Material Selection 1

[deaconp]

I am cuently working on a North sea Project which I inhereted a couple of months ago. In somebodys wisdom they decided to use two housings made from ST-ST which at temperature mate and Gall the threads. Has anybody any Ideas on alternative materials I may use possibly self lubricating to avoid the two housings galling, yet holding the same material properties of 17-4 PH ST-ST H150-M

HRC = 27-33
min yield = 75 ksi
min tensile = 115 ksi

I have looked at a number of materials but help would be gratefully recieved cheers

 

[boo1]

When two metal surfaces are rubbed together under heavy pressure, and without lubrication, it is expected that galling (or even seizing) may result. For it to occur there usually has to be sufficient force along with materials that are susceptible to galling. Lack of lubrication increases the tendency to gall. The "button and block" galling test has been used to evaluate the adhesive wear resistance of various stainless steels under nonlubricated conditions (Schumacher 1977). The lightest load that caused galling was used to calculate the "threshold galling stress". Threshold galling stresses for selected stainless steel combinations shown in the chart are stainless steel with stainless steel of the same type.

Typically we have found when we utilize alloys of different hardness we have a design that exhibits excellent resistance to metal galling. Recommend a design uses 17-4 PH and A286. Some also utilize silver plate on the nut to minimize galling.

please rate if this is helpfull
Boo1

 

[MadMango]

There is a European material called 3CR12... nearly the same corrosive resistance as SST, but wuth the workability of mild steel. "Happy the Hare at morning for she is ignorant to the Hunter's waking thoughts."

 

[awol]

I assume ST-ST is stainless steel and yes, expect galling every time from sliding under pressure. Do you have the room to incorporate a sacrificial metal to take the rubbing and provide smooth operation? Use a bronze insert against the Stainless Steel, maybe self adjusting for removing the freeplay. Otherwise investigate a teflon coating (heat applied) to sacrifice or a flame spray (titanium oxide) to be harder than the stainless steel. Hope these ideas help.

 

[Hush]

17-4PH especially in the low hardness heat treats like H1150 have a high tendency to gall. try a liquid or gas nitride treatment on the mating surfaces. Personally I'd find an alternative to 17-4. It has a tendency to be susceptible to stress corrosion cracking in high chloride and hydrogen rich environments. Without mentioning names, one of the large wellhead equipment manufacturers ran into a lot of trouble in the North Sea using 17-4 for subsea housings. They had to replace them all at a considerable cost. Will 410 or F6NM work, or try a low alloy with a 625 clad.

 

[boo1]

Agree with the above, I should have listed Nitronics 60 and A286 alloys