Although numerous conditions may cause or lead to service failures, the responsibility for a failure can generally be assigned to one of five classifications. Those classifications are broadly described as:
- Base metal defects
In some instances, the responsibility can be related to a combination of several of these classifications.
Although the majority of failures occur gradually, they generally occur by cracking or corrosion or sometimes a combination of both. Upon crossing the wall thickness of the pressure vessel, tank or piping component, a surface opening develops through which some of the steam, liquid, or gas carried in the system may leak. This, in effect, produces a warning of the potentially hazardous condition. Very rare are failures where partial cracking across the wall occurs, followed by a sudden pipe rupture, or where a sudden rupture occurs which is not preceded by detectable prior cracking. The sudden rupture type failures are viewed with extreme concern as they may result in injury to personnel or loss of life, in turn becoming extremely costly to the plant. Most service failures are related to the notch-brittle behavior of certain steels.
When failures occur by cracking, three stages are typically defined. These stages are:
The initiation of a failure involves the conditions leading to causing the initial cracking. Such cracking may be of a submicroscopic or microscopic size, or maybe even visible to the eye. Three conditions are typically involved in crack initiation:
- Structural defects present in the original components.
- Metallurgical defects present in the original components.
- Defects introduced during service in otherwise sound materials.
Structural defects may range from atom sized dislocations in the metal, which cannot even be observed under the highest power microscopes, to major metal discontinuities visible to the naked eye. Many gross defects, such as base metal inclusions, metal porosity, or slag may not reduce the service of life of the pressure vessel, tank or piping component. However, many other visible surface and invisible sub-surface defects can result in service failures.
Metallurgical defects represent differences in properties of sufficient magnitude to become a potential cause of failure. Such conditions can occur within the same metal. For example, material defects can occur where localized heat treatment produces an area of high hardness adjacent to an area of low hardness. Metallurgical notches also can represent a higher hardness level in the heat affected zone adjacent to base metal or weld metal. Localized cold work or residual stresses are also included. Most apparent metallurgical notches occur between dissimilar metal combinations, as clad materials, weld overlays, dissimilar metal welds, etc.
Defects introduced during service involve cracks, corrosion pits, grooves, or general corrosion occurring at locations where the material itself was structurally and metallurgically sound in the originally erected condition.
The initiation and extension of the crack to a critical size is defined as the initial growth stage. The extent depends on factors such as the mechanical and metallurgical properties of the material, the nature of the initial defect, the size and thickness of the vessel or pipe, and the stress level caused by the residual and external stresses and loads. In inherently brittle materials, the growth stage may be incredibly small. Whereas a ductile material, the crack may extend into the wall thickness to a significant extent.
The propagation stage involves the unstable stage beyond the growth stage when the crack extension becomes more rapid. Such propagation may be continuous or intermittent. It may be extremely rapid, as often occurs in a brittle material. In a ductile material, such crack propagation is generally slow and may take days, months, years, or even decades before it navigates through the wall thickness of the pressure vessel or pipe. Sometimes crack propagation and ductile material may occur slowly in the earlier stages and then continue very rapidly in the final stages. Thus, such failures may have the appearance of brittle failures, even though the mode of fracture is primarily of the shear type. In these instances, a severe mechanical or thermal shock or external load often triggers the final sudden rupture.
As industry experts, Thielsch Engineering has been identifying damage locations and preventing catastrophic failures for over 30 years. For more information on service failure classifications, damage mechanisms, or effective methods to identify possible damage in your high-temperature high-pressure components, contact Peter Kennefick at email@example.com.