Spray Welding on Press-Fit Seats

By jml7 Date 03-05-2002 14:02

Hello,

I just posted this in the general section, and then discovered this section, where the question is more appropriate.

I'm wondering if building up a press-fit seat is an appropriate application for spray welding. I don't know much about it, but based on what I've seen I believe the cold process is preferable to not damage the 4140 annealed stock of the shaft. The application is a narrow gauge railroad wheel, where the diameter of the press-fit seat is 4" and the interference is supposed to be 0.004" but ended up as 0.002". If spray welding is acceptable here I could save the railroad museum several hundred dollars for buying a new axle. If this is structurally acceptable, would the surface be machinable (I'm assuming that more than 0.001" would be added and it would have to be machined down again)? There is a local shop with spray welding capability so I will inquire with them as well.

Thanks in advance,

Jason

By chall

Date 03-05-2002 17:06

It may be more appropriate to electroplate the surface. Someone may be able to shed more light, but I believe you could have the shaft successfully chrome or nickel plated to achieve your 0.004" interference fit. Charles Hall

By Niekie3 (***)

Date 03-05-2002 17:31

When you say "spray welding", I assume you are referring to what we call in South Africa "metal spraying".

You can certainly metal spray the shaft and then machine it back down again. You are talking about a "cold" spray technique. Theoretically all the metal spray processes are "cold" in that the moulten metal droplets impinge on a nominally "cold" surface and due to their velocity would tend to form a mechanical bond to the substrate. Obviously the surface being impinged will also heat up some, but I do not think that it will have any significant effect on your material's mechanical properties.

My experience with metal spraying is that the lower velocity processes do not result in such a good bond. As such, High Velocity Oxy Fuel (HVOF) metal spraying is the best.

If you want to expand that shaft by the small amount that you mentioned, I may have a better idea. More than likely, your shaft has quite a lot of retained austenite in it. (Depending on the heat treatment it has seen.) If you again let the shaft go through a tempering treatment, you will more than likely find that it has expanded because the retained austenite has transformed to ferrite or martensite. Both of these structures have a lower density than austenite. Obviously you will have to again perform final machining on all your critical dimensions.

Hope this helps

Regards

Niekie Jooste

By RonG

Date 03-05-2002 20:13

I think he means HVOF (High Velocity Oxy Fuel) (Jet-Coat).

Not really welding, the particals are propelled at ultra speed all most like explosive welding, No high temp heat is applied to the shaft and will not alter the properties of the shaft material.

It will do the job very well, We use if for very critical applications, difference being we, Jet-Coat the bore of the wheel (maybe not practical in this application) but we do the wheels for reasons not related to material properties.

By MBSims

Date 03-05-2002 23:17

If you use thermal spray, you will have to undercut the shaft slightly to prepare the surface or grit blast. You can spray the surface without undercutting, but you may wind up peeling the sprayed surface when you try to do the press fit. You may want to machine a mockup and try it before you do it on the actual part.

Marty