Proper Selection of Low Alloy Electrodes

Low alloy electrodes are often selected to match the properties of the steel to be welded rather than matching the exact chemical composition of the steel. These properties (i.e., strength, toughness, creep, and corrosion resistance) reflect the type of service for which the steel is intended. The letternumber suffix of the electrode classification gives an indication of that service. Whenever possible, the electrode should be selected on the basis of the appropriate strength levels and the intended service of the weldment. Service Conditions - The large family of steels that are sold in the as rolled, controlled, cooled condition have a 50,000 psi minimum yield point and 70,000 psi minimum tensile strength. Electrodes that deposit low hydrogen weld metal of those strength levels are used to weld them. Some of the low alloy high strength steels are intended for use at subzero temperatures. Nickel bearing low hydrogen electrodes (C1, C2, C3 types) are available for such low temperature applications. Chromium molybdenum low alloy steels are used for moderately high temperature service. Piping, tubing, boilers, etc., that are used extensively in power generation plants, are fabricated from these steels. Chrome-moly hydrogen electrodes (B1, B2, B3, etc.) are produced to weld these steels. Many bridges and outdoor structures are constructed from “weathering” grade steels. These are low alloy steels that, on exposure to the atmosphere, develop a thin, tightly adhering layer of rust that prevents further rusting and eliminates the need for painting. Low alloy electrodes with additions of chromium and copper are available for welding these steels. (Note – most weathering steel is welded with 1% Ni, 8018-C3 electrodes). Quenched and tempered low alloy steels usually develop high strength with good toughness. These types are used where a substantial saving in the weight of the structure is important. Quite often, but not exclusively, this steel is used by the military. One of the more exotic applications for quenched and tempered low alloy steels is in the fabrication of the pressure hulls for nuclear submarines. The “m” series of high tensile low hydrogen electrodes is intended to weld these steels. High tensile line pipe for the transmission of oil and gas is being used with greater frequency today. Low alloy cellulosic electrodes of the 7010 and 8010 variety are used for filed welding. Joint Design – In fillet welding of high strength quenched and tempered steels, toe cracking alongside the welds (see Figure 1) is frequently a problem. The toe cracking is caused by the high strength weld metal having a higher yield point and tensile strength than the steel. When the weld area shrinks on cooling from the welding temperature, something must give, and because the yield and strength levels of the steel are lower than those of the weld metal, cracking occurs in the heat affected zone of the steel. The solution to this problem is to use a lower strength weld metal and increase the fillet size to meet the weld joint strength requirements. Figure 1. Toe Cracking With a somewhat lower strength weld metal as the filler, the yield point of the weld metal is reached during the shrinkage on cooling. The weld metal stretches without overloading in the heat affected zone of the steel and there is no cracking. Figure 1. Toe Cracking Equipment – The electrode selected will operate only on the appropriate power source. Table 1 lists the type of current for which each class of electrodes is designed. Table 1. Current Requirements for AWS Electrode Classes Electrode Class Current EXX10-X* DCRP EXX11-X AC or DCEP EXX13-X AC or DC either polarity EXX15-X DCEP EXX16-X AC or DCEP EXX18-X AC or DCEP EXX20-X AC or DCEN(horizontal fillet) AC or DC either polarity(flat) EXX27-X AC or DCEN(horizontal fillet) AC or DC either polarity(flat)