Partially Nitriding a Weldment - 4140 Weld Question

[mewhg]

I was given a drawing from our home office in the UK for a lever type prying tool that is to be a weldment fabricated out of S275 steel (A36 equiv) and a 7/8 inch diameter EN19A pin (4140).

The drawing shows the 4140 annealed pin welded to the A36 piece so it sticks out the side as a cantilever. The pin will be loaded in bending and needs to have abrasion resistance as it will rub with considerable pressure on another component that it is designed to pry on.

There is a dotted box around the pin area where the designer wants this section Nitrided .008-.012" deep. He says the 4140 will take the nitriding well but the weld and A36 piece will not so the pin will come out with a hard skin that will be abrasion resistant yet the weld area will be ductile and not subject to cracking.

I have a couple of concerns about this approach. One is that welding the pin will require a pre-heat to 400 F and a post weld slow cool period and possibly a post weld stress relieve. This will add cost to the tool and I am afraid that the mom-and-pop type shops we use for this work will not do the pre-heat and we will wind up with cracked welds.

The other is that the nitride process cannot be cheap and this will add even more cost to the tool.

I think eventually the entire tool will be an investment casting out of 4320 that will be case hardened but for right now I have to work with the weldment and pin.

I have some ideas such as welding a Stressproof or Fatigueproof pin without heat treat and forgoing the Nitride step but I don't think Stressproof or Fatigueproof is weldable. I am also considering a pin from e.t.d. 150 that is welded using pre-heat but still forgoing Nitriding.

Any suggestions or comments would be greatly apprecisted.

Bill

 

[swall]

I would be concerned about using a nitrided pin or a carburized pin for this situation. My interpretation of what you have told us is that the pin will see a lot of contact stress in use--this would tend to crack and chip a nitride case and possibly a carburized case as well. What I would consider would be to make the pin out of an air hardening tool steel, weld it to the lever (using a pre-heat, of course) and then air blast cool from welding while the pin is still orange/red hot. Then give the whole thing a temper at around 700-750F.

 

[mewhg]

One potential solution: Our heat treater suggested partially carburizing the pin BEFORE welding. About 2/3 of the pin would be carburized and the remainder treated to prevent carbon absorption during the cauburizing process. Pin would be quenched and tempered to 56 RC on the caurburized surface. This way we could be welding on the non-hardened area of the pin. 8620 would be a good candidate for this approach I believe.

Bill

 

[swall]

The partially carburized pin idea has some potential. It is very easy to mask off one end of the pin with a steel cap to keep the atmosphere from carburizing one end. Will the pin be butt welded to the lever or will it be inserted into a hole in the lever and then welded? One company I worked for used to make pivot pins for tractors by inserting an induction hardened pin into a hole in a lever and then mig welding on the backside.Hardly any heat got to the wear surfaces of the pin.In my previous post, I neglected to mention that unless the welding process was oxy-acetylene, the air hardening tool steel pin would likely not get hot enough for a direct air quench after welding. The pin would have to be reheated after welding, either with a torch or in a furnace and then air blast cooled.

 

[mewhg]

Swall,

The pin will be set into a "notch" and welded around the sides. I like your idea of the air hardening pin. I wonder how ductile the weld would be to that pin? I would think the induction hardened pin would crack at the weld but perhaps if it was a tight or drive fit into the lever that backside weld would see little stress.

thanks

 

[unclesyd]

You might want to look at some of the wear and abrasion resistant steels that are weldable. Most can be flame hardened. One we have used extensively is 'Astralloy V'. I would definitely contact them for information.

http://www.astralloy.com/products/

 

[mewhg]

That Astralloy looks like an interesting product. I can't tell though if it's weldable though. I have a call into Astralloy.

thanks Unclesyd