Weld Crack

What are the materials and what type of tubing are you using?

Tube is ASTM A513 1008/10 Type dia 38.1mm, t1.7mm

Stamping is ASTM A607 G50 HSLA t2.7mm

Possum,

In addition to the other suggestions and advice, you might take a critical look at the tubing. The low carbon DOM (drawn over mandrel) tubing is cold drawn and electric resistance welded (with all flash removed). In many applications it is preferred over seamless because of its excellent OD and ID concentricity. Each tube must be tested for soundness of weld but.....

You might examine a cross section microstructure of the crack and see if it just happens to align with the tubing weld.

Hallo..

Usually it is difficult to predict and imagine the actual process unless it is seen in person and hence exact remedy cannot be suggested. However I am sharing some of thoughts(probable causes and remedies) based on my experience.Pl see if you find any clue to solve the problem.

Let us first understand how the 'weld cracks' are generated.:-Whenever we weld any two parts;local heat is generated. The steel of that local area expands due to heating. The co-efficient of expansion of steel is 0.000016. So you can imagine the expansion tendency of localised area; which leads to stress. When there is lot of welding in a certain area (which is your case) then there is lot of localised stress generation. Further, a cooling is also taking place and hence the weld area undergoes 'thermal shocks' and thereby 'stresses'. This leads to weld crack.

This is usually the mechanism of welding cracks.

The crack always developes at weakest section. If the crack is consistantly at same place then it must be by 'design' (of product or weld process) and not by 'default'.

Having known this the remedies could be focussing to the 'reasons' and eliminate or reduce it to possible extent.

Some of remedies usually adopted are:-

a) While welding, first weld at centre and then gradually go to ends. This allows material to temperorily expand and then contract while cooling. This reduces stress level.

b) Avoid continuos welding. Welding of alternate flanges and alternate ends, allows the weld to cool down.Intermittant welding reduces cracking because there is 'lesser dis-similar stressing'.

c) Preheat the parts and then weld. This reduces thermal shock and thereby welding stresses.

d) Allow the component to cool slowly after welding. This is achievable by controlling room temp, or immerse weld portion in mica, put it in low temp furnance etc.

The generation of crack is always during cooling,after welding is over. Controlled and slow cooling therefore reduces cracking tendency.

While programming of rototic welding, the relevent parameters mentioned above are generally considered.hence in your case, it may not be cracking by robotic welding.

Pl study robotic welding sequence minutely and replicate it manually.

Pl update us once you get a solution,for our knowledge.

Wish you all the Best and all the success.

Excellent explanation, thank you!

This is for Qqberci:

R_U for real or just joking? This is a serious discussion U know!

Thanks all for the input.

"The crack always developes at weakest section" I agree, but there are exactly 8 welds per fixture, and 2 fixtures, 16 joints total. All welds are the same parameters, wire speed, gas, travel speed, and torch alignment. So would not this crack appear at any one joint?

For "a" and "b": Customer requires that the weld be one continuous for aesthetic.

"c" Pre heat I agree would probably help, but we do not have the luxury of time for this part. If we did this we would have to re-quote to the customer, increasing cost, therefore probably lose this job. Cost is always an issue.

"d" component is cooled naturally at room temperature.

If we get this figure out I will update all. I have been in the welding industry now for around 15yrs, and I have never ran into this scenario.

By the explanation u have given , it looks like a restrain or stress cracks. These sort of cracks happens at random and above 550⁰c. You can observe under microscope to see whether they traveled across the boundaries between the grains ( intergranular) or not. The fractured surface may also show whether it has bluish or black scale.

They may indicate the cracks might be developing at the end of solidification range, when the last portion is still liquid and the mass of the metal is unable to deform without cracking. At this stage , when the vibrational or contraction stresses are imposed on the metal a fissure develops when it is subjected to high localized contractions or shrinkage stresses .

Coupled with the joint restraint ( in your case as U are doing continuous robo- welding) it is really the presence of certain undesirable factors promotes the phenomenon.

Since you are using thin walled sections and MIG process, I assume it could be either 0.8 or 1.0 mm wire with Argon gas, the joint do not call for any pre heating ( except for thick sections) at all taking into consideration all the parameters are same. But the joint may need staggering ( indexing - in your case) of the welds.

I presume the crack might have developed from the 4th to 7th weld ( stresses would have built up by then) where the restrain will be maximum ( in between the weld joints) due to continuous welding. Either you can attack 1st, 4th, 7th ( leaving 2 joints in between) or 1-3-5-7 ( with one joint interval) to allow cooling of the welds in between ( may be time consuming initially ) and avoid stress concentration and complete the joint .

This method was adopted at our end successfully for certain auto components. Since you have 2 fixtures you can still stagger them better .

Also please check for root penetration, fusion where the cracks were developed as any LP or LF ( due to more gap – due to fit etc) would have aggravated the problem.

Sridhar

I forgot to add. Since you have only 50 mm length weld and done continuously the chances are more for stress concentration as there is absolutely no time for the weld joint to cool intermittently.

sridhar

Make sure you have the correct wire for the material, if the material is harder than the wire it will crack. Another thing you may have to do is preheat it, preheating it should keep it from cracking. Good Luck

I do agree with Mr.Suhas Gokhale,General Manager,Supreme Group,Nasik,India

Rajeev

ESAB INDIA LTD

If you can't make heating after every weld, I think you have to keep a little break after each welding. The other thing you have to check the base material, bacause sometime there are some cracks in it.

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Dear Possum, (nice puppie!)

Could you please post/send some sketch with thickness and materials indication?

Order of the welds and a closer picture of the crack might be useful.

CU,

Chefdechocolate

The program needs to be adjusted. Visibly, two things happened;

1. The welder velocity is too fast parts were inadequatly heated/cooled

2. The attachment may have been crudely fitted

Start the welds from the ends and stop them in the middle with the second part of each weld overlapping the first then there will be no cracking

h2om@hotmail.com

I would be checking material certs to determine potential chemistry issues.

I would also review my filler materials as well.

Are you quenching or allowing to air cool?

The weld heat affected zone (HAZ) may be the issue here. Typically weldable materials do not crack unless you are inducing excessive shrinkage stress or the material chemistry is off.

Also, you may consider altering your shielding gas and weld process. I suggest looking at pulse welding systems as heat input is reduced while maintaining spray transfer and very good weld cosmetics.

The tube dia. d must be ERW-with weak weld at pipe plant.

Answers:

Simplest- Do Position 1 (lengthwise gusset to the pipe) MIGweld on the pipe's ERW.

OR

i) Weld 2 brackets to bottom footplate leaving gap between brackets =d+1mm. Let it COOL down!

ii)Use 2 /4 MIGwelding machines with 2/4 wire feeds to weld this with pipe.

Just thinking out load.

The welding operation has been going on for 1.5 years without this problem?

The welding materials are the same?

The welding perimeters are the same?

The welding equipment is the same and has been tested?

I would look at possible leaks in shielding gas lines or clogs.

I would test my shielding gas and lines for moisture or contamination.

I would check the wire for contamination.

1 would suspect possibly tubing that will not meet minimum requirements.

I would also look for possible grounding or surge problems that aren't consistent.

Branson