Other Metallurgical Properties of Metals

In addition to the properties already described on Piping & Fabrication, other characteristics of metals can have an important effect on the design process. These may profoundly affect the uniformity, achievable level, or stability of mechanical strength and ductility over long periods of usage. and the rest of Other Metallurgical Properties of Metals is here.

Grain Size
Upon solidification from the molten state, metals take crystalline form. Rather than a single, large crystal, the material consists of many small crystals that initiate independently and nearly simultaneously from separate nuclei sites. These individual crystals are called grains, and their outer surfaces are called grain boundaries. Grains form initially during the solidification process, but they may also reform, grow, or rearrange while in the solid state.

FIGURE A3.15 Sketch illustrating individual grain growth from nuclei and dendrites

Some properties of many engineering metals are very dependent on grain size (Fig. A3.15). For example, austenitic stainless steels, such as Type 304 (18% Cr-8% Ni-Fe), possess excellent creep strength when the material has a coarse grain structure, but very poor strength with fine (small) grains. If this same austenitic material is plastically cold-worked, these grains will become distorted and possess high levels of lattice strain and residual stress. Subsequent heat treatment can cause the crystal lattice to reform unstrained grains initiating at lattice defects which act as nuclei. The process, called recrystallization, results in an initially very small grain size as the nucleated stress-free grains begin to grow. If heavily strained material is placed into elevated temperature service at temperatures sufficient to cause recrystallization, it will initially exhibit good creep strength until the grains begin to reform, upon which the result is very poor creep rupture strength. The material will only return to its prestrained creep-strength level if additional heat treatment is performed, resulting in further grain growth.

Grain size is a material characteristic that is sometimes directly inspected in the base material testing and certification process. The test entails retrieving a piece of the material and then metallographically polishing and etching the specimen with a weak acid solution, which reveals the grain boundaries under magnification.

The test is described in ASTM Specification E 112. Grain size can be measured and reported a number of ways. The most commonly used method involves reporting grain size as an ASTM grain size number (n), corresponding to the exponent of the following equation:

ASTM has correlated this grain size number, which increases as grain diameter decreases, to a series of photographs representing the grain structure at 100 magnifications. The grain size number can then be estimated by visual comparison. Examples of this comparative standard are shown in Fig. A3.16. Fine grained carbon and low alloy steels tend to possess better notch toughness and ductility than coarse grained steel. As noted earlier, as operating temperature increases into the creep regime, engineering material strength properties are usually enhanced with coarser grains. Although this is an oversimplified (and perhaps overstated) rule of thumb, it is important for the engineer to take grain size into account for critical structures.