Re: [MW:17877] Preheat & Interpass

Dear Pandithan,

Please refer to AWS D1.1 clause 5.6

On Mon, Jun 3, 2013 at 8:24 PM, c sridhar <sridhar305@yahoo.com> wrote:

 

            Mr. Pandian,

            Preheat is applied for four primary reasons:

·  It slows the cooling rate in the weld and base material, producing a more ductile metallurgical structure with

   greater resistance to cracking.

·  The slower cooling rate allows any hydrogen present to diffuse out harmlessly, reducing the potential for

   cracking.

·   Preheat reduces shrinkage stresses in the weld and adjacent base material, particularly important in highly

   restrained  joints.

·   It raises some steels above the temperature at which brittle fracture would occur in fabrication. Additionally,

   preheat can help attain specific mechanical properties, such as notch toughness.

Minimum preheat depends on code requirements, section thickness, base-material composition, joint restraint,

ambient temperature, filler-metal hydrogen content, and previous cracking problems. We can also increase the

minimum value if necessary.

 

In general, preheat is usually not required on low-carbon steels less than 20/25 mm thick. However, as the diffusible

hydrogen level of the weld material, amount of restraint, or section thickness increase, the need for preheat rises.

Welding codes will also typically specify minimum values for preheat temperature, which may or may not be adequate

to prohibit cracking in every application. Conservative minimum preheat requirements prescribed by AWS  D1.1

may not be adequate for highly restrained connections.

To control the mechanical and micro structural properties of weldments, inter pass temperature is just as important as,

if not more important than, preheat temperature. Yield and ultimate tensile strengths of the weld metal depend

greatly on inter pass temperature. There are two aspects to inter pass temperature: the minimum, and maximum, value.

The minimum Inter pass temperature should be at least as high as the minimum preheat temperature.

It may be important to impose control over the maximum inter pass temperature when certain mechanical weld metal

properties are required.A maximum inter pass temperature can reduce weld strength and at the same time result

in a finer grain structure and improved Charpy V notch-toughness at transition temperatures. However, when inter pass

temperatures exceed approximately 260⁰ C, this trend is reversed and notch toughness gets reduced.

On sensitive base materials, a minimum inter pass temperature to be specified to prevent cracking, and a maximum

inter pass temperature in order to provide adequate mechanical properties. To maintain this balance, we have to consider

the time between passes, base-material thickness, preheat temperature, ambient conditions, heat transfer characteristics,

and heat input from welding.

Maintain inter pass temperature at 25 mm from the weld toe and within the first 30 cms of its start. Apply preheat from the

back side of the joint so as to completely "soak" the base material. Weldments with smaller cross-sectional areas naturally

tend to "accumulate" inter pass temperature: as welding continues, the temperature of the part increases. 

Preheat  and  high  inter pass  temperatures  can  have a  negative  effect  on  the mechanical   properties   or  Corrosion

resistance  of  some  alloys   like Austenitic manganese (14 % Mn) steel, Austenitic Stainless Steels, Duplex stainless steels etc.

 

Hope above details will be of help to you.

              C Sridhar.             

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Best regards,
 
Tarek Yehia

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