The main reason behind that during welding of spot weld it has been occurred inter-metallic stage in between joint and also affect the joint strength.

Better, we can avoid inter-metallic during welding based on suitable filler materiel according to the dissimilar.

Presence of martensite itself is the reason for cracking in that region. During a tensile testing you are facilitating the crack propogation which results in lower tensile stresses. Martensite should be avoided and you need to design the process accordingly

The only correct answer in that of Rijesh Malayathodi. Martensite presents deter the failure because of crack propagation. Martensite is fragile!

Thank you all for your valuable comments. Still I have some doubts regarding the same. 

If the martensite formation is the only reason, why do crack propagate through Fe-Mn-C-Al steel fusion zone rather than DP fusion zone?

How  martensite is forming in fully austenitic steel with 18% Mn and 0.6%C? 

Siva,

I too agree that presence martensite itself causes failure due to its brittleness. But crack propogation is observed in austenitic steel interface in ur case is quite uncommon. I suspect that ur austenitic steel seems to be more complex and there may be possibility of forming some other detrimental second phases during welding which would have caused the poor mechanical property. I suggest you to go for further intensive charecterization on both the interface to identify the real cause of the failure.

Ashok k

Thank you for your answer Mr. Ashok.

Actually I have done EPMA and EBSD. Segregation of Mn and C are observed from EPMA results, but its a common phenomenon for both similar and dissimilar welding of high Mn austenitic steel.  

What is the chance for the formation of β-Mn? I read some where that,  β-Mn can cause the easy propagation of crack. 

Thank you in advance. 

Siva Prasad 

SP, 

        What s the C content in your DP steel ?

Can you please check for interfacial microstructure. Do you reveal the presence of type II boundary in this region? (Disbonding). You can perform non-standard charpy (C. Pan - Direct fractography technique) test and then can assign the reason. Martensite has high YS and UTS so during gradual loading of CTT (slow strain rate) its failure cannot be truly possible provided there is no any pre-existing defect. But if impact (charpy), yes then we can assign the martensite presence. Please refer the following link and if possible perform the test.

http://dx.doi.org/10.1179/174328405X43135

Rijesh Sir,

C content in DP steel is 0.073.

Such low C content cannot give you martensite on fast cooling and you cant expect martensite in DP fusion zone . 0.6 % C on the other side is prone for martensite formation during fast cooling. Thats the reason why crack propogate through austenetic steel

Thank you Dr. Mukhtar.

I measured the nugget diameter and indentation. I could not observe any change in strength with nugget diameter. Even the weld with 6.4 mm dia show inferior tensile strength compared to that with a  dia of 4.8.  

Regards