Designing Weldments. Группа авторов
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Название: Designing Weldments
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Техническая литература
isbn: 9781119865827
isbn:
2.1.4 Low Temperature Properties
Lowering the temperature of metal profoundly affects fracture behavior, particularly of metals that have bcc structure. Strength, ductility, toughness and other properties are changed in all metals when they are exposed to temperature near absolute zero. The properties of metals at very low temperatures are of more than casual interest, because pipeline, welded pressure equipment and vessels are expected to operate satisfactorily at levels below room temperatures. For example, moderate sub-zero temperatures are imposed on equipment for de-waxing petroleum and for storage of liquefied fuel gases and pipelines. Much lower temperatures are involved in cryogenic services, metal temperature –100oC (–150 oF) and below. The cryogenic service may involve storage of liquefied industrial gases like oxygen and nitrogen. Down near the very bottom of the temperature scale, there is a real challenge for metals that are used in the construction of equipment for producing and containing liquid hydrogen and liquid helium, because these elements in liquefied form are increasingly important in new technologies. Helium in liquefied form is only slightly above absolute zero, which is 1 Kelvin (–273.16oC or –459.69oF).
Absolute zero (1oK) is the theoretical temperature at which matter has no kinetic energy, and atoms no longer exhibit motion. Man has yet to cool any material to absolute zero, so it is not known how metals would behave when cooled to this boundary condition.
However, metal components have been brought to the temperatures very close to absolute zero, hence it presents a special challenge to metals and welded components as they would be required to serve in this extremely low temperature.
On cooling below room temperature every metal will reach a temperature where the kinetic energy will be reduced to nil. The atoms of the element will move closer and the lattice parameters would become smaller. All these changes would affect the mechanical properties of the metal.
With above information on the physics of metal in mind, let us review the behavior of an un-notched specimen without flaws. It may be pointed out that in real life, there is no material without flaw, every material has some flaw in it, and hence the assumption to a material without flaw is more of hypothetical in nature. It is the flaw that has to be considered as initiator of the material behavior in the given environment.
Consider the graph in Figure 2.2 below, 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 sub-ambient temperature dependence of yield strength σo (Rp0.2) and ultimate tensile strength σu in a bcc metal is shown in Figure 2.2 below.
Figure 2.2 Low temperature behavior of metals.
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 to 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.
2.1.4.1 Metal Strength at Low Temperature
As we have seen as temperature is lowered from room temperature 75oF (24oC or 297oK) to absolute zero 1oK the atoms of an element move closer together by dimensions easily compounded from the coefficient of thermal expansion. Number of changes occurs as a result of this smaller lattice parameter. For example, the elastic module increases. In general the tensile strength and yield strength of all materials increase as the temperature is lowered to the extent that at NDT the yield and Tensile strength are equal (σo = σu). The change in these properties is variable in degree for different metals but change does occur.
When the temperature of low carbon or low alloy steel is lowered the corresponding increase in strength of metals is attributed to an increase in resistance to plastic flow. Since plastic flow is strongly dependent upon the nature of the crystalline structure, it would be logical to assume that metals with the same kind of structure would react in similar manner.
Because toughness tends to decrease as temperatures are lowered, especially for bcc-structured material like steel. Testing is often carried out to measure and monitor this property of steel. The most frequently used test specimen is notched-bar impact specimen, despite the shortcomings of this test. The popularity of impact test is due to its long established position in standards and relatively easy procedure available with laboratories to test standard Charpy V-notch specimen.
2.1.5 Elevated Temperature Properties
The behavior of metal at the elevated temperature assumes importance for primarily following reasons.
1 1. Welding operation involves heat, and higher temperature, it is essential that we understand the changes in the metal’s mechanical properties, and predict its behavior under induced strain during welding. Going through the cycle of heating and cooling can significantly alter the properties of base metal, and the weld itself.
2 2. Metals are formed and shaped to be useful for the specific structural applications. It is important that the metal so formed and worked on possess the desired. During hot working the metal may develop flaws that may subsequently reduce the strength of the structure.
3 3. May weldments are exposed to high temperature service.
Strength of the metal reduces as the temperature rises, the elastic modulus is reduced, and plastic deformation is more active.
A weldment during welding experiences rapid rise of temperature to its melting point, and then it rapidly cools. If the structure or its any member is restrained the hot weld metal and the HAZ area goes through complex thermal strains. This can lead to weld or HAZ or both cracks, or distortion occurs in the structure, due to the shrinkage stress.
Apart from the normal strength and ductility parameters are not very effective in predicting the behavior of metal at elevated temperature. Other properties such as time at the elevated temperature, and the applied stress give a new parameter to consider, the creep property of the metal. The metal at high temperature and under stress continues СКАЧАТЬ