The fact that the cracks start at the fillet weld raises a question: do they protrude also into the fillet weld? If so, the repair is more difficult.
There's another question to answer: how deep the cracks are? Do they reach the other side of the duct plate? If so, the repair is also more difficult.

Let's be optimistic and assume that the answer to both questions is NO.
The duct thickness is low and my advice is not to use a grinding disk but a mounted stone that you can fix on a pneumatic grinder or an electrical drilling machine.
If you don't know what a mounted stone is, it's a small stone (2 inches diameter at the most) that has a short steel shaft (one inch or so) coming off its center. The shaft is tightened into the grinder or driller tips. The mounted stone periphery shape can vary: it can be flat or like an angle. In this case use an angle shaped stone.
Grind lightly the cracks until they disappear. Let's hope that you won't need to grind across the whole plate thickness.
After the cracks have been ground off, apply weld onto the resulting holes.

Warning! Don't use a silicon carbide stone (trade name Carborundum). The carbon content of the stone will deplete the chrome content of the 410 SS. Use instead an alumina stone.
Giovanni S. Crisi
Sao Paulo - Brazil

GT = Gas Turbine? Which part of the gas turbine does this duct attach to? Depending on where it is located, gas turbines develop a lot of heat.  410 SS tends to develop very high hardness levels in the as-welded condition with very little ductility.  The heat affected zone would be sensitive to vibration fatigue, thermal fatigue (such as heat up and cool down during starts & stops) or stress corrosion cracking (if moisture is present). I've seen 410 SS shaft sleeves on hydro plant turbines crack in ambient river water environments.  If the cracks tend to follow the toe of the weld in the HAZ, it is likely fatigue. If they they to wander and are branching, could be SCC. A photograph would help. 400 F minimum preheat helps keep the cooling rate and hardness down.

Boon
I am not knowledgeable in Stainless, not enough room in my brain I guess.
But once a long time ago, I was asked to repair the inner stack liner of the forward Smoke Stack on the destroyer I was on. We had been to the Yards while R&R'ing in Hawaii just a couple of weeks before and they had done a quick repair on it and buttoned it up and we headed back to the gun line.
Seems the heat they applied while welding the patches on caused the original Stainless to start cracking out from their welds. And the stresses from a combination of things like the constant winding up and down of the fans for the force draft boilers and the continuous firing of the 5" guns fore and aft were not helping. Seems the Officers quarters were in close proximity and the vibrations were disturbing their sleep.   We secured  numbers 1 & 2 Boilers and I crawled in.
The only way I could stop these cracks in their trek across the plate was to drill holes at the end of the crack and then weld them up.
It worked.
I was just a 19 year old kid, @ 6 months out of boot camp, the only weldor on-board and knew even less about stainless then than now.\

But felt reminiscent of that place and time ever since I first read your post.

Good Luck
Marshall