Sigma Phase Formation
What Is Sigma Phase in Stainless Steel?
Sigma phase (commonly denoted as σ-Phase) formation in stainless steel refers to the precipitation of a hard, brittle, chromium-iron intermetallic compound that develops in duplex, ferritic & austenitic stainless steels. The presence of sigma phase significantly degrades both mechanical properties and corrosion resistance, making its control essential in stainless steel fabrication and service applications.
How Does Sigma Phase Formation Occur?
The sigma phase formation process begins when stainless steel is subjected to prolonged exposure in a specific temperature range, typically between 550 °C and 900 °C, allowing chromium, molybdenum and iron atoms to diffuse and form the intermetallic σ phase. The formation of sigma phase in stainless steel is linked to time-temperature interaction, the diffusion of alloying elements (mainly chromium), and the prior presence of ferrite phases. Slow cooling or extended hold times in the critical temperature window significantly increase the risk of sigma phase formation in stainless steel.
Effects of Sigma Phase Formation on Stainless Steel
The effects of sigma phase on stainless steel are detrimental. The impact of sigma phase on properties includes high hardness, reduced toughness, increased brittleness & decreased ductility- leading to embrittlement and diminished corrosion resistance. Because the sigma phase consumes chromium & molybdenum from the surrounding matrix, the protective chromium-rich passive film is significantly compromised. The weakening of mechanical and metallurgical properties means that even relatively small amounts of sigma phase formation can lead to serious degradation in stainless steel performance and lead to premature failure.
Prevention of Sigma Phase Formation
To avoid sigma phase formation in stainless steel, engineers implement key controls: maintaining alloy compositions with limiting ferrite content, applying solution annealing at sufficiently high temperatures to dissolve sigma phase, followed by rapid quenching, and avoiding prolonged exposure in the temperature range favorable for sigma phase precipitation. Using controlled welding procedures that limit excessive ferrite transformation and cooling delays.
Detection and Testing of Sigma Phase
Detecting the sigma phase typically involves a combination of metallographic examination under high magnification and mechanical tests, while metallographic evaluation consists of chemical etching techniques to reveal its distinct morphology. The sigma phase testing methods include optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). In practice, impact testing (such as the Charpy V-notch test) is also used to assess embrittlement due to sigma phase in stainless steel, while the Magnetic or ferrite measurement techniques can also be used as indirect indicators. These methods enable reliable identification of sigma phase and evaluation of its impact on material integrity.