Galling - Stellite 6 on Stellite 6

[howkers]

The usual loaded pin galling pressure test runs at relatively low surface velocities. I am told by one of our valve suppliers that they had extensive difficulties with self-mated Stellite 6 in water when the opeating speed of the valve was increased to slightly over 1 ft/sec. Below this it was OK.

This velocity doesn't seem particularly fast to me and I have always regarded Stellite 6 on Stellite 6 as an excellent anti-galling pair. None of the local metalurgists has heard of velocity being a factor in galling; however the valve man says it was definitely confirmed by extensive trials with different material pairs and velocities.

Does anyone else have any experience of surface velocity being an important variable in resisting galling in Stellite or other materials.

 

[unclesyd]

It's hard to give a definitive answer to your question as normally there are a considerable number of variables that have to be considered.
Whether the alloy is self mated or against another alloy.
The matings and pressure between the two components usually determine the threshold galling stress and the relative speed of the components will determine the wear rates.
An example with Stellite 6B is the pressure to produce galling on 316 SS is only 8 ksi, while the pressure on 304 SS is around 47 ksi. This means that if the 316 and 6B touch there is possibility that the couple will gall.
If you have a specific case please post if and we can probably come up with a little more information.

 

[CdotS]

From ASM Handbook Volume 18 - Friction, lubrication and wear technology:
To reduce galling
1. Lubricate where posisble
2. Keep load, temperature and SPEED as low as possible
3. Keep surface finish between 0.25 to 1.75 um (10 to 70 miro inch)
4. Use nitriding, carburizing, hard facing or shot peening
5. Increase surface area, clearance, etc.

ASTM G98 button-on-block test is generally done at low speeds (I do not know the exact speed as I do not have access to ASTM standards). Your speed of 1 ft/sec is less than 1 m/s which I use as a magic number when it comes to sliding wear. My gut feeling is that your surface is smoother than required and the load is excessive.

Although Stellite on Stellite is okay, avoid using similar metal combinations. Change to dissimilar metal combinations or use a coating on one of the surfaces with a leaner alloy. This way, you can save money also.

Best wishes.

 

[EdDanzer]

All materials when running against another material, similar or different, will have a P for pressure and a V for velocity. This P/V value is available for some materials, but must be found for others by testing. Lubrication type and amount will effect the P/V value. Temperature of operation can have a major effect on P/V also.