Ductile temperature

The ASME Codes recommend that hydrostatic tests be run at a temperature that is usually above the nil-ductility temperature of the material. This is, in effect, a pressure-temperature treatment of the vessel. When tested in the relatively ductile condition above the nil-ductihty temperature, the material will yield at the tips of cracks and flaws and at points of high residual weld stress. This procedure will... 

Earlier work had shown that lower-molecular weight compatibilizers have a more significant effect on reducing the ductile-brittle transition than higher-molecular weight ones. Eurther, the brittle-ductile temperature. I m, increases with the increase in loading velocity. 

Excessively low temperature may involve the hazard of brittle fracture. A vessel that is out of use in cold weather could be at a subzero temperature and well below its nil-ductility temperature. In startup, the vessel should be warmed slowly and uniformly until it is above the NDT. A safe value is 38°C (100°F) for plate if the NDT is unknown. The vessel should not be pressurized until this temperature is exceeded. Even after the NDT has been passed, excessively rapid heating or cooling can cause high thermal stresses. 

With the reference toughness curve approach, the RT m index first came into use as the reference nil-ductility temperature which is determined in accordance with the ASME Code, Section III, Subsection NB-2331. The reference toughness Km curve and the Ki curve for static crack initiation later came into use as part of Section XI where the Km curve was called the crack arrest Xia curve. Thus, the RTndt reference temperature index has become the key material parameter in determining the allowable (P-T) limits for plant operation and for evaluating RPV integrity as the result of extreme transients such as PTS. Note that several years ago, the concept of a different, directly measured fracture toughness Master Curve approach was accepted in the ASME Code based on the index parameter RTjq. This development is covered in detail in Chapter 10. 

Another valued result in determining performance is obtained by studying impact behavior as a function of temperature. Materials that behave in a ductile fashion at room temperature become brittle at a low temperature. This transition in mechanical behavior is known as the Brittle to Ductile Temperature. Figure 5 gives an example of the temperature dependence of the toughness of nylon 6 modified with core/shell particles. 

Figure 1337 Schematic comparison of the brittle-ductile temperature transition in four different tests ( ) Hertzian indentation (lower transition), (2) plastic-elastic indentation (upper transition), (3) Double cantilever beam (lower transition) and (4) notched bar (upper transition). (Reproduced from Puttick, K.E. (1980) The correlation of fracture transitions. ). Phys. D, 13, 2249. Copyright (1980) Institute of Physics.)...

The percentage of dull and bright areas in the cross section of tested speci-.mens at a given temperature is a measure oFtfie ductffity at failure (Fig. 4.10c). It is also helpful in comparing Ihe ductility of tw6 ste els at a ven temperature. as..w.elLasjletemrining e gtHmdi oLt est..tej[np. atute.with..m the nil ductility temperature. 

The curve of ductility (for example, elongation in a tensile test) as a function of temperature generally has an S shape similar to that shown in Fig. 3.3. The material shown here has considerable ductility at higher temperatures but is brittle at low temperatures. All materials have a ductility-temperature dependence represented by at least some portion of Fig. 3.3. Between temperatures Ti and T2 a transition from ductile to brittle behavior takes place. The brittle transition may be wide or narrow, may occur in any temperature range or, for some materials, may not occur at all. If the material is either ductile or brittle over the entire temperature range, then the curve consists only of the portion above T2 or below Ti. For example, face-centered cubic metals show only the curve above T2 and brittle materials, such as glass, show only the curve below 7 2. 

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