Saw Welding pokemarks.

[Dinesh Somwanshi]

Dear sir,

We are observing an pokemarks on Saw welding weld bead continuously.

The flux has used by baking.

Pls guide.

Dinesh V Somwanshi
Api 570, Cswip 3.1,Bgas 2
Authorised Certified Piping, Welding, Painting Inspector.
00919881344135

[Nilesh Pathare]

Dear Dinesh,
We have also experienced the pokemarks on SAW beads.
We analysed the reason due to powdery flux. Due fine powder in flux the gases are not able to escape prior to solidification. If you observe the slag you will find the ditches on slag surface.
1. Pokemarks on filling passes are not a problem. Ensure  welder is cleaning the bead thoroughly prior to deposit next pass.
2. For final pass ask the welder to pass the baked flux through the mesh and use only granular flux for final pass.
3. Reduce the amount of flux heap on weld bead. Pls. Use optimum quantity of flux to submerge the arc.

Regards,
Nilesh Pathare.

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[Prakash Hegde]

Hi! Some time you req to bake flux at higher temp of 350c with bed height less than 50mm Iij) during weldng flux bed height should not be excesive Iii) check the flux grain size distribution Regards Hegde

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From: Dinesh Somwanshi <som...@gmail.com>;
To: materials-welding <material...@googlegroups.com>;
Subject: [MW:25369] Saw Welding pokemarks.
Sent: Sun, Sep 11, 2016 12:49:43 PM

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[Kannayeram Gnanapandithan]

Reduce current, travel speed and increase arc voltage. Reduce flux burdens and use begin flux

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[Alan Denney]

In the UK we call them gas flats. Please see the comments in the article from TWI - The Welding Institute.

 

http://www.twi-global.com/technical-knowledge/job-knowledge/submerged-arc-welding-consumables-part-1-087/

 

Alan Denney

AKD Materials Consulting Ltd

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[c sridhar]

Pock marking or gas flatting are depressions in the surface of a weld with voids in the slag directly over 

them. Both the depression in the weld surface and void in the slag are caused simultaneously by a gas

bubble. This gas bubble (oxygen and/or carbon monoxide) forms in the weld puddle and must escape 

through the slag to the atmosphere.

Pock marking can be controlled by following manners: 

• Killing agent Silicon is very effective in eliminating/reducing pock marking in most cases. This is because 

  it decreases  the amount of gas produced in the weld puddle. An example is given below:

  2FeO + si (scale) heat 2Fe + Si0₂ (slag) 
  Silicon decreases the 0₂ available to react with carbon to produce gas. 

• The silicon needed to control pock marking can be increased without changing the welding procedure by 

   using a higher silicon electrode. 

• The second source of silicon is the flux. Increasing the amount of flux melted will increase the silicon in the

   weld puddle.The most effective method of increasing the amount of flux melted is to increase the arc voltage. 

  This increases the amount of flux melted for a given amount of electrode melted. 

•  An increase in welding current increases the amount of electrode melted. This will normally increase pock 

   marking due to a decrease of silicon in the weld puddle. Increases in current must be matched by comparable

   increases in voltage to keep an adequate level of silicon in the weld metal. 

• The flux is a greater source of silicon than the electrode. However, procedure limits the maximum usable voltage.

   For this reason, high silicon electrodes are very helpful to increase the weld puddle silicon. 

• The joint will also have an effect on the pock marking. Narrow joints do not permit as much flux to melt for the 

  same current and voltage. This increases the pocking tendency.The second important factor in determining the 

  level of pock marking is the slag freezing (solidification) rate. The gas bubbles that form in the weld puddle must 

  have time to escape through the liquid slag. If the slag freezes (solidifies) too rapidly, some of the gas is trapped 

  between the weld and slag and forms a pock mark. 

Sridhar.

 

 

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From: 'Prakash Hegde' via Materials & Welding <material...@googlegroups.com>
To: materials-welding <material...@googlegroups.com>
Sent: Sunday, 11 September 2016 7:47 PM

Subject: Re: [MW:25370] Saw Welding pokemarks.

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