The selection of materials for piping applications is a process that requires consideration of material characteristics appropriate for the required service. Material selected must be suitable for the flow medium and the given operating conditions of temperature and pressure safely during the intended design life of the product. Mechanical strength must be appropriate for long-term service, and resist operational variables such as thermal or mechanical cycling. Extremes in application temperature can raise issues with material capabilities ranging from brittle fracture toughness at low temperatures to adequacy of creep strength and oxidation resistance at the other end of the temperature spectrum.
In addition, the operating environment surrounding the pipe or piping component must be considered. Degradation of material properties or loss of effective load-carrying cross section can occur through corrosion, erosion, or a combination of the two. The nature of the substances that are contained by the piping is also an important factor.
The fabricability characteristics of the materials being considered must also be taken into account. The ability to be bent or formed, suitability for welding or other methods of joining, ease of heat treatment, and uniformity and stability of the resultant microstructure and properties all of a given piping material contribute toward or detract from its attractiveness and economy. The selection process should lead to the most economical material that meets the requirements of the service conditions and codes and standards that apply.
Applicable design and construction codes such as the ASME Boiler and Pressure Vessel Code and the ASME B31 Pressure Piping Code identify acceptable materials for piping systems within their jurisdiction. These codes specify the design rules, allowable design stresses, and other properties required to accomplish the design task. However, the information supplied is generally only adequate and intended to assure safe operation under the thermal and mechanical conditions expected under steady-state and sometimes (as in nuclear construction) cyclic operation. These codes do not directly and explicitly address the many other environmental and material degradation issues that should be considered by design and materials engineers in arriving at a piping system that is not only safe to operate but will offer long-term, reliable service and function. Thus, simply designing to ‘‘the Code’’ when selecting materials can sometimes lead to premature end-of-life of piping system components.
This chapter will attempt to identify the important metallurgical characteristics of piping materials and how they can affect or be affected by operation of all of the other materials available to the engineer. Carbon and low-alloy steels come closest to being the ideal construction material. #Little_PEng