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Section 2.2
Welding Materials
Last Revised: 11/04/2014
There is quite a lot to fully understanding welding materials, but in keeping with the theme that simple is better for beginning engineers, we will endeavor to stay focused on those pieces of information necessary to get a jump start into designing welds. Be aware that there is much more to the topic than what is presented in this text. Discussion about the types of welds and their capacities are left for our discussion on the design of welds later on.
The welding process and its materials are governed by the American Welding Society's (AWS) specification AWS D1.1: Structural Welding Code - Steel. The AISC specification covers design procedures. The design procedures are highly dependent on the process and material requirements of AWS D1.1.
The Basic Welding Process
Welding is the process of uniting two metal parts by melting the materials at their interface so that they will bond together. A filler material is typically used to join the two parts together. The parts being joined are referred to as base metal and the filler is referred to as weld metal. Since structural welding is typically done by an electrical arc process, the weld metal is typically supplied via weld electrodes, sometimes known as welding rods.
We'll discuss this more later, but the quality of weld is highly dependent on a number of factors, one of which is the nature of the base metals and the weld metal used to join them.
The AWS has developed specifications for what metals can be connected by what electrodes. This information is contained in AWS D1.1. For this text, we will work with basic information however, you should spend time studying AWS D1.1 if you are planning on designing welds professionally. For now, use either a 60 ksi or 70 ksi electrode for steels with Fu in the neighborhood of 60 ksi (ASTM A36 steel and others) and 70 ksi electrodes for steels with Fu between 65 ksi (ASTM A992 and others) and 75 ksi. The AWS specification also has provisions for developing weld procedures for other materials and configurations. In practice, you should consult the AWS specification for selecting the electrodes to be used with your selected materials.
Weld Electrodes
As will be discussed later, we will focus on two primary forms of structural welding: The Shielded Metal Arc Welding (SMAW) and Submerged Arc Welding (SAW) processes. The electrodes used for these two processes are very different in their makeup.
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Figure 2.2.1 |
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The one thing that you need to know at this point is that part of the designation of an electrode is its specified minimum tensile strength, Fu. The main criteria we will use for selecting weld metals is to find one that somewhat matches the Fu of the base metals.
For the SMAW process, the electrode designation takes the form: E__XX, where the blank contains the Fu of the base metal and the XX is a number of more importance to the welder than the engineer. For example, and E60XX electrode has an Fu of 60 ksi. SMAW process electrodes are available in Fu values of 60, 70, 80, and 100 ksi. Figure 2.2.1 shows the electrode ends that attach to the welding machine for some E70XX electrodes.
For the SAW process only the first one or two significant figures of Fu is used. SAW process electrodes go by the designator F_XX. The common versions include F6XX, F7XX, F8XX, F10XX, F11XX which have Fu values of 60, 70, 80, 100, and 110 ksi respectively.
We will go into electrode selection and use more in the chapter on weld design.