Selection Of Pipe Material For Low-Temperature Service
Similar consideration applies to the aboveground pipe and components. Aboveground valves flanges and pipes are more exposed to the weather and are also carrying the similar product. Therefore, they have greater propensity to face low temperature in their service lives. The following questions must be asked and answered: Are they insulated? Are they heated? Is there any possibility of depressurization that would lead to extensive temperature reduction, etc? There is a multiplicity of factors that affect the understanding of the material behavior in extreme stress conditions. All possible factors must be identified and addressed.
The questions we have tried to explore are more complex than this discussion which is an attempt to simplify the basic understanding of the subject. This discussion is intended to bring out the importance of the subject and direct readers to available resources for material selection issues.
Important Additional Information
The sub-ambient temperature dependence of yield strength σo (Rp0.2) and ultimate tensile strength σu in a bcc metal is shown in Figure 1. Consider the graph, the material is ductile until a very low temperature, point A, where Y.S. equals the UTS of the material (σo = σu). Point A represents the NDT temperature for a flaw-free material. The curve BCD represents the fracture strength of a specimen containing a small flaw (a < 0.1mm). The temperature corresponding to point C is the highest temperature at which the fracture strength σf ≈ σo. Thus point C represents the NDT for a specimen with a small flaw.
The presence of a small flaw raises the NDT of steel by about 200°F (110°C). Increasing the flaw size decreases the fracture stress curve, as in curve EF, until with increasing flaw size a limiting curve of fracture stress HJKL is reached. Below the NDT the limiting safe stress is 5,000-8,000 psi (~35 to 55 MPa).
Above the NDT the stress required for the unstable propagation of a long flaw (JKL) rises sharply with increasing temperature. This is the crack-arrest temperature curve (CAT). The CAT curve defines the highest temperature at which unstable crack propagation can occur at any stress level. Fracture will not occur for any point to the right of the CAT curve.
The temperature above which elastic stresses cannot propagate a crack is the fracture transition elastic (FTE). The temperature defines the FTE, at the point K, when the CAT curve crosses the Yield Strength, σo curve. The fracture transition plastic (FTP) is the temperature where the CAT curve crosses the Ultimate Tensile Strength σu curve (point L). Above this temperature, the material behaves as if it is flaw-free, for any crack, no matter how large, cannot propagate as an unstable fracture.
Ramesh Singh is a senior principal engineer for Gulf Interstate Engineering, 16010 Barkers Point Lane, Houston, TX 77079. He specializes in materials, welding and corrosion. He graduated from California Coast University with a master of science degree (2003) in engineering management and gained his basic metallurgical education (1984) from Air Force Technical Institute in India. He is registered with the Engineering Council in the UK and is a member of The Welding Institute, Cambridge UK. He is a NACE member and has served as secretary and vice chair of the NACE Houston chapter. firstname.lastname@example.org.
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