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
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
>
>
>
>
--
> To post to this group, send email to materials-welding@googlegroups.com
> To unsubscribe from this group, send email to materials-welding-unsubscribe@googlegroups.com
> For more options, visit this group's bolg at http://materials-welding.blogspot.com/
> The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
>
--
>
To post to this group, send email to materials-welding@googlegroups.com
>
To unsubscribe from this group, send email to materials-welding-unsubscribe@googlegroups.com
>
For more options, visit this group's bolg at http://materials-welding.blogspot.com/
>
The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
>
PREM S NAUTIYAL
CELL : 9820313278
 |
To post to this group, send email to materials-welding@googlegroups.com
To unsubscribe from this group, send email to materials-welding-unsubscribe@googlegroups.com
For more options, visit this group's bolg at http://materials-welding.blogspot.com/
The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
No comments:
Post a Comment