Fig. 3 — The factors that can combine to produce cracking. strength, as the resulting weld metal would be quite crack sensitive. When using such consumables, ensure you maintain tight welding controls. On the other hand, fillet welds or partial-joint-penetration (PJP) groove welds on higher-strength steels often can be made successfully and economically by “overwelding” with an “undermatching” filler metal. You can determine the strength of a fillet weld or PJP groove weld by its length, throat size, and filler-metal strength. You can compensate for using a lower-strength filler metal by increasing the weld’s leg size (and resulting throat size) and/or increasing the length. The lowerstrength filler metal would provide such benefits as better ductility, improved weldability, and generally lower cost. In general, matching strength filler metals are often used for low-carbon steels or medium- and high-carbon steels when the weld is in tension. Undermatching strength filler metals are often preferred for high-strength base materials. Using “overmatching” filler metals 54 WELDING JOURNAL / APRIL 2015 Fig. 4 — Common alloy designators for low-alloy filler metals. is generally discouraged and rarely occurs, as these materials could produce welds with more residual stresses and higher susceptibility to hydrogeninduced cracking (HIC). Additional Requirements for Filler Metals Beside strength level, additional filler metal requirements arise, depending on the welding application. Low Hydrogen Sometimes, it’s necessary to specify low-hydrogen filler metals, particularly for use with higher-strength steels. Hydrogen is naturally absorbed into liquid weld metal. Over time, it diffuses out of the solidified weld metal. However, it also can potentially cause cracking. Specifically, cracking can occur when you have a combination of three factors: 1) Elevated hydrogen levels, 2) weld stress/highly restrained joints, and 3) sensitive base material microstructure (high strength, martensitic) — Fig. 3. Sources of hydrogen include hydrocarbons (contaminants on the steel’s surface, such as oil, rust, etc.); excessive moisture in the air and/or shielding gas; and condensation on the filler metal’s coating or flux, in which moisture becomes bound. Note that although condensation also occurs on the steel portion of filler metal, moisture does not become bound to steel. It simply evaporates as it is heated. Therefore, maintaining a certain diffusible hydrogen rating is only a concern with covered electrodes, submerged arc fluxes, and cored wires (because of condensation on the inner flux through the wire’s seam). That is why they have specific instructions for storage and handling. On the other hand, solid-steel electrodes (gas tungsten arc rods, gas metal arc wires, and submerged arc wires) are all considered very low hydrogen and do not require any special packaging or storage other than to prevent Table 1 — Example of a Particular Carbon-Steel Filler Metal’s Published CVN Properties Mechanical Properties — As required per AWS A5.20/A5.20M: 2005 Yield Strength Tensile Strength Elongation % Charpy V-Notch MPA (ksi) MPa (ksi) J (ft lbf) @ –18°C (0°F) @ –29°C (–20°F) @ –40°C (–40°F) Requirements AWS E71T-1C-JH8 27 (20) min. Not Specified AWS E71T-9C-JH8 400 (58) min. 480–655 (70–95) 22 min. Not Specified 27 (20) min. 27 (20) min. AWS E71T-12C-JH8 480–620 (70–90) Not Specified 27 (20) min. Typical Results As-Welded with 100% CO2 485–535 (70–77) 540–585 (78–84) 25–28 135–193 (100–143) 91–164 (67–121) 57–133 (42–98)
Welding Journal | April 2015
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