DEGRADATION OF MATERIALS IN SERVICE

Continue again with Piping & Fabrication and this time we will talk about Degradation of Materials in Service and still being part of Piping Materials.

A number of metallurgically based processes can occur in steels which contribute to loss of engineering strength, and even premature failure. Several of these are addressed in the following paragraphs.

Aging of Properties
A number of steels that have accumulated considerable service time are known to have experienced changes in their properties, usually to their detriment. This phenomenon has been called aging, and occurs in materials that are heat-treated or cold-worked to achieve high strength levels and to be used at elevated temperatures.

These materials are potentially more susceptible to failure after the condition has developed. ‘‘Aging’’ in this case should not be confused with the same term used to represent the purposeful heat treatment performed to some types of nonferrous alloys. In the context being addressed here, aging refers to that normally very slowly progressing metallurgical reaction that occurs in a number of alloys while at operating temperatures for extended periods of time. Some specific types of this behavior (i.e., temper embrittlement and ‘‘885’’ embrittlement) are addressed in the paragraphs that follow.

Components that experience considerable service time contain materials that have aged with time. The materials of special interest are those that regularly experience higher operating temperatures; for example, ferritic steels above 900_F (482_C) and austenitic stainless steels at or above 1000_F (538_C).Astudy sponsored by the ASME Boiler and Pressure Vessel Code attempted to identify and quantify these effects. The effort was the result of concerns for near end-of-life seismic loadings in elevated-temperature nuclear boilers.

Data gathered from a number of sources have shown that the room and elevated temperature yield strengths of both ferritic and austenitic steels may degrade after history, and service temperature. The most severe degradation occurs in weld regions. Due to the extensive use of Cr–Mo steels in the petrochemical and power boiler industries, most of the studies have concentrated on this family of materials.

It has been recognized for some years that a steel’s susceptibility to temper embrittlement is due to the existence and amount of the trace elements antimony (Sb), phosphorous (P), tin (Sn), and arsenic (As), with P and Sn having the greatest effect. Other elements that may contribute to reduced toughness are silicon, manganese, and copper. Beneficial effects can be gained by additions of molydbenum and aluminum. The obvious dependency of temper embrittlement severity and chemistry has led to the development of a number of embrittlement factors.

Bruscato made the first attempt to combine the effects of various elements into a single factor, known as the embrittlement factor X, which is expressed as follows:

The concentration of elements is in parts per million (ppm). Miyano and Adachi arrived at a J-factor defined as:

Finally, Katsumata et al, asserted that the following embrittlement factor (E.F.) was appropriate for 21⁄₄ Cr–1Mo and 3 Cr–1Mo steels.

All of these are useful in assessing the relative susceptibility of various steel compositions to temper embrittlement. In all cases, the larger the factor, the more susceptible the particular heat of steel is to embrittlement. The type of heat treatment applied to the materials may also affect a material’s susceptibility to temper embrittle. For example, a number of experimenters have confirmed that 21⁄₄ Cr–1 Mo alloy steels’ susceptibility increases as austenitizing temperature used during its heat treatment increases. Inversely, susceptibility is lowest after an intercritical hold, at a temperature between the lower and upper critical temperature (Ac1 and Ac3, respectively). This effect is believed to be associated with the grain size achieved during the hold time, a larger grain being more detrimental. Although intercritically treated materials are less susceptible to temper embrittlement, they are weaker in the as-heat-treated condition.

In a parallel fashion, the degree of embrittlement, as measured by loss of toughness or shift of nil ductility temperature, is decreased if the material is more substantially tempered prior to the embrittling treatment. In this context, ‘‘tempered’’ represents the planned heat treatment that typically follows a normalizing or austenitizing and rapid quenching operation.Alonger or higher temperature temper results in a softer, less strong, and more ductile condition, usually accompanied by good fracture toughness.

Luckily, the temper embrittled condition is reversible. Heat treatment for short periods of time at temperatures well above the upper critical will result in reestablishing nearly virgin properties in these materials.