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[MW:2231] Re: CHEVRON CRACK

By Materials & Welding
My dear Ali,
1. Heat Input is not the highest in SAW but in more SMAW.
2. On Cheveron Cracking, please see a brief from some of my notes.

Chevron cracking: This type of cracking is unfamiliar in process piping and is mainly confined to thick-wall multipass welds deposited in carbon and carbon-manganese steels and is associated with fabrication of pressure-retaining facilities and heavy structural fabrications. The cracking mechanism is attributed to mishandled / sloppily stored submerged arc flux, which has, therefore, been allowed to absorb moisture. During welding, this moisture dissociates into hydrogen and oxygen. The oxygen may form oxides within the weld, or recombine and evaporate. Hydrogen gets trapped within the weld metal as it cools down and, if a sufficient quantity of hydrogen is available, cracking is likely to occur. The cracks take the shape of a chevron or a V, hence the name chevron cracking.

 Hydrogen or stress are known to trigger these crack (HIC, SCC).

The marks are formed in brittle failures in plates due to the way the fast moving crack front propagates along the plate as illustrated in the fig. 
The "chevron" marks point to the crack origin.
The exact thickness limits for this behaviour are connected with the fracture toughness of the steel, temperature, etc., depending on the
specific steel and its condition.
However, a common NDE may not be able to readily detect or determine a steel's / process's susceptibility to these cracks. To find if a steel is sensitive to this, you must use fracture toughness  measurement methods like CTOD, etc.
Note : Please see attachment for the fig, I can not post it here.
Best regards,
Shashank Vagal


--- On Sat, 23/5/09, Ali Asghari <asghariali@rocketmail.com> wrote:

From: Ali Asghari <asghariali@rocketmail.com>
Subject: [MW:2228] CHEVRON CRACK
To: materials-welding@googlegroups.com
Date: Saturday, 23 May, 2009, 9:37 AM

 
IN FABRICATION SPECIFICATION OF MY PROJECT SAID THAT:FOR THICKNESS ABOVE 25mm IN SAW,WELD SHALL BE EXAMINED FOR DETECTION OF CHEVRON CRACK BY ULTRASONIC.
ACCORDING TO HEAT INPUT IN SAW IS HIGHER THAN SMAW,WHY IT SAID ONLY IN SAW?




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