Can i use stainless steel as a furnace roof. Furnace temperature will be max upto 850°C. If so which ss grade will be the best in this situation. Thanks
Most grades of stainless steel can handle 850 degrees without problem. Even the cheap ones such as 304. Although at that temperature it will be glowing bright red. It will discolor and some degradation will occur over time. Especially if it is exposed to abrasive particulate exhaust, or if it's stressed due to thermal cycling. The solution for this is usually to make it thicker. No matter what you choose, though. It will not last forever.
316 SS is a good combination of quality and price.
For super high temp situations, 309s and 310s steel is usually used.
As a side note, chimney liners are stainless steel. 316 steel is usually used if higher temps are expected. 304 is used for cheap or wood only fires. They are designed to be able to withstand temperatures of over 1000 degrees peak, and both can run around 500 degrees for years with no issues.
Ignoring the fuel source for the furnace (such as vanadium in fuel oils causing rapid oxidation); What do you want the answer to be? For minimum stress as I expect a roof to be , ferritic like 430 or 446 should be lower cost. Otherwise any of the "18-8" types like 304 (HF as a casting) would be good. I would avoid 316 , 317 because of possible rapid oxidation in stagnant locations ( trapping of moly oxide). The "best" of ordinary stainless are 309 and 310; they are the industry standard for furnace hardware. Nearly always as castings , HH and HK ( higher carbon types of 309 and 310). For hardware a supplier might even offer HH at the same price as HF because it is so common. If you use any 300 series stainless that is not a casting, use H grade, higher carbon for better strength. L grades would be a mistake.