Sigma Phase Embrittlement | Materials And Corrosion Control

Damage Description

o   Alloy composition, time and temperature are critical factors.

o   In susceptible alloys, the primary factor that affects sigma phase formation is the time of exposure at elevated temperature.

o   Sigma phase occurs in ferritic, martensitic, austenitic and duplex stainless steels when exposed to temperatures in the range of 1000°F to 1750°F (538°C to 954°C). Embrittlement can result by holding within or cooling through the transformation range.

o   Sigma forms most rapidly from the ferrite phase that exists in 300 Series SS and duplex SS weld deposits, but usually more slowly in the 300 Series SS base metal (austenite phase).

o   The 300 Series SS can exhibit about 10% to 15% sigma phase.
Cast austenitic stainless steels can develop considerably more sigma.

o   Formation of sigma phase in austenitic stainless steels can also occur in a few hours, e. g. when it is subjected to a postweld heat treatment at 1275°F (690°C).

o   The tensile and yield strength of sigmatized stainless steels increases slightly compared with solution annealed material.
This increase in strength is accompanied by a reduction in ductility and a slight increase in hardness.

o   Stainless steels with sigma can normally withstand normal operating stresses, but upon cooling to temperatures below about 500°F (260°C) may show a complete lack of fracture toughness.

Materials & Equipment

a) 300 Series SS wrought metals, weld metal, and castings. Cast 300 Series SS including the HK and HP alloys are especially susceptible to sigma formation because of their high (10-40%) ferrite content.

b) 400 Series SS and other ferritic and martensitic SS with 17% Cr or more are also susceptible (e.g., Types 430 and 440).

c) Duplex stainless steels.

Equipment:

·         Stainless steel cyclones, piping ductwork and valves in high temperature FCC Regenerator service.

·         300 Series SS weld overlays and tube-to-tubesheet attachment welds can be embrittled during PWHT treatment of the underlying CrMo base metal.

·         Stainless steel heater tubes are susceptible and can be embrittled.

Control Methodology

·         The lack of fracture ductility at room temperature indicates that care should be taken to avoid application of high stresses to sigmatized materials during shutdown, as a brittle fracture could result.

·         The 300 Series SS can be de-sigmatized by solution annealing at 1950°F (1066°C) for four hours followed by a water quench. However, this is not practical for most equipment.

·         Sigma phase in welds can be minimized by controlling ferrite in the range of 5% to 9% for Type 347 and somewhat less ferrite for Type 304. The weld metal ferrite content should be limited to the stated maximum to minimize sigma formation during service or fabrication, and must meet the stated minimum in order to minimize hot short cracking during welding.

·         For stainless steel weld overlay clad Cr-Mo components, the exposure time to PWHT temperatures should be limited wherever possible.

Monitoring Techniques

·         Most cases of embrittlement are found in the form of cracking in both wrought and cast (welded) metals during turnarounds, or during startup or shutdown when the material is below about 500°F (260°C) and the effects of embrittlement are most pronounced.

Inspection Frequency

KPIs

References