Copyright Goodheart-Willcox Co., Inc. Chapter 8 Manual Welding Techniques 135 Welding Steels Using DCEN (Straight) Polarity DCEN polarity is used for welding steels because the higher concentration of heat input into the base material results in deeper penetration. DCEN polarity is the best option for obtaining full penetration welds and controllable weld bead profi les. With DCEN polarity, two-thirds of the arc’s heat is concentrated on the workpiece and one-third is concentrated on the electrode. Since the majority of the heat is concen- trated on the work and not on the electrode, smaller electrodes can be used without worrying about elec- trode deterioration. Electrodes Used on Steels For a long time, 1% or 2% thoriated electrodes were recommended for welding steel. When thorium is added to tungsten in small amounts, it increases the electrode’s current-carrying capabilities. Thoriated electrodes do not work well with AC current and are not recommended for use with the newer inverter- type machines. If thoriated electrodes are overheated, the end of the electrode can begin to vaporize and spit small amounts of the electrode into the weld. Grinding thoriated tungsten electrodes results in radioactive dust that can be hazardous. These electrodes usually have a yellow or red color band near the top of the electrode, depending on the percentage of thorium. Today, electrodes with more desirable character- istics are available. Lanthanum is a “rare earth” mate- rial that is not radioactive. Lanthanated electrodes work well for AC or DC currents, have good current- carrying capacities, and can be used universally, eliminating the confusion of selecting an electrode. Lanthanated electrodes usually have a gold, black, or blue color band near the top of the electrode. Other types of electrodes, such as ceriated, zirco- niated, and pure tungsten, all have some desirable traits. Ceriated tungstens usually have a gray color band, zinconiated tungstens usually have a brown color band, and pure tungstens usually have a green color band. Research the base materials to be welded to determine which electrode is best suited for your application. More detailed information about different electrode types and when they should be used was presented in Chapter 3, Auxiliary Equipment and Systems. Amperage ranges must be considered when selecting the size of the electrode. Higher welding amperages require larger electrodes. When preparing an electrode for welding steel, keep in mind that different tip confi gurations result in different bead profi les. For welding steel, it is gener- ally recommended to sharpen the electrode to a point and slightly fl atten the tip. Pointed tips direct the arc in a more concentrated area. Flattening the very tip of the electrode helps concentrate the arc even more. Shielding Gases for GTAW on Steels Pure argon gas can be used for welding metal up to 1/8″ (3.2 mm) thick. When thicker materials are being welded, helium should be added to the shielding gas to improve weld pool control and pene- tration. Helium increases the temperature of the arc, resulting in deeper penetration. The following stan- dard shielding gas mixes can be used: • 75% argon–25% helium • 50% argon–50% helium • 25% argon–75% helium Pure helium gas can be used for automatic welding of steel or steel alloys. Gas purity must be suffi cient to prevent hydrogen pickup. Hydrogen causes porosity and cracking. Techniques for Welding Steels and Steel Alloys 1. Clean the oxide scale from the weld joint imme- diately prior to welding. Clean the entire weld area to bright metal. 2. Select a welding rod that has the required prop- erties. When welding porosity-prone material, always use a welding rod that contains deoxi- dizers to prevent porosity. 3. Preheat low-alloy steels and tool steels to prevent cracking in the weld and heat-affected zones. 4. Weld porosity-prone materials by maintaining the arc on the molten metal. Do not hold the arc on the base metal. On crack-sensitive materials, do not make concave welds. These welds are prone to cracking through the centerline. 5. On multipass welds, always remove any scale on the surface of each pass to reduce the possibility of oxide entrapment. 6. Always fi ll craters on crack-sensitive materials. 7. When welding tool steels, make small stringer beads, (Figure 8-10A), to reduce the expansion of the base metal and prevent cracking. Imme- diately after welding, the weld metal can be peened to reduce shrinkage as the metal cools.