Preheat for pipelines is typically done because EXX10 type cellulosic SMAW stick electrodes are used which are high in moisture which leads to hydrogen in the weld. The preheat is intended to aid removing hydrogen from the weld so the hydrogen assisted cracking (aka cold cracking, delayed cracking, HIC, etc) does not occur. Preheat is also used to slow the cooling rate and improve the mechanical properties (increase toughness and lower hardness). Preheat is also used to remove moisture, however the above two conditions are the primary reasons to use preheat.
Most plant welding is done with low hydrogen type welding. Since there is very little hydrogen, preheat is not needed to aid with removing
On Fri, Oct 8, 2010 at 10:25 PM, Hemant Wadibhasme <hemant.wadibhasme@gmail.com> wrote:
Dear all,i am workig in cross country pipeline,as per WPS the preheating temp. for carbon steel (Gr. X56, X65) is 100 degree, and in plant for carbon steel the preheating temp. is 10 degree that means there is no requirement of preheating, somebody has told me in pipeline the preheating is done only to remove the moisture on the surface of metal otherwise there is no any requirement of preheating, is it proper reason, pls share your experience.
--Thanks and Regards
Hemant F Wadibhasme
Sr. QA/QC Engineer.
Larsen and Toubro
ECC Division.Contact No. 09727083772
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