I will only say Inconel does not like heat, as in preheat or high interpasses.  In fact it hates it and will let you know how badly it hates it by cracking on you.

There are probably a million "tricks" to welding inconel, it's not hard but it does demand some respect.

Give a little time studying things here: http://www.specialmetals.com/index.php

EDIT:  Re-read your post after already hitting the send key.  Your problem well could be shrinkage related (flanges having a much heavier wall thickness than the pipe / tube so you may have helped yourself by preheating but I surely wouldn't go as high as 200.

Mike.

Welcome to the forum!

While inconel 600/62 is referred to as a "superalloy",  when the rubber meets the road, the actual welding is pretty similar to your 300 series stainless.

You might find the pool slightly more "sluggish" (low thermal conductivity),  and you might even find your welding arc is a rather greenish tint, but don't let any of this spook you.

Things like pre-heat and stress relief should be scheduled by people other than you.. "should be"   You should have a clear plan of attack provided via WPS.

Your bead pattern is really nice and consistant..  On the very bottom pic I noticed what might be a slightly concave fillet profile on the tube to heavy plate.  This might be a cause for consern related to centerline cracking.  A concave fillet profile reduces the effective throat of the weld, and when you have the great difference in thickness, the cooling rates come into play and the reduced throat may give way to the stress.

Brushing and simple green may be ok as a part of your preweld prep, but I would suggest a wipe with acetone or trichloroethene right before you weld..  I would *NOT* trust simple green to not leave a residue.

Torch angles, gas flow, dip technique are all the same as stainless... BUT... Gas shielding is more critical with inconel.. Keeping the hot filler wire tip under the cover gas is much more important.. Clipping the end of your wire before you begin a weld is a good technique all the time.. But more important with superalloys.

Dedicate your prep tools for inconel...  Don't use a wire brush that has carbon steel or aluminum residue.

Backing gas is critical.. If there is *any* sugar on your melt thru... you have a problem that needs to be fixed.

I really like the fixture you made, welds are pretty.. Your a real pro.

I can't add much more than what the guys have already done, but you asked about the black soot.  It's pretty typical to see it when welding with Argon/Helium mixes regardless of material, but I have noticed it on Inconel (82) with Argon shielding.  I've not noticed it inhibit wetting or anything in multipass welds, but if you an clean it off, do so.

Im guessing this a exhaust system, ive welded similiar systems out of the same material and filler. Our testing consisted of 1100 deg C, cyclical testing. at those conditions the 600 material would slowly have cross-sectional material loss. The Engineers were happy enough for the number of testing we got out of exhaust system. Ive read that 625 would hold up more to the cyclical temperatures we were tetsing at.
We didnt use backing gas, 35-45 amps on schdl 40 pipe and took my time on those flanges (no preheat). no bevels
if there were failures they would mostly occur near the brackets that were restricting exspansion and contraction of the system.
The soot at the toes are possibly can be  from the lubrication used to manufacture the filler rod. Ive noticed  that from some nickel filler material there would be an unusual amount of "gargage" floating in the puddle and more residue at the toes of the weld. I wiped down rod with Acetone and problem gone.
thanks for your post most detailed ive seen

"I noticed on my stop points im getting a little darker color/soot looking spot. "

That's just the leading edge of the soot you're seeing beside the welds. 

I just noticed a Terrible One sticker under your outlet.  You ride?  4130 is what got me into TIG welding.

It's not necessarily about having a "thicker" throat; rather it's about having a "weld size" that meets the requirement set forth in the engineers design.

A concave throat on a 1/8" fillet is not as strong as one with a flush finish.  If the cross section were to be measured, and the triangle from the root to the toes (even if the leg length were to be correct) were to exceed the concavity of your weld, the weld might be undersized.

There are times when concave welds are required, but engineers design weld sizes that are strong enough to do the job and still be concave.

Most AWS DX.X code books have a nice graphic that describes convexity vs Concavity vs flushness and how these shapes relate to acceptability.

I am always a little concerned when engineers leave it up to the welder to develop the WPS. Not because you aren't capable of doing the work, but the welders have enough on their plate and may be missing part of the bigger picture and service conditions to make informed decisions.  I'm glad you spoke to your boss about getting in someone with experience to design the first few weldments or WPS if no one in house has experience designing inconel joints. I've spent a good portion of my engineering career coming in to redesign joints or fix mistakes created by engineers without a welding engineering background.

All that being said, from a welding perspective it welds very similar to 300 series SS. Nickel alloys tend to be more "sluggish" than mild steel and even stainless, what this means is you generally want to open up your bevels a bit to give you more penetration on the root pass, especially if you aren't taking a mirror or Xray to the root. This means more of a knife edge and away from a land. Some people also will also increase the root gap.

As mentioned having a concave fillet is bad from a stress point of view, this puts the weld centerline in tension, this also tends to magnify other centerline cracks like solidification cracks. Which bring me to my next point that cleanliness is more important than carbon steels, it's important to keep low melt point alloys like sulfur, lead, aluminum etc away from the weld. This is usually done by using dedicated grinding wheels, wire brushes etc. We also do a solvent wipe before welding with isopropyl or acetone to remove oils.  Centerline cracking may be prevented or reduced by having  a weld bead that's flat instead of concave or convex.  If you've having an issue welding to those flanges that are signficantly thicker than the pipe, that would be a case for a preheat, however it should be kept low.  I can't remember numbers but off the top of my head I would keep it under 250F, really it's just to bring it up so the thermal expansion of the two pieces is similar.

Now from an engineering perspective.  You mentioned these things are going through thermal cycling, then it's important to have 100% penetration    I can't stress this enough, un welded roots even if they aren't needed for strength purposes are the enemy of heat cycled welds. these will cause hairline fractures to open up due to the thermal expansion forces.  This just increases the importance of getting good penetration. This also means that in fillet welds the vertical leg usually needs to be beveled or double beveled and welded 100% instead of leaving an unfused root.  Some of that is a function of temperature.

In other regards if you are welding inconel to carbon steels than a lot of those concerns go out the window as the service conditions probably won't be corrosive or thermally cycled.

Good luck, you're showing a lot of craftmanship and dedication to the trade. The acquired knowledge part takes time, but you're doing great.

Welcome the the forum Thrashman!

Give some specifics about your project.

Acutal thicknesses of each part.

Joint details and positions

Base and filler metals specifics.

Process you are using.

Restraining fixtures?

Are you using a WPS?

Are you compliant to a code?  If yes; Which?

A pic of the part

A pic of the cracks.