The variety of piping system materials currently in use is extensive and continually growing. The purpose of this section is to provide a brief overview of the common engineering properties of those materials and to describe how those properties influence the design process. For the most part, discussions of specific material characteristics will be limited to plain-carbon and low-alloy steel piping materials. Many of the concepts discussed, however, are applicable to virtually all piping materials.
Strength
Most piping design codes relate the allowable working stresses for materials to their yield strength or ultimate tensile strength at the working temperature. For example, the allowable working stresses for materials used for construction in accordance with ASME B31.1, Power Piping, are developed using rules defined in the ASME Boiler and Pressure Vessel Code, Section II, Materials. At any temperature below the creep range, those rules require that the allowable working stress be set at a value no greater than the lowest of the following alternatives:
One-fourth of the specified minimum tensile strength at room temperature
One-fourth of the tensile strength at operating temperature
Two-thirds of the specified minimum yield strength at room temperature
Two-thirds of the yield strength at operating temperature.
As the temperature of most common pressure-retaining materials increases from ambient, their tensile and yield strengths decrease. Application of the above rules ensures that the decreasing strength of piping materials, with increasing temperature, is reflected in the allowable stresses used for design.
At temperatures within the creep range, the allowable working stress is set at a value equal to the lowest of the following:
100 percent of the average stress for a creep rate of 0.01 percent/1000 h
67 percent of the average stress for rupture at the end of 100,000 h
80 percent of the minimum stress for rupture at the end of 100,000 h
When carbon steels are exposed to temperatures greater than 775 deg F (413 deg C) for long periods, the carbide phase may convert to graphite. Graphitization causes steels to experience brittle fracture at stress levels well below their short-term rupture strength. In recognition of this phenomenon, the ASME B31.1, Power Piping, Code provides the following warning statement in the allowable-stress tables:
Upon prolonged exposure to temperatures above 775 deg F (413 deg C), the carbide phase of carbon steel may be converted to graphite.
For temperatures in excess of 775 deg F (413 deg C), chromium-molybdenum low-alloy steels or high-alloy stainless steels may be used. These steels offer almost complete freedom from graphitization and enhanced creep-rupture resistance. ASME B31.1 allows the use of these materials at temperatures up to 1200 deg F (649 deg C).
Toughness
Toughness, or ductility, is the ability of a material to resist impact, to withstand repeated reversals of stress, or to absorb energy when stressed beyond the elastic limit. Steel is normally considered to be a ductile material. Contrary to expectation, however, steels sometimes rupture without prior evidence of distress. Under certain conditions, steel may shatter just as glass. In piping, however, this behavior generally occurs only at low temperatures.
The transition temperature for any steel is the temperature above which the steel behaves in a predominantly ductile manner and below which it behaves in a predominantly brittle manner. Steel with a high transition temperature is more likely to behave in a brittle manner during fabrication or in service. It follows that a steel with a low transition temperature is more likely to behave in a ductile manner. Therefore, steels with low transition temperatures are generally preferred for service involving severe stress concentrations, impact loading, low operating temperatures, or a combination of all three.
Table B2.6 indicates the low-temperature limitations of various piping materials.
Low-alloy steels may be used at low temperatures 0 deg F(-18 deg C) when they have a Charpy keyhole impact value of at least 15 ft . lb (2.1 kg . m) at the lowest design temperature. Austenitic stainless steels with limited carbon content, copper and copper alloys, and aluminum do not experience transitions in impact strength from ductile to brittle fracture and, therefore, may be used for low temperatures without pressure-rating penalties.
Low-temperature piping is generally covered with insulation which, in addition to limiting heat transfer, helps provide protection from external impact. This, how-ever, is not sufficient insurance against the type of damage that could result if a pipe should fracture.
Corrosion resistance
Considered as a material property, corrosion resistance is a measure of a piping system material’s relative inertness to chemical attack from a specific process fluid at the system’s normal operating temperature, or its environment (see earlier section ‘‘Corrosion’’). The importance of considering the system’s operating temperature cannot be overemphasized. It is well known that many chemical reactions are highly temperature-dependent. A particular piping system material could be virtually immune to chemical attack by a specific corrodant at one temperature, while proneto excessive attack by the same corrodant at a higher temperature.
Within this context, then, it is clear that there is no such thing as a universally corrosion-resistant material. All common piping system materials react with some process fluids (corrodants) at certain temperatures. Therefore, when one is pursuing a ‘‘corrosion-resistant’’ material for a specific application, the objective is to identify a material whose corrosion rate in the presence of a specific corrodant is negligible, or at least acceptable, over the design life of the piping system.
It is important also to consider the effect corrosion may have on the process fluid. Under certain conditions, the dissolution of the base metal or the corrosion products into the process stream may require economic or technical considerations that go beyond the piping system’s pressure-containing parts. In some cases, the major consideration in choosing a piping system material may be the preservation of the chemical purity of the process fluid. Such is usually the case in choosing piping system materials that handle food products and piping used in many chemical process operations.