Dear Friends,
we need to repair the fillets of the screws of heavy oil screw pumps . This screws are weared out the abrasive action of sand found in the oil formation. Originally the screws are PTA coated with Deloro40, a Ni based Stellite/Deloro material having a hardness of 40 HRc. The coating function is to increase the wear resistance of the screw material and mostly to create a protection against galling as the screw and the body rubs one against each other in some operational conditions.
We have repaired some of this screws by TIG and MIG welding procedure using the same original Stellite Deloro40 material. We found this procedure highly time consuming as the welding procedure calls for a lot of heat control on the screw surface as the Deloro welded surface is very sensitive to crack formation. Actually our costumer is asking for a harder wear resistant material,so we guess if after having builded up the screw surface with 309 Stainless would be safe to spray a Tungsten Carbide HVOF thin (5 to 8/1000") coating. The only threat is if a HVOF thin Wc coating will resist the impact between the pump body and the screw and the galing action between the surfaces. Does anybody have some experience/opinion on this application?[color=#000080]

Hi,
Galling test of the coating would lead to a proper selection of material and process.You can do the test as per ASTM G 98 or an alternative simpler method (ref.Surface Wear- Analysis, Treatment, and Prevention,a book by ASM-International, p68-69) as developed by us. The load at which the galling or siezure occurs with the counterbody is called galling load.Galling resitance is expressed in terms of galling load. Some manufacturers of consumables, particularly powders (for spray fuse & PTA) include galling load data of the coating in their test certificate or/and data sheet.You can ask your consumable supplier to recommend suitable coating materials for your application with the galling load data.
ram chattopadhyay

HI HVoiF,

I used to repair these screws a lot. what I do is building up the screw surface with 309 stainless steel stick first, then coat the sueface with Ni-Gr-B-Si powder by spray-fuse torch. This process will take you a little more time but save a lot of money.(spray-fuse torch is much cheaper than HVOF)

Thanks Derek, I am worried for the possible impact between the screw and the pump body due the extended lenght of the screw shaft. The costumer says that sometimes for changes in the fluid being pumped (water instead of oil or gas instead of oil) the screws oscillates rapidly and some impact occurs between the screw and the pump body. What do you think about it,would a spray and fuse coating be safe for this "schock loading"? The pump manufacturer makes the screw PTA Deloro 40 coated. Any suggestion?

derek Wrote:HI HVoiF,

I used to repair these screws a lot. what I do is building up the screw surface with 309 stainless steel stick first, then coat the sueface with Ni-Gr-B-Si powder by spray-fuse torch. This process will take you a little more time but save a lot of money.(spray-fuse torch is much cheaper than HVOF)

I never face this problem before because the screw I repaired is part of rubber extruder. There are few cracks on the surface of screw sueface after spray-fuse because the hardness of the coating is HRC60(you can choose HRC40 powder). The joint between substrate and coating is metallic joint, so I think that the "schock loading" will not hurt the coating. You can do some test before use this process such as impact wear test.

Greetings from Indonesia, actually sounds like a very tough application, and I would expect extremely difficult to solve the problem based on laboratory tests and trials. I expect the original design, using Deloro 40 material was a half measure, a trade off of hardness and corrosion resistance that the OEM selected for general purpose.

Your application with the sand is obviously much more challenging. So based on the information so far, and taking some guesses, I would suggest.....

Laser Cladding using a 60 or 65% Macrocrystalline WC in a NiCrSiB matrix of about 50HRc. Although I do not have a Laser in house, my associates and colleagues have, and can apply the above coating to about 300 to 500 micron with little risk of cracking.

The geometry of the part, the substrate, etc will make it almost impossible to get a WC or Hard NiCSiB coating using spray, fuse. If you have a very large oven, with inert gas you might try oven brazing of the WC-NiCrBSi Alloys, but still consider really challenging.

Questions:

1. What is the normal depth of the wear
2. How fast does the wear occur, days?
3. How large, is the part, size, weight?
4. Who is the OEM of the part, country of origin?

Hi,
Problems you are facing are entirely due to your selecting high dilution welding process, like TIG & MIG instead of originally used PTA process.In TIG & MIG you can land in a dilution of 30% max, allowing 30% of iron getting in your first layer, around 9% in second layer and 2.7% in 3rd layer.Your coating thickness limitatation would allow you to restrict single layer deposition, thus your coated surface chemistry would be far off from that of Delloro 40 thus leading to hardness, wear and galling problem.
If you are using PTA dilution can be restricted in first layer within 3%, and the low HAZ combined with low dilution would lead to less tendency to crack with preheat and slow cooling.
In addition to Dell 40, there are several Co- & Ni-based stellites/stellite substitutes available including Ni-Cr-B-Si-C + WC (as suggested in last letter) & HSS +TiC, all of which have been used successfully for this type of application by PTA process using powder materials.Laser is a costly alternative to PTA. Further info you can get in my books.
ram chattopadhyay