Sigma Phase

A brittle chromium-molybdenum intermetallic that forms in duplex and high-alloy stainless between roughly 600 and 1000 °C, destroying toughness and corrosion resistance.

Why duplex welding procedures are strict

Sigma phase can form in minutes at temperature. A few percent is enough to drop Charpy values below acceptance and to open the steel to pitting. That is the reason duplex has both a minimum and a maximum heat input, and an interpass ceiling.

How it is checked

ASTM A923 — Method A (metallographic screening), Method B (Charpy impact) or Method C (ferric chloride corrosion test). We can supply duplex and super duplex plate with A923 testing on the certificate.

Related terms

Duplex Stainless Steel
Ferrite Content

Where this applies

Duplex & Super Duplex Plate

Need help applying this?

Stancor Stainless engineers answer specification questions directly. Contact us.