Failure Analysis on Impeller
The object
An impeller is a mechanical component used primarily in pumps and turbines. It is a rotating part within a housing that is designed to generate flow or motion within a fluid (liquid or gas) or to transfer energy from a driving source to its surroundings. When used within a pump, the impeller is responsible for creating pressure or flow in the pumped liquid. Centrifugal pumps, for example, use impellers to increase water pressure or liquid flow. These components can go through various failure modes:
- Wear | Over time, the impeller may experience wear due to continuous contact with the fluid. This can lead to reduced performance or decreased efficiency.
- Erosion | If the pumped fluid contains abrasive particles, the impeller may erode over time, impairing its ability to pump or convert energy.
- Corrosion | Exposure to corrosive fluids can cause chemical damage to the impeller, weakening it and reducing its service life.
- Stalling or surging | In some situations, an impeller may stall or experience surging when fluid flows unevenly through it. This can cause structural damage or reduce efficiency.
- Overheating | In high-speed applications, impeller overheating due to poor lubrication or inadequate design can lead to deformation or breakage.
- Imbalance | Any loss of balance in the impeller can cause unwanted vibration, which in turn can lead to failure or breakage.
Purpose of the survey
The impeller under analysis suffered a fracture at one of the vanes with the fracture surface oriented along two different planes, the first of which is transverse to the impeller duct, while the second is tangent to the wall of the vane itself. The purpose of the investigation is tothoroughly investigate the type of fracture, characterizing its surfaces to determine the failure mechanisms and formulate hypotheses on the possible causes that led to the damage. The sample under investigation consists of an impeller shown in Figure 1, which suffered a fracture at one of the vanes. The fracture surface is oriented along two different planes, the first of which is transverse to the impeller duct, while the second is tangent to the vane wall itself, as shown in Figure 2. Specifically, the second section originates at the radius of the connection between the vertical wall and the base surface of the duct and joins the first portion at about 30mm from the beginning of the duct itself, terminating at the horizontal connection radius at vane foot.
The analyses
In addition to verifying the chemical composition of the material used to make the component, analytical work focused on characterizing the material in the vicinity of the fracture, identifying its microstructure and formulating hypotheses about possible failure mechanisms by means of fractographic investigation.
Examination of the fracture surface highlights the presence of a smooth macro morphology characterized by beach lines converging toward two points that can be seen in Figure 2. This part of the surface affects the entire second portion of the damaged area, while the rest of the fracture zone appears dull and more irregular in appearance.
The flat portion of the fracture surface shows a flat transgranular morphology on which two families of striations converging toward two different points are evident.
Origin points are characterized by the presence of cavities that create discontinuities in the material.
The horizontal portion of the fracture surface exhibits a mixed transgranular morphology characterized by microdimples and portions affected by cleavage. Such morphologies are typical of a crash failure.
The microstructure detected on the sample on the fractured paddle is shown in micrograph No. 1.
Islands of ferritic phase dispersed in austenitic matrix with ferrite percentage around 15 percent are observed.
No secondary phases and precipitates are detected at the grain edge.
The fracture surface at the origins has a smooth and regular profile as shown in micrograph No. 2, and the microstructure is similar to the rest of the sample.
Micrograph No. 3 depicts the structure detected on an undamaged pallet.
Ferritic islands in austenitic matrix are still observed, similar to the damaged vane.
The results
The material from which the fractured impeller is made is classifiable as an A351 CF3M (SAE 316L) type cast stainless steel, although its Cr content is slightly lower than the minimum value of the intended fork.
The study of the fracture surface indicates that the fracture occurred due to fatigue, with identification of two different points of origin, located at the radius of connection between the vertical vane wall and the duct bottom surface.
The fatigue crack initiation stresses are duct opening (concordant with centrifugal force). The density of striations indicates that the load cycle to which the vane was subjected was of modest amplitude.
The propagation affects the entire part of the fracture surface tangent to the vane profile, while the part of the surface orthogonal to the duct constitutes the part affected by the crash failure, which occurred brittlely. The microstructure of the sample is in accordance with the material used, and no anomalous phases were detected. Examination of the points of origin highlights only the presence of microcavities that may have promoted the failure but are not believed to be the main cause.
Given the characteristics of the fracture and the location of the damaged surface, it is believed that the nucleation of the crack can be attributed to the concentration of stresses at the foot of the blade due to geometric causes (too small radius of connection, unlike other areas where this part is more connected).
Conclusions
The impeller fracture occurred due to fatigue. The main initiating factor was the too-small blade-foot fillet radius, which resulted in a high stress concentration.
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