Welding 4140 or 8620 to A36 or A572 Grade 50 plate

[apMechE]

I am looking at welding 4140 or 8620 annealed, not prehardened, to mild steel. From what I've read, 8620 would be the better choice due to its lower amount of carbon. All I find is that preheat is recommended for the prehardened forms... is that needed for the non-prehardened materials? The size of the shaft will be 6'' being welded to .5'' to 1'' thick material.

 

[weldstan]

Lower preheating temperature can be used for the 8620. The weld to the 6" diameter shaft should be post weld heat treated. Successful welds can be made without PWHT using low hydrogen electrodes like E7018 H4. I assume that you will be fillet welding the the shaft to the plate or will the weld be full penetration through the plate thickness?

 

[weldstan]

If this is a rotating part, PWHT!!!

 

[apMechE]

Yes, fillet welding to the plate. It only pivots or turns to fold up the machine for transport purposes. The shaft is greatly over sized and not as concerned with wear due to the low frequency of spinning. Could the post weld HT be done simply with a torch?

 

[weldstan]

The plate will then be removed after transport? If so, I would not PWHT, espeacially with a torch. I might give it a dehydrogenation treatment though.
Why not mechanically connect for transport?

 

[apMechE]

No, I was mentioning why it probably wouldn't need to be heat treated or hardened there after... the pin always stays in the joint and only rotates when putting the machine in transport position. The pin will always be welded to the plates and always slipped in a bronze sleeved joint. So 8620 HR annealed should be preheated prior fillet welding and post weld heat treated?

 

[weldstan]

Assuming that the "pin" is the shaft and is lightly loaded, the 8620 to carbon steel weld need not be subject to PWHT. A 300 F preheat is recommended with preheat comncemntrated on the on the 8620 pin. Weld with E7018-H4.

 

[apMechE]

Thanks... I guess backing up a bit, if hardness and wear isn't an issue, but just want something relatively strong to weld to using typical wire... what low carbon grade would you select?

 

[weldstan]

I have given you the welding filler metal with the SMAW "stick" process. Do you intend to use another welding process? If so, which process will yuo use?

 

[apMechE]

Preferably weld with a MIG and wire. So recommended gas mixture and wire.

 

[tbuelna]

As others noted you can weld 8620 round bar to A36 mild steel plate if you use the proper filler wire, preheat, etc. But it would not be practical to heat treat the 8620 material after welding.

Why don't you consider designing your part to use a bolted connection between the A36 plate and the 8620 shaft? This way you can machine the shaft from heat treated 8620 material.