It is the usual practice in shipyards, to straighten or minimize the
distorted items in hull plates.
As per my experience , temperature has to be kept below 400 degree Celsius
to avoid the grain growth and phase transformations in low alloy steels.
This shall be applicable first time only & beyond the first time , it has
be proved by testing of hardness in actual item by mock up tests.
Pls see the below extracted from the ABS Modu Code (2010)
"Fairing and Flame Shrinking
Fairing by heating or flame shrinking and other methods of correcting
distortion or defective workmanship
in fabrication of main strength members within the midship portion of the
vessel and other plating which
may be subject to high stresses is to be carried out only with the express
approval of the Surveyor. These
corrective measures are to be kept to an absolute minimum when the
higher-strength steels are involved,
due to high local stresses and the possible degradation of the mechanical
properties of the base material.
See 2-4-1/1.9.Fairing and Flame Shrinking"
Regards,
K.Babu
Singapore
----- Original Message -----
From: "hpi001" <pieper-qsi@kpnmail.nl>
To: "Materials & Welding" <materials-welding@googlegroups.com>
Sent: Monday, August 09, 2010 3:38 PM
Subject: [MW:6386] Re: Flame straightening of distorted weldments
Dear Mr. Vagal,
Weld distortion is caused by shrinkage of the weld at which the
material sometimes becomes internal residual tensile stresses above
the materials yield strength. Because of that it will cause elongation
in small areas which causes the distortion. In some cases you can
straighten the structure by heating small local spots in a specific
temperature, during cooling down the material will shrink to a smaller
dimension as before the heating and therefore will cause some residual
tensile stress in the structure by which the distortion can be
reduced. But be aware, this is a very special procedure and you need
someone with a lot of experience to execute such properly. Beside that
you will also introduce extra tensile stresses into your structure
which is not desirable especially in case where fatigue can be a
problem due to dynamical loads. I know that a lot of shipyards still
use this technology for straighten there plate fields for a ship hull
after the stiffeners are welded against it but the people who execute
such are really professionals with many years of experience.
Sometimes technical gas suppliers have also such professionals
available who can support their clients to execute such, perhaps you
can contact your welding gas supplier to see if they have someone with
experience in this direction.
Best Regards,
Herman Pieper
On 9 aug, 03:42, Shashank Vagal <nach_...@yahoo.com> wrote:
> Hi members,
> During and after welding distortion due to myriad reasons (wrong welding
> sequences, incorrect travel speed etc) usually occurs in the weldments.
> There is a definite methodology to restore this distortion and bring it
> within the limits of dimensional tolerances. I have seen some experts in
> this field who just apply torch flame to bring it to red heat - some
> typical portions of the affected parts and then apply water as coolant and
> lo! the material straightens out. What is the science behind it? I guess
> all is done below 300 deg C as judged from the colour of the spots.
> Can some one guide me what it is all about, what factors rule here? What
> are the risks involved if not done properly?
> Thankfully,
> Shashank Vagal
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