Free Machining Steels For Threaded Connections 4

[mrfailure]

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

http://www.lehightesting.com

 

[dgallup]

I suppose it all depends on the strength requirements. I've certainly put lots of screw threads into 303 fixtures, some as tapered pipe threads for gas or fluid ports, others as mounting holes. Never had a problem but most of these are low pressure/force. I wouldn't try to make a 303 bolt.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[swall]

I would hypothesize that since the inclusions run longitudinally and tensile cracks in a fastener form in the transverse direction,
the propensity for crack formation is not as dire as you propose.

 

[mrfailure]

There is probably not much loss of strength due to longitudinal inclusions on a coupling load. I wonder more about the short ligaments of intact material between sulfides - would free machining steels be more prone to crack initiation or propagation than nonresulfurized material at the same loads?

Aaron Tanzer

http://www.lehightesting.com

 

[swall]

With fractured components made from free machining steels (mostly 416 in my case) one thing that stands out upon SEM exam is the prevalance of dimple rupture in both the ligaments of "sound material" (for lack of a better term) as well as the large macro dimples initiated by the inclusions. So, in that sense, the material behaves in a somewhat ductile manner with tensile loading or bending. In failures from circumferencial stresses, inclusions play a prominent role--think split or burst couplings, shot gun barrels, etc. Ultimately keep in mind that these are low strength materials and lower your expectatins accordingly.

 

[TVP]

AT,

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):

http://www.bossard.com/index.cfm?ap...fOcOAAAAoMIjDQ==&bcsi_scan_filename=index.cfm

 

[EdStainless]

With stainless the FM grades (with just a few exceptions) have poorer corrosion resistance than the non-FM grades. So strength aside you would be more likely to have crevice corrosion in the bolted connections.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[mk1979]

now,this overcome in free cutting steel by modification the mrphology of MnS by modifiers like Ca. may be your product is not developed.