High Strength Alloy steel used for a low temperature application 2

[franck]

Hi guys,

Once again I would need your point of view. Really, I have an application for which I need to use high strength screws to connect some 6061-T6 aluminum plates. I selected the following ones:

Holo-Krome 1960 series (high grade alloy steel hardened and tempered).

My problem is that those screws are subjected to a low level temperature -140C (-220F) and under vacuum condition. I am very familiar with stainless 18-8 for which I never had an issue at low temperature. I am not too concerned about the brittleness since the loads are going to be applied very slowly and progressively (very slow thermal loading).

However, I am not sure how will the strength evolve at low temperature. My spin is that it is going to increase. Am I right?

Regarding the vacuum, those screws will be cleaned by ultrasonic so contamination should not be a problem.

Any thought would be greatly appreciated.

Franck

 

[metengr]

franck;
Understanding that the quenched and tempered alloy steel fastener will be at service temperatures below the ductile to brittle transition temperature for this material (meaning brittle fracture behavior), exposure to service temperatures below -50 deg C will affect mechanical properties.

To generalize, for certain carbon and low alloy ferritic steels, the affect of low service temperature will increase the yield strength and the tensile strength of the fastener. Certain alloys see a more dramatic increase in yield versus tensile strength at low temperatures (below -50 deg C). Fatigue strength can increase. However, the ductility of the material will rapidly decrease as the temperature drops below -50 deg C.

For your information, the attached web site depicts these behaviors at low temperatures;

http://www.key-to-steel.com/Articles/Art48.htm

 

[franck]

Thanks Metengr!

 

[CoryPad]

You should not use ferritic steels below -55 [°]C (-65 [°]F). At these low temperatures, ferritic steels have very low fracture toughness.

For low temperature + vacuum conditions, you should replicate what NASA uses. Their preferred fasteners for these conditions are alloy 718, A286, or MP35N, with silver plating and possibly molybdenum disulfide added for lubrication. SPS Technologies is a supplier of these fasteners:

http://www.spstech.com/aero/

By the way, if you are using high strength fasteners with high preload, you will need to ensure that you have adequate bearing area to avoid deforming the relatively low strength 6061 aluminium alloy.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

franc;
Aside from the mechanical property affects at low temperature, I totally concur with CoryPad on this issue. I would not use the Holo-Krome high strength fastener for this specific application.

 

[TVP]

I agree with my esteemed e-colleagues on this one-- Holo-Krome steel fasteners are not acceptable. For low vacuum conditions you will need to be careful of coatings, plating, and/or lubricants due to outgassing.

 

[franck]

Thanks all for your excellent feedback! Very Much Appreciated!

Franck