Designing Equipment For Low Temperatures – Part 1
Related Content: *Note* there is now a Part 2 of the Effects of Low Temperature on Performance of Steel & Equipment
When designing equipment for low-temperature applications, it is important to keep in mind that low temperatures can adversely affect the tensile toughness of many commonly-used engineering materials. Tensile toughness is a measure of a material’s brittleness or ductility; it is often estimated by calculating the area beneath the stress-strain curve. Ductile materials absorb significant amounts of impact energy before fracturing, resulting in tell-tale deformations. Brittle materials, on the other hand, tend to shatter on impact. In general, materials with high ductility (i.e. a tendency to deform before fracturing) and high strength have good tensile toughness. However, depending on the material, tensile toughness can be very sensitive to temperature changes. Many materials experience a shift from ductile to brittle behaviour if the temperature is lowered below a certain point. The temperature at which this shift occurs varies from material to material. It is sometimes defined as the temperature at which the material absorbs 15 ft*lb of impact energy during fracture. It is commonly known as the “ductile-to-brittle-transition” temperature (DBTT), the “nil-ductility transition” temperature, or the “15 ft*lb transition” temperature.
As designers, we ideally want the DBTT to be as low as possible for low-temperature design. Metals such as aluminum, gold, silver, and copper have an FCC (face-centred cubic) crystal lattice structure, and most do not experience a shift from ductile to brittle behaviour. Other metals, such as iron, many steels, chromium, and tungsten, have a BCC (body-centred cubic) crystal structure and experience a sharp, often non-linear shift in ductility. Note that austenitic stainless steels, such as SAE 200- and 300-series (e.g. 316 stainless), have an FCC structure and do not experience a ductile-brittle transition. On the other hand, ferritic and martensitic stainless steels, such as SAE 400-series (e.g. 416 stainless), have a BCC structure and do experience a ductile-brittle transition.
DBTT values are usually determined through a standardized Charpy (or similar) impact test at varying temperatures. Because of the nature of this test, these values may not be representative of conditions that the material will experience from a design standpoint. Several factors may cause the DBTT to increase, including material thickness, the rate at which the device is loaded (i.e. strain rate), defects in the material microstructure (often the result of alloying), and the presence of stress concentrations (i.e. notches, sharp corners, etc.) in the device. According to the ASTM A514 standard, QT-100 steel has excellent low-temperature properties, but is not recommended for structural use below -46ºC, as it can become very brittle. Many components on a crane have even a higher DBTT. For example, many blocks are rated with a service temperature of -20 degrees C.
What does this mean in the field?
Obviously people are using this equipment below temperatures of -20 so right off the bat they are assuming liability for doing so. It also means that an engineer cannot accurately predict the mode of failure in the low-temp zone. Any brittle failure will be catastrophic but the failure wont necessarily be predictable. It will be from a random impact, dynamic loading or propagate out of a crack or nick.
- Low Temperature and the Performance of Steel and Equipment – Part 2
- High Tensile Steel in Design
- Sparta Products and Services