Arc Welding Processes Handbook. Ramesh Kumar Singh

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      1

      Introduction to Welding Processes

      1.1 Synopsis

      The chapter introduces the most common welding and joining processes, by discussing the acceptable definition of welding, and the elementary understanding of skill development steps required to be a welder or a welding machine operator.

1.2 Keywords

      Joining processes, definitions, welding, arc welding, arc efficiency, heat, heat affected zone (HAZ), solidification.

1.3 Welding

      When we speak of welding, various images comes to our minds. Depending on persons’ knowledge and experience with the process that can be various, simple or complex. But one thing that can be common to all those images and pictures is that the process of joining two pieces of metal to create a useful object.

      This establishes one aspect of the term welding, that is, that the welding is a metal joining process. Let us explore a little more about what is welding, and how it is different from other Joining processes?

      There have been discussions and sometimes arguments on describing if welding is an art or a science. Mundane as it might appear the question is pertinent and, in my experience, some well-meaning experts often miss the point as to which part about the term “welding” they are referring about to support their arguments. Welding as the physical and practical part of joining two materials in most part is an art, it requires dexterity in hand, and hand-eye coordination to do a good job. However, the study of the heat and melt flow solidifications prediction, prediction of material behavior under heating and cooling cycles associated with the term welding is a science, an essential pat under the science of physics. Hence it is both an art, and a science of joining metals by use of adhesive and cohesive forces between metals by welding, brazing, and soldering some of these joining processes produce metallurgical bonds. Person with the balanced knowledge of both science, and art parts of welding is expected to do much better work on either side of the argumentative divide. Further we get into the depth of the study, the line of separation from art to physics starts to become more evident.

Both process metallurgy and physical metallurgy is involved in welding. Welding is a unique metallurgical activity as it involves a series of metallurgical operations similar to metal production, like steelmaking and casting but in a rapid succession and on a very small scale. In science side of welding the thrust of the study is on the materials behavior during application of localized heat, and cooling and solidification physics.

1.4 Defining Welding

      The AWS definition for welding is “a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone and with or without the use of filler material”.

      Welding is often compared in a very rudimentary way with casting. The comparison with casting involves the fact that in welding a volume of molten metal is solidified (cast) within the confines of a solid base metal (mold). The base metal may have been preheated to retard the cooling rate of the weld joint just as in casting molds are preheated to slow down cooling and reduce “Chilling” of the casting. Upon solidification, the weld deposit or casting can be directly put into service, as the welds are often used in as-welded condition or may be heat-treated or worked further on as required. However, such comparison is not an accurate depiction of welding process, nor it is a fair comparison. For example, in welding the base metal “mold” is part of the weld, unlike the mold of a casting, which is removed after solidification, so unlike casting process, what happens to the “mold” is of significant importance in welding. Unlike casting, in welding the solidification and the nucleation of weld metal takes effect on the basis of the base metal grain structure that is just adjacent to the molten metal of welding and a unique set of metallurgy is created in the base metal that is heated to above austenitic temperature range, this small band of base metal is called heat affected zone (HAZ).

      Welding involves small area relative to the full size of the structure, the base material. Thus, a weld is a very small mass of metal, mostly two metals that are heated very rapidly by intense heat and cooled rapidly, this rapidly heated and cooled small area often overlap each other in succession to create yet another complex metallurgical condition. The dissipation of heat is by all three modes; Conduction, Radiation and Convection. Often the large surrounding mass of colder base metal is heated by conduction process, which is the major source of heat transfer from weld. The heating and after welding the cooling process are dynamic, equilibrium conditions are seldom seen in conventional welding operations, in fact welding conditions represent a great departure from equilibrium. That is the reason weld zones often display unusual and verity of structures and properties, all this within the confines of a very small area affected by welding process.

      It is thus important that a welding personnel have a very good understanding of “Heat” in welding. The understanding of the heat generation and physics of welding are important steps in making of a good welding engineer, and it helps being a good welder as well.

Welding is carried out based on a well thought out and specific plan in order to attain the required material properties. Many regulatory and industrial specifications have well developed process to get the plan in activation. Such plans are called Welding Procedures, and a well laid out sequence of operation is established for the welding qualifications, of both the procedure’s ability to meet required metallurgical and mechanical properties and also a welder’ ability to repeatedly produce the quality of weld desired through that welding procedure. Following is a brief discussion on welding procedures and their role in welding application.

1.5 Welding and Joining Processes

There are number of different approaches to welding, some of them are near universal in their application to most common materials, and are capable of adjusting to number of variables to be used on different positions, and conditions, while others are very specific and are no so universal in their application. With the welding we have included some other material joining processes that are in fact not a welding process. These are very often encountered in the industrial environment, and are often demanded that an accomplished welder knows how to use these processes. The Figure 1.1 below shows various welding and joining process.

The Table 1.1 below list s various welding and joining processes grouped as per the mode of energy used for that specific welding process. The table also includes other joining process that do not use Electric as the source of energy for joining. And there is other that are distinguished by the way they transfer the molten metal in to the metals being joined.

1.6 Arc Welding

      The arc welding group includes eight specific processes, each separate and different from the others but in many respects similar. An introduction to those basic arc welding processes is presented here for some of those most common first-generation arc welding processes. Note that further variations have been made in some of these processes, some of them are discussed further in the book, but there are others that are proprietary developments, the information is covered СКАЧАТЬ