pre heating of C.S

Date 04-20-2006 16:23

The process of welding alone will heat up the thinner materials under 19mm enough to drive out the hydrogen to prevent hydrogen induced cracking, but the thicker materials act as a heat sink and quench the molten pool before all this takes place and cracking can occur at the root. The 3/4" (19mm) is just a place to start, they have figured out over many years of welding that 19mm and above need some extra heat to make sure all will go well, yet under 19mm no preheat is necessary.
John Wright

By CHGuilford (****)

Date 04-20-2006 17:26

Just to add to John's post - 19mm is not a magic number - it is a lower thickness limit that has been determined to be the point where the omission of preheating could be a problem. Most likely that is a conservative number , but realize that cracking can occur in any thickness depending on metal chemistry and other considerations. On the other hand, even thicker steels could possibly be welded successfully with no preheating at all, but that is required to be proven before it can be allowed.

Chet

By Ajirajnatarajan Date 04-21-2006 07:50

THE POINT OF MY QUESTION IS WHY IS IT 19MM?WHY CAN'T IT B 18.5 MM OR ANY OTHER THICKNESS.?

By billvanderhoof (****)

Date 04-21-2006 07:59

I suspect that you are actually asking why not a nice even number like 20 mm. Without knowing for sure I bet that if you go look at old codes, at some time in the past the code said 3/4 inch, was then converted to metric, almost exactly 19 mm.
Bill