Boring Bar stiffness

No,stiffness will be the same. The modulus of elasticity is pretty much the same for all grades and all heat treat levels of steel. What will change is the amount of flex you can have before the bar will take a permanent set.
CAMCOMPCO wrote:

Heat treating will have no effect on stiffness. The modulus of elasticity (Young's Modulus) of mild steel is roughly the same as that of tool steel, and soft tool steel has the same stiffness as hardened tool steel. To get more stiffness you would need to go to another material -- tungsten carbide or some tungsten alloy, for the most part.
That's a good question, though, and one that is assumed incorrectly by many new metalworkers.
-- Ed Huntress

In a recent post on this question, a couple of addiitonal comments were made:
Solid tungsten carbide is much better than carbon steel, but such boring bars are very expensive and/or hard to make.
4140 is a steel that is stiffer than carbon tool steel, and, considered by one poster, to be the minumum acceptable boring bar material.
From the above information and from that already posted on the subject, I decided to use 4140 (because I can get it) and go for the largest diameter I can possibly get into the hole I want to bore.
Pete Stanaitis ---------------------
CAMCOMPCO wrote:

When you say "tool steel", do you mean ALL steels that anyone would consider "tool steels"? Or do you mean carbon tool steels?
Pete Stanaitis ----------------------
Ed Huntress wrote:

4140 is NOT stiffer than mild steel where stiffer is defined as force divided by deflection. With proper heat treating you can make it 5 or 6 times STRONGER. This allows you to put some reasonable force on it without bending it.
spaco wrote:

Pete, 4140 is a good tool material but it isn't any stiffer than mild steel. It *will* take much more load before taking a permanent bend, but it won't do anything for you in terms of stiffness.
-- Ed Huntress

All steels. As someone else posted, they all fall in a very narrow range of stiffness, even the higher alloy steels, and the differences are so slight that you wouldn't notice any difference in practice.
This whole thing is a common misconception, Pete. Making steel harder or stronger doesn't make it stiffer, until you reach the yield limit of the particular alloy and it takes a permanent bend. Up to that bending point, though, all grades of steel are pretty much the same.
FWIW, stainless is somewhat *less* stiff than other steels. I think the difference is around 10%. Stainless has a lot of alloying elements in it (chromium and nickel, mostly), enough to reduce its stiffness by a noticeable amount.
-- Ed Huntress

Nope. Just make your tool as robust and short as possible.