titanium galling potential between threads parts 1

[aroundhere]

Ive been thru just about every other material and coating threads to prevent galling etc.... I want to use titanium for both mating parts for weight reasons, but not sure if I can get there. I have never had the need to screw titanium onto titanium, so I didnt know if there is a preferred coating, plating, process etc... Or, is it just something to be avoided? Acme threads. Again, this is for no other reason than weight reduction and corrosion. thanks

 

[TVP]

Can you provide some information on the size of these components (externally threaded member and internally threaded member)? Also, are these parts meant to be continuously actuated against each other as in a lead screw mechanism, or are these part of a static, preloaded joint? If the latter, is this connection regularly loosened and re-tightened?

Depending on these details, there are varying degrees of coatings and lubricants that can be used. My first thoughts are as follows:

Externally threaded member should be coated with either phosphate-fluoride (SAE AMS2486) or anodized (SAE AMS2488 Type 2). Internally threaded member coated with baked-on dry film lubricant containing MoS[sub]2[/sub] (MIL-PRF-46010 or SAE AS5272). Some products to consider include Everlube ® 9002, Sandstrom #099 WATERBORNE Solid Film Lubricant, Tiolube® 20/20, and Molykote® 3400A AERO. Use a suitable lubricant or paste during assembly, such as Dow-Corning Molykote® G-N Metal Assembly Paste, Bostik Never-Seez®, Loctite ® Heavy Duty Anti-Seize Lubricant, etc.

 

[CoryPad]

Rolled threads are much better for resistance to galling compared to cut threads.

If the threads slide against each other frequently, then consider case hardening. SAE AMS2775B identifies a liquid salt bath process that improves galling:

http://standards.sae.org/ams2775b/

 

[moltenmetal]

Parts can be hardened by a number of surface treatment means. Boriding is one, and we've used it on both threaded parts and on rubbing items such as ball valve seats etc. There are others.

 

[aroundhere]

Its actually a stub acme, around 3.00-8p Downhole tool so will be broke and made up possibly once a day or once a week, so must be durable and last. 350F with downhole environment. Could see h2s, c02... I have never tried titanium on titanium, so this is new to me. Usually weight isnt an issue. Thanks for any feedback

 

[racookpe1978]

It's counter intuitive, but remember that galling is a function of the very-localized pressure across the "joint" - the little bitty mating areas of the two chemically-identical threads themselves.

A coarse threads will resist better than a fine thread.
Rolled threads - mentioned above - are not cut and so are both smoother and have muh less minute "freash" cut places to start galling. The tapped (female) threads are always cut, so look there first for the nitriding or chemical treatment.
Always use a pressure-and-temperature compatible permanent lubricant.
A "loose-fit" ("bad fit" actually) pair of threads will be less likely to gall. You might even try a mis-matched thread fit between male and female threads so the threads mechanically yield a little plastically rather than "weld" to each other chemically across the surface.
Try a larger screw thread: both male and female will be larger (heavier) but the screw hole will be larger and somewhat make up for the bigger screw heads. If the clamped parts are very thin, this won't help much, but calculate it anyway.
If the threaded parts can be economically sacrificed on dis-assembly (like rivets always are!) then who cares if they gall?

 

[mighoser]

Salt bath nitriding isn't an option for Titanium because the molten salts attack Ti. Titanium anodizing is a good option for threads since its only a few tenths thick. You could add solid film lubricant on top of the anodize. For SFL, I've had the best luck with Sandstrom 9A which is sprayed and baked on. Its around .001-.002 thick but can be burnished in with a roll threading die/tap.

 

[tbuelna]

Seriously, are you saying weight is an issue with a 3" diameter buttress thread joint on a down-hole component? If you truly feel the need to use titanium for both parts of the threaded connection, then I would suggest using a coating like VitroLube.

 

[aroundhere]

tbuelna

Weight is usually not a issue at all. The fact that I havent made a tool out of ti in the 15 years of designing downhole strings, is proof that it isnt common. Thanks for the tip

 

[tbuelna]

aroundhere-

I apologize if my post seemed abrupt, so I'll try again.

You mentioned that the reason for using titanium for both parts of your threaded connection was due to a weight concern. Since there are stainless steel alloys that have a fairly close tensile strength/density ratio as titanium alloys, without the galling surface contact limits presented by titanium-on-titanium screw threads, why not consider using one of these materials instead? By the time you apply a sufficient knock-down factor on the surface bearing stress levels to prevent galling of the titanium buttress threads, you'll likely find that there are heat treatable stainless steel alloys that will result in a similar or lower weight assembly.