How to Eliminate Centerline Crack in SAW?

Check for the flux(moisture & reuse of unutilised flux during welding) you are using while welding, I too had faced the same thing some time before. That can also be because of ambient temperature. please do inform if you find this helpfull.

This is a thick piece of base metal!

1. Are you preheating?

2. What Joint Geometry is used

3. Single ot Multipass

4. If Multipass are interpass temps controlled

Suggestions:

1. Adjust Travel Speed (Slower?)

2. Increase Electrode Size while lowering Voltage

3. Increase Joint Angle

You will porbably have to find the correct combination of these things through trial and error. I suspect heat diferentals are your problem with "T" so large. Also; release from fixture as soon as practicable after completion of welding.

How many runs 1 or 2? What prep is being used? Type of joint?

Are you sure it is not centerline segregration?

● Preheating is very necessary especially in case of welding materials with higher thicknesses to minimize and control the effect of rapid cooling of weld puddle due to rapid of heat transfer due to the huge of mass around the welded joint. Preheating requirement is as per ASME VIII-1, Nonmandatory Appendix R.

Note: Applying preheat can directly solve the problem of centerline cracking, where the sides of welded puddle start to solidify (due to rapid cooling rate) where the center of welded puddle remain in molten phase in which the shrinkage shall cause cracking at centerline.

● Also, interpass heating between every two welding passes is very essential. In addition to PWHT (if needed per code) is very essential as per ASME VIII-1, Table UCS-56.

● See the attached file for actual WPS & PQR (including preheat, interpass & PWHT) for welding of ASME SA516 Grade 70 using SAW for test coupon with thickness t = 14 mm which used to qualify welding of 2t : http://www.4shared.com/office/-nJs6I8E/WPSs__PQRs.html

Try this one if it helpsssstr

In thick section welding of SAW process,cracks may appear due to one or more of the following reasons.

a) Rapid cooling of the weld deposit, b) Heavy restraint in the joint c) Excessive dilution of weld with parent metal, resulting in pick up of carbon and other alloy elements and consequent hardening, d) excessive hydrogen in the derived from moisture , grease, oil etc.

If the cracks are due high dilution,Welding procedure must be modified to reduce the metal penetration through,

1) Increase the electrode size (you have not indicated the size) to 4.00 mm or so.

2) Deposit first pas with SMAW- E 7018- 4.00 mm and later with SAW.

3) The bevel angle be ensured to 60⁰ minimum to ensure the width is greater than the depth of the bead.

4) Abnormal bead shape is also sighted as cause of cracking.i.e.,

a. Cracking could be due to large width and low depth of a bead.

b. Cracking could also be due to small width and high depth ratio, i.e., we call it as a 'bead factor' and W/P >1. (where W is bead with, P is total penetration) If it is less than 1,due to stress concentration and other factors a center line crack is sure to appear if not at the time of welding, but it can occur in service also.

Trust above details will help to overcome your problem.

sridhar.

I am using...

Plate Thik.= 25 mm (SA516 Gr 70)

Code: ASME Sec- IIC , All Weld Assembly

Process :SAW

Wire Dia: 4 mm (EM12k)+ Metal powder addition seperatly ( 95.5 % Fe powder + 4.5 % alloy powder)

A=550 , V=30, TS =50 cm/min.

which element is better to add with powder to prevent cnterline crack?

pls reply....

Your weld parameters are OK. May be stick out could be 40 to 60 mm. Metal powder addition helps to increase the volume of weld bead and reduces the no. of layers.

As mentioned earlier, please check the bead width to depth ratio.

Second point is check for basicity of the flux. Basicity index is very important for agglomerated fluxes and it is the ratio of basic and acidic oxides.It is kept in between 2.2 to 3.2.and higher the basicity, lower are the oxygen and sulphur contents of the weld metal and higher its notch toughness. More basicity makes the slag removal difficult.

Hence check up the batch test certificate for the values. If details.are not mentioned, ask your manufacturer for same.

Sulphur is one element which may cause grain segregation (center line cracking) during the welding and neutralized by additional ratios of manganese.

But in your case we do not expect this problem if the wire & flux manufactures are one and same. They take care of the end chemistry for every batch production.Do not mix different wires & fluxes.

sridhar.

Sir,

I have chcked Chemistry of my above mention weld.

it contain,

C=0.004 apprx.

Si=.53, Mn=1.75

B=>0.0017

Boron % is more and Carbon % is low comparative to other weld chemistry.

So what will be the reason for crack?

Also if u have any knowledge about metal powder addition in SAW, pls guide me or send some litrature about it.

We have given enough hints for the said problem. Check up the bead factor as detailed earlier. The cracks can also appear if the joint is under contraction stress or if the joint is under restraint.Check up the bevel angle, joint fit up. If the cracks were seen in the first weld pass remove them before proceeding with further passes. Check up and maintain proper preheat and inter pass temperatures.

The chemistry is OK and cracks do not occur due to meal powder additions. Boron is a trace element only.

sridhar

Check for storage condition of your flux & metal powder - any humid environment could lead to moisture absorption and resulatnt H2 in to metal bead- cracking.

Check also for your plate centreline defect/segregation if any at weld area where it is craking in weld . - Macro-examination of cross section by aid etching will reveal the defect if any , or go for micro-structuural examination .

A small correction from my end. Examination of the defective area (cu out a small portion - 10*10*25 mm size or as required by the code of testing) will help. Analise it for its Micro structure, hardness etc after polishing and enlarging the grain structure through a microscope.This could be done at any analytical testing lab..

This will help to conclude the reasons for cracking.

sridhar.