440C Stainless fastener problem 7

[PrecisionT]

Hi All,

I am having a major problem with a 440C fastener heat treated to 53-55HRC. It does have a sharp corner at the bottom of the slot so not sure if this is causing a stress riser or if I am just overlooking something but I have about a 10% fail rate. When put under lite torque (just by hand screw drivers) the heads seem to shatter on the male side only. Sometimes they can set there for 1 hour or more and then fail. But we have put some under decent pull force to try and create a fail and could not. What am I missing?

Thanks in advance,

 

[PrecisionT]

Cloa,

Thank you

tbuelna,

Thanks that helps...... It does really look like PH13-8Mo is a very good alternative to what I am looking for. I will do some samples in that and test them.

 

[EdStainless]

13-8 has good toughness and corrosion resistance.
You should really try to stay with aging temp of 1025 or higher (1025, 1050, or 1075) for best toughness and decent SCC resistance.

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

 

[tbuelna]

PrecisionT,

EdStainless makes a good point about SCC resistance of 13-8 or Custom 465 and aging below cond H1025. You did not state what the end use was for these fasteners, but for aerospace structural applications SCC is a huge concern. Generally, 13-8 or Custom 465 would not be used age hardened to cond. H950 due to the aircraft industry's very conservative position on SCC. However, for your particular application you might prefer to sacrifice some SCC resistance for higher strength.

Attached is some reference material on SCC considerations of 13-8 and Custom 465.

Lastly, there is one other corrosion resistant fastener material that has good SCC resistance and very high strength (>250 ksi UTS), MP35N multiphase alloy. Unfortunately it is also very expensive.

Best regards,
Terry

 

[PrecisionT]

EdStainless and tbuelna,

Thanks, you have been a wealth of info and it is greatly appreciated!

And this is actually for the cutlery industry believe it or not, so strength and corrosion are the big factors for my customer.

Again, Thanks all

 

[tbuelna]

A cutlery product? Cripes!

For a fastener on a knife handle, why wouldn't you simply increase the thread diameter of the fastener and use a lower strength material? I can understand the need for corrosion resistance in the fastener, but the desire for extremely high strength seems a bit excessive.

There are carbon stainless alloys like 420HC that are excellent for knife blades. But 440C stainless is not a good choice for threaded fasteners.

Best regards,
Terry

 

[PrecisionT]

I agree with you tbuelna, if it was my product it would not be my choice.......but........

And there biggest reason for the change was actually the HRC. They had a lot of buggered screws in assembly and wanted something in the low 50's.

Thanks,

 

[Screwman1]

I would switch to a different drive system than a slot if the problem is damaged parts during assembly. Go with something like an internal hex or a Torx and the damage during assembly will go away.

 

[PrecisionT]

Screwman,

This is what we are now leaning towards...... The reason we didn't really want to do that is from a maintenance stand point...... but give and take I guess.