Heating SS With O/A

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Ok at work, one of the smart guys dropped a massive stainless steel shaft, this shaft has plates running from one end to the other, with a large shafe that goes through the centre of it.......the plates that run from one end to the other are about 85mm width and probably 2 metres long.... on these plates there are smaller playes, they are about 120mm long and aprox 50mm wide and 6mm thick....they are basically scraper plates that scrape off paper that gets fed off a belt or something, which these stainless steel plates (120x50mm x 6mm which have 20mm of stellite welded onto the scraping edge, then are machined back smooth) scrape off the paper....To the question..... the shaft was bent out 2.2mm at one end of the shaft and .30mm at the other end, so what we did was heat the long strips of plate in certain spots, until it was glowing red and then cooled it down with water.... this moved the outside rolled plate down a few mm, so we got the shaft back to a tolerance of .35 on the 2.2mm side...What i am wondering, is where we heated it, which would have to have been at least 10 places, would this cause that heated area to be weaker then the opposing areas, and could cracks and faults form there, because the properties of the metal have been changed by the extreme heat we put into it.... or would this have just heat treated these areas?sorry for the long *** explaination, but yea, just thought id roughly explain what im talkign about, cheers"One World...One Muay Thai...Muay Thai Never Dies..."
Reply:Chromium is what makes stainless steel corrosion resistant.  Heating can damage the corrosion resistance of SS.  Heating SS in the range of 500 to 800 C will cause carbide precipitation, also called sensitization.  The degree of carbide formation depends on the temperature and the time at temperature.  Carbon combines with chromium to form carbides that form along the grain boundaries in the steel.  The amount of free chromium in the grains, along the grain boundaries, is reduced, and this leaves an unprotected path for corrosion to penetrate into the steel and lead to failure.This is called intergranular corrosion, because it follows a path along the grain boundaries.If your heating and cooling process was fairly rapid, the degree of sensitization may be minimal.  It was a good idea to water cool this, to avoid lingering in the 500 to 800 C range.If you totally screwed this up, say it's totally sensitized, and needs to perform in a highly corrosive environment, the only way you can reverse the carbide precipitation is to "solution anneal" the entire weldment in a controlled atmosphere furnace.If this thing is not being used in a wet corrosive environment, maybe carbide precipitation doesn't matter, and maybe it didn't need to be stainless steel in the first place.
Reply:Depending on the alloy, you may have annealed it though.I work with 304 all the time, and the 300 series alloys can be annealed by heating to red, then immediately quenching in water. I do this all the time, intentionally, when forging stainless. And boy, can I tell the difference between mill finish 304, which has been work hardened by rolling, and annealed 304. I often hand bend the stuff, using a hossfeld, and the annealing makes it a lot easier to bend, thats for sure.So if the shaft bends again, it might have been annealed by your heat.It doesnt make it any softer, but it makes it bend easier.