Fatigue is the progressive localized permanent structural change that occurs in a material subjected to repeated or fluctuating strains at stresses having a maximum value less than the tensile strength of the material. Therefore failures that occur under cyclic loading are termed fatigue failures. These can be vibration stresses on blades, alternating bending loads on shafts, fluctuating thermal stresses during start-stop cycles, etc. There are two types of fatigue: low cycle fatigue (LCF) and high cycle fatigue (HCF). Traditionally, low cycle fatigue failure is classified occurring below 104 cycles, and high cycle fatigue is above that number. An important distinction between HCF and LCF is that in HCF most of the fatigue life is spent in crack initiation, whereas in LCF most of the life is spent in crack propagation because cracks are found to initiate within three to 10 percent of the fatigue life. HCF is usually associated with lower stress, while LCF usually occurs under higher stress. Most failures of rotating turbomachinery are due to high cycle fatigue. These failures generally occur at loads that when applied statically would produce little affect. Fatigue occurs in three stages: I) Crack initiation II) Crack propagation III) Final fracture overload
Fatigue is a particularly insidious failure mechanism. A fatigue failure spends 80-90% of its life in crack initiation. This means a fatigue crack is not detectable by NDE inspection until the crack propagation stage which is the last 10-20% of its life. A component can be reaching the end of its life and yet show no physical evidence in which to identify impending failure. Typical fatigue fractures are relatively flat and smooth and in some cases show beachmarks that indicate successive positions of the advancing crack tip. Fatigue is most commonly transgranular with striations, which each one indicates a single cycle of stress.
Corrosion Fatigue and Stress Corrosion Cracking
Corrosion fatigue is the combined action of repeated or fluctuating stress and a corrosive environment to produce cracking. The corrosion is typically localized and takes the form of pitting. These pits act as stress risers and when combined with an alternating stress can lead to a fatigue failure. Figure 5 shows a pit from which a fatigue crack is propagating. Corrosion fatigue can occur on both steam turbines and compressors but is more commonly seen on steam turbine blades.
Stress Corrosion Cracking is a mechanical-environmental failure process in which sustained tensile stress and chemical attack combine to initiate and propagate fracture in a metal part. Therefore in order for SCC to occur three variables are required: 1) Tensile stress 2) Corrosive environment 3) Susceptible material
Removal of any one of the above variables will prevent SCC from occurring. SCC usually initiates at stress risers such as pits due to the high peak stresses present. However, the absence of pits does not exclude the possibility the failure mechanism was SCC. SCC can be transgranular or intergranular depending on the material and the corrosion media. Cracks typically appear branched but can be straight depending on the applied stress involved. Both steam turbines and centrifugal compressors can be subjected to SCC. Unwanted carryover contaminants can lead to SCC in high stressed components such as impellers and steam turbine blades/disks.
SCC is of particular concern on last stage steam turbine disks and blades. Blades are typically manufactured from 12Cr martensitic stainless steel such as AISI 403 which can be susceptible to corrosion in a chloride environment. Chlorides such as NaCl can lead to pitting and SCC of steam turbine blades. Sodium Sulfite (Na2SO3) is used as an oxygen scavenger but has the potential to decompose into hydrogen sulfide. Hydrogen sulfide is a corrosive compound that can lead to sulfide stress cracking and promote hydrogen embrittlement.
SCC is most commonly seen on steam turbine disk blade root attachments. The low alloy steels of steam turbine rotors are susceptible to SCC in the presence of hydroxides such as NaOH. The crack morphology of SCC on rotor materials is typically always intergranular.
Erosion – Liquid or Solid Particle
Erosion is a loss of material due to mechanical interaction between the surface and liquids or particles. The liquids/particles when accelerated in steam or process gas can impact components resulting in the removal of small amounts of material. Eventually pits and microcracks form at the surface. When cyclic stresses are present the pits and microcracks will create stress concentrations from which fatigue cracks can initiate. Erosion is particularly problematic for steam turbine blades. Steam turbines operate at higher temperatures and with a more closed gas path compared to centrifugal compressors. Solid particle erosion can occur on steam turbine blades due to the exfoliation of scales from piping during transient conditions. As the latter stages of a steam turbine increase in wetness the blades become susceptible to liquid droplet erosion. The damage typically takes the form of removal of material from the blade leading edge. Compressor impeller and hot gas expander blades can also experience erosion damage as well.
Creep is time dependent strain occurring under stress. After a period of time, creep may terminate in fracture by creep rupture. In turbomachinery, creep is a concern on components which operate under stress at high temperatures. Evidence of creep damage in the high temperature regions of blade attachment areas of turbine rotors has been observed. Creep damage takes the form of isolated then oriented cavities which link up to form cracks.
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