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.
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.
From: 'Prakash Hegde' via Materials & Welding <materials-welding@googlegroups.com>
To: materials-welding <materials-welding@googlegroups.com>
Sent: Sunday, 11 September 2016 7:47 PM
Subject: Re: [MW:25370] Saw Welding pokemarks.
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 |
From: Dinesh Somwanshi <som.din@gmail.com>;
To: materials-welding <materials-welding@googlegroups.com>;
Subject: [MW:25369] Saw Welding pokemarks.
Sent: Sun, Sep 11, 2016 12:49:43 PM
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 http://www.linkedin.com/groups/MaterialsWelding-122787?home=&gid=122787&trk=anet_ug_hm 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. --- You received this message because you are subscribed to the Google Groups "Materials & Welding" group. To unsubscribe from this group and stop receiving emails from it, send an email to materials-welding+unsubscribe@googlegroups.com. To post to this group, send email to materials-welding@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/materials-welding/CAELgRrV4EZNPkvz-JBo28_a39f9k3T-tXf07YFRGEjFL%2BqrxLQ%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout. |
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