APRIL 2015 / WELDING JOURNAL 53 wide range of welding procedures, to produce sound, crack-free welds, typically without the need for extra preheat or postweld heat treating (PWHT) operations. On the other hand, medium- to high-carbon and low-alloy, highstrength steels typically have poorer weldability and yield welds that are more crack sensitive. This means you have a smaller selection of possible filler metals and will need to have tighter control of your welding procedures. Also, you might need to slow down the weldment’s cooling rate by using a minimum preheat and minimum interpass temperature control (for multipass welds). If the weld cools too fast, it could develop an exceptionally hard, but brittle, microstructure (i.e., martensite), which may crack from internal stresses. Some type of PWHT operation may also be required. Filler Metal Selection If you know your steel’s classification and grade number, you may be able to reference various resources designed to help you select the appropriate filler metal. As an example, Lincoln Electric publishes Filler Metals Selection Guide (C1.50), which is available on its website — Fig. 1. This guide lists recommended filler metals by arc welding process for several of the more common ASTM and API classifications and grades of steel. If a reference guide is not readily available, then you’ll need to determine what type of steel you have, as well as any mechanical and/or chemical property requirements, welding code requirements, or application requirements. Based on these factors, you then can choose an appropriate filler metal. Carbon Steels Carbon steels can be divided into three groups: low-carbon, mediumcarbon, and high-carbon steels. Lowcarbon (mild) steel has 0.05–0.30% carbon by weight. This group is, by far, the highest quantity produced and most widely used. It is tough, ductile, easily welded, and ranges in tensile strength from about 30 to 70 ksi (207 to 483 MPa). Of note regarding filler metal, the general rule is to select one of “matching” strength (see lead photo). Matching does not mean the strengths of the base metal and filler metal must be exactly the same. Rather, the filler metal’s minimum tensile strength should be at least as high as that of the base metal. Often any 60 or 70 ksi (414 or 483 MPa) series tensile strength filler metal will work from a matching tensile strength standpoint. When joining two different grades of steel with different strength levels, always use a filler metal that matches the strength of the weaker steel. That way, the weld metal’s strength will always be as strong as the loweststrength material around which the part was designed. Using a higherstrength filler metal that matches the stronger base plate is not necessary and would likely be more difficult to weld. Lower-strength filler metals usually offer easier weldability and typically cost less. The general guideline for filler metal selection is to always use the lowest-strength filler metal possible for best ductility and weldability — Fig. 2. Medium-carbon steels have 0.30–0.45% carbon by weight, while high-carbon steels have 0.45–0.75% carbon by weight. Steels with 0.75– 1.50% carbon by weight can be considered very high-carbon steels. Mediumand high-carbon steels are strong, hard, and not easily welded. As the carbon content increases, the weld metal can become brittle. These grades of steel will likely require the use of preheat, PWHT, and more tightly controlled welding. As a rule of thumb, if the steel’s points of carbon, carbon equivalency, or hardness level (Rc scale) equal 30 or higher, you should preheat the steel before welding. Regarding filler metal, you may need to use one of matching strength, particularly when the connection is in tension and is a complete-jointpenetration (CJP) groove weld. Matching strength filler metal could be an 80, 90, 100, 110, or 120 ksi (552, 621, 690, 758, or 827 MPa) filler metal. Rarely do you see filler metals with more than 120-ksi (827-MPa) tensile Fig. 1 — An example of a filler metal reference document. Fig. 2 — These tensile specimens represent examples of relative high and low ductility. The tensile specimen on the top shows poor ductility with minimal elongation. The specimen on the bottom shows a greater percentage of elongation.
Welding Journal | April 2015
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