[TheForge] 4140 vs 4340

[Grant Neil Sarver]

Not so much annealling that is a concern, but hardening. Even with pre-heating
the area of the 4140 ajacent to the weld has been elevated to over critical
temperature and rapidly cooled. I don't like the term "post-heat" so much as
temper. Most people understand that tempering is done AFTER the part has cooled
to room temperature. Post-heating seems to infer just keeping it hot after
welding - not the sae thing at all. I weld heat treated alloys all the the time
even H-13 and A-2 etc. I heat to just below the temperature the piece was drawn
at, allow to cool to room temperature and then re-temper at the the original draw
temperature. In the case of H-13 this means about 1000F pre-heat and draw. I have
very good sucess with 11018 although 7018 works O.K. too.

Commercially supplied 4140 "HT" is generally around 32 Rc. A little soft for
hammer dies, but not bad.

grant

Dave Brown wrote:

> At 22:54 04/04/02 +0930, you wrote:
> >Well I just bought some 4140 after reading this discussion, its for my air
> >hammer dies. The bar I bought is 38mm square and Im just going to grind it a
> >little then weld it to the die plate, this bar is already heat treated and
> >tempered by manufacturer, my question is welding it, does this stuff weld
> >ok? Id like to keep the factory heat treatment intact so am not sure about
> >preheat etc, I was going to use 7018 rod.
>
> Shannell,
>
> Over on the Principal Metals site under Alloy Steels and then to 4140, it
> says the following about welding:
> "Weldable by all of the conventional methods. Note that welding with the
> alloy in the heat treated condition will affect the mechanical properties
> and a post weld heat treatment may be needed."
>
> For annealing it says:
> "Annealing is done at 1600 F followed by slow furnace cooling."
>
> As for hardening, it says:
> Hardens by cold working, or heating and quenching.
>
> So, not being afraid to hazard a guess:  At 38mm (1.5") I think you have
> enough mass that you won't anneal anything far from the welding line.  Any
> moderate annealing that might take place will probably work harden
> relatively quickly.  But the working faces of your 4140 will probably stay
> just fine.
>
> Note:  This is an educated guess on my part.  I could be just as wrong as
> right.  But I'm sure someone else will chime in if I'm all wet.  Right Grant?
>
> Dave Brown
>
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