Mr. Kumar
Weld preheating protects structures made of metals which have a high conductivity rate. These are metals such as carbon steel, copper and aluminum alloy, which conduct heat quickly and suck heat away from weld metals causing them to cool faster than normal. When this happens, the weld and parent metals do not have the time to fuse properly so once again you end up with an uneven joint.
What weld pre heating does is it reduces the rate at which this heat exchange takes place. As a result the cooling rate of the deposited weld metal allows it to become more evenly fused with the parent metal.
Weld preheating uses a carefully adjusted application of heat to areas affected by the welding process. This controlled application is what allows the fusing of weld and parent metals to happen smoothly and to preserve the integrity of the metal. It also properly removes the components which cause weld porosity and hydrogen cracking. Therefore weld preheating is the key to prevention of frequent and costly repairs.
Pre-heating even with 100 deg C, prepares metal to make it more receptive to welding. The importance of preheating increases with the thickness of the base metal because of the rapid self quench capability, and with the rigidity of the welded structure because of the derived constraints. In general the higher the preheat temperature and the lower the heat input, the conditions are more favorable for limiting martensite formation and its hardness, hopefully contributing to higher quality welds
The minimum preheating temperature to be assured to avoid cracking depends on the following factors:
1.Carbon equivalent - More than 0.40% and if you are using E 6013 type electrodes
2.Carbon content - More than 0.35% and if you are using same Rutile coated electrodes
3.Condition of base metal - Prior to welding like dampness or in cold condition etc.
4.Thickness of base material - More than 20 mm or above.
5.Constraint level - Higher restrain level in the joint fit up
6.Hydrogen available risk. - Due to moisture level, sea-coastal area or if the humidity level on the higher level
7.Alloy content - Cr-Mo- and other elements which are stress risers and may cause cracks due to martensite formation in the HAZ ..
But, if it is a CS with low carbon level and if the thickness is less than 20 mm Pre heating can be avoided..
Use E 7018/ E 7016 type electrodes if required and avoid pre-heating at critical areas..
Pre heating can done by various methods including gas burners. It is an additional operation and fabrication cost may go up. Hence , avoid it
at the lower segment and if they still insist up on it, it could be done at extra payment if not covered by your agreement.
Sridhar.
From: Naresh Kumar <snareshkumar30@yahoo.com>
To: "materials-welding@googlegroups.com" <materials-welding@googlegroups.com>
Sent: Sunday, 28 October 2012 10:02 PM
Subject: [MW:15791] Tensile strength and Preheating
Dear Experts,
I need some information about preheating. As per our client specification, if the tensile strength value above 450mpa,100 C preheating require for any thickness. Is it really essential ?
Some specifications: Material: SA 105( asper ASME IX-Min.UTS Vaule -485mpa) + SA106 B, Size:OD:33.4, Thk = 6.35 mm, As per test certificate pipe UTS value is 515mpa.
Our client suggestion is to do preheating for all joints.
Thanks & Regards,
Kumar
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