes, Prentice Hall, New Jersey.
Kou, S 2003, Welding metallurgy, John Wiley & Sons, New Jersey.
Miller Welds 1982, Submerged arc welding, Web.
Singh, R 2012, Applied welding engineering, Butterworth-Heinemann, Oxford.