Category: Materials

Guidelines to Determine MPT for Existing Vessels

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Note that “minimum pressuring temperature” (MPT) is the same as “minimum design metal temperature” (MDMT). Most vessels built since 1969 to Company specifications should already have MPT on the vessel drawings, or on Safety Instruction Sheets (SISs). For carbon and low alloy steel vessels without an MPT specified, Figure UCS-66, along with materials of construction […]

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Variables Which Affect Notch Toughness

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Temperature. The ferritic steels (carbon, low alloy, and 400 series stainless) undergo a ductile-to-brittle transition as temperature is lowered. Each of these steels has a ductile-to-brittle transition temperature range. Above their transition temperature range these steels are tough; in and below the transition temperature range, they can fracture in a brittle manner. Section 523 discusses […]

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Relationship of Flaw Crack Size, Stress, and Notch Toughness

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Relationship of Flaw Crack Size, Stress, and Notch Toughness

Fracture mechanics uses stress analysis techniques to define a stress intensity factor (KI) which is proportional to the product of stress and the square root of flaw size. Fracture occurs when the stress intensity factor exceeds a critical value. For a given material, the critical value is a function of temperature, loading rate, and thickness. […]

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Fracture mechanics

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Fracture mechanics is an analytical tool for quantitatively relating the factors controlling susceptibility to brittle fracture. Fracture mechanics is used only in critical applications justifying in-depth engineering analysis. The principal factors to be considered in fracture mechanics are: 1. Flaw size—The basic theory of fracture mechanics is that brittle fracture initiates at preexisting flaws (cracks). […]

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Transition Temperature Approach to Fracture Control

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Transition Temperature Approach to Fracture Control

The ferritic steels (carbon, low alloy, and 400 series stainless steels) all undergo a ductile-to-brittle transition as temperature is lowered. Each of these steels has a ductile-to-brittle transition temperature range. Above their transition temperature range these steels are tough; at and below the transition temperature range, they can fracture in a brittle manner. Toughness is […]

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Design to Prevent Brittle Fracture

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Susceptibility of structures to brittle fracture depends on: 1. Preexisting flaw size 2. Tensile stress level 3. Fracture toughness of the material Flaw size and stress level are controlled by design, fabrication, and inspection in accordance with the ASME code. Toughness is controlled by material selection, also in accordance with the ASME Code. To prevent […]

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Definition of Brittle Fracture

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Brittle fracture is a sudden, often catastrophic failure which is inherent to “brittle” materials (discussed below). It involves little or no deformation (yielding), and it has been experienced in steel structures such as pressure vessels, tanks, pipes, ships, bridges, beams, and other similar structures, often of welded construction. The most well known incidents were those […]

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Sulfide Stress Cracking

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The martensitic stainless steels are especially susceptible to sulfide stress cracking. Welds are difficult to soften with heat treatment and are, therefore, susceptible to cracking. Low carbon grades, like Type 410S, are used to limit weld zone hardness. This cracking is prevented by controlling weld strength and hardness. These requirements are covered by PVM-MS-4748.

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Stainless Steels Above 1000°F

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At elevated temperatures, all stainless steels with high chromium contents will develop some “sigma phase” which causes embrittlement at lower temperatures. Sigma phase is very hard, nonmagnetic, and brittle. The composition of sigma phase varies depending on the alloy from which it formed. Sigma phase normally does not affect the steel’s elevated temperature properties but […]

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