"Stress Corrosion Cracking of 416 Pump Shafts"

Question:

"We have several pump shafts that have failed in the field. They are used in well water with a reportedly high concentration of chlorides and sulfates. One laboratory examined the failed shafts, and determined that they failed due to the stress corrosion cracking mechanism. Can anyone offer an opinion as to the appropriateness of the grade UNS S18200 (18Cr-2Mo), a ferritic stainless with moly, as a potential replacement for the 416? Any other suggestions? A reasonable level of machinability is needed (shafts are drilled on the ends), and therefore the S18200 has been suggested. 1. Metallographic examination revealed pitting on the failed shafts. 2. Report refers to presence of "classic hook" feature associated with corrosion of stainless steel by chlorides. This is apparent on micrographs at 70X. 3. Report claims that the pitting, in conjunction with the applied stress on the shafts, has led to fracture via the SCC mechanism. Nothing is said about the specific type of SCC. What is mentioned is that the pitting has provided a crack initiation site for the fractures. 4. The shafts are cold-finished type 416 hexagons. The bars are annealed, and cold-worked to a tensile level of about 110 ksi, and yield of about 98. The bars must be capable of withstanding 100 ft-lbs. of torque without permanent set. The aforementioned mechanical properties have traditionally allowed for satisfaction of this requirement. It should also be noted that this problem has only occurred in this one particular service environment (water with higher than normal chloride and sulfate levels). Thousands of pumps with these shafts have been used successfully for years. Even in this environment, the failure rate isn't that bad. Unfortunately, the report that I have is not a good faxed copy, and I can't get better. Any ideas regarding my thoughts 18Cr-2Mo as a good selection in light of the need for corrosion resistance and some level of decent machinability? Or, would perhaps a duplex grade be better (such as type 329)?"

Answer:

For your application where increased corrosion resistance is needed, compared with 416SS, as well as strength, I suggest considering either the duplex stainless steels, i.e., 329SS, 2205, or the martensitic precipitation hardenable grades such as 17-4PH. I prefer the above compared with the ferritic stainless steels.

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