thank you dear metalguy.I don't know exactly the analysis , but the environment is the exaust gas from gas turbine.In ASM handbook it is noted that the maximum service temperature in air for continuous service for 304 ss is 925 centigrade degree. does it mean that it is not sesitized in this temperature?
That high a temp. involves scaling resistance. You probably don't have to be concerned with sensitization in an exhaust situation--scaling is the concern. Therefore 700 deg. C shouldn't be a problem.
The range for sensitization of 304 is 800-1500F (427-815C). If the S.S. is heated or cooled rapidly through this temperature range it will not sensitize. but if it is held within this temperature range it will sensitize and loose some of its corrosion resistance by becoming susceptible to intergranular corrosion in certain environments. I am not sure about your environment but someone here will likely answer that for you.
I believe the max temp of 925C referenced in ASM is with respect to usable mecahnical properties without respect to corrosion resistance. This does not sound like a good application for 304 S.S.
You should look at alloys with better oxidation resistance. Higher Cr and Ni will help with a better oxide layer and keeping it from flaking off.
I wouldn't be a fan of using 409 at that temp. Have you done it? Does it work? Why do you want to change?
= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
The 700C exhaust temp seems a little high for steady state operation for industrial gas turbines but not during startup. 409 SS is being used for industrial turbine exhausts as well as 3XX series. Superior resistance to oxydation will be obtained with 304 SS over 409 SS. If steady state operation is 700C, I would recommend a higher Chrome containing alloy like 309 SS.
439 does sound like a good option. Higher Cr and still no Ni so the cost should be reasonable.
I have seen units that are cold every night and used for max power every morning and afternoon. These tend to have more severe cyclic oxidation issues.
= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
Go to the AK Steel web site. They have developed the greatest number of high temp ferritic alloys. They have good data sheets. This duty would be a slow ground ball for their 18 SR. It's an version of 439 optimized for this very application.
thank you all my friends .the problem is that we used 304 ss instead of 409ss wrongly in casing panels of hrsg system and changing will cost much. so i wanted to assure myself that it will not cause problem.
The allowable stress for 304 ss at 700 deg C will exceed the allowable stress for 409 ss. The only problem other than cyclic oxidation of the 304 ss is the coefficient of expansion for 304 ss is greater than 409 ss. If this was not taken into account by design, the casing will warp and pull away at rigid attachment locations. This could result in tears to the casing. If your thermal expansion is acceptable with 304 ss, I would leave it alone and periodically monitor the condition of the casing.
Maryam1,
*IF* 409 ss is OK for your particular application, then 304 should do fine. Keep in mind that 409 is just about the cheapest grade that can be called "stainless"--it has just enough Cr and nothing else in it.
I heat treat 305series stainless steel the parts are quite small approx 0.2500 squares x 0.017 thick, my problem is that they stick (fuse) to each other.
I use a temperature of 1850f and a hydrogen atmosphere (9999% pure) the same problem accurse using a vacuum furnace.
I understand way the SS sticks but cant help put think that there’s a cure for this.
I have tried mixed percentages of atmospheres such as Nitrogen and Argon blended
with H2 but this only creates a discoloration that cant be tolerated, I have also tried with some success Aluminum Oxide power but this Alo2 tends to embed itself in the base material and this is another problem.
I have been in the heat treat industry for over 35 years and have not solved this one yet.
What you're doing is called sintering. In the stainless production business we too are familiar with the problem. If one anneals a coil of stainless it will weld to itself wrap to wrap unless it has an oxide coating. So we just continuously annealed instead.But,if you have to avoid the sticking why not coat them with a fine slurry of some stable oxide that will prevent metal to metal contact and still be easily removed after heat treat.
mcquire, Thanks for the feed back, I have tried hundreds of buffer materials the problum with them is that they remain on the base material even after a ultrasonic clean.
I run LPC and SEM and show a small level of particals remainimg that will effect the parts performance I refer to them as hard particals that can disloge while the part is in servive, this micro partical will damage the machine.
Im not at liberty to reveal the machines function but I have spent years looking for the solution that will allow me to anneal this material approx ~1mil per day and yield no sintering effect.
305ss: When I was in the atmosphere/vacuum heat treat business, we used a product called fiberfrax sandwiched between components to separate them and prevent diffusion/sintering.
This fiberfrax came in sheets about 1/32" thick in a clothlike form in a random pressed condition. I seem to recall that in some intances there was some slight adherance of particles to the metal. This material might no longer be available because I believe it is made of asbestos type material.
Other times we used another product also a woven clothlike material that was more glossy (the fiberfrax was dull matt-like finish). The shiny material I think is fiberglass and was thicker (about 3/32")than than fiberfrax.
