The properties of metals can be divided into five general groups: (1) mechanical, (2) physical, (3) corrosion, (4) optical, and (5)nuclear. The specific properties in each of these groups are divided into structure-insensitive properties and structure-sensitive

properties. This distinction in properties is commonly made in most textbooks on metals to emphasize the considerations that should be given to reported property values. See Table P-8.

Structure-insensitive properties are well defined properties of a metal. They do not vary from one piece of metal to another of the same kind. This is true for most engineering purposes, and is verified by the data obtained from standard engineering tests. These properties can often be calculated or rationalized by consideration of the chemical composition and the crystallographic structure of the metal. They are commonly listed in handbooks as constants for the particular metals.

Structure-sensitive properties are dependent not only on chemical composition and crystallographic structure, but also on microstructural details that may be affected in subtle ways by the manufacturing and processing history of the metal. Even the size of the sample can influence test results obtained for a structure-sensitive property, and they are likely to vary to some degree if there are differences in the treatment and preparation of the samples.

 

The most important mechanical properties in the design of weldments, with the exception of elastic moduli, are structure-sensitive. Consequently, single values of these properties must be accepted with reservation. It is not uncommon for plates or bars of a

metal, which represent unusual sizes or conditions of treatment, to have significant deviations in mechanical properties from those published for the metal. Also, the mechanical properties, as determined by standard quality acceptance tests in an American Society of Testing and Materials (ASTM) specification, do not guarantee identical properties throughout the material represented by the test sample. For example, the direction in which wrought metal is tested (longitudinal, transverse, or through-thickness) may give significantly different values for strength and ductility. Although the physical and corrosion properties of metals are considered to be structure-insensitive for the

most part, some of the values established for these properties apply only to common polycrystalline metals.

Metals are appraised for the following mechanical characteristics: modulus of elasticity, elastic limit, yield strength, tensile strength, fatigue strength, ductility, fracture toughness, low temperature properties and elevated temperature properties.

Physical properties that may require consideration in designing or fabricating a weldment are: thermal conductivity, melting temperature, thermal expansion and contraction, and electrical conductivity.

Corrosion resistance is often an important consideration; particularly since weld joints often display corrosion properties that differ from the remainder of the

weldment.

 

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