SAE 1144 shaft fracture surface morphology
SAE 1144 shaft fracture surface morphology
(OP)
Hello everyone, I'm looking for some information about the type of failure and fracture surface appearance on this particular case: an sprocket shaft made of SAE 1144 (resulfurized steel) fails at de keyway showing long circunferential cracks in a way that makes me think that corresponds to a fatigue process. The thing that keeps me thinking is the faceted fracture surface that I´ve never seen in the cases of fatigue with which we have dealt before. Notice the sings of corrosion. Does anyone have any experience on some similar case? Thanks!

















RE: SAE 1144 shaft fracture surface morphology
RE: SAE 1144 shaft fracture surface morphology
RE: SAE 1144 shaft fracture surface morphology
In general 'free machining' steels have very poor fatigue properties.
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P.E. Metallurgy, Plymouth Tube
RE: SAE 1144 shaft fracture surface morphology
RE: SAE 1144 shaft fracture surface morphology
RE: SAE 1144 shaft fracture surface morphology
RE: SAE 1144 shaft fracture surface morphology
This component seems to be part of a gear drive. If the system was properly designed, including the shaft connection, I would expected a fatigue failure to occur first at the gear tooth root fillets which tend to have high tensile stress levels. Since the bearing journals and gear teeth seem to be in good shape, I don't think you need to change material. 1144 has advantages in terms of inherent mechanical strength, low cost, and machinability, so I'm assuming that's why you chose to use it in the first place. What you need to do is apply an appropriate type of corrosion protection to the exterior shaft surfaces, and redesign your sprocket/shaft connection so that it is rigid and there is no possibility of relative movement at the contact interface.
RE: SAE 1144 shaft fracture surface morphology
RE: SAE 1144 shaft fracture surface morphology
RE: SAE 1144 shaft fracture surface morphology
http://www.emersonindustrial.com/en-US/documentcen...
RE: SAE 1144 shaft fracture surface morphology
I suspect that what you think is corrosion product is really oxide generated by fretting, wear between two surfaces. The area surrounding the fracture appears to have been rubbed, indicating possible wear. Corrosion of the fracture would not leave red oxide on the machined surface like your pictures show.
RE: SAE 1144 shaft fracture surface morphology
You appear to have a design problem with your shaft/sprocket connection. So to correct the situation you need to modify the shaft/sprocket connection design. mrfailure points out that the shaft surface degradation is likely due to fretting rather than environmental conditions. Fretting occurs when there is relative motion at contacting metal surfaces. Simply changing the shaft material will not likely resolve the problem.
Keys are a miserable way to transfer torque in a shaft connection. They are only really suitable for indexing purposes. In order to prevent fretting with your shaft/sprocket connection, you need to modify it to have enough of an interference fit such that there will not be any relative movement at the contact interface for the torque force being transmitted.
RE: SAE 1144 shaft fracture surface morphology
This fracture pattern is intergranular fracture above along the prior austenite grain boundaries (which is where the sulphide stringers form)
You will also see this pattern in weld sensitised areas of rolled steel.
Cheers,
RE: SAE 1144 shaft fracture surface morphology
Longitudinal 250X (Dimensional reference is mistaken)
Tranversal 250X
Transversal 1000X
RE: SAE 1144 shaft fracture surface morphology
That is banding!
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P.E. Metallurgy, Plymouth Tube