what is this structure 2

[safaeng]

hi
This is a resistance spot weld of steel (2 mm thickness)

process:
squeez-->weld(KA,Cycles)-->hold(40 cycles)

material:
C Mn Si P S Cr
0.15 1.02 0.17 0.012 0.011 0.004

Ni Mo Al Co V Ti Cu
0.02 0.009 0.037 0.002 0.003 0.001 0.03

In general
the resistance weld structure consists of 4 different areas
1) Base metal, 2) Intercritical HAZ, 3) coarse grain HAZ and 4) fusion zone.

I got pictures by myself

Anyone can help me with detecting phases there?
I mean which phases do you recognize at each pic?
I etched it with Nital solution. I think some areas are over-etched.

1- base metal:

http://i22.servimg.com/u/f22/12/92/10/76/base10.gif

I think it is ferrite matrix with pearlite. can you see any carbides there? because we have Ti and V.

2-Intercritical HAZ,

http://i22.servimg.com/u/f22/12/92/10/76/haz-5010.jpg

3-coarse grain HAZ(near the Haz and fusion zone)

http://i22.servimg.com/u/f22/12/92/10/76/haz-we10.jpg

4- fusion zone(near the center of weld nugget)
Is this martensite structure with ferrite(white)?

http://i22.servimg.com/u/f22/12/92/10/76/weld-110.jpg

Thank you in advanced

 

[metengr]

safaeng;

1- base metal:

Mostly ferrite + colonies of carbides with little to no pearlite. You can see banding of the structure from fabrication.

[/quote]2-Intercritical HAZ,[/quote]
Ferrite + upper bainite + carbides (dark islands)

3-coarse grain HAZ(near the Haz and fusion zone)

Bainitic and grain boundary ferrite + carbides

4- fusion zone(near the center of weld nugget)

Widmanstätten ferrite and martensite

 

[safaeng]

Thank you very much dear metengr!
We had discussions about this, before

So, please kindly, would you teach me how you differ between bainite and martensite there?

================================================
as we discussed earlier, I did a modified weld schedule on this material.

new schedule:
squeeze--> weld1 -->Hold (pretty long)-->weld2(60% of weld1)-->hold2 (10cycles)-->release

and here is the micro structure. Note that the fusion zone here is softened about 50 HV (350 HV) and the HAZ (400HV) is harder than the fusion zone. (I know this from microhardness testing).

so what are these phases here?

1- Base metal to HAZ areayou can see the micro hardness effect there )

http://i22.servimg.com/u/f22/12/92/10/76/haz-ba10.jpg

2- middle of the HAZ:

http://i22.servimg.com/u/f22/12/92/10/76/haz-mi10.jpg

3-fusion zone to HZA:

http://i22.servimg.com/u/f22/12/92/10/76/haz-we11.jpg

4-Coarse grain HAZ:

http://i22.servimg.com/u/f22/12/92/10/76/haz-we12.jpg

5- fusion zone:

http://i22.servimg.com/u/f22/12/92/10/76/weld-111.jpg

http://i22.servimg.com/u/f22/12/92/10/76/weld-510.jpg

 

[metengr]

safaeng;
Realizing you recognize the various phases in steel, what I would do is use the web site below and generate a CCT diagram based on chemical composition. You can adjust the cooling rates depending on distance from the weld region to determine what phase constituents should appear.

http://calculations.ewi.org/vjp/secure/TTTCCTPlots.asp

 

[safaeng]

thanks,I have seen that before. I don't know the cooling rates at different regions to use the CCT diagram.
all I know is that the cooling rate at the center of the fusion zone is about 3000K/S.

This would be great if you could see the pictures of my second post here and make some comments on them, like you did about the first one. So I can compare them to make some results from it.