Pre Heating : To control the formation of H2 cracks. Control the effects of expansion & contractional stresses.
PREHEATING
What is Preheat?
Preheating involves heating the base metal, either in its entirety or just the region
surrounding the joint, to a specific desired temperature, called the preheat temperature, prior
to welding/cutting. Heating may be continued during the welding process, but frequently the
heat from welding is sufficient to maintain the desired temperature without a continuation
of the external heat source. The interpass temperature, defined as the temperature
between the first and last welding passes, can not fall below the preheat temperature.
Why Preheat?
There are four primary reasons to utilize preheat:
(1) it lowers the cooling rate in the weld metal and base metal, producing a more ductile
metallurgical structure with greater resistance to cracking;
(2) the slower cooling rate provides an opportunity for any hydrogen that may be
present to diffuse out harmlessly without causing cracking;
(3) it reduces the shrinkage stresses in the weld and adjacent base metal, which is especially
important in highly restrained joints and
(4) it raises some steels above the temperature at which brittle fracture would occur in
fabrication. Additionally, preheat can be used to help ensure specific mechanical properties,
such as notch toughness.
When Should Preheat be Used?
In determining whether or not to preheat, the following array of factors should be
considered: code requirements, section thickness, base metal chemistry, restraint, ambient
temperature, filler metal hydrogen content and previous cracking problems.
If a welding code must be followed, then the code generally will specify the minimum preheat
temperature for a given base metal, welding process and section thickness. When there are no
codes governing the welding, one must determine whether preheat is required, and if so,
what preheat temperature will be appropriate. In general, preheat usually is not required on
low carbon steels less than 1 in, (25 mm) thick. However, as the chemistry, diffusible
hydrogen level of the weld metal, restraint or section thickness increases, the demand for
preheat also increases. There are several methods to determine the required preheat
temperature for a given base metal and section thickness that will be discussed in the next
section.
Regards,
Rupesh A. Jambhale
Inspection Department,
Mott MacDonalds Consultants (India) Pvt Ltd., Mumbai
-----Original Message-----
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of redbull
Sent: 26 October 2009 12:45
To: Materials & Welding
Subject: [MW:3544] Reason for preheating
Dear friends,
Is there any other reason for preheating of weld joints other than Code requirements (B31.3). any metallurgical requirement etc.,
Regards,
Mohan.
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