Flame straightening of distorted weldments

[Shashank Vagal]

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

[Ashfaq Anwer]

Dear Shashank,

Hot adjustment of a weldment depends upon many factors like type of weldment, degree of distortion, material of construction and application of weldment as to which environment it has to be exposed. Normally, for pressure piping, welding distortion is not allowed to be removed by applying heat with flame and then to quench. All such distortions are recommended to be removed by cutting the weld and then to re-do.

But yes to cater the piping misalignment during fit-up before welding, hot adjustments are made with uniform heating in a temperature range of 500 deg C to 700 deg C for carbon steels and then cooling is done is normal air. There also exists criteria for low alloy steels and this practice is usually prohibited for austenitic stainless steels as it impairs the chemical and mechanical properties.

The best way forward for you is to consider the factors as mentioned above and if you decide going for hot adjustment to cover the degree of distortion, then you must carry out post heat treatment (if applicable) and then to examine the whole area with RT and UT examination.

Regards,

Ashfaq

thepetrostreet.com

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[hpi001]

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

[PREM SHANKARDUTT NAUTIYAL]

Hi

Thermal or Oxy-acetylene Flame straightening is used to remove distortion in materials having sufficient thermal expansion and plastic deformability like steels, nickel, aluminium, copper and titanium.
The area to be straightened is heated @600 - 650 degrees C and suddenly cooled with water or air. Faster rate of heating of selected areas and establishing a steep thermal gradient is important for success of the operation.

It works on the principle that: heated area shrinks on cooling, more than it has expanded because of the constraints of the unheated area. The shrinkage forces which are induced during straightening exceed the elastic limit of the material and pull the material in the desired direction.

As this involves heating material surfaces near to the transformation temp. and faster cooling is involved detrimental metallurgical effects can occur. But steels having low carbon content (0.2%) are not affected by flame straightening.their mech.properties remain same.
In carbon steels, there is a possibility of crack formation and embrittling effect. hence temp. should not exceed 650 degree C.
For quenched & tempered steels temp. should not exceed 590 degree C.

Regards

Prem Nautiyal
On Mon, 09 Aug 2010 10:46:06 +0530 wrote

>Dear Shashank,
Hot adjustment of a weldment depends upon many factors like type of weldment, degree of distortion, material of construction and application of weldment as to which environment it has to be exposed. Normally, for pressure piping, welding distortion is not allowed to be removed by applying heat with flame and then to quench. All such distortions are recommended to be removed by cutting the weld and then to re-do.
But yes to cater the piping misalignment during fit-up before welding, hot adjustments are made with uniform heating in a temperature range of 500 deg C to 700 deg C for carbon steels and then cooling is done is normal air. There also exists criteria for low alloy steels and this practice is usually prohibited for austenitic stainless steels as it impairs the chemical and mechanical properties.
The best way forward for you is to consider the factors as mentioned above and if you decide going for hot adjustment to cover the degree of distortion, then you must carry out post heat treatment (if applicable) and then to examine the whole area with RT and UT examination.
Regards,
Ashfaq
thepetrostreet.com

On Mon, 9 Aug 2010 07:12:56 +0530 (IST), Shashank Vagal 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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PREM S NAUTIYAL
CELL : 9820313278

[Arunachalam A]

Dear Shasank,

Please see the attached file.

arun

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[Kathalingam Babu]

Hi

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

Dear Mr. Vagal,

Best Regards,

Herman Pieper

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[Bathula Raghuram (Mumbai - PIPING)]

following is an extract from Shell DEP

FINAL CORRECTIONS

the following correction methods are acceptable, if approved by the Principal:

1. Local heating or cooling;

2. Application of force with local heating.

The following restrictions apply to the heating, cooling and forcing of piping made from carbon steel, low alloy steel or austenitic stainless steel:

For carbon steel which does not require a PWHT, the maximum temperature during alignment corrections shall be 600 °C. Temperature-indicating crayons or contact thermometers shall be used to measure the maximum temperature. If temperature indicating crayons are used, a margin of 50 °C shall be taken to allow for measurement inaccuracy, i.e., the reading with the crayon shall not exceed 550 °C. Forcing may be applied if necessary. Cooling in still air shall be applied.

For carbon steel which requires a PWHT, the maximum temperature during alignment corrections shall be the maximum PWHT temperature. Temperature indicating crayons or contact thermometers shall be used to measure the maximum temperature. If temperature-indicating crayons are used, a margin of 50 °C shall be taken to allow for measurement inaccuracy, i.e., the reading with the crayon shall not exceed the maximum PWHT temperature minus 50 °C. Only heating with application of local force shall be used and no quenching shall be used. The area shall either be heated in full compliance with the PWHT procedure or the area which is heated (and corrected) shall be post weld heat-treated.

For 0.5 Mo and Cr-Mo steels, the maximum temperature during alignment corrections shall be the maximum PWHT temperature. Preferably, contact thermometers shall be used to measure the maximum temperature, but temperature-indicating crayons may also be used. If temperature-indicating crayons are used, a margin of 50 °C shall be taken to allow for measurement inaccuracy, i.e., the reading with the crayon shall not exceed the maximum PWHT temperature minus 50 °C. Forcing may be applied if necessary. Cooling in still air shall be applied. Random hardness measurements shall be taken and the hardness shall not exceed 248 HV10.

For austenitic stainless steel, the maximum temperature during alignment corrections shall be 650 °C. Temperature-indicating crayons or contact thermometers shall be used to measure the maximum temperature. If temperature-indicating crayons are used, a margin of 50 °C shall be taken to allow for measurement inaccuracy, i.e., the reading with the crayon shall not exceed 600 °C. The duration of heating shall be kept as short as possible and no forcing shall be applied.