Cracked weld: what did I do wrong?

I have those headers I'm trying to repair. The headers are SS and they end with a flange that looks to be cast (either cast iron or cast steel). From the factory, the stainless tubes were welded to the inside of the flange. The spare flange I have has the tube cut flush to the flange. So my plan was to cut a new piece of stainless steel tube and weld it to the rest of the header, then to the flange.

Now I'm no pro welder or anything, just a guy in his garage trying to fix his stuff. I have a MIG/flux-core and I'm using 0.030" steel flux core wire.

When I tacked it, I noticed some tacks cracked on the tube-flange joint. I didn't think anything of it and started welding. I was being careful not to put too much heat in it so I was welding maybe 0.5" to 0.75" at a time. Some of the welds went okay like this one:

But on two occasions I did the pass and heard a loud "pop" right after lifting my helmet. The welds cracked:

What caused this? Poor fitment? Do I need to heat the flange before welding it? Now that I have 2 cracks like that, can it be fixed? Should I take that to a pro?

The other joint went okay, no cracks.

If it's really dissimilar, you may not have heat on one side and it caused the weld to crack because of that.  
 

if you can preheat the piece first, toaster, over, propane torch, that may help to keep this from happening.  Get both sides to around 350+ degrees, then weld.

I'm no welding expert, but welding a casting is a special kind off flavor I've never been able to master.  The expansion rates of the two different metals are vastly different and its fine when it's hot and squishy, but as it solidifies and shrinks you get problems.

Others will chime in with better advice, but it has something to do with preheating the casting to a dull glow, then keeping heat on it to let it cool very slowly.

For steel to cast steel, get something with high nickel content.  There is special rod for castings, and special stuff for dissimilar. I am most familiar with stick, but there may be wire, too. Get a recommendation from a local weld supply shop.

Some preheating of the casting and a filler rod with some extra nickel like a 308L rod can help a lot with the cracking.

That weld bead wants to get smaller as it cools. It also tries to pull along the two pieces of metal that you welded together

But the casting does not want to come along for the ride as the weld bead cools and contracts, and in fact probably wants to do it's own contracting in a different direction.  This sets up a tug-of-war where the weld bead is the rope. The rope is going to break in this case.

My guess is the cast part and the steel part are cooling at different rates causing the crack.

Preheat, weld, bury in vermiculite so it will cool slowly. I use a turkey cooker burner for most preheating. 

The preheat like everyone else said is critical, but the cool down is what's killing your weld.  I've had luck using the torch post-weld to keep the heat up, and slowing pulling it back or spreading the heat out to try to slow the cooling process down. The longer you can make the cool down process the better for dissimilar materials. 

Another "trick" is to peen the crap out of the weld as it cools...sharp end of a chipping hammer maybe in your case.

Flange needs to be hot, as mentioned.