Benefits Gained with Low-Hydrogen Filler Metals Using filler metals with low hydrogen designations helps minimize one source of hydrogen in the welding process Choosing the right filler metal is as much a matter of matching the chemical and mechanical properties of the material being welded as it is considering the available equipment, operator skill set, and service conditions the finished product will encounter. The climate where the welding takes place is also a factor. In certain industries, including pipeline, offshore fabrication, and heavy equipment manufacturing, where increasingly higher strength materials are being used for service in extreme climates and/or where welding takes place in a very hot or cold environment, using a filler metal with low hydrogen levels is imperative. High-strength, low-alloy steels, typically those above 70 ksi, are especially prone to hydrogen-induced cracking — Fig. 1. Using low-hydrogen filler metals, along with implementing the appropriate pre-/interpass and/or postweld heat treatments and minimizing residual stresses in joint design, can help mitigate that risk. Low-hydrogen filler metals are those classified with less than 4 mL of diffusible hydrogen per 100 g of weldment. The most common for highstrength steel applications are shielded metal arc welding (SMAW) electrodes, gas metal arc welding (GMAW) wires, gas-shielded flux cored arc welding (FCAW) wires, and metal cored wires with an H4 designator (for example, American Welding Society AWS E80C-Ni1 H4). The Dangers of Hydrogen Hydrogen is unavoidable in the welding process. In addition to being present in filler metals, hydrogen can also enter the weld pool from the atmosphere and the base material. In exceptionally cold environments, the opportunity for hydrogen to cause problems is of special concern. The critical cooling rate of the weld is faster in colder temperatures, resulting in hydrogen becoming more readily trapped in the completed weld. The increased presence of hydrogen in hot, humid environments is also a factor to consider; the more moisture that is in the air, the greater the chance of hydrogen entering the weld pool. The action by which hydrogen-induced cracking occurs is fundamentally the same no matter how it enters the weld pool. As a highly mobile element, hydrogen is able to diffuse into the atmosphere when the weld pool is still at an elevated temperature. Upon the weld cooling, however, it becomes more difficult to escape, resulting in the hydrogen becoming trapped in the weld and migrating to the grain boundaries and the heat-affected zone (HAZ). When enough hydrogen collects in the weld, and the right amount of stress builds up in a crack-sensitive microstructure, then cracking can occur. Hydrogen-induced cracking typically occurs at temperatures below 600°F (commonly around 300°F or below) and appears within a day or two of completing the weld. It is especially prevalent in high-strength, low-alloy steels because of the presence of martensite, a more brittle microstructure that naturally occurs in steel formation and that can also result from rapid cooling after the welding process. Low-Hydrogen Filler Metal Options Using filler metals with low hydrogen designations helps minimize one source of hydrogen in the welding process — Fig. 2. In today’s marketplace, there are several options available for welding high-strength steels, each with its own characteristics that can also benefit the welding process. Shielded metal arc welding electrodes featuring an 18 classification are a good choice for low-hydrogen applications that require portability. These include AWS E7018, E8018, and E9018 SMAW electrodes, which all include an H4 designation. These products, in addition to offering low hydrogen levels, are usually user-friendly. They have a forgiving arc and low spatter levels and offer good penetration. Some low-hydrogen SMAW electrodes may feature R designators (for example, AWS E7018 H4R), which indicate the product meets additional moisture-resistant requirements, making it even less prone to picking up hydrogen, which could result in cracking. To obtain this designation, the SMAW electrode must have successfully undergone testing that proves it is moisture resistant (within a specified range) after being exposed to 80°F temperature and 80% relative humidity for 9 h. While each has its own benefits and limitations, solid, flux cored, and metal cored wires with low hydrogen levels can also benefit high-strength welding applications that are more crack sensitive. Choosing among these options depends on how low of hydrogen levels are required for the job, as well as 66 WELDING JOURNAL / APRIL 2015 BY DERICK RAILLING
Welding Journal | April 2015
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