Base Metals and Filler Metals

There is a lot of thing that we have to know on the piping system, and Piping & Fabrication try to share the knowledge of the piping. Hope that will be useful to all of us, and let's continue with this post.

Base Metal. Base metal is one of the essential variables for welding qualification. Because there are so many base metals to be welded, ASME Section IX has established a system of P Numbers and Group Numbers. Each base metal is assigned to a specific P Number depending on characteristics such as composition, weldability, and mechanical properties. Each P Number is further subdivided into Group Numbers depending on fracture toughness properties. See Table A6.3. When a procedure is qualified with a base metal within a particular P Number, it is also qualified for all other base metals within that P Number. When fracture toughness is a requirement, qualification is limited to base metals within the same P Number and Group Number. For example: A 106 Gr. B pipe is P No. 1 Gr. No. 1, while an A 105 flange is P No. 1 Gr. No. 2. Since both are P No. 1, qualification on either qualifies both when fracture toughness is not a factor. However, should fracture toughness become a requirement, a separate qualification would be required for each to itself and to each other.

Filler Metals. Electrodes, bare wire, wire-flux combinations, and consumable inserts which form a part of the finished weld are classed as filler materials. Most are covered by AWS and ASME specifications. See ASME Section II Part C.29 When the filler material is part of the electric circuit, it is designated as an electrode. If it is fed externally and melted by the heat of the arc, it is designated as a rod. Coated electrodes for SMAW come in straight lengths. Bare rods for GTAW come in straight lengths or spools. Electrode wire for GMAW and SAW are in spools or coils, while composite electrodes for FCAW are in spools. Each specification incorporates a system of identification so that the filler materials manufactured by different suppliers which have equivalent characteristics are identified by the same number.

For qualification purposes, they are classified in ASME Section IX with F Numbers and A Numbers. Changes in filler metal from one F Number or A Number to another require requalification. One of the problems associated with coated electrodes for SMAW is the introduction of hydrogen into the arc atmosphere and finished weld, resulting in hydrogen induced cracking. To minimize this problem, low-hydrogen-type coatings are used, but these can absorb moisture from the atmosphere. Once a sealed can of electrodes is opened, the electrodes should be stored in an oven at about 250 to 350°F (120 to 175°C) or other temperature recommended by the manufacturer. Once removed from the oven, low-hydrogen electrodes should be maintained at 175°F (80°C) minimum until consumed. Baking to remove moisture is recommended for electrodes which have been out of the oven for several hours. Refer to the manufacturers’ recommendations.

A problem associated with welding of fully austenitic stainless steel is microfissuring. To combat this problem the chemical composition of the filler material is adjusted to produce a weld deposit with small amounts of ferrite. ASME III requires that filler materials used in welding austenitic stainless steels contain a minimum of 5 percent ferrite. Ferrite, however, can be a problem at cryogenic and high temperatures. For cryogenic services the weld metal may not possess the fracture toughness capabilities of the base metal, and the ferrite content should be kept as low as possible. Alternatively, fully austenitic fillers may be required, but these are more crack-sensitive. For very high temperatures ferrite in the weld may convert to a brittle phase called sigma. For this reason applications over about 800°F (427°C) usually require a minimum of 3 percent ferrite for weldability but not exceeding 7 percent to minimize sigma formation.