Hi Phatkat
Alex if you still want to make a set of rods i can get you the steel 8ins long by 75mm wide and 30mm thick 4340 it comes heat treated 30RC and they will cut it depending which way you want the grain to to run $65 each.
Graham Russell

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Jesus that's cheap as chips! I actually did some ringing around this arvo and managed to get a much better price than previously quoted too. I couldn't get hold of 4340 cut plate (I used to get 4340 from Interlloy, and Bohler... bohler shut their sydney office and interlloy only stock round bar now) but while I was doing the rounds I managed to get 4140 plate in the size I needed for 4 rods for only $96.30! I was blown away, seems I was getting pretty baddly middle-manned in the past. So I could easily produce rods for the same price as the Ebay ones I posted last night, wouldn't be making a whole lot of margin on them, but it's just possible.

<Edit> and that's for billet rods... if I were to make forging dies and pay to have them forged, I doubt I could do it at the same price anymore by the time their machined and supplied with fasteners. I don't know that there'd be enough call to make it worth while making dies. Who knows. Oh, and 4340 usually comes as supplied at between 269 and 331HB which is between 28 and 32HRc (ish).

Is 4140 the right material though, definitely cheaper but my limited understanding tells me that it is harder to properly heat treat? And has less tensile strength?

Yeah it's fine, 4340 is slightly better, it has a slightly higher tensile rating as I mentioned earlier, as well as a better Yeild, and its other mechanical properties are generally a little better suited, but 4140 is still fine.

You'll notice for example the UTS ratings "range" from 850 - 1000MPa for 4140, and ""Range"" for 4340 is from 930-1080MPa. Those numbers are averages based on random sampling.... So the numerical average UTS for 4140 is 920MPa and the numerical average for 4340 is 1005MPa.... you're looking at bugger all difference there. And often you might pick up 4140 that sits above the bottom of the range for 4340 and at the same time 4340 thats lower than the upper range for 4140. Does that make sense? And as far as annealed hardness goes, 4340 is slightly higher.... you're looking at 248 - 302HB for 4340 and 223-277HB for 4140 (if you want that in the Rockwell C scale... 20HRc to 28.8HRc for 4140.... v's... 24.2 - 32.1 for 4340)... That said most places supply 4340 as hardened and tempered... but only barely you can still machine it very easily. Interlloy quote 28-36HRc for their 4340.

As far as hardenability goes they're much the same. Neither is any easier or harder to harden.. They're both water or oil hardening (water if you want it harder though).. both can be nitrided, though 4340 needs longer immersion because of the high nickel content.

But there isn't a huuuuuuuge difference between them. Certainly not enough to worry about.

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