I can’t tell you when they turn to carbon
But most fuels we use in steam cars vaporize between 400 to 450-F
And almost all cars I have worked on have two long of a vaporizing coil and there mounted in the wrong place.
They should be above the superheater and as close to the boiler as possible.
The burners run between 2500 to 3000-F the water in the boiler at 600 psi is only 486-F and most cars run at 500 psi or less.
You be the judge.
Yes that’s the way mine is built but it has straight vaporizer that goes right across the boiler only 18 inches long.
I think there would be a lot less trouble with a wide ring and both the vaporizer and superheater were in the ring. The vaporizer above the superheater next to the boiler. It’s the coolest place in the combustion chamber.
The few I have had the leeway to modify I moved the vaporizer as high as I could and built new branch forks. It has been a learning curve.
Not many want to make changes. Most all the vaporizer I have built for people is what they want and mostly the same as the old one.
If I had my choice I would use 4130 instead of stainless and make the hotdog with the same stuff.