Category Archives for Materials
Guidelines to Determine MPT for Existing Vessels
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 … Continue reading
Variables Which Affect Notch Toughness
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 … Continue reading
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 … Continue reading
Fracture mechanics
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 … Continue reading
Steel Selection for Pressure Vessels Subject to Autorefrigeration
Autorefrigeration should be considered when selecting steels, as described below. In some liquid services such as LPG, a leak could reduce the pressure and cause a drop in temperature of a pressure vessel and its contents as the liquid boils … Continue reading
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 … Continue reading
Design to Prevent Brittle Fracture
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. … Continue reading
Definition of Brittle Fracture
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, … Continue reading
Sulfide Stress Cracking
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 … Continue reading
Stainless Steels Above 1000°F
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 … Continue reading