Meaning
Brittle crystalline structures that form in high-alloy steels at elevated temperatures degrade the toughness of industrial process piping. The development of an intermetallic sigma phase occurs between six hundred and nine hundred and fifty degrees Celsius, compromising both structural and corrosion resistance. This phase forms in duplex and austenitic stainless steels, stopping at the boundary of non-ferrous alloys.
Microstructural Formation
High chromium and molybdenum contents accelerate the precipitation of this undesirable phase during thermal exposure. In affected alloys, intermetallic sigma phase growth depletes the surrounding metal matrix of these corrosion-resistant elements. Long exposures during welding or high-temperature service are the primary causes.
Restricting heat input during welding minimizes the risk of formation.
Mechanical Impact
Embrittlement of the metal occurs rapidly even with small volume fractions of the precipitate. Toughness and impact energy decrease, making the components susceptible to brittle failure during thermal cycles or pressure surges. In addition, the depletion of chromium adjacent to the precipitates creates localized zones of low corrosion resistance.
This phenomenon leads to rapid intergranular corrosion in service. It creates an elevated risk of catastrophic pipe rupture.
Detection Methods
Non-destructive metallographic replication reveals the presence of these structures on the outer surfaces of pressure vessels. This test verifies the integrity of long-service high-temperature piping.
