Originally Posted by
joeavaerage
Hi,
peteeng is right, any steel will do.
You could go to the trouble and expense of using hardened and tempered 4340, but it would be exactly the same stiffness as a the same part made in mild steel. Sure
the 4340 would be way stronger but we never come within a fraction of 1% of the strength....its all about stiffness.
Mild steel is obviously way cheaper than 4340 but more importantly can be welded without problem. When steel is welded it really needs Thermal Stress Relief (TSR)post welding, otherwise it will
move months and years later, especially if its machined. Vibratory Stress Relief is a much cheaper alternative and works OK in many situations, but TSR is still
the gold standard. TSR is not cheap!
May I suggest find out whom the heat treaters are in you area, and especially what size oven they have.
As an example when I was building my mill the only local heat treater's oven was 700mm long, ergo my axis bed castings are 695mm long. It cost $6NZD/kg....and that was prior to COVID!
You NEED to find out what capability you have access to and what its going to cost before you make any plans or you could face an unanticipated budget hurdle which will screw your
project.
My machine is steel and cast iron......both inclined to rust, but aside from a little surface discolouring it does not, I'm guessing the water soluble oil in the coolant, and I use flood coolant
a lot, inhibits the rust. All-in-all I'd say rust is a non issue, having the means to make full penetration welds and subsequent TSR are what you need to concentrate on.
Craig