Phat Kat wrote:
I suspect, (and I says suspect, I don't know for sure off the top of my head but will look into it and let you know) that as is the case with the idea that its a "rust proofing", that yes, there is a measurable difference in tensile strength, but its not as dramatic as the improvement in surface hardness... It might only be a few Mpa... I don't know I'll have a look and come back.
Alrighty I went through a few of the books at work that go into nitriding and none of them provide any specific data or numbers on any improvement in tensile strength. Infact, older books make
no references to increase in fatigue life... Infact, no
publication makes reference to increase fatigue life, though I have heard others mention it too.
They'll all tell you that you can get between 800 HV and 1200 (and some even 1500). But no other data.
Interestingly the Bohler Heat Treatment book states in regards to Nitriding:
"It has for some time been a practice to nitride tool steels intended for special applications in order to increase their wear life. Nitriding increases the surface hardness which improves the wear resistance of the tools.... It should be borne in mind, however, that a nitrided layer is exceedingly brittle, so that it will prove only advantageous for jobs where no shock is involved"As hardness goes up, almost all other mechanical properties go down (Sheer strenght, tensile strength, ductility...). About the only one that "increases" is brittlness.
EN40B, is an alloy steel. Its alloyed with Manganese, Chromium and Molybdenum to increase its shock resistance. The appeal with alloy steels is that they can be case hardened and remain tough. However, case harden it too deep, and it will still become brittle.....
(For eveyone else reading, Nitriding, Tufftriding and all other forms of Nitrocarburizing are all "case hardening" processes, they create a hard layer or skin around a softer core.)
..... back to where I was going with mechanical properties..... By only case hardning it to a shallow depth, you're allowing the core to remain tough, meaning that you now effectively have the best of both worlds, a tough but wear resistant materal. The deeper you harden it, the less tough it and the more brittle it becomes.
<edit 2> I would infact go so far as to say, nitriding seems to do nothing for torsional fatigue.
Matt wrote:
as a side note (because someone mentioned corrosion resistance) we`ve left a pile of cranks out in the weather in the paddock (Literally for years & years & years) & the well hardened ones don`t have rust on them , except where the wear on the journals have broken the hardening crust, or where they have been chipped/banged etc... i should take a photo of them all as it really is THAT obvious
Matt, I don't doubt that for a second. EN40B and most steels that are used for this app have a high(ish) chromium content which gives it a fantastic resistance to corrosion. Nowww.... if there are any bright spark smart arses reading this, they'll pull me up right now and say "OI! YOU! you can't nitride steel that does have at least
some Cr in it! So whats with this crap about rusty nitrided Dies?"..... and you'd be right, but if it has an appropriate amount of other elements such as Moly, you can get away with having less Cr in it. I was half wrong, it wasn't dies that I had seen it was pillars for high volume press tools. I may even have some in the shed
... when it cools down I'll go have a look and take some snaps if I can.
<edit> I'm not saying nitriding does nothing for rust resistance.... just that its not to be confused with rust proofing... nitrided surfaces hold up 5 times better than non treated ones
back to topic now...
but you could never make a crank out of tool steel could we. Sure you could, if you had $5k for the billet, then machining and design on top to do a V8 crank.
This is epic