Pelloux’s law of metal failure states that: "It always breaks where it’s welded".
Why?
The filler metal used for welding may have a different composition and be softer or harder than the adjacent base metal. Also, the surrounding heat affected zone (HAZ) may be is softer than base metal due to phenomena such as tempering or recrystallization. Conversely in steels, due to a high cooling rate, some regions of the HAZ also may be harder than the base metal.
The welding process may introduce discontinuities. See "Understanding weld discontinuities" The Fabricator, June 12, 2003:
http://www.thefabricator.com/WeldingInspection/WeldingInspection_Article.cfm?ID=603
For a discussion of weld cracking also see:
http://content.lincolnelectric.com/pdfs/knowledge/articles/content/weldcracking.pdf
Ken Russell has quoted Pelloux’s law in two of his dozens of brief case histories published in Design News magazine. See "The case of the wayward weld", Design News, September 22, 2003: http://www.designnews.com/article/ca321648.html
Russell is an emeritus professor at MIT, as is Pelloux. Pelloux received ASM’s Albert Sauver Achievement Award "for pioneering contributions in the areas of fatigue crack propagation, micromechanisms of deformation and fracture, and quantitative microscopic analysis of fracture processes in structural engineering alloys."
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There can be many reasons that's why the metal failure occur when welding is done. it may be possible that incorrect selection of consumables and parameters done that may lead to lower weld toughness. So, try to employ experienced and skilled steel fabricator so that you can't face such problem.
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