Shielded metal arc welding is often the first choice for a great variety of welding applications because the equipment is inexpensive, portable, and versatile. The process provides a wide range of electrodes, parameters, and techniques that can have a positive influence on the cost of welding.
In some instances, choosing a welding process other than shielded metal arc welding is the best economic decision. Other processes may be used to advantage in the following situations:
1. When the weldment requires a large volume of filler metal and results in increased labor and material costs, unless only a few joints will be welded.
2. When welding light-gauge sheet, which is more economically welded with flux-cored or metal cored electrodes.
One way to evaluate the economic performance of a joining process for a weldment is to determine the cost per unit length of the weld must be computed. This calculation requires approximating the cost of filler metal, equipment and labor. In addition, consideration must be giving to special circumstances, for example, if an accelerated completion date impacts the cost of financing.
To compute the labor cost, the operating factor (defined as the arc time divided by the total working time; for SMAW it is typically 30%) and the deposition rate (the amount of weld metal added to the joint per unit of arc time) must be known or approximated. To compute the electrode material cost the price must be known, the amount (total weight) of electrode approximated from the joint volume, and the deposition efficiency (the ratio of the weight of metal deposited to the weight of electrode used, typically 0.60 for SMAW). Deposition rates for the particular electrodes, if not printed on the packaging, are readily available from the manufacturer. Figure 2.19 shows some typical values.
The convention within industry for the shielded metal arc welding process is to report the deposition efficiency with no adjustment for stub loss, that is, simply state the amount of metal deposited divided by the amount of electrode consumed. This approximate deposition efficiency must then be adjusted by using a measure for the expected stub loss. The computation to adjust the deposition efficiency for stub loss is based on the original length of the electrode; the efficiency increases with increased length. Table 2.4 shows the approximate deposition efficiency for the various standard electrode lengths when adjusted for stub loss.