Damaged Impellers in a Rotary Pump

Egon Kauczor, Staatliches Materialprüfungsamt an der Fachhochschule


From: Case Histories in Failure Analysis, Vol 1, P.M. Unterweiser, Ed., American Society for Metals, 1979

Abstract: Two damaged impellers made of austenitic cast iron came from a rotary pump used for pumping brine mixed with drifting sand. On one of the impellers, pieces were broken out of the back wall in four places at the junction to the blades. The fracture edges followed the shape of the blade. Numerous cavitation pits were seen on the inner side of the front wall visible through the breaks in the back wall. The back wall of the as yet intact second impeller which did not show such deep cavitation pits was cracked in places along the line of the blades. The microstructure consisted of lamellar graphite and carbides in an austenitic matrix, and was considered normal for the specified material GGL NiCuCr 15 6 2. It was concluded that the cause of the damage was porosity at the junction between back wall and blades arising during the casting process. Cavitation did not contribute to fracture but also could have led to damage in the long term in the case of a sound casting. It is therefore advisable in the manufacture of new impellers to take care not only to avoid porosity but also to use alloy GGL NiCuCr 15 6 3, which has a higher chromium content and is more resistant to cavitation.

Keywords: Alloy cast iron; Impellers; Rotary pumps

Material: GGL NiCuCr 15 6 2 (Gray (flake or lamellar graphite) cast iron)

Failure type: Cavitation wear


The two damaged impellers made of austenitic cast iron came from a rotary pump used for pumping brine mixed with drifting sand. On one of the impellers, pieces were broken out of the back wall in four places at the junction to the blades. The photograph in Fig. 1 shows that the fracture edges follow the shape of the blade. Numerous cavitation pits can be seen on the inner side of the front wall visible through the breaks in the back wall, The back wall of the as yet intact second impeller which did not show such deep cavitation pits was cracked in places along the line of the blades, as shown in Fig. 2.
Figure 1

Fig. 1  Breaks in the back wall of impeller I and cavitation pits on the inner surface of the front wall $\genfrac{}{}{0.1ex}{}{2}{3}$ ×

Figure 2

Fig. 2  Cracks in the back wall of impeller II following the shape of the blades. $\genfrac{}{}{0.1ex}{}{2}{3}$ ×

The microstructure illustrated in Fig. 3 showing lamellar graphite and carbides in an austenitic matrix can be considered normal for the specified material GGL NiCuCr 15 6 2.
Figure 3

Fig. 3  Microstructure of a specimen from impeller I. Etched with V2A pickle. 500 ×

A specimen for metallographic examination was taken from one of the fracture edges. It was found that the material in this region was in the final stages of disintegration (Fig. 4). A further section was taken through an as yet unbroken region, the essential portion of which is shown at low magnification in Fig. 5. On the left marked ← cavitation pits can be seen on the internal surface of the front wall and on the right marked south east arrow casting pores in the region corresponding to Fig. 4. It should be mentioned here that even when the specimens were cut carefully with a water cooled cutting wheel the stem in the brittle porous zone crumbled away and only at the third attempt was it possible to obtain a suitable whole specimen for examination. A section through a cracked region of the second impeller revealed a porous zone in the corresponding position.
Figure 4

Fig. 4  Porous zone under a fracture edge of impeller I (shrinkage zone in Fig. 5). 20 ×

Figure 5

Fig. 5  Cavitation pits (←) and porous zones south east arrow in an unetched transverse section from an as yet unfractured region of impeller I. 2.5 ×

Fig. 6 shows a section of a cavitation region. The absence of corrosion products confirms that the surface here has been damaged by cavitation.
Figure 6

Fig. 6  Microsection from a cavitation zone. 100 ×

From the results of the investigation it can be concluded that the cause of the damage is the porosity at the junction between back wall and blades arising during the casting process. Cavitation has not contributed to fracture but could also have led to damage in the long term in the case of a sound casting. It is therefore advisable in the manufacture of new impellers to take care not only to avoid porosity but also to use alloy GGL NiCuCr 15 6 3 which has a higher chromium content and which is more resistant to cavitation.

Related Information

Y. Chen, Cavitation Erosion, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 2002, p 1002–1012