liquid metal embrittlement-HRSP sheet cracking while brazing 1

[kkg123]

3 mm HRSP sheet has exctruded hole in which coppeer tube is brazed with 25 % silver brazing rod.cracking occurring while brazing.
can some one throw light on liquid metal embrittlement.how to avoid it the extuded hole cracks up after brazing. i heard about thumb rule to give radius on die punch will solve this problem.
will any body share information

 

[unclesyd]

Can you give a little more information about your problem?
How often does this happen?
One problem that worries me is how straight the crack is.

What is the clearance between the hole in the plate and tube?

Are you monitoring the temperature during brazing, as the tube looks like it has been extremely hot?

What is the exact alloy and flux?

How is the heat for brazing supplied?

Can you give some physical measurements like plate material and thickness, hole size and clearance, and tube OD.

 

[redpicker]

What does HRSP mean?

rp

 

[unclesyd]

HRSP is hot rolled steel plate in most places

 

[btrueblood]

kkg123, I am replying to your post in the general ME forum here, instead, as it is probably a more appropriate place for it. Unclesyd has given you some things to think about, and you should be able to research LME on rolled steel both on the web, or (better) via the green books (AMS handbooks).

My story has to change a bit, if that is not a high-nickel-content alloy. Low-alloy steel is susceptible to LME from molten copper (or tin), though. The three necessary ingredients for LME are a solvent/solute pair (and/or the temperature necessary for the pair to exist), stress, and time.

To cure the problem: annealing to remove some of the residual forming stress will help. Redesigning the joint to reduce the residual forming stress may help, but I wouldn't count on it being 100% effective. Also, you can try reducing the heat applied to the joint during brazing (as Unclesyd hints at), perhaps a lower melt point braze alloy, in the interests of reducing the amount of copper that dissolves into the braze alloy as well as reducing (hopefully) the amount of time involved.

 

[kkg123]

can LME phenomenon be avoided?

kedarg6500

 

[EdStainless]

I'll second (third?) the theme of heat reduction. Use no more than is needed.
You also need to look at the flux that you are using. It may be that the attack is from the flux and not the braze alloy.

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Plymouth Tube

 

[swall]

In my experience, you are more likely to have a LME problem during a re-braze.