Yes, sigma is an intermetallic compound, but it is a real chemical compound, rather like a carbide, except that it forms from two metals, like chromium and molybdenum. I can guarantee you that steels, like the 316 you mentioned, will form sigma in a LOT less time than 24 hours. As a matter of fact, this 316 will form sigma in about 20 MINUTES at 1450 degrees F. A 317 will form sigma in about 10 minutes at 1650F. A 254 SMO, a highly alloyed pure austenitic material will form sigma in less than a minute at this same 1650 F. To say 24 hours is a guideline for sigma formation at 1000-1700 is a misnomer. The higher the temperature, the quicker the sigma formation. Sigma definitely forms quicker in the moly-bearing steels, whether it is an austenitic or pure austenitic. Depending on the chemistry of the steel and the temperature exposure time is what determines sigma formation. The kenitics are what they are. Depending on the kinetics, a 304 or a 308 can form sigma just as quickly as a 309 or any of the other grades you mentioned. At some higher temperatures, sigma will form in a matter of minutes while it takes other grades much longer times at lower temperatures. The stabilizing agents like columbium and titanium and niobium will have greater effects on preventing sensitization than sigma. These stabilizing agents are normally such a lower addition to the steel that they have minimal effect on sigma. It is the moly that has the greatest effect, with or without stabilizers.
A great explanation.
Best regards - Al
A few other points if I may. One of the greatest problems when concerning yourself with sigma is not so much the formation of it as an absolute and immediately upon procedure qualification, but the fact that the nucleation kinetics are more difficult to overcome (at a given temperature) than the growth kinetics. Once the nucleation 'mountain' so to speak (or bump in the ground with some alloys and at some temps) is overcome-once the lattice is established- growth can proceed rather easily depending of course upon the temperature.
What does this mean? Most often (though there are certainly exceptions that merit concern especially as Chuck made clear with higher alloys) sigma (and let us not forget its bretheren chi-which is often overlooked yet maintaining very similar responses mechanically and chemically) in its initial formation will effect immediate properties very little. There is work out there verifying that as much as 1 or 2 volume percent may not effect tensiles or bends. At lesser volumes it will generally be almost undetectible mechanically and quite often even through corrosive testing. In even lesser volume percents it can even go undetected visually through photomics. Especially if the exact proper etching agent is not used. But, once the nucleation has taken place the growth is facilitated more easily, and depending upon how close the service temp is to critical temps, can reduce service life considerably. Therefore, even if immediate post qualification testing reveals no problems, the long term effects can be quickly manifest.
In my opinion, the thinking behind testing for sigma in some alloys is actually quite similar to corrosive testing as a whole. Short term tests with an eye towards long term effects.
Good discussion.
Good discussion . Tks for the responses
I did a quick reveiw of an article I had handy, and I need to add something and make a correction of myself. The diagram you are looking for is called TTP diagram (Time Temperature Precipitation). They manifest as C-curves.
The other thing I wish to add is that carbide precipitation can effect the nucleation and growth of sigma as well, since carbon helps to stabilize austenite as opposed to sigma. The idea being a carbon depleted austenite microsturcture will find an equilibrium in sigma much easier, if I may put it crudely.
So my thought is, even if sigme is not formed during the welding regime, if excessive carbides are, they may lead to accelerated sigma formation in service.
How much carbide precipitation is too much?
I don't know. Not sure that question even has an answer given the huge diversity of service regimes. Just don't let the SS get too hot, even if its a fully autenitic like 310.