[CnC_BoY]

I'm wondering what is the practical difference between a 1018 and a HT alloy 4140 ACME rod.. Wich would be better?

[lakeside]

if you are going to kept the rod in an area were you could have moister or high humidity you may want the black oxide the stainless would be over kill unless it used for a food or medical application I think the b7 is for black.0007(but I'm not sure) 4140 is a lot harder nickel

[DareBee]

WOAH Lakeside -
That is quite a tangent, there was no mention of coatings or SS in the question, It must be the Sam Adams goggles ;-)

CNC Boy
1018 is standard cold rolled steel it is soft. Usually rated in Rockwell B or Brinell hardness scale. I once threw a piece in the hardness tester and I believe it came up -20 Rc on the other hand 4140 HT is an alloy steel and is 28 Rc. This is a huge difference between the two.
The alloy steel will resist corrosion better than CRS.
CRS machines easier.
CRS is less expensive.
To answer which would be better - better for what?
Generally the alloy steel is better but depends on what it is being used for and whether you can justify the difference in cost.

[CnC_BoY]

thanks guys for the fast and precise answer