TWI JOB KNOWLEDGE


21


Avoiding weld imperfections High hardness in the HAZ makes this type of stainless steel very prone to hydrogen cracking. The risk of cracking generally increases with the carbon content. Precautions which must be taken to minimise the risk include: using low hydrogen process (TIG or MIG) and ensure the flux or flux coated consumable is dried (MMA and SAW) according to the manufacturer's instructions; preheating to around 200 to 300ºc. Actual temperature will depend on welding procedure, chemical composition (especially Cr and C content), section thickness and the amount of hydrogen entering the weld metal; maintaining the recommended minimum interpass temperature. Carrying out post-weld heat treatment, e.g. at 650-


750ºc. The time and temperature will be determined by chemical composition. Thin section, low carbon material, typically less than


3mm, can often be welded without preheat, providing that a low hydrogen process is used, the joints have low restraint and attention is paid to cleaning the joint area. Thicker section and higher carbon (> 0.1%) material


will probably need preheat and post-weld heat treatment. The post-weld heat treatment should be carried out immediately after welding not only to temper (toughen) the structure but also to enable the hydrogen to diffuse away from the weld metal and HAZ.


Duplex stainless steels Duplex stainless steels have a two phase structure of almost equal proportions of austenite and ferrite. The composition of the most common duplex steels lies within the range 22-26% Cr, 4-7% Ni and 0-3% Mo normally with a small amount of nitrogen (0.1-0.3%) to stabilise the austenite. Modern duplex steels are readily weldable but the procedure, especially maintaining the heat input range, must be strictly followed to obtain the correct weld metal structure.


Avoiding weld imperfections Although most welding processes can be used, low heat input welding procedures are usually avoided. Preheat is not normally required and the maximum interpass temperature must be controlled. Choice of filler is important as it is designed to


produce a weld metal structure with a ferrite-austenite balance to match the parent metal. To compensate for nitrogen loss, the filler may be overalloyed with nitrogen or the shielding gas itself may contain a small amount of nitrogen. ■


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