Sigma Phase

There is always a concern about duplex stainless steels and the presence of sigma phase. This is an intermetallic phase that can precipitate due to poor heat treatment or to excessive heat inputs during welding. Sigma phase causes a loss in both toughness and corrosion resistance.

Provided care is taken during manufacture and fabrication, sigma phase should not be a problem. However, when it occurs, the question is often asked, how much is acceptable. There is not a simple answer to this question, because the effect of sigma phase depends on its size. The larger the diameter of the sigma particle, the greater is the area around it that is denuded in chromium and molybdenum, and generally, the more detrimental is its effect on properties. However, a wide range of particle sizes is possible and it is better to conduct fitness for purpose tests if sigma is suspected.

The most common tests are a Charpy impact toughness test at the desired temperature (often -46 or -50°C) and an ASTM G48 method A corrosion test. The minimum Charpy energy has been suggested as 70J for parent metal, as it will decrease further on welding, and 35J for welds. In the corrosion test, the following temperatures have been recommended, with the pass criteria being no pitting and a weight loss < 4g/m2.

Recommended ASTM G48 test temperatures for duplex stainless steels.

Posted on: 21st Dec 2016

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