API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

2-152 API RECOMMENDED PRACTICE 581
22.10 Figures
STEP 1 :Do administrative controls
prevent pressurizing below some
temperature T min ?
Yes
Use this T min
No
STEP 2: Determine T min, the Minimum of:
• Design Temperature
• Upset Temperature
Design
Temperature
Operating
Temperature
STEP 3: Determine T ref
Use T ref 28C(80F) unless the actual ductile to
britltle transition temperature is known.
STEP 4: Calculate (T min - T ref )
STEP56:Calcualtion the Damage Factor using
Table 22.3
Use (T min - T ref ) in the lookup.
Figure 22.1—Determination of the 885 °F Embrittlement DF
23 Sigma Phase Embrittlement DF
23.1 Scope
The DF calculation for components subject to sigma phase embrittlement is covered in this section.
23.2 Description of Damage
Sigma phase is a hard, brittle intermetallic compound of iron and chromium with an approximate composition
of Fe 0.6 Cr 0.4 . It occurs in ferritic (Fe-Cr), martensitic (Fe-Cr), and austenitic (Fe-Cr-Ni) stainless steels when
exposed to temperatures in the range of 593 °C to 927 °C (1100 °F to 1700 °F). The rate of formation and
the amount of sigma formed are dependent on chemical composition of the alloy and prior cold work history.
Ferrite stabilizers (Cr, Si, Mo, Al, W, V, Ti, Nb) tend to promote sigma formation, while austenite stabilizers
(C, Ni, N, Mn) tend to retard sigma formation. Austenitic stainless steel alloys typically exhibit a maximum of
about 10 % sigma phase, or less with increasing nickel. However, other alloys with a nominal composition of
60 % Fe, 40 % Cr (about the composition of sigma) can be transformed to essentially 100 % sigma. A
transformation vs time curve for such a Fe-Cr alloy showed 100 % conversion to sigma in 3 hours at 747 °C
(1377 °F). Conversion to sigma in austenitic stainless steels can also occur in a few hours, as evidenced by
the known tendency for sigma to form if an austenitic stainless steel is subjected to a PWHT at 691 °C
(1275 °F). Sigma is unstable at temperatures above 899 °C (1650 °F), and austenitic stainless steel
components can be de-sigmatized by solution annealing at 1066 °C (1950 °F) for 4 hours followed by a
water quench.