Selection Of Pipe Material For Low-Temperature Service

By Ramesh Singh, Senior Principal Engineer, Gulf Interstate Engineering, Houston | June 2009 Vol. 236 No. 6

The selection of material for any specific environment is directly dependent on the material’s properties, especially those properties that are affected by that special environment.

Metal properties are classified in terms of Mechanical, Physical and Chemical properties. These are further subdivided into Structure Sensitive or Structure Insensitive properties. The following table describes these properties.

Table 1: Metal properties.

In this article, we are concerned only with the structure-sensitive mechanical properties of metal. Metals are favored as a construction material because they offer a combination of mechanical properties that are unique and not found among non-metals. Metals are generally strong and many can be loaded or stressed to very high levels before breaking. One property of metals of interest is their capacity to exhibit a high degree of elastic behavior in their early load-carrying capacity. This is a very important property for effective use of the metal as a construction material. When these metals are loaded beyond their elastic range they exhibit another set of important properties called ductility and toughness. These properties and how they are affected by change in temperature are the point of this article.

Pipeline Steels
We will focus on carbon and low-alloy steels. It may be noted that the bulk of the material that is used in conventional pipeline engineering comes from this generic group. Aptly, it is the ductility and toughness of these metals and how they are affected by the variation of temperature that is our subject. The emphasis is made on the variation under low temperature. For this purpose it is essential to know what is meant by these metal properties and by low temperature. The following definitions are understood by fracture mechanics.

Ductility is defined as the amount of plastic deformation that metal undergoes in resisting the fracture under stress. This is a structure-sensitive property and is affected by the chemical composition.

Toughness is the ability of the metal to deform plastically and absorb energy in the process before fracturing. This mechanical and structure sensitive property is the indicator of how the given metal would fail at the application of stress beyond the capacity of the metal, and whether that failure will be ductile or brittle. Only one assessment of toughness can be made with some reasonable accuracy from ordinary tensile testing, and that is the metal displays either ductile or brittle behavior. From that it can be assumed that the metal displaying little ductility is unlikely to display a ductile failure if stressed beyond its limits. The failure in this case would be brittle.

The temperature of metal is found to have profound influence on the brittle/ductile behavior. The influence of higher temperature on metal behavior is considerable. The rise in temperature is often associated with increased ductility and corresponding lowering of the yield strength. The rupture at elevated temperatures is often intergranular, and little or no deformation of the fractured surface may have occurred. When lowered below room temperature, the propensity for brittle fracture increases.

ASTM E 616 defines some of the terminology associated with Fracture Mechanics and Testing, such as: