and greater ductility than the roomtemperature form of ferrite. When more than 16% chromium is added to the steel, the roomtemperature crystalline structure, ferrite, is stabilized and the steel remains in the ferritic condition at all temperatures. Hence the name ferritic stainless steel is applied to this alloy base. When more than 17% chromium and 7% nickel are added to the steel, the high-temperature crystalline structure of the steel, austenite, is stabilized so that it persists at all temperatures from the very lowest to almost melting. Austenitic stainless is commonly referred to as the “chrome-nickel” type, and martensitic and ferritic steels are commonly called the “straight chrome” types. Certain alloying elements used in stainless steels and weld metals behave as austenite stabilizers and others as ferrite stabilizers. The most important austenite stabilizers are nickel, carbon, manganese, and nitrogen. The ferrite stabilizers are chromium, silicon, molybdenum, and niobium. Balancing the alloying elements controls the quantity of ferrite in the weld metal. Austenitic grades are more readily and satisfactorily welded than those that contain less than 5% nickel. Weld joints produced in austenitic stainless steels are strong, ductile, and tough in their as-welded condition. They do not normally require preheat or postweld heat treatment. Austenitic grades account for approximately 80% of the stainless steel welded, and this article focuses heavily on them. How do I choose the correct stainless filler metal? If the base material in both plates is the same, the original guiding principle used to be, “Start by matching the base material.” That works well in some cases; to join Type 310 or 316, choose the corresponding filler type. To join dissimilar materials, follow this guiding principle: choose a filler to match the more highly alloyed material. To join 304 to 316, choose a 316 filler. Unfortunately, the “match rule” has so many exceptions that a better principle is, “Consult a filler metal selection table.” For example, Type 304 is Fig. 2 — To improve wet out in GMAW applications, consider using a welding wire with silicon, such as 308LSi or 316LSi. JULY 2016 / WELDING JOURNAL 33 Table 1 — Stainless Steel Types and Their Chromium and Nickel Content Type % Chromium % Nickel Types Austenitic 16–30% 8–40% 200, 300 Martensitic 11–18% 0–5% 403, 410, 416, 420 Ferritic 11–30% 0–4% 405, 409, 430, 422, 446 Duplex 18–28% 4–8% 2205
Welding Journal | July 2016
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