By -
Date 08-26-2005 23:16
Good Afternoon Mr. Roberts,
When ferrite containing austenitic SS weld metals are exposed to temperatures above 800F, a variety of microstructural changes will occur. One is the carbide precipitation that takes place at the austenite-ferrite interphase boundaries. Growth of these carbides are dependent on the exposure time and temperature.
304L, along with the other austenitic SS, when exposed to temperatures in the 900-1650F range, various intermetallic phases can form, the main one being sigma. To say that sigma contains moly, as I understand your statement, is a bit confusing. Sigma is a product of the chemical composition, the ferrite content, and the time at the sigma temperature range. While it is true that generally, the sigma phase transformation occurs more rapidly in the moly bearing SS, the amount of ferrite is a driving force in determining the amount of sigma. We know that moly, along with the Cr content is a strong ferrite former. The higher the Cr and Moly content, the higher the ferrite content becomes and therefore, the more susceptability to sigma. To say that "sigma contains moly" might mean that moly will help accelerate the formation of sigma due to the fact that moly promotes ferrite. That is why a duplex material with it's usual 50% ferrite and 50% austenite is so prone to sigma, because of the ferrite levels, and can form sigma within a matter of one minute in the 1600F range. To say that the residual moly in a 304L could be enough to accelerate sigma is sort of a misunderstanding. In the 304L, it is the amount of chromium that is the driving force in forming sigma. If the Cr is sufficient enough to form the amount of ferrites, then yes, sigma will form. A 316 series SS is normally more prone to sigma than a 304 because the ferrites are generally higher in the 316 due to the Cr and the moly. But remember, a 304 SS has more Cr than a 316 SS, so a low moly 316 can actually have the same of less ferrites than a 304 series SS. Only on special circumstances should a SS go through the "normal" PWHT cycles. The only recommended heat treatment for a 304, and virtually all the other austenitic SS is a quench anneal at 1050C. As in all cases, everything should be considered when deciding on a PWHT of austenitic SS. Hope this answers your question as I understood it.
Chuck