SA 213 TP 304L

1. at 500^oC+ Cr starts seggregating as Chromium carbide and that reduces the properties on the steel as well as the absence of Chromium is likely to make it more prone to corrosions (sensitizated)

2. Approximately 475⁰C is the temperature where one encounters temper embrittlement.

Since it is a 304 series I agree you may observe chromium segregation. This effect may be a problem if the material is submitted several times to the temperature, because this effect also depends on time to occur. The result is that, as mentioned, corrosion may arise from the sensitized areas. You have at your side this SS is low C, so, less prone to chromium carbide formation. Over time you may observe pitting corrosion or corrosion in key areas like joints, welds, etc. But I do not think temper embrittlement will be a problem, once this is austenitic stainless steel.

That is correct - carbide precipitation (which can lead to detrimental corrosion/mechanical problems) is a function of carbon levels, temperature, and time.

For the low carbon 304L, please see the popular stainless-carbide diagram with the well known "nose" function. And note that it would take just under 10 hours at 600 deg C for the 0.03% carbon "L" grade stainless to start forming the undesirable elements.

http://www.staff.ncl.ac.uk/s.j.bull/mmm373/WELDMAT/img016.gif

So if you don't see any gross mechanical deformations of your heater (and don't continue with this overheating practice) you will be fine.

A final question, is this heater classified as a pressure vessel or is it only at almospheric pressure?

Sir This heater is designed as a pressure vessel according to ASME Section VIII Div-1 for Design pressure -10 Bar and 300 Degree C.

By restricting the carbon content to 0.03% maximum, the 304L or 316L, ( any low carbon) type steels are rendered sufficiently immune to carbide precipitation in the sensitising temperature range of 425-870⁰C. These steels can be welded, stress relieved if required or used without loss of corrosion resistance.

However if the steels remain in these sensitising temperature range for extended period of service, carbide can precipitate in significant quantities to cause intergranular corrosion.

These steels re usually exployed below 425⁰ C.

Similarly the stabilized steels have sufficient amount of columbium + Tantalum or Titanium in relation to carbon. These steels also exbits higer resistance to corrossive atmosphere in the sensitising tempearature of 425-870⁰C...

Again, sensitation occurs under certain conditions, under some media, or during service at about 650⁰C and not all the times.

sridhar