Free Machining Steels For Threaded Connections
Free Machining Steels For Threaded Connections
(OP)
I have long held the personal opinion that free machining steels (e.g. 1144, 1215, or 303 stainless) were not suitable for threaded connections because the density of sulfide (and other) inclusions had the effect of creating a structurally porous material loaded with easy crack paths. These steels make crack formation much easier in the threaded connections than their non-free machining counterparts. I would like opinions on this admitted bias.
Aaron Tanzer
www.lehightesting.com





RE: Free Machining Steels For Threaded Connections
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RE: Free Machining Steels For Threaded Connections
the propensity for crack formation is not as dire as you propose.
RE: Free Machining Steels For Threaded Connections
Aaron Tanzer
www.lehightesting.com
RE: Free Machining Steels For Threaded Connections
RE: Free Machining Steels For Threaded Connections
You need to separate externally threaded fasteners from internally threaded fasteners. No consensus standard (ISO 898-1, SAE J429, etc.) for steel externally threaded fasteners allows grades like 1144 or 1215, even for low strength/property class applications. SAE J429 does allow Grade 4 studs (threaded from cold drawn bar, 0.25-1.5") to have S up to 0.13%, which means that 1137, 1141 and other resulfurized grades besides 1144 can be used. Mechanical properties (YS/UTS/% el) for a tensile specimen are 100 ksi/115 ksi/10% min. ISO 3506-1 does allow Type 303 to be used for low-strength Class A1-50 externally threaded fasteners, but the mechanical properties are quite low: 210 MPa/500 MPa/0.2d (d is thread diameter).
For internally threaded fasteners, all of the consensus standards allow free machining grades for low strength/property classes. For ISO 898-2, property classes 4, 5, and 04 can use 1213, 1215, and 12L14. SAE J995 allows rephosphorized and resulfurized steels for Grade 2 and Grade 5 nuts. Since nuts are loaded in shear rather than tension, heavily included steels are deemed acceptable. Bossard has a nice technical manual on fasteners that shows some of this information (ISO standards):
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RE: Free Machining Steels For Threaded Connections
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Plymouth Tube
RE: Free Machining Steels For Threaded Connections