heat treatment 304l

By - Date 08-24-2005 18:55

Heat treating 304L for 1-3 hours can cause the steel to become sensitized. Sensitization occurs when the steel is held in the range of 800-1500F for the time you stated. During this time, chromium carbides will precipitate to the grain boundaries. Held at temperatures of 1600F for 3 hours can cause the 304L to experience sigma phase, which is an embrittlment of the steel. This will compromise the strength and the corrosion resistance. The recommended heat treatment of a 304 is a complete quench anneal at approximately 1050C.

Chuck

By billvanderhoof (****)

Date 08-25-2005 05:45

For those looking for that book of tables now- 1050C =1922F
Bill

By GRoberts (***)

Date 08-26-2005 21:38

Hello Chuck,
A question for you on the sigma embrittlement at 1600F for 304L sst. My understanding is that sigma contains moly- is that correct? So would a stainless steel like 304L that would only have residual moly form enough sigma to acutally embittle? For some reason that I do not know, one of our customers specifies a 1600F PWHT for a project they have with -200F CVNs. Most of the imact tests got 30-40 ft-lbs.

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