WHAT EVERY ENGINEER SHOULD KNOW ABOUT WELDING
From amperage to preheat, there’s more to welding procedures than meets the eye
By Duane K. Miller, P.E.
ITHIN THE WELDING INDUS TRY, THE TERM
“W E L D I N G P ROCEDURE S PECIFICATION ” (or WPS) is used to signify the combination of variables used to make a certain weld. At a minimum the WPS (or “Welding Procedure” or simply “Procedure”)consists of: process (Shielded Metal Arc Welding [SMAW], Flux Cored Arc Welding [FCAW], etc.); electrode specification (AWS A5.1, A5.20, etc.); electrode classification (E7018, E71T-1, etc.); electrode diameter; electrical characteristics ( (AC, DC+, DC-); base metal specification (A36, A572 Gr. 50, etc.); minimum preheat and interpass temperature; welding current (amperage)/wire feed speed; arcvoltage; travel speed; position of welding; post weld heat treatment; shielding gas type and flow rate; and joint design details. The welding procedure is somewhat analogous to a cook’s recipe: It outlines the steps required to make a quality weld under specific conditions. EFFECTS OF WELDING VARIABLES The effects of the variables are somewhat dependent on the welding process being employed, butgeneral trends apply to all the processes. It is important to distinguish the difference between constant current (CC) and constant voltage (CV) electrical welding systems. Shielded metal arc welding is always done with a CC system, while flux cored welding and gas metal arc welding generally are performed with CV systems. Submerged arc may utilize either. • Amperage is a measure of the amount ofcurrent flowing through the electrode and the work. It is a
primary variable in determining heat input. Generally, an increase in amperage means higher deposition rates, deeper penetration, and more admixture. The amperage flowing through an electrical circuit is the same, regardless of where it is measured. It may be measured with a tong meter or with the use of an electrical shunt. The role ofamperage is best understood in the context of heat input and current density considerations. For CV welding, an increase in wire feed speed will directly increase amperage. For SMAW on CC systems, the machine setting determines the basic amperage, although changes in the arc length (controlled by the welder) will further change amperage. Longer arc lengths reduce amperage. • Arc voltage isdirectly related to arc length. As the voltage increases, the arc length increases, as does the demand for arc shielding. For CV welding, the voltage is determined primarily by the machine setting, so the arc length is relatively fixed in CV welding. For SMAW on CC systems, however, the arc voltage is determined by the arc length, which is manipulated by the welder. As arc lengths are increased withSMAW, the arc voltage will increase, and the amperage will decrease. Arc voltage also controls the width of the weld bead, with higher voltages generating wider beads. Arc voltage has a direct effect on the heat input computation. • The voltage in a welding circuit is not constant, but is composed of a series of voltage drops. Consider the following example: assume the power source delivers a totalsystem voltage of 40 volts. Between the power source and the welding head
Modern Steel Construction / May 1997
or gun, there is a voltage drop of perhaps 3 volts associated with the input cable resistance. From the point of attachment of the work head to the power source work terminal, there is an additional voltage drop of, say, 7 volts. Subtracting the 3 volts and the 7 volts from theoriginal 40 leaves 30 volts for the arc. This example illustrates how important it is to ensure that the voltages used for monitoring welding procedures properly recognize any losses in the welding circuit. The most accurate way to determine arc voltage is to measure the voltage drop between the contact tip and the work piece. However, this may not be practical for semiautomatic welding, so voltage...
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