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Metallurgical failures can be extremely costly, resulting in replacement costs, downtime, and in the worst case, injury or loss of life. Therefore, preventing as many of these failures as possible, is of significant interest and importance to industry. One important factor in preventing failures is to carefully select the materials for each component. Each material type has dominant failure modes. The more one knows about a selected material, the better one can predict the risks that each type of material holds. It is worth noting that there is some correlation between the failure modes and the types of materials, i.e. materials designed for high-temperature service are more likely to have temperature-induced failures because they are specifically selected to be in high-temperature environments. However, the overall effect should be a better understanding of the risks facing any given part.
Carbon and Alloy Steel
Carbon and alloy steel make up the vast majority of products used by the oil, gas, and petrochemical industries. Therefore it is not surprising that these same steels make up the majority of the failures analyzed by KML. A sample ferrite-pearlite microstructure is shown in Figure 1, although there are a variety of possible microstructures in carbon and alloy steels. Of these failures, the dominant failure mode is fatigue (~23.6% of failures), followed by corrosion (~20.5% of failures), Environmentally Assisted Cracking (EAC) (~15.8%), Overload (~11), Material Defect (~11), Wear, Erosion, and Erosion Corrosion (~8.7%), Operator Error (~5.5), Heat Damage (Creep, Overheating 3.2%) and other failure modes (~0.7%) round out the failure modes, observed by KML, in carbon and alloy steels.
Figure 1: Banded ferrite-pearlite microstructure of a plain carbon steel pipe.[/caption]
The distribution of failure modes generally matches with the overall failure modes (discussed in our previous article, “The Causes of Metallurgical Failures: How Things Break”) which is generally expected considering the high prevalence of carbon and alloy steels in the industry. However, it is worth noting that there are deviations from the overall data. Most significantly, corrosion is more prevalent among carbon and alloy steels than among the overall materials population. Unlike the majority of the other materials, carbon and alloy steels have no significant innate corrosion resistance.
Stainless steels are generally used instead of carbon and alloy steel for their corrosion, and sometimes heat, resistance. Much like carbon and alloy steels, stainless steels are produced with a number of microstructures, compositions, mechanical properties, and levels of corrosion and heat resistance. The most common failure mode observed by KML for stainless steels is fatigue (~27.1% of failures), followed by Overload (~20.8%), EAC in the form of stress corrosion cracking (SCC) (~18.8%), Corrosion (~12.5%), Material Defect (~12.5%), Wear, Erosion, Erosion Corrosion (~4.2%), and heat damage in the form of sensitization, (~4.1%) make up the remaining failure modes observed for stainless steels. The prevalence of SCC in stainless steels is notable due almost entirely to their susceptibility to chloride SCC at extremely low Cl concentrations, as few as 10 ppm. This is especially true for austenitic stainless steels, whose microstructure is shown in Figure 2. Duplex stainless steels are also susceptible to chloride SCC, but at significantly higher Cl concentrations. Their microstructure is shown in Figure 3. When compared to carbon and alloy steels, stainless steels tend to be substantially less likely to fail from corrosion. This implies that when stainless steels are used to prevent corrosive failures, they are often successful.
Figure 2: Austenitic microstructure of a 316 stainless steel.[/caption]
High Nickel Alloys
In the oil, gas, and petrochemical industries, high nickel alloys are principally used for several reasons: heat resistance, resistance to a highly corrosive environment, or a combination of these two factors. These alloys contain greater than 25 wt. % nickel; an example microstructure is shown in Figure 4. The combination of the material properties and the conditions in which high nickel alloys are used, severely alters the failure modes that are observed in these alloys. The most prevalent failure mode in high nickel alloys is heat damage, most often in the form of creep (~28.6% of failures), followed by fatigue (~19.1%), EAC (~19.1%), other failure modes, including metal bonding and cyclic hardening (~14.3%), Corrosion (~9.5%), Erosion Corrosion (~4.7%), and Overload (~4.7%). The high prevalence of heat (creep) damage is largely due to the use of high nickel alloys in furnace tubes. They can often experience temperatures above design, due to process variations. This results in damage to the materials and can occur in a shorter timeframe than the desired 20-year design life. The low occurrence of corrosion failures, despite the highly corrosive service environments that many high nickel alloys are exposed to, demonstrates the ability of these alloys to withstand corrosive attack.
Figure 3: Ferrite-Austenite microstructure of a lean duplex stainless steel.[/caption]
Copper and Aluminum Alloys
In the industries served by KML, Copper and Aluminum based alloys are most often used in bearing components, which significantly alters the failure modes that these alloys suffer from, in KML’s experience.
An example microstructure is shown in Figure 5. Those failure modes are Operator Error, generally in the form of improper installation or improper/insufficient lubrication (~37.5%), Other mechanisms, including debonding (~25%), Wear (~12.5%), Overload (~12.5%), and corrosion (~12.5%). It should be noted that these failures are rare enough that KML likely has not had enough statistically significant samples to observe.
Figure 4: As-cast dendritic microstructure with CR enrichment on the grain boundaries of a Ni-Cr-Fe alloy.[/caption]
Other materials (Polymers, ceramics, and other metals including cast, ductile, and grey iron) make up approximately 3% of the failures that are analyzed by KML. As KML sees so few of these failures, there is not a statistically significant distribution of the failure modes to present.
This article was written by Lab Director and Chief Metallurgist, Dr. Michael Hoerner, for a series published by KnightHawk Materials Lab.