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

2-134 API RECOMMENDED PRACTICE 581
Low temperature/low toughness fracture of steel is affected by the following.
a) The applied loads. Brittle fracture is less likely at low applied loads.
b) The material specification. Some materials are manufactured to have good fracture properties or
toughness properties. Materials are often “qualified” for use by performing an impact test. This test
measures the energy needed to break a notched specimen.
c) Temperature. Many materials (especially ferritic steels) become brittle below some temperature called
the brittle-ductile transition temperature or reference temperature. Brittle fracture is typically not a
concern above 149 °C (300 °F).
d) Weld residual stresses and PWHT.
e) Thickness of the component.
The goal of the low temperature/low toughness fracture assessment is to rank components by their relative
POF with respect to fracture. This assessment will take into account the thickness, the material type, the
PWHT, and temperatures.
20.3 Screening Criteria
If both of the following are true, then the component should be evaluated for susceptibility to brittle fracture.
a) The material is carbon steel or a low alloy steel; see Table 20.1.
b) If minimum design metal temperature (MDMT), T MDMT , or minimum allowable temperature (MAT), T MAT , is
unknown, or the component is known to operate at or below the MDMT or MAT under normal or upset
conditions.
20.4 Required Data
The basic component data required for analysis are given in Table 4.1, and the specific data required for
determination of the DF for brittle fracture are provided in Table 20.2.
20.5 Basic Assumption
Brittle fracture requires the coincident presence of a crack-like defect, application of sufficient stress, and a
susceptible material. The susceptibility to failure by brittle fracture can change due to in-service conditions.
20.6 Determination of the DF
20.6.1 Overview
A flow chart of the steps required to determine the DF for brittle fracture is shown in Figure 20.1. The
following sections provide additional information and the calculation procedure.
20.6.2 Inspection Effectiveness
Low temperature/low toughness fracture is prevented by a combination of appropriate design and operating
procedures. When low temperature/low toughness fracture does occur, it almost invariably initiates at some
pre-existing crack-like defect. From the initiation point, a crack will grow unstable, resulting in a serious leak
or sometimes complete catastrophic rupture of the component. Theoretically, an inspection to locate and
remove such pre-existing defects would reduce the POF. However, the initiating defect can be very small
and need not be exposed to the surface where it could be found. For this reason, inspection for such defects
is generally not considered to be an effective method for prevention of brittle fracture.