Alloy Steel Welding
Alloy steel is a name applied to a range of alloys with different welding characteristics depending on the specific alloy. The alloy steels most frequently encountered are Ni steel, Cr-Mo and Cr-Mo-V steels, and some high strength alloys containing Cr, Ni, Mo and other elements. In general, these materials can be welded without problems, provided the necessary precautions are taken. As the carbon and alloy content of a steel (its carbon equivalent) increases it becomes progressively more difficult to weld. This is due to phase transformations that take place during rapid cooling from elevated temperature. Steels with high carbon equivalents are hardenable, and even a relatively slow cooling rate may produce a hard HAZ and the steel may suffer hydrogen cracking. Therefore, preheat, interpass temperature control and, possibly, post weld heat treatment must be considered before welding begins to counter these effects. All common arc processes are used for alloy steels, the most frequently used being MMA, MIG, FCAW, MCAW, SAW, and TIG. The consumables are selected to closely match the steel being welded and the shielding gas used (where required) depends on the process and the steel. Using a low hydrogen process like MIG, or low hydrogen MMA, MCAW, or FCAW consumables, combined with preheat, can help prevent hydrogen cracking. Other welding problems presented by alloy steel include several different types of cracking. For example, contaminants like sulphur and phosphorous can contribute to solidification cracking and another form of cracking, known as lamellar tearing, caused by bands of inclusions can impair the through-thickness properties leading to failure. Welding these materials may present possible electrical, radiation, compressed inert gases, and noise hazards plus generation of welding fume.
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