Cracking mechanism at 475°C
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Does anyone know the embrittlement mechanism at 475°C? I’ve never understood what substance is produced that causes stainless steel to become brittle
Bipolar weld metal containing a high amount of ferrite phase (with a volume fraction exceeding 15%~20%) experiences a significant reduction in plasticity and toughness, as well as embrittlement, when heated to 350~500°C. Since the embrittlement rate is fastest at 475°C, it is referred to as “475°C brittleness”. The higher the ferrite content, the more severe this embrittlement becomes. Welds that have suffered \"475°C embrittlement\" can have this issue eliminated through quenching at 900°C. When stainless steel welded joints are used over a long period at temperatures ranging from 375 to 875°C, an Fe-Cr intermetallic compound is formed, known as the σ-phase. The σ-phase is hard and brittle, with a hardness of up to 68 HRC. As a result of the precipitation of the σ-phase, the impact toughness of the weld decreases sharply; this phenomenon is known as \"σ-phase embrittlement\". It is generally believed that the σ-phase develops from ferrite; when the volume fraction of ferrite in steel exceeds 5%, the σ-phase forms rapidly. Therefore, for stainless steel materials operating at high temperatures, in order to prevent the formation of the σ phase, it is necessary to control the ferrite content. To eliminate the σ-phase that has formed and restore the toughness of the welded joint, the welded joint can be heated to 1000–1050°C and then rapidly cooled. The σ phase generally does not form in the welds of 1Crl8Ni9Ti stainless steel. From the encyclopedia.
API571: The temperature at which sigma phase embrittlement occurs is higher than the embrittlement temperature of 475°C. The sigma phase embrittlement typically takes place in the range of 1000°F–1750°F (538°C–954°C). It is not clear whether the sigma phase forms in the microstructure at the 475°C embrittlement temperature
This post was last edited by hlcl on 2016-6-15 at 13:40. Please refer to API571 4.2.5, but it does not specify the sigma phase. API581 states that it is caused by the formation of CR-P intermetallic compounds. Then it’s not a sigma phase.
Could you send me API581 to email 506616037@qq.com? Thank you.
I just downloaded the Chinese version from the Internet and am learning it*. Thank you!