198 Modern Welding Copyright Goodheart-Willcox Co., Inc. Transfer (STT®). Miller Electric Manufacturing Com- pany calls their process Regulated Metal Deposition (RMD™). Figure 8-4 shows the current during a short circuit cycle of the modifi ed short circuiting transfer process. Inductance is the property in an electric circuit that slows down the rate of the current change. Increas- ing inductance in a welding machine slows down the increase of the welding current. Decreasing the induc- tance increases the rate of change of the welding current. Current rises too rapidly when too little induc- tance is used. The maximum current will be high. The pinch force is so great that the molten metal at the end of the electrode explodes, resulting in a great deal of spatter. Current does not rise fast enough when too much inductance is used. The molten end on the elec- trode is not heated suffi ciently because the maximum current is too low. An ideal short circuiting transfer rate and pinch force can be obtained by properly balancing the inductance and slope. Shielding gas also has an effect on short circuiting transfer. Carbon dioxide (CO2) can be used as a shielding gas for short circuiting transfer of carbon and low-alloy steels. CO2 produces greater penetration but creates more spatter than an inert shielding gas. Mixtures of argon and CO2 are often used because they provide a good combination of improved penetration with mini- mal spatter. Welding stainless steel with short circuiting transfer usually requires a mix of three gases. A typical mixture is 90% helium, 7 1/2% argon, and 2 1/2% CO2. Inert shielding gases are required for all nonfer- rous base metals. Nonferrous base metals are those in which iron is not the main element. This includes all metals except steels, steel alloys, and cast irons. Add- ing helium to argon increases the penetration. Mix- tures containing only argon and helium are used only on nonferrous base metals. Modifi ed Short Circuiting Transfer Advancements in inverter power source technology have made modifi cation of the short circuiting trans- fer process possible. Inverter welding machines can alter or modify the current output of the power sup- ply. This ability allows an inverter power source to be used for modifi ed short circuiting transfer. The power source uses feedback information to determine when to change the current from a low background current to a high peak current and to a near zero current. Peak current is necessary to establish the arc after a short circuit. Background current maintains the arc and is signifi cantly less than the peak current. The average of these two currents is less than the current value set during the standard short circuiting process covered earlier. This results in less heat introduced into the weld joint and signifi cantly reduced spatter. Different power source manufacturers have their own names for this modifi ed process. The Lincoln Electric Company calls their process Surface Tension A B C D E A B C D E A B E A C D A Peak time Peak current Pinch current Background current Tail-out speed The Lincoln Electric Company Figure 8-4. The graph at the top of this figure shows the changes in the welding current during a single STT® short circuit. The high-speed photos in the center of the figure correspond to the points in the graph above. The graph at the bottom of the figure shows the changes in voltage and current and their effects on the electrode, arc, and weld pool.